Data Management System

Abstract

To provide a data management system that can read information stored in a ferroelectric recording medium at high speed with high recording density and can have high expandability.SOLUTION: A data management system according to the present invention is for storing data on a high-speed communication network, including: at least one external storage device having a data management unit including an internal storage device and a ferroelectric recording medium, in which the data management unit stores data on the high-speed communication network in the external storage device, and stores metadata used for reading the stored data in the internal storage device.SELECTED DRAWING: Figure 30

Description

[Technical Field]

[0001] This invention relates to a data management system. [Background technology]

[0002] Ferroelectric recording media, such as hard disk drives and various recording media, are capable of repeatedly recording information by changing the polarization of a ferroelectric material. Ferroelectric recording media are ultra-high-density recording media that achieve high recording capacity by utilizing the spontaneous polarization of the ferroelectric material generated by the application of an external electric field, while incorporating a ferroelectric layer. Because ferroelectric recording media can achieve high capacity, the development of ferroelectric memory devices equipped with ferroelectric recording media is being considered.

[0003] Patent Document 1 discloses a dielectric recording and regeneration device that applies an alternating electric field to a dielectric material constituting a dielectric recording medium and regenerates information recorded on the dielectric recording medium due to the nonlinear dielectric properties of the dielectric material.

[0004] Patent Document 2 discloses an information storage device in which a recording medium having an electrode layer and a ferroelectric layer provided on an insulating substrate is mounted on a spindle, and a head slider attached to a head assembly is held in a state where it is floating a predetermined amount above the surface of the recording medium, thereby recording and reproducing information on the recording medium. It also discloses the use of a semiconductor sensor for reading the information. Furthermore, as materials for the ferroelectric layer, perovskite materials such as lead titanate, barium titanate, strontium titanate, and strontium barium titanate, as well as lithium tantalate and lithium niobate are disclosed.

[0005] Patent Document 3 discloses an information recording and reading head that has an annular guard surrounding the vicinity of the tip of the probe to prevent dust from contacting and colliding with the probe, and uses a piezoelectric material as a means of movement to move the probe in a direction substantially perpendicular to the recording surface.

[0006] Patent Document 4 discloses a recording / playback head comprising a projection erected on a support member such that its tip faces a dielectric recording medium, the projection having a ridge at its tip, and the projection being formed using a mold formed by anisotropic etching.

[0007] Patent document 5 discloses a memory device comprising a dielectric laminate formed by stacking a ferroelectric material and a paraelectric material.

[0008] Patent Document 6 discloses a dielectric recording and playback head that applies a voltage corresponding to the data between the probe and each bias electrode, thereby forming a polarization domain between the probe and each bias electrode having a polarization direction parallel to the surface of the dielectric recording medium, and recording four types of data in a predetermined location on the dielectric recording medium to record multi-level information. [Prior art documents] [Patent Documents]

[0009] [Patent Document 1] Japanese Patent Publication No. 2004-14016 [Patent Document 2] Japanese Patent Publication No. 2007-272961 [Patent Document 3] Japanese Patent Publication No. 2004-171622 [Patent Document 4] Japanese Patent Publication No. 2005-158117 [Patent Document 5] Japanese Patent Application Publication No. 9-307073 [Patent Document 6] Japanese Patent Publication No. 2004-178750 [Overview of the Initiative] [Problems that the invention aims to solve]

[0010] Storage services utilizing ultra-high-speed communication technology are evolving. In storage services, various devices such as computers and communication terminals are connected to storage via the Internet, and various information is shared. Generally, optical fibers are used for these communications, and the communication speed exceeds 10 Gbps (gigabits per second).

[0011] Also, in environments where it is difficult to lay optical fibers or in mobile bodies, there is a shift to wireless and mobile communications of 10 Gbps or more, so-called 5G, and further, application to 6G of 100 Gbps or more is also being considered.

[0012] In these storage services, as storage, mainly recording media such as HDDs and flash memories (SSDs) are used. The transfer speed in HDDs is generally about 1 Gbps, and the transfer speed in SSDs is generally about 3 Gbps. Therefore, it is difficult for a single storage to meet the strict input / output requirements of ultra-high-speed communication. Also, the capacity required for storage is constantly increasing.

[0013] Also, with global warming becoming a major social issue, there is concern about the increase in power consumption accompanying the expansion of storage service use. Therefore, there is a demand for an efficient storage with low energy consumption per unit storage capacity and capable of reducing environmental impact.

[0014] In a ferroelectric memory device, by stacking a large number of high-recording-density ferroelectric recording media and rotating them at high speed, high transfer speed and high storage capacity can be achieved.

[0015] However, no data management system has been proposed that can utilize the characteristics of such a ferroelectric memory device.

[0016] An aspect of the present invention aims to provide a data management system that can read information stored in a ferroelectric recording medium at high speed with high recording density and has high scalability.

Means for Solving the Problems

[0017] One aspect of the data management system according to the present invention is a data management system for storing data on a high-speed communication network, comprising a data management unit including an internal storage device and at least one external storage device having a ferroelectric recording medium, wherein the data management unit stores the data on the high-speed communication network in the external storage device and stores metadata used for reading the stored data in the internal storage device. [Effects of the Invention]

[0018] One aspect of the data management system according to the present invention is capable of high recording density, high-speed reading of information stored on a ferroelectric recording medium, and high expandability. [Brief explanation of the drawing]

[0019] [Figure 1] This is a partial cross-sectional view showing the structure of a ferroelectric recording medium. [Figure 2] This is a perspective view of a ferroelectric recording medium. [Figure 3] This is a cross-sectional view showing an example of a ferroelectric recording medium inserted into the spindle shaft of an information recording device. [Figure 4] This figure shows an example of the relationship between the radius of curvature of a conductive probe and the electric field strength. [Figure 5] This is a schematic diagram illustrating the process by which polarization reversal in the ferroelectric layer spreads from the central part directly beneath the conductive probe to the peripheral part. [Figure 6] This diagram shows the data region and servo information region of a ferroelectric recording medium. [Figure 7] This is a cross-sectional view showing an example of a conventional ferroelectric recording medium inserted into the spindle shaft of an information recording device. [Figure 8] This is a perspective view showing a ferroelectric memory device. [Figure 9] This is a perspective view showing the configuration of the head assembly as seen from below. [Figure 10] This is a cross-sectional view showing an example of the configuration of a probe slider. [Figure 11] This is a cross-sectional view showing another example of the probe slider configuration. [Figure 12] This is a partially enlarged cross-sectional view of Figure 11. [Figure 13] This is a partially enlarged cross-sectional view of Figure 11 from a different direction. [Figure 14] This is a magnified view of a section of Figure 11 as seen from below. [Figure 15] This is a cross-sectional view showing the configuration of a conductive probe. [Figure 16] This figure shows an example of a method for manufacturing a conductive probe. [Figure 17] This figure shows an example of another method for manufacturing conductive probes. [Figure 18] This is a perspective view showing other configurations of the conductive probe. [Figure 19] This is a perspective view showing other configurations of the conductive probe. [Figure 20] This is a cross-sectional view showing other components of the conductive probe. [Figure 21] This is a cross-sectional view showing an example of the configuration of a probe slider. [Figure 22] This is an explanatory diagram showing an example of another method for manufacturing conductive probes. [Figure 23] This is a cross-sectional view showing other components of the conductive probe. [Figure 24] This is an explanatory diagram showing an example of another method for manufacturing conductive probes. [Figure 25] This is an explanatory diagram showing the displacement of the conductive probe. [Figure 26] This is a cross-sectional view showing the configuration of the ferroelectric recording medium drive unit. [Figure 27] This is a diagram showing voltage waveforms. [Figure 28] This is a cross-sectional view showing an example of another configuration of the probe slider. [Figure 29] This is an explanatory diagram showing an example of the configuration of a conventional magnetic head slider. [Figure 30]This figure shows the configuration of the data management system according to this embodiment. [Figure 31] This figure shows an example of the configuration of an external storage device. [Figure 32] This is an explanatory diagram showing an example of the data connection relationships stored on a ferroelectric recording medium. [Modes for carrying out the invention]

[0020] Embodiments of the present invention will be described in detail below. For ease of understanding, the same reference numerals are used for identical components in each drawing, and redundant explanations are omitted. Furthermore, the scale of each component in the drawings may differ from the actual scale. In this specification, the "~" indicating a numerical range means that the values ​​before and after it are included as the lower and upper limits, respectively, unless otherwise specified.

[0021] In describing the data management system according to an embodiment of the present invention, a ferroelectric recording medium and a ferroelectric storage device provided in the data management system will be described.

[0022] <Ferroelectric recording media> A ferroelectric recording medium will now be described. Figure 1 is a partial cross-sectional view showing the configuration of a ferroelectric recording medium, Figure 2 is a perspective view of the ferroelectric recording medium, and Figure 3 is a cross-sectional view showing an example of the ferroelectric recording medium being inserted into the spindle shaft of an information recording device. As shown in Figure 1, the ferroelectric recording medium 10 comprises a substrate 11, an electrode layer 12, a ferroelectric recording layer 13, a protective layer 14, and a lubricant layer 15, with the electrode layer 12, ferroelectric recording layer 13, protective layer 14, and lubricant layer 15 stacked on the substrate 11 in this order. A conductive probe 17, attached to the side of a probe slider 16 facing the ferroelectric recording medium 10, is positioned near the surface (main surface) 101 of the ferroelectric recording medium 10, and the conductive probe 17 performs the recording (writing) and playback (reading) of information to the ferroelectric layer 131 contained in the ferroelectric recording layer 13.

[0023] As shown in Figure 2, the ferroelectric recording medium 10 is formed in a disc shape with an opening 10a in the center of its main surface. As shown in Figure 3, the spindle shaft 18 of the ferroelectric recording medium drive unit, which rotates the ferroelectric recording medium 10, is inserted into the opening 10a of the ferroelectric recording medium 10, thereby fixing the ferroelectric recording medium 10 to the spindle shaft 18.

[0024] As shown in Figure 3, the ferroelectric recording medium 10 has a ferroelectric recording layer 13 (ferroelectric layer 131, paraelectric layer 132) on both the upper and lower surfaces of the substrate 11, allowing information to be recorded on both surfaces (double-sided recording). However, it is also possible to have a ferroelectric recording layer 13 on only one surface, either the upper or lower surface of the substrate 11, allowing information to be recorded on only one surface (single-sided recording).

[0025] [substrate] The substrate 11 has the function of holding the electrode layer 12, the ferroelectric recording layer 13, the protective layer 14, and the lubricant layer 15.

[0026] The electrical properties of the substrate 11 are not particularly limited and may be either an insulator or a conductor.

[0027] Examples of insulators that can be used include glass, silicon, magnesium oxide (MgO), and sapphire.

[0028] As conductors, metal materials such as aluminum and its alloys, chromium, platinum, gold, silver, and iron, and oxides such as indium oxide (InO2) can be used. In addition, silicon to which conductivity has been imparted by doping can be used.

[0029] If the substrate 11 is a conductor, the substrate 11 can function as an electrode layer 12. Therefore, if an electrode layer 12 is not placed on the substrate 11, it is preferable that the material constituting the substrate 11 is a conductor.

[0030] The substrate 11 is preferably characterized by minimal waviness, high smoothness, and low fluttering when rotated at high speed.

[0031] The thickness of the substrate 11 is not particularly limited as long as it achieves its purpose, and for example, 100 μm to 1 mm is preferred.

[0032] When selecting materials to make up the substrate 11, it is preferable to match the lattice constant of the material making up the ferroelectric layer 131 with the lattice constant of the material making up the substrate 11 within a range of ±10%, taking into consideration lattice matching with the ferroelectric layer 131. This increases the crystallinity of the ferroelectric layer 131 and increases the recording density of the ferroelectric recording medium.

[0033] Furthermore, it is preferable that the crystal system of the material constituting the substrate 11 and the crystal system of the material constituting the ferroelectric layer 131 be the same, and it is also preferable that the crystal type of the material constituting the substrate 11 and the crystal type of the material constituting the ferroelectric layer 131 be the same. It is acceptable for either the crystal system or the crystal type to be the same, but most preferably both the crystal system and the crystal type are the same. This increases the crystallinity of the ferroelectric layer 131, and increases the recording density of the ferroelectric recording medium.

[0034] Here, crystal systems include triclinic, monoclinic, orthorhombic (orthorhombic), tetragonal, hexagonal, and cubic systems, depending on the classification that defines the crystal symmetry. It is preferable to use the same material for both the substrate 11 and the ferroelectric layer 131.

[0035] Furthermore, crystal structures can be classified based on the close-packed structure of the crystal, including simple cubic lattice structure, face-centered cubic lattice structure, body-centered cubic lattice structure, hexagonal close-packed structure, diamond structure, white tin type structure, graphite structure, A15 type structure, sodium chloride type structure, cesium chloride type structure, zincblende type structure, wurtzite type structure, nickel arsenide type structure, lead monoxide type structure, fluorite type structure, pyrite type structure, cuprite type structure, rutile type structure, cadmium iodide type structure, bismuth fluoride type structure, rhenium oxide type structure, Ni4Mo type structure, Al4Ba type structure, calcium boride type structure, CaCu5 type structure, corundum type structure, perovskite type structure, ulmanite type structure, spinel type structure, silver phosphate type structure, CuAuI type structure, K4 crystal structure, etc. It is preferable that the crystal structure of the material constituting the substrate 11 and the material constituting the ferroelectric layer 131 be the same.

[0036] For example, if hafnium oxide is selected for the ferroelectric layer 131, the hafnium oxide has an orthorhombic, fluorite-type structure, with lattice constants of 5.30 Å for the a-axis, 5.11 Å for the b-axis, and 5.10 Å for the c-axis. However, it is known that the crystal system of hafnium oxide changes from the stable monoclinic phase to the tetragonal phase and then to the cubic phase. Since these crystal systems do not exhibit ferroelectric properties, it is important to change the cubic hafnium oxide to an orthorhombic phase through heat treatment or other means to make it a ferroelectric material.

[0037] Therefore, the material constituting the substrate 11 is preferably 4.6 Å to 5.8 Å in lattice constant so as to be lattice-matched within ±10%, and its crystal system is preferably one of orthorhombic, monoclinic, tetragonal, or cubic, more preferably orthorhombic or cubic. Furthermore, it is preferable to use a material with a fluorite-type crystal structure. Examples of such materials include silicon, Ge, Pd, and CeO2. Silicon has a lattice constant of 5.4 Å for the a-axis, b-axis, and c-axis, and is cubic. Ge has a lattice constant of 5.7 Å, and is cubic. Pd has a lattice constant of 5.0 Å, and is cubic. CeO2 has a lattice constant of 5.4 Å, and is cubic.

[0038] [Electrode layer] The electrode layer 12 can be provided on the substrate 11. The electrode layer 12 can be provided on the underside of the ferroelectric recording layer 13 (opposite side from the conductive probe 17) and function as a counter electrode for the conductive probe 17 that reads and writes information to the ferroelectric recording layer 13.

[0039] The material constituting the electrode layer 12 is preferably the same crystal system and crystal type as the ferroelectric layer 131, or both, considering lattice matching with the ferroelectric layer 131 constituting the ferroelectric recording layer 13. This increases the crystallinity of the ferroelectric layer 131 and increases the recording density of the ferroelectric recording medium.

[0040] Preferably, the lattice constant of the material constituting the electrode layer 12 is lattice-matched with the lattice constant of the material constituting the ferroelectric layer 131 within a range of ±10%. This increases the crystallinity of the ferroelectric layer 131 and increases the recording density of the ferroelectric recording medium.

[0041] The ferroelectric layer 131 is preferably a single-crystal film.

[0042] The material constituting the electrode layer 12 can be appropriately selected according to the material constituting the ferroelectric layer 131. For example, metallic materials such as aluminum, chromium, platinum, gold, silver, and iron, or oxides such as InO2 can be used.

[0043] For example, if hafnium oxide is selected for the ferroelectric layer 131, as described above, the material constituting the electrode layer 12 should have a lattice constant of 4.6 Å to 5.8 Å such that it is lattice-matched within ±10%, and the crystal system should preferably be one of orthorhombic, monoclinic, tetragonal, or cubic, more preferably orthorhombic or cubic. It is also preferable to use a material with a fluorite-type crystal structure. Examples of such materials include Ge and Pd. Ge has a lattice constant of 5.7 Å and is cubic. Pd has a lattice constant of 5.0 Å and is cubic.

[0044] Here, since metallic materials are more resilient to lattice strain than oxides, even if the ferroelectric layer 131 and metallic materials differ in crystal system, crystal type, and lattice constant, the resulting lattice strain can be easily relieved. From this perspective, the effect of improving the crystallinity of the ferroelectric layer 131 is greater in the substrate 11 than in the electrode layer 12.

[0045] The electrode layer 12 can be fabricated by forming a conductive thin film on the substrate 11 using the materials constituting the electrode layer 12 by any formation method such as sputtering or vapor deposition.

[0046] The thickness of the electrode layer 12 is not particularly limited as long as it achieves its purpose, and for example, 10 nm to 500 nm is preferred.

[0047] [Ferroelectric recording layer] The ferroelectric recording layer 13 is provided on the upper surface of the electrode layer 12 and has the function of recording information. The ferroelectric recording layer 13 includes the ferroelectric layer 131 and may also include other layers.

[0048] The ferroelectric layer 131 has the function of recording information. The ferroelectric layer 131 is not particularly limited as long as it is a ferroelectric material exhibiting ferroelectricity, but an oxide ferroelectric material is preferred from the viewpoint of electrical properties.

[0049] Examples of oxide ferroelectrics include lead titanate (PbTiO3), lead zirconate (PbZrO3), barium titanate (BaTiO3), lithium niobate (LiNbO3), lithium tantalate (LiTaO3), and hafnium oxide (HfO2). Among these, hafnium oxide is particularly preferred. Oxide ferroelectrics such as lead titanate, lead zirconate, barium titanate, lithium niobate, and lithium tantalate have a tetragonal perovskite crystal structure, which is complex and requires a high deposition temperature. On the other hand, hafnium oxide has an orthorhombic crystal system fluorite structure, which is a binary system and simpler than the perovskite crystal structure, allowing for deposition at a lower temperature.

[0050] If the ferroelectric layer 131 contains hafnium oxide, it may contain additives or a mixed crystal of hafnium oxide and zirconium dioxide (ZrO2) (Hf x Zr 1-x It is preferable that it contains O2.

[0051] When the ferroelectric layer 131 contains hafnium oxide and additives, examples of additives include silicon (Si), aluminum (Al), gadolinium (Gd), yttrium (Y), lanthanum (La), strontium (Sr), etc. These may be used individually or in combination of two or more.

[0052] The additive content is preferably in the range of 1 atom% to 20 atom%, more preferably in the range of 3 atom% to 17 atom%, and even more preferably in the range of 5 atom% to 15 atom%. If the additive content is within the above preferred range, the film formation temperature during the formation of the ferroelectric layer 131 can be reduced, and the amount of additive used can be reduced.

[0053] The method of adding additives to hafnium oxide is not particularly limited, and any method may be used as appropriate.

[0054] The ferroelectric layer 131 is a mixed crystal of hafnium oxide and zirconium dioxide (Hf x Zr 1-x If O2 is included, Hf x Zr 1-x The value of x in O2 is preferably between 0.3 and 0.6.

[0055] The ferroelectric layer 131 is preferably composed of a single crystal, but may also include an amorphous structure having short-range order. The ferroelectric layer 131 may consist only of an amorphous structure having short-range order, or it may include a single-crystal region. Short-range order refers to the short-range order between atoms constituting the amorphous structure, specifically, the ordered nature of the number of nearest neighbor atoms (the number of closest atoms), the bond distance between atoms, the bond angle between atoms, etc. On the other hand, long-range order refers to the order between atoms located far apart among the atoms constituting the crystal, specifically, the regularity of the number of nearest neighbor atoms, the bond distance, the bond angle, etc., in a range far exceeding the interatomic distance, and the general term for materials having this structure is a single crystal. When the ferroelectric layer 131 includes an amorphous structure having short-range order, polarization reversal due to crystal lattice strain occurs in the region containing the amorphous structure, allowing information to be recorded. Furthermore, since the region containing the amorphous structure does not contain grain boundaries or lattice defects, the wider the region forming the amorphous structure, the wider the recording area of ​​the ferroelectric layer 131 becomes.

[0056] The length of the short-range order in the ferroelectric layer 131 is preferably 2 nm or less. The length of the short-range order is the length of the region having short-range order between atoms, and refers to the length in the vertical and horizontal directions between atoms with respect to the ferroelectric recording medium surface. The length of the short-range order is also called the vertical, horizontal, and height of the short-range order. The bit size of current hard disk drives (HDDs) is approximately 10 nm in length and width on the magnetic recording medium surface, and that region is composed of several magnetic particles. In order for a ferroelectric recording medium to have a higher recording density than a magnetic recording medium, the bit size of the ferroelectric recording medium needs to be smaller than the bit size of the magnetic recording medium. In order for several recording regions to be included within one bit region of the ferroelectric layer 131, the length of the region having short-range order (i.e., corresponding to the magnetic particles contained in the magnetic recording medium) is preferably 2 nm or less.

[0057] Amorphous structures with short-range order of 2 nm or less can be confirmed by X-ray diffraction, electron microscopy, and electron diffraction. Specifically, such structures do not show clear crystals when observed with an electron microscope, and electron diffraction yields a blurred intensity distribution called a halo pattern. Similarly, X-ray diffraction also yields a halo pattern at the locations of crystal planes exhibiting short-range order. When the ferroelectric layer 131 is heated above the deposition temperature (approximately +200°C above the deposition temperature), the halo pattern changes into a signal with a sharp peak. This is because the short-range order present in the ferroelectric layer 131 changes to long-range order upon substrate heating.

[0058] It is preferable to match the lattice constant of the short-range order of the amorphous structure with the lattice constant of the material constituting the substrate 11 within a range of ±10%. This enhances the short-range order of the ferroelectric layer 131 and increases the recording density of the ferroelectric recording medium 10.

[0059] The thickness of the ferroelectric layer 131 is appropriately selected within the range of 1 nm to 1000 nm in total. If the thickness of the ferroelectric layer 131 is within the above range, polarization reversal can be induced in the ferroelectric layer 131, and the voltage required to reverse the polarization of the ferroelectric layer 131 can be suppressed.

[0060] Furthermore, the thickness of the ferroelectric layer 131 is preferably 1 nm to 30 nm in total, more preferably 3 nm to 25 nm, and even more preferably 5 nm to 20 nm, from the viewpoint of causing polarization reversal in the ferroelectric layer 131 while keeping the voltage required for polarization reversal of the ferroelectric layer 131 as small as possible.

[0061] The actual thickness of the ferroelectric layer 131 is preferably determined comprehensively from the following perspectives.

[0062] From the viewpoint of recording density for ferroelectric recording media, if the bit length in the track direction is, for example, 10 nm, the film thickness of the ferroelectric layer 131 is preferably 1 to 5 times the bit length, i.e., 10 nm to 50 nm. If the bit length is 1 nm, the film thickness is preferably 1 nm to 5 nm. This is because, since one bit in the ferroelectric layer 131 is composed of a three-dimensional ferroelectric material, the ratio of its length, width, and height is empirically stable when it is within the range of 1 to 5 times.

[0063] From the perspective of reading sensitivity with a conductive probe, when tunnel current is used for reading, if you want to increase the reading sensitivity, the thickness of the ferroelectric layer should be increased, and if you want to decrease the reading sensitivity, the thickness of the ferroelectric layer 131 should be decreased. That is, since ferroelectrics are insulators and have a large band gap, the thicker the thickness of the ferroelectric layer 131, the greater the energy band barrier, making it more difficult for tunnel current to flow, and thus the sensitivity of the conductive probe that detects this needs to be increased. On the other hand, if the thickness of the ferroelectric layer 131 is reduced, tunnel current flows more easily, and the sensitivity of the conductive probe can be decreased.

[0064] Furthermore, when using interatomic forces for reading, the amount of charge in the ferroelectric layer 131 increases as the film thickness of the ferroelectric layer 131 increases, so the reading sensitivity of the conductive probe that detects this can be reduced. On the other hand, when the film thickness of the ferroelectric layer 131 decreases, the amount of charge in the ferroelectric layer 131 also decreases, so it becomes necessary to increase the sensitivity of the conductive probe.

[0065] From the perspective of leakage current in the ferroelectric layer 131, as the thickness of the ferroelectric layer 131 increases, the amount of charge in the ferroelectric layer 131 also increases, thus reducing the effect of leakage current and lowering the refresh (rewrite) frequency. On the other hand, as the thickness of the ferroelectric layer 131 decreases, the amount of charge also decreases, increasing the effect of leakage current and thus increasing the refresh frequency.

[0066] From the standpoint of the crystallinity of the ferroelectric layer 131, the crystallinity tends to increase as the thickness of the ferroelectric layer 131 increases.

[0067] From the viewpoint of the smoothness of the growth surface of the ferroelectric layer 131, the thinner the film thickness of the ferroelectric layer 131, the smoother the surface tends to be.

[0068] Known methods can be used to form the ferroelectric layer 131, including, for example, sputtering, CVD, sol-gel, and laser ablation. Among these, sputtering and CVD are preferred.

[0069] To improve the crystallinity of the ferroelectric recording layer 13, it is preferable to raise the film deposition temperature to around 500°C. However, when the film deposition temperature exceeds 500°C, the ferroelectric layer 131 tends to become polycrystalline, and the growth surface also tends to become rough. Furthermore, the types of substrates 11 that can be used are also limited. By using sputtering and CVD as film deposition methods, the film deposition temperature can be suppressed, thereby suppressing polycrystallization of the ferroelectric layer 131 and reducing the roughness of the growth surface.

[0070] When using sputtering as the method for forming the ferroelectric layer 131, it is preferable to use high-frequency sputtering and reactive sputtering, which utilize plasma assistance to lower the gas temperature in the deposition region while increasing the electron temperature. When using CVD as the method for forming the ferroelectric layer 131, it is particularly preferable to use plasma CVD, EACVD, and organometallic CVD (MOCVD). This makes it easier to change the cubic hafnium oxide to an orthorhombic crystal system.

[0071] As described above, information is recorded in the ferroelectric layer 131. The principle for recording and retaining information in the ferroelectric layer 131 is as follows: The ferroelectric material constituting the ferroelectric layer 131 has the property that its polarization direction changes when an electric field exceeding its coelectric field is applied. Furthermore, once the polarization direction of a ferroelectric material is changed by the application of an electric field, it has the property of maintaining that polarization direction even after the application of the electric field is stopped (spontaneous polarization). By utilizing these properties, information can be recorded and retained in the ferroelectric layer 131. For example, the overall polarization direction of the ferroelectric layer 131 is pre-aligned to one direction perpendicular to the surface of the ferroelectric recording medium 10. Then, an electric field exceeding the coelectric field is locally applied to the ferroelectric layer 131 in a direction perpendicular to the surface of the ferroelectric recording medium 10. As a result, after the polarization direction of the part to which the electric field was applied is reversed, the reversed polarization direction is maintained even after the application of the electric field is stopped.

[0072] For example, if the information to be recorded is binary digital data consisting of "0" and "1", the bit state "0" is associated with the downward polarization direction, and the bit state "1" is associated with the upward polarization direction. In this case, an electric field only needs to be applied to the ferroelectric layer 131 when recording the bit state "1". In this way, information can be recorded and stored in the ferroelectric layer 131.

[0073] On the other hand, the method for reproducing the information recorded as polarization direction in the ferroelectric layer 131 will be described later.

[0074] The ferroelectric recording layer 13 may be made up of multiple ferroelectric layers 131 stacked together.

[0075] The ferroelectric recording layer 13 preferably has the smallest single recording area (hereinafter sometimes simply referred to as the "recording area") in which multi-level information containing three or more values ​​is recorded (multi-level recording) in the ferroelectric layer 131 by a ferroelectric memory device described later. The multi-level recording in the recording area is regenerated by the ferroelectric memory device. By making the information recorded in the recording area multi-level, the recording density of the ferroelectric layer 131 can be increased.

[0076] Multi-level recording is a method of recording three or more levels of information in the smallest single recording area. For example, in the magnetic recording layer of a magnetic recording medium such as an HDD, the smallest single recording area is composed of two magnetic poles, either north or south. On the other hand, it is preferable that the smallest single recording area of ​​the ferroelectric recording layer 13 is composed of three or more levels.

[0077] Information is recorded in the ferroelectric layer 131 by polarization. For example, suppose the surface side of the ferroelectric layer 131 is spontaneously polarized to positive and the back side to negative. In this case, if a positive electric field of a certain strength or greater is generated at the tip of a conductive probe 17 facing this location, the polarization of this location can be reversed, with the surface side becoming negative and the back side becoming positive.

[0078] A needle-shaped, sharpened conductive electrode is generally used for the conductive probe 17. Figure 4 shows an example of the relationship between the radius of curvature r at the tip of the needle-shaped electrode and the electric field strength E generated in the space at its tip. As shown in Figure 4, the radius of curvature r at the tip of the needle-shaped electrode and the electric field strength E generated in the space at its tip are inversely proportional; the smaller the radius of curvature r, the larger the electric field strength E becomes. The conductive probe 17 is usually conical in shape, and its tip is microscopically spherical. The radius r is smallest at the very tip, and gradually increases towards the periphery. Therefore, the electric field strength E is highest directly below the tip of the conductive probe 17 and decreases towards the periphery. Consequently, when the voltage applied to the conductive probe 17 is gradually increased, the polarization reversal of the ferroelectric layer 131 spreads from the central part directly below the conductive probe 17 to the periphery.

[0079] Figure 5 is a schematic diagram illustrating the process of polarization reversal in a ferroelectric recording layer. As shown in Figure 5, a conductive probe 17 is placed opposite the surface of a positively spontaneously polarized ferroelectric layer 131 in a non-contact state (see Figure 5(a)). When a positive voltage is applied to this conductive probe 17, and the electric field strength generated in the space at the tip of the probe exceeds the polarization reversal potential of the ferroelectric layer 131, the area directly beneath the conductive probe 17 first undergoes negative polarization reversal (see Figure 5(b)). Then, as the applied voltage is further increased, the negative polarization reversal progresses from directly beneath the conductive probe 17 to the surrounding area (see Figures 5(c) and 5(d)).

[0080] Here, the five consecutive charged positions shown in Figure 5 are considered the smallest single recording area. In this case, the number of positive charges within this single recording area is 5 in Figure 5(a), 4 in Figure 5(b), 2 in Figure 5(c), and zero in Figure 5(d). This number of positive charges allows for the recording of 4-level multi-level information. Writing this multi-level information to this smallest single recording area is performed by the simplest single write operation in a ferroelectric memory device.

[0081] The multi-level information recorded in the ferroelectric layer 131 (multi-level information) is read and reconstructed by the ferroelectric memory device. This reconstruction (reading) of multi-level information is performed by the simplest single read operation in the ferroelectric memory device.

[0082] Methods for reproducing the information recorded in the ferroelectric layer 131 include, for example, a method that utilizes the fact that the dielectric constant of the ferroelectric layer 131 differs depending on the polarization direction of the ferroelectric layer 131, a method that detects a weak tunnel current flowing between the conductive probe 17 and the electrode layer 12, and a method that detects the interatomic force between the conductive probe 17 and the ferroelectric layer 131. In any of these methods, multi-level recorded information can be reproduced from the charge amount of the smallest single recording area of ​​the ferroelectric layer 131.

[0083] In other words, when employing a method that utilizes the fact that the dielectric constant of the ferroelectric layer 131 differs depending on the polarization direction of the ferroelectric layer 131, the greater the difference between the amount of positive charge and the amount of negative charge within a single recording area, the greater the difference in dielectric constant.

[0084] When employing a method that detects a weak tunneling current flowing between the conductive probe 17 and the electrode layer 12, the tunneling barrier of the ferroelectric layer 131 changes according to the polarization direction and amount. The tunneling current injected from the electrode layer 12 also changes, and by detecting this change, the information recorded in the ferroelectric recording medium 10 can be reconstructed.

[0085] When employing a method to detect the interatomic force between the conductive probe 17 and the ferroelectric layer 131, the electric force (Maxwell stress) between the ferroelectric layer 131 and the conductive probe 17 fluctuates depending on the polarization direction and amount of the ferroelectric layer 131. By detecting this change along with the interatomic force, the information recorded in the ferroelectric recording medium 10 can be reproduced.

[0086] In a ferroelectric memory device, it is preferable that position information (also called "servo information") for detecting the relative position of the conductive probe 17 and the ferroelectric recording medium 10 in the track direction on the ferroelectric recording medium 10 is recorded in the ferroelectric layer 131. In this case, it is preferable that the ferroelectric layer 131 has a servo information area for recording the servo information and a data area for recording and reproducing data, which are arranged alternately at regular intervals in the circumferential direction of the track. As a result, the conductive probe 17 can detect its position using the servo information during the reproduction of recorded data.

[0087] Figure 6 shows the data region and servo information region of the ferroelectric layer 131. Figure 6(a) is a plan view of the ferroelectric layer 131, and Figure 6(b) is an enlarged view of the rectangular region A in Figure 6(a). As shown in Figures 6(a) and 6(b), the ferroelectric layer 131 may have a data region 131A and a servo information region 131B on one surface of the disc-shaped electrode layer 12. In Figure 6(a), the region indicated by lines extending radially from the center is the servo information region 131B, and the region between the radial lines is the data region 131A. As shown in Figure 6(b), the data region 131A has a regular, annular shape.

[0088] As shown in Figure 6(b), the servo information area 131B includes the burst information area 131B-1, the address information area 131B-2, the preamble information area 131B-3, and the reference signal information 131B-4. In Figure 6(b), the conductive probe 17 is shown as moving from left to right, but the order in which the burst information area 131B-1, the address information area 131B-2, the preamble information area 131B-3, and the reference signal information 131B-4 are provided may be changed as appropriate.

[0089] The burst information area 131B-1 records burst information and other data to position the conductive probe 17 in the center of the recording track.

[0090] The address information area 131B-2 records address information including track information (radial information) and sector information (circumferential information) that indicate the address of the data area 131A.

[0091] The preamble information area 131B-3 and the reference signal information area 131B-4 record preamble information used to identify the point in the recording track where the data area 131A transitions to the servo information area 131B.

[0092] In the ferroelectric layer 131 shown in Figure 6(a), a conductive probe 17 moving circumferentially on the surface reads the preamble information in the preamble information area 131B-3 and prepares to read the address information. Then, in the address information area 131B-2, the conductive probe 17 reads the address information in the data area 131A and the burst information in the burst information area 131B-1 to fine-tune the track position (radial position). After that, the conductive probe 17 can record and reproduce information in the data area 131A.

[0093] The servo information area 131B preferably includes reference signal information 131B-4 that indicates a reference for the signal level of the multi-level recorded information. For example, in the case of the four-level recording in Figure 5, the reference signal information 131B-4 may record the four types of charge quantities shown in Figures 5(a), 5(b), 5(c), and 5(d). Alternatively, the reference signal information 131B-4 may record the two types of charge quantities shown in Figures 5(a) and 5(d), with Figures 5(b) and 5(c) being 1 / 3 or 2 / 3 of the difference in signal levels between Figures 5(a) and 5(d).

[0094] The conductive probe 17 reads the reference signal information 131B-4 in the servo information area 131B, thereby determining the signal level of the multi-level recording. Using this determined signal level, the multi-level information recorded in the data area 131A is then reconstructed.

[0095] As shown in Figure 1, it is preferable that the ferroelectric recording layer 13 includes a paraelectric layer 132.

[0096] The paraelectric layer 132 is provided on the electrode layer 12 side of the ferroelectric recording layer 13, and is preferably provided between the ferroelectric layer 131 and the electrode layer 12.

[0097] If the ferroelectric recording layer 13 is composed of only a single layer of ferroelectric layer 131, the ferroelectric layer 131 will be placed in contact with the electrode layer 12. In this case, the charge from the polarized ferroelectric layer 131 may leak into the electrode layer 12, potentially causing the information recorded on the ferroelectric layer 131 to be lost.

[0098] In particular, the region of the ferroelectric layer 131 in the early stages of growth, that is, the region near the interface with the electrode layer 12, is prone to deterioration of crystallinity during film formation, and is likely to become, for example, polycrystalline or amorphous. As a result, charge is easily leaked from these polycrystalline or amorphous regions to the electrode layer 12. This is because the amorphous structural portion formed near the interface of the ferroelectric layer 131 is completely amorphous, lacking both long-range and short-range order, and therefore causes charge leakage similar to that at grain boundaries in polycrystalline materials.

[0099] By providing an insulating paraelectric layer 132 between the ferroelectric layer 131 and the electrode layer 12, charge leakage from the ferroelectric layer 131 can be suppressed.

[0100] Furthermore, the provision of an insulating paraelectric layer 132 between the ferroelectric layer 131 and the electrode layer 12 has the effect of increasing the tunnel current from the ferroelectric layer 131.

[0101] Ferroelectric materials are insulators with a large band gap, making it difficult for tunnel currents to flow. However, by making the ferroelectric material a thin film, the tunnel barrier can be reduced, and by forming a junction structure with a conductive electrode layer, a weak tunnel current can be allowed to flow due to the electronic state at the junction. Furthermore, by adding a paraelectric layer to this junction, creating a junction structure where ferroelectric, paraelectric, and conductor are joined in this order, band bending occurs at the interface between the ferroelectric and paraelectric materials, lowering the tunnel barrier due to the charge, making it even easier for tunnel currents to flow.

[0102] As the paraelectric layer 132, known materials can be used as the paraelectric material. Preferably, oxides, nitrides, carbides, borides, and silicides are used as the paraelectric material. These may be used individually or in combination of two or more.

[0103] Examples of oxides include alumina, zirconia, yttrium-stabilized zirconia, silicon dioxide, titanium dioxide, cerium oxide, titanium dioxide, lead oxide, yttrium oxide, barium oxide, chromium oxide, iron oxide, and lanthanum oxide.

[0104] Examples of nitrides include titanium nitride, silicon nitride, chromium nitride, and aluminum nitride.

[0105] Examples of carbides include titanium carbide, tungsten carbide, boron carbide, silicon carbide, and chromium carbide.

[0106] Examples of borides include titanium boride, iron boride, and neodymium boride.

[0107] Examples of silicides include molybdenum silicide.

[0108] Furthermore, the paraelectric material constituting the paraelectric layer 132 is preferably one or both of the same crystal system and / or crystal structure as the ferroelectric layer 131, taking into consideration lattice matching with the substrate 11 and electrode layer 12.

[0109] It is preferable that the lattice constant of the paraelectric material constituting the paraelectric layer 132 be lattice-matched with the ferroelectric material constituting the ferroelectric layer 131 within a range of ±10%, similar to the substrate 11 and the electrode layer 12. This facilitates the growth of the ferroelectric layer 131 on the paraelectric layer 132 and improves the crystallinity of the ferroelectric layer 131.

[0110] For example, if hafnium oxide is selected for the ferroelectric layer 131, as described above, the material constituting the paraelectric layer 132 is preferably 4.6 Å to 5.8 Å in size so that its lattice constant is lattice-matched within ±10%. The crystal system is preferably orthorhombic, monoclinic, tetragonal, or cubic, and more preferably orthorhombic or cubic. It is also preferable to use a material with a fluorite-type crystal structure.

[0111] Examples of such materials include cerium oxide (cubic crystal system, fluorite-type structure, lattice constant 5.4 Å), silicon (cubic crystal system, diamond-type structure, lattice constant 5.4 Å), 10(Y2O3)-90(ZrO2) (cubic crystal system, fluorite-type structure, lattice constant 5.1 Å), aluminum oxide (trigonal crystal system, corundum-type structure, lattice constant 4.8 Å), and titanium oxide (tetragonal crystal system, rutile-type structure, lattice constant 4.6 Å).

[0112] The effect of the paraelectric layer 132 in enhancing the crystallinity of the ferroelectric layer 131 through lattice matching with the ferroelectric layer 131 is as high as that of the substrate 11, and higher than that of the electrode layer 12. Furthermore, as mentioned above, band bending occurs at the interface between the ferroelectric layer 131 and the paraelectric layer 132 in a direction that lowers the tunnel barrier, and this band bending is strongly influenced by the lattice strain at the interface. That is, since lattice strain increases the energy at the interface, this increased energy may mask the energy state due to the aforementioned band bending. Therefore, in order to detect the weak tunnel current flowing between the conductive probe 26 and the ferroelectric layer 131, it is important to improve the lattice matching between the paraelectric layer 132 and the ferroelectric layer 131 and reduce the energy rise caused by lattice strain.

[0113] The thickness of the paraelectric layer 132 is preferably 1 nm to 100 nm, and more preferably 5 nm to 50 nm from the viewpoint of suppressing charge leakage from the ferroelectric layer 131.

[0114] If the thickness of the paraelectric layer 132 is within the above preferred range, polarization reversal can be caused in the ferroelectric layer 131, and charge leakage from the ferroelectric layer 131 can be suppressed, preventing charge from leaking out of the ferroelectric layer 131. Furthermore, if the thickness of the paraelectric layer 132 is within the above preferred range, tunnel current can be made to flow more easily, thus increasing the tunnel current.

[0115] Furthermore, since the increase in the distance between the ferroelectric layer 131 and the electrode layer 12 is suppressed, the voltage required to reverse the polarization of the ferroelectric layer 131 can be reduced. Therefore, it is possible to suppress the increase in the voltage required to reverse the polarization of the ferroelectric layer 131 while suppressing the increase in the film thickness of the ferroelectric recording layer 13.

[0116] Furthermore, from the viewpoint of increasing the tunnel current from the ferroelectric layer 131, the film thickness of the paraelectric layer 132 is preferably 1 nm to 30 nm, and more preferably about the same as the film thickness of the ferroelectric layer 131. In this case, the film thickness of the ferroelectric layer 131 is also preferably 1 nm to 30 nm in total. It is preferable that the film thickness of the paraelectric layer 132 is equal to the total film thickness of the ferroelectric layer 131. If there is a difference between the film thickness of the paraelectric layer 132 and the total film thickness of the ferroelectric layer 131, it is preferable that the paraelectric layer 132 is thinner than the total film thickness of the ferroelectric layer 131, and the difference between the film thickness of the paraelectric layer 132 and the total film thickness of the ferroelectric layer 131 is preferably 10 nm or less.

[0117] [Protective layer] The protective layer 14 is provided on the upper surface of the ferroelectric recording layer 13. The protective layer 14 has the function of protecting the ferroelectric recording layer 13 from external sources, and can reduce damage to the ferroelectric recording layer 13 even if the ferroelectric recording medium 10 comes into contact with the conductive probe 17, etc.

[0118] In order to avoid impairing the information recording and playback functions of the ferroelectric recording layer 13, the protective layer 14 is preferably made of an insulating material with a low dielectric constant.

[0119] Suitable materials for the protective layer 14 include oxides such as silica, alumina, zirconia, titania, magnesium oxide, and aluminum oxide; nitrides such as silicon nitride, aluminum nitride, titanium nitride, and boron nitride; carbides such as silicon carbide and boron carbide; diamond-like carbon films; and polymer insulating materials. From the viewpoint of protecting the ferroelectric recording layer and suppressing its deterioration in durability, a material with high hardness is preferred.

[0120] The thickness of the protective layer 14 is preferably 0.5 nm or more. On the other hand, considering the voltage required for the ferroelectric material of the ferroelectric layer 131 to undergo polarization reversal, the thickness of the protective layer 14 is preferably 50 nm or less, more preferably 30 nm or less, and even more preferably 20 nm or less.

[0121] As shown in Figure 3, it is preferable that the ferroelectric recording medium 10 does not have a ferroelectric recording layer 13 (ferroelectric layer 131, paraelectric layer 132) and a protective layer 14 around the opening 10a. That is, it is preferable that the ferroelectric recording layer 13 and the protective layer 14 are provided in a region other than the central opening 10a and its surroundings in the ferroelectric recording medium 10. The region around the opening 10a refers to a range of about 10 mm in length from the inner circumference of the opening 10a, depending on the size of the substrate 11, the size of the opening 10a, etc. By providing the ferroelectric recording layer 13 and the protective layer 14 in a region other than the opening 10a and its surroundings, the ferroelectric recording layer 13 and the protective layer 14 will not come into contact with the spindle shaft 18, and the leakage of charge from the ferroelectric layer 131 to the spindle shaft 18 through the protective layer 14 can be suppressed.

[0122] The ferroelectric recording medium 10 has a structure in which a ferroelectric recording layer 13, a protective layer 14, and a lubricant layer 15 are laminated on both surfaces of the substrate 11 and the electrode layer 12. The spindle shaft 18 to which the ferroelectric recording medium 10 is attached is cylindrical with a step at the top that reduces in diameter. The ferroelectric recording medium 10 is placed on this step, and a mounting bracket 19 is placed on top of it and screwed to the spindle shaft 18, thereby fixing the ferroelectric recording medium 10 to the spindle shaft 18. At the point where the ferroelectric recording medium 10 is fixed to the spindle shaft 18, the ferroelectric recording layer 13, protective layer 14, and lubricant layer 15 are not provided, and the substrate 11 and the spindle shaft 18 are in direct contact. The spindle shaft 18 is connected to the ferroelectric layer 131 via the substrate 11 and constitutes part of the circuit that reads and writes information to the ferroelectric layer 131.

[0123] For example, as shown in Figure 7, when a ferroelectric recording medium having an electrode layer 12, a ferroelectric layer 131, and a protective layer 14 covering the entire surface of the substrate 11 is mounted on a spindle shaft 18, the ferroelectric layer 131 is insulating, so even if the ferroelectric layer 131 is in contact with the spindle shaft 18, it is not possible to read or write information to the ferroelectric recording medium 10. However, because the ferroelectric recording layer 13 and the protective layer 14 are thin, it is possible to establish electrical contact with the substrate 11 by allowing the convex portion 181a of the mounting surface 181 of the spindle shaft 18 to penetrate the ferroelectric layer 131 and the protective layer 14 and bite into the electrode layer 12, thereby creating electrical contact between the substrate 11 and the spindle shaft 18.

[0124] When the ferroelectric recording medium 10 is installed in a ferroelectric memory device and used for a long period of time, the protective layer 14 deteriorates due to moisture and dirt inside the ferroelectric memory device, reducing the insulating properties of the protective layer 14, which may cause charge leakage from the ferroelectric layer 131 that constitutes the ferroelectric recording layer 13. Although the degree of deterioration is less because the ferroelectric recording layer 13 is covered by the protective layer 14, the ferroelectric recording layer 13 also deteriorates in the same way as the protective layer 14, and charge leakage from the ferroelectric layer 131 is possible. In this embodiment, as shown in Figure 3, by forming the ferroelectric recording layer 13 and the protective layer 14 in areas other than the opening 10a and its surrounding region of the ferroelectric recording medium 10, it is possible to prevent charge leakage from the ferroelectric recording layer 13 from the protective layer 14 to the spindle shaft 18.

[0125] As shown in Figure 1, the lubricant layer 15 may be provided on the surface of the protective layer 14 to suppress wear caused by contact with the conductive probe 17.

[0126] As a lubricant used in the lubricant layer 15, saturated fatty acids such as stearic acid, dyes such as phthalocyanine, and fluororesins such as perfluoropolyether (PFPE) are preferred, with fluororesins such as PFPE being particularly preferable due to their good lubricity.

[0127] In order to avoid impairing the information recording and playback functions of the ferroelectric recording layer 13, it is preferable to use an insulating material with a low dielectric constant for the lubricant layer 15.

[0128] Thus, the ferroelectric recording medium 10 can achieve lattice matching between the lattice constant of the material constituting the ferroelectric layer 131 and the lattice constant of the material constituting the electrode layer 12 within a range of ±10%. This enhances the crystallinity of the ferroelectric material constituting the ferroelectric layer 131. As a result, the ferroelectric material contained in the ferroelectric layer 131 becomes a single crystal, eliminating grain boundaries and thus reducing their influence. Since polarization reversal of the ferroelectric material contained in the ferroelectric layer 131 is caused by crystal lattice strain, the single crystal nature of the ferroelectric material in the ferroelectric layer 131 increases the region in the ferroelectric layer 131 where polarization reversal of the ferroelectric material occurs. Therefore, the ferroelectric recording medium 10 can have a high recording density.

[0129] Furthermore, in the case of the ferroelectric recording medium 10, if the substrate 11 is a conductor, the substrate 11 can also serve as an electrode because it is conductive. In this case, the substrate 11 can function as an electrode layer 12, and an electrode layer 12 becomes unnecessary. Therefore, the ferroelectric recording medium 10 can achieve lattice matching within ±10% of the lattice constant of the material constituting the ferroelectric layer 131 and the lattice constant of the material constituting the substrate 11 without providing an electrode layer 12 on the substrate 11. Thus, as described above, the ferroelectric recording medium 10 can increase the region in the ferroelectric layer 131 where ferroelectric polarization reversal occurs, and thus can have a high recording density.

[0130] The ferroelectric recording medium 10 can also have a single-crystal ferroelectric layer 131. This eliminates the grain boundaries of the ferroelectric layer 131, thereby further increasing the recording capacity per ferroelectric recording medium 10.

[0131] The ferroelectric recording medium 10 includes a ferroelectric layer 131, which has an amorphous structure with short-range order. The length of the short-range order is set to 2 nm or less, allowing the lattice constant of the amorphous structure to be matched with the lattice constant of the material constituting the substrate 11 within a range of ±10%. Because the ferroelectric layer 131 has an amorphous structure with short-range order, polarization reversal due to crystal lattice strain can be generated in the region with short-range order, and crystal grain boundaries can be reduced. Therefore, the ferroelectric recording medium 10 can use a wide area of ​​the ferroelectric layer 131 as the recording area. Furthermore, by setting the length of the short-range order of the ferroelectric layer 131 to 2 nm or less, the storage area can be increased, and thus the recording density can be increased. Thus, the ferroelectric recording medium 10 can have a high recording density.

[0132] Furthermore, if the ferroelectric layer 131 has an amorphous structure with short-range order, the deposition temperature of the ferroelectric layer 131 can be further reduced compared to when the ferroelectric layer 131 is formed as a single crystal film. As a result, the growth surface of the ferroelectric layer 131 becomes smooth, and the surface of the ferroelectric recording medium 10 can be made smooth. Therefore, the ferroelectric recording medium 10 can reduce the spacing loss with the conductive probe 17, and thus the recording density can be increased.

[0133] Furthermore, because the ferroelectric recording medium 10 has an amorphous structure with short-range order in the ferroelectric layer 131, the deposition temperature of the ferroelectric layer 131 can be further reduced, thus increasing the types of substrates 11 that can be used.

[0134] The ferroelectric recording medium 10 includes an amorphous structure with short-range order in its ferroelectric layer 131, which makes it easier to thin the ferroelectric layer 131. As a result, the ferroelectric layer 131 can easily form a smooth surface. Therefore, the ferroelectric recording medium 10 can have enhanced smoothness.

[0135] Furthermore, by including an amorphous structure with short-range order in the ferroelectric layer 131 of the ferroelectric recording medium 10, the grain boundaries of the ferroelectric layer 131 can be reduced, thereby increasing the recording capacity per sheet of the ferroelectric recording medium 10.

[0136] The ferroelectric recording medium 10 may have a silicon substrate 11 and a hafnium oxide ferroelectric layer 131. This improves the lattice matching between the substrate 11 and the ferroelectric layer 131 and enhances the crystallinity of the ferroelectric layer 131. Therefore, the ferroelectric recording medium 10 can reliably achieve a higher recording density.

[0137] The ferroelectric recording medium 10 has a ferroelectric layer 131 made of a mixture of hafnium oxide and one or more additives selected from the group consisting of silicon, aluminum, gadolinium, yttrium, lanthanum, and strontium, or a mixed crystal of hafnium oxide and zirconium dioxide (Hf x Zr 1-x It can be formed with O2)(x is 0.3 to 0.6). As a result, the ferroelectric layer 131 is formed in the above mixture or mixed crystal containing hafnium oxide, which allows the deposition temperature to be lowered when forming the ferroelectric layer 131 on the substrate 11, so that the ferroelectric layer 131 can be formed on the substrate 11 at a low temperature. In general, the higher the temperature at which the ferroelectric layer 131 is formed, the easier it is to form polycrystals. In this embodiment, the ferroelectric can increase the single-crystal region, so the single-crystal region of the ferroelectric layer 131 can be further increased. Therefore, the ferroelectric recording medium 10 can have a higher recording capacity.

[0138] Furthermore, by increasing the single-crystal region of the ferroelectric layer 131, the ferroelectric recording medium 10 can be made thinner, and the ferroelectric layer 131 can form a smooth surface over a wider area, thereby further improving the smoothness of the ferroelectric layer 131.

[0139] The ferroelectric recording medium 10 can have an additive content within the range of 1 atomic% to 20 atomic%. This allows the ferroelectricity of the ferroelectric layer 131 of the ferroelectric recording medium 10 to be enhanced.

[0140] The ferroelectric recording medium 10 can have a ferroelectric layer 131 with a thickness of 5 nm to 1000 nm. This allows the ferroelectric recording medium 10 to induce polarization reversal in the ferroelectric material contained in the ferroelectric layer 131, and also reduces the voltage required for polarization reversal of the ferroelectric material, thereby reducing the load on the ferroelectric layer 131.

[0141] The ferroelectric recording medium 10 includes a paraelectric layer 132 between the ferroelectric recording layer 13 and the electrode layer 12. The paraelectric layer 132 may contain one or more paraelectric materials selected from the group consisting of oxides, nitrides, carbides, borides, and silicides. During the formation of the ferroelectric layer 131, the initial growth region, i.e., the region near the interface with the electrode layer 12, is polycrystalline or amorphous. As the film thickness grows, the crystallinity increases, and finally the surface becomes single-crystal. Therefore, the charge of the ferroelectric layer 131 is prone to leaking from the polycrystalline or amorphous region near the interface to the substrate 11. The paraelectric layer 132 can suppress the leakage of charge from the ferroelectric layer 131 to the substrate 11. Thus, the ferroelectric recording medium 10 can suppress the leakage of charge from the ferroelectric layer 131, and therefore can suppress the loss of information recorded on the ferroelectric layer 131.

[0142] Furthermore, the ferroelectric recording medium 10 can match the lattice constant of the material constituting the paraelectric layer 132 within ±10% of the lattice constant of the material constituting the ferroelectric layer 131. This further enhances the crystallinity of the ferroelectric layer 131 and suppresses charge leakage from the ferroelectric layer 131.

[0143] Furthermore, the ferroelectric recording medium 10 can have a paraelectric layer 132 with a thickness of 1 nm to 100 nm. This allows the ferroelectric recording medium 10 to induce polarization reversal in the ferroelectric layer 131 while further enhancing the effect of suppressing charge leakage from the ferroelectric layer 131.

[0144] The ferroelectric recording medium 10 has a paraelectric layer 132 with a thickness of 1 nm to 30 nm, and a ferroelectric layer 131 with a thickness of 1 nm to 30 nm. The thicknesses of the paraelectric layer 132 and the ferroelectric layer 131 are approximately equal, or the thickness of the paraelectric layer 132 is thinner than the combined thickness of the ferroelectric layers 131, making the difference in thickness between the paraelectric layer 132 and the ferroelectric layer 131 10 nm or less. As a result, when the ferroelectric recording medium 10 is applied to a ferroelectric memory device, it can increase the weak tunnel current flowing between the conductive probe 17 and the ferroelectric layer 131.

[0145] The ferroelectric recording medium 10 has an opening 10a in the center of the substrate 11, and a protective layer 14 can be provided in the area other than the opening 10a and its periphery. Since the protective layer 14 does not come into contact with the spindle shaft 18, even if the protective layer 14 deteriorates over time, the ferroelectric recording medium 10 can suppress the leakage of charge from the ferroelectric layer 131 to the spindle shaft 18 through the protective layer 14. Therefore, the ferroelectric recording medium 10 can suppress charge leakage from the ferroelectric layer 131, thereby suppressing the loss of information recorded on the ferroelectric layer 131 and suppressing the adverse effects of deterioration of the protective layer 14 over time.

[0146] Furthermore, the ferroelectric recording medium 10 can have a ferroelectric recording layer 13 in areas other than the opening 10a and its surroundings. This prevents the ferroelectric recording layer 13 from coming into contact with the spindle shaft 18. As a result, even if the ferroelectric recording layer 13 and protective layer 14 deteriorate over time, the ferroelectric recording medium 10 can suppress the leakage of charge from the ferroelectric layer 131 to the spindle shaft 18 through the ferroelectric recording layer 13 and protective layer 14. Therefore, the ferroelectric recording medium 10 can more reliably suppress charge leakage from the ferroelectric layer 131, thereby more stably suppressing the loss of information recorded on the ferroelectric layer 131 and more effectively suppressing the adverse effects of deterioration of the ferroelectric recording layer 13 and protective layer 14 over time.

[0147] <Ferroelectric memory device> A ferroelectric memory device will now be described. The ferroelectric memory device includes the ferroelectric recording medium 10 described above as the ferroelectric recording medium. Figure 8 is a perspective view showing the ferroelectric memory device according to this embodiment. As shown in Figure 8, the ferroelectric memory device 100 includes the ferroelectric recording medium 10, a head assembly 20, a ferroelectric recording medium drive unit 30, a probe drive unit 40, a control unit (not shown), and a recording / reproduction signal processing unit 50, all of which are housed in a housing 60. The ferroelectric memory device 100 may include multiple ferroelectric recording media 10. Details of the ferroelectric recording medium 10 are omitted because the ferroelectric recording medium described above is used.

[0148] [Head Assembly] As shown in Figure 8, the head assembly 20 is fixed to a fixed shaft in the ferroelectric memory device 100 and includes an actuator arm 21, a suspension arm 22, and a probe slider 23.

[0149] Figure 9 is a perspective view showing the configuration of the head assembly 20 when viewed from below. As shown in Figure 9, lead wires 24 for writing and reading signals are wired to the suspension arm 22. One end of the lead wire is electrically connected to a conductive probe 26 incorporated in the probe slider 23, and the other end is connected to an electrode pad 25.

[0150] As shown in Figure 8, the actuator arm 21 has a hole at one end for fixing to the fixed shaft of the ferroelectric memory device 100, and the suspension arm 22 is connected to the tip end.

[0151] As shown in Figure 8, the suspension arm 22 has an actuator arm 21 connected to one end, and a probe slider 23 attached to its tip.

[0152] (Probe slider) As shown in Figure 8, the probe slider 23 is located at the tip of the suspension arm 22.

[0153] Figure 10 is a cross-sectional view showing an example of the configuration of the probe slider 23. In Figure 10, the arrows in the +X and -X axes represent the sector direction of the ferroelectric recording medium 10, the +Y and -Y axes represent the track direction of the ferroelectric recording medium 10, and the Z axis represents the direction opposite to the recording surface of the ferroelectric recording medium 10. As shown in Figure 10, the probe slider 23 is equipped with a conductive probe 26 at its tip.

[0154] Figure 11 is a cross-sectional view showing another example of the configuration of the probe slider 23. The direction of the arrows in Figure 11 is the same as in Figure 10.

[0155] As shown in Figure 11, the probe slider 23 may include a conductive probe 26 at its tip and a piezoelectric element 27 provided between the probe slider 23 and the conductive probe 26. The piezoelectric element 27 has a first piezoelectric element 27A and a second piezoelectric element 27B. The first piezoelectric element 27A drives the conductive probe 26 in the +Z axis direction and the -Z axis direction, and the second piezoelectric element 27B drives the conductive probe 26 in the +Y axis direction and the -Y axis direction. In addition to the first piezoelectric element 27A and the second piezoelectric element 27B, the piezoelectric element 27 may also have a third piezoelectric element 27C and a fourth piezoelectric element 27D, as described later.

[0156] Figure 12 is a partially enlarged cross-sectional view of Figure 11, Figure 13 is a partially enlarged cross-sectional view of Figure 11 as seen from another direction, and Figure 14 is a partially enlarged view of Figure 11 as seen from the bottom. As shown in Figures 12 to 14, an electrode 28A-1 is provided between the conductive probe 26 and the first piezoelectric element 27A, and an electrode 28A-2 is provided between the first piezoelectric element 27A and the second piezoelectric element 27B. Electrodes 28A-1 and 28A-2 form a pair, sandwiching the first piezoelectric element 27A on both the upper and lower surfaces in the Z-axis direction of the first piezoelectric element 27A.

[0157] Furthermore, a second piezoelectric element 27B is provided between the probe slider 23 and electrode 28A-2, and electrodes 28B-1 and 28B-2 are provided on the Y-axis side of the second piezoelectric element 27B. Electrode 28B-1 is provided on the +Y-axis side of the second piezoelectric element 27B, and electrode 28B-2 is provided on the -Y-axis side of the second piezoelectric element 27B, with electrodes 28B-1 and 28B-2 forming a pair with the second piezoelectric element 27B in between.

[0158] Electrode 28A-2 is connected to wiring L11, electrode 28B-1 is connected to wiring L12, electrode 28B-2 is connected to wiring L21, and electrode 28A-1 is connected to wiring L22. Electrodes 28A-1, 28A-2, 28B-1, and 28B-2 are each connected by wiring to electrode pads 28C located on the lower surface of the probe slider 23, further outward in the Y-axis direction than electrode 28A-2.

[0159] Here, electrode 28A-2 is used to polarize the first piezoelectric element 27A, but at the same time it is in contact with the second piezoelectric element 27B, and may also polarize the second piezoelectric element 27B. To prevent this, it is preferable to provide an insulating layer with a low dielectric constant between electrode 28A-2 and the second piezoelectric element 27B, and it is preferable that the dielectric constant of this layer be 1 / 100 or less of the dielectric constant of the second piezoelectric element 27B.

[0160] Furthermore, while piezoelectric effects include the piezoelectric longitudinal effect, the piezoelectric transverse effect, and the piezoelectric thickness sliding effect, the electrode positions shown in Figures 12 and 13 utilize the piezoelectric longitudinal effect together with the first piezoelectric element 27A and the second piezoelectric element 27B. However, when using other effects, each electrode can be provided on a different surface of the piezoelectric element.

[0161] Furthermore, when the first piezoelectric element 27A is driven in the +Z axis direction or the -Z axis direction, the first piezoelectric element 27A will also be displaced in the +Y axis direction or the -Y axis direction due to the piezoelectric transverse effect. It is preferable to compensate for this displacement of the first piezoelectric element 27A in the +Y axis direction or the -Y axis direction by driving the second piezoelectric element 27B in the +Y axis direction or the -Y axis direction.

[0162] (Conductive probe) The conductive probe 26 has the function of recording and reproducing information on the ferroelectric recording layer 13 of the ferroelectric recording medium 10. The conductive probe 26 is a needle-shaped conductive electrode used in scanning probe microscopes such as scanning tunneling microscopes (STMs) and atomic force microscopes (AFMs).

[0163] As shown in Figure 11, the conductive probe 26 is provided on the lower surface of the second piezoelectric element 27B via an electrode 28A-1, and is preferably provided at a position offset from the center of the lower surface of the second piezoelectric element 27B toward the track direction of the ferroelectric recording medium 10 (+Y axis direction or -Y axis direction). For example, as shown in Figure 14, the conductive probe 26 is preferably provided on the electrode 28B-2 side, which is located toward the +Y axis direction from the center of the lower surface of the second piezoelectric element 27B. Alternatively, although different from Figure 14, the conductive probe 26 may be provided on the electrode 28B-1 side, which is located toward the -Y axis direction from the center of the lower surface of the second piezoelectric element 27B.

[0164] The conductive probe 26 is made to levitate and move on the surface of the ferroelectric recording medium 10. For recording and reproducing information on the ferroelectric recording medium 10, it is preferable to rotate the recording medium at high speed, like an HDD, and use the airflow generated on the surface of the recording medium to levitate the probe slider 23, and then record and reproduce information using the conductive probe 26 attached to the probe slider 23. That is, a needle-shaped conductive probe 26 is attached to the probe slider 23, and this probe slider 23 is made to levitate and move on the surface of the ferroelectric recording medium 10 at the nano level, so that the needle-shaped conductive probe 26 is brought very close to the surface of the ferroelectric recording medium 10 to record and reproduce information.

[0165] For example, metals such as tungsten, molybdenum, and platinum can be used as the material for forming the conductive probe 26.

[0166] Figure 15 is a cross-sectional view showing the configuration of the conductive probe 26. As shown in Figure 15, the conductive probe 26 has, for example, a substrate 261 provided on the piezoelectric element 27, and a sharpened needle-shaped electrode 262 formed on the substrate 261. The substrate 261 and the needle-shaped electrode 262 may be formed integrally from the same material, or they may be formed from different materials.

[0167] An electrostrictive element may be used instead of the piezoelectric element 27. Both piezoelectric elements and electrostrictive elements have the common function of being displaced when an electric field is applied, but the direction of displacement of a piezoelectric element changes depending on the direction of the electric field, whereas an electrostrictive element only stretches and does not contract. Also, piezoelectric elements generate an electric charge in accordance with the stress, whereas electrostrictive elements do not generate an electric charge even when stress is applied. Since both piezoelectric elements and electrostrictive elements have the common function of being displaced when an electric field is applied, they can be used similarly in ferroelectric memory devices.

[0168] The needle-shaped electrode 262 is a cone-shaped electrode, and its width and height are, for example, several nanometers to several millimeters.

[0169] The smaller the curvature of the tip of the needle electrode 262, the higher the electric field strength at the tip, and the lower the voltage applied to the needle electrode 262, which is advantageous when recording information on the ferroelectric recording medium 10. Also, it becomes easier to bring the tip of the needle electrode 262 closer to the ferroelectric recording medium 10, which is advantageous when reading information. The radius of curvature of the tip of the needle electrode 262 is preferably a few nanometers or less.

[0170] The conductive probe 26 is scanned over the surface (recording surface) of the ferroelectric recording medium 10. The conductive probe 26 is brought very close to the surface (recording surface) of the ferroelectric recording medium 10. Then, an electric field exceeding the coercive field of the ferroelectric layer 131 is applied from the conductive probe 26, reversing the polarization direction of the ferroelectric layer 131 located directly beneath the conductive probe 26. This applied voltage is converted into a pulse signal whose level changes according to the information to be recorded, and while applying the voltage to the ferroelectric recording medium 10 via the conductive probe 26, the position of the conductive probe 26 relative to the ferroelectric recording medium 10 is moved in a direction parallel to the surface of the ferroelectric recording medium 10. In this way, information can be recorded as the polarization state of the ferroelectric layer 131 of the ferroelectric recording medium 10.

[0171] The method for reproducing the information recorded on the ferroelectric recording medium 10 will be described later.

[0172] The conductive probe 26 can be manufactured using any manufacturing method. A manufacturing method for the conductive probe 26 may include, for example, the steps of forming a dot-shaped mask on the surface of a conductive material, etching the conductive material to obtain a cone-shaped needle electrode, and removing the mask. This results in the formation of a conductive probe 26 having a cone-shaped, sharpened needle electrode 262 on a substrate 261 made of etched conductive material.

[0173] An example of a method for manufacturing a conductive probe 26 will be described. Figure 16 shows an example of a method for manufacturing a conductive probe 26. As shown in Figure 16, a dot-shaped mask 71 is formed on the surface of a conductive material 260 (see Figure 16(a)), and then the conductive material 260 is etched (see Figure 16(b)). Because the etching of the conductive material 260 is delayed in the area where the mask 71 is placed, the mask 71 lifts off, and a substantially conical needle-shaped electrode 262 is formed beneath the mask 71 (see Figure 16(c)). Subsequently, by cutting out the conductive material 260 around the needle-shaped electrode 262, a conductive probe 26 having a conical needle-shaped electrode 262 formed on a substrate 261 is formed (see Figure 16(d)).

[0174] Furthermore, if the process of cutting out the needle-shaped electrode 262 is performed by machining, the needle-shaped electrode 262 may be damaged. In such cases, it is preferable to etch the conductive material 260 after cutting out the area where the needle-shaped electrode is to be formed.

[0175] Furthermore, if the conductive material 260 used in the above process is an insulator or semiconductor material, a conductive film may be formed by coating the surface of the manufactured needle-shaped electrode with Au (gold) or the like using a sputtering method, thereby making the needle-shaped electrode conductive.

[0176] Furthermore, the conductive probe 26 can be manufactured using other manufacturing methods. Figure 17 shows an example of another manufacturing method for the conductive probe 26. As shown in Figure 17, a photoresist 72 is applied to the surface of the conductive material 260 (photoresist coating step (see Figure 17(a))).

[0177] Next, the photoresist 72 is etched in a circular shape to form a mask 72A having circular fine through-holes 72a in the photoresist 72 (mask formation step (see Figure 17(b))).

[0178] Next, metal 73 is deposited onto the surface of the conductive material 260 in the through hole 72a and onto the mask 72A to form a needle-shaped electrode 262 that is formed in a substantially conical shape (needle electrode formation process (see Figures 17(c) and (d))).

[0179] At this time, as the amount of metal 73 deposited on the mask 72A increases, the through-holes 72a become blocked and the width of the through-holes 72a narrows. As a result, the metal 73 deposited at the bottom of the through-holes 72a of the mask 72A is deposited widely at the bottom, and the deposition area gradually narrows as it moves upwards, ultimately forming a needle-shaped electrode 262 deposited in a roughly conical shape (see Figure 17(d)).

[0180] Next, the mask 72A is removed to obtain a conductive probe 26 having a needle-shaped electrode 262 (conductive probe fabrication process (see Figure 17(e))). By removing the mask 72A and cutting out the conductive material 260 around the needle-shaped electrode 262, a conductive probe 26 having a cone-shaped needle-shaped electrode 262 is obtained on a substrate 261.

[0181] The conductive probe 26 can be used in various forms. The embodiments of the conductive probe 26 will be described below.

[0182] ((First aspect)) As shown in Figures 18 and 19, the conductive probe 26A is preferably formed in the shape of a triangular pyramid or a square pyramid. If anisotropic etching is performed using a single crystal of the conductive material, it is easy to form a top formed by any crystal plane (texture plane).

[0183] For example, if the conductive material 260 uses a (100) plane of a single crystal having a diamond structure such as Si, by processing the (100) plane as a crystal plane, the conductive probe 26A can be easily formed into a square pyramid with a sharp apex, consisting of four (111) equivalent planes. Therefore, when the conductive probe 26A is approximately a triangular pyramid or approximately a square pyramid, it is possible to create a conductive probe with a sharper apex compared to when it is approximately a cone.

[0184] Furthermore, if the conductive probe 26A is approximately a triangular pyramid or a square pyramid, it can have a sharper apex compared to the conductive probe 26, which is approximately a cone.

[0185] Furthermore, in order to equalize the electric field distribution generated at the tip of the conductive probe 26A during writing, and to stabilize the tunnel current flowing between the conductive probe 26A and the ferroelectric layer 131 during reading, it is preferable that the conductive probe 26A be rotationally symmetrical about an axis passing through its tip.

[0186] The conductive probe 26A can be manufactured using any manufacturing method. For example, in the manufacturing method of the conductive probe shown in Figure 16 above, the shape of the mask 71 formed on the surface of the conductive material 260 is made triangular or square. As a result, as shown in Figures 18 and 19, a conductive probe 26A having a triangular pyramidal or square pyramidal needle-shaped electrode 262A can be formed on the substrate 261.

[0187] Furthermore, it is preferable to use a method in which anisotropic etching is performed on the conductive material 260 using a single crystal to form a apex formed by an arbitrary crystal plane (texture plane). For example, if the conductive material 260 is a (100) plane of a single crystal having a diamond structure such as Si, the conductive probe 26A will consist of four (111) equivalent planes, and a square pyramid with a sharp apex can be formed with good reproducibility.

[0188] For anisotropic etching in this case, it is preferable to use reactive ion etching (RIE) using an etching gas such as SF6, and wet etching using KOH as the etchant.

[0189] Furthermore, the conductive probe 26A can be manufactured using other manufacturing methods. As an alternative manufacturing method for the conductive probe 26A, for example, in the mask formation step of the conductive probe manufacturing method shown in Figure 17 above, the shape of the through-holes 72a formed in the photoresist 72 is made triangular or square, and a mask 72A having triangular or square through-holes 72a is formed in the photoresist 72. By making the shape of the through-holes 72a triangular or square, a needle-shaped electrode 262 in the shape of a roughly triangular pyramid or square pyramid is formed on the conductive material 260. As a result, a conductive probe 26A having a needle-shaped electrode 262A in the shape of a triangular pyramid or square pyramid on a substrate 261 is obtained, as shown in Figures 18 and 19.

[0190] ((Second aspect)) Figure 20 shows a cross-sectional view of another configuration of the conductive probe 26. As shown in Figure 20, the conductive probe 26B has a substrate 261 made of a conductive material, a recess 261a formed in the substrate 261, and a needle-shaped electrode 262 formed in a cone shape in the recess 261a, and it is preferable that a part of the needle-shaped electrode 262 protrudes from the surface (main surface) 261b of the substrate 261. The surface 261b of the substrate 261 refers to the main surface of the substrate 261, excluding the recess 261a.

[0191] Since the conductive probe 26B has a needle-shaped electrode 262 that is mostly covered by the substrate 261, damage to the needle-shaped electrode 262 when the conductive probe 26B accidentally comes into contact with the ferroelectric recording medium 10 can be reduced. In addition, vibration and deformation of the needle-shaped electrode 262 caused by airflow generated by the rotation of the ferroelectric recording medium 10 can be suppressed.

[0192] Figure 21 is a cross-sectional view showing an example of the configuration of a probe slider 23 equipped with a conductive probe 26B.

[0193] A method for manufacturing the conductive probe 26B will now be described. The method for manufacturing the conductive probe 26B includes the steps of: applying a photoresist to the surface of a conductive material; patterning the photoresist to form a mask having fine through-holes; etching the conductive material within the holes to form recesses on the surface of the conductive material; depositing a metal film on the bottom of the recesses of the conductive material formed in the through-holes; and removing the photoresist to obtain a needle-shaped electrode formed in a cone shape at the bottom of the recesses of the conductive material, with a portion of the needle-shaped electrode 262 protruding from the conductive material. This gives the conductive probe 26B.

[0194] Figure 22 is an explanatory diagram showing an example of a manufacturing method for conductive probe 26B. As shown in Figure 22, a photoresist 82 is applied to the surface of conductive material 260 (see Figure 22(a)), and after patterning, a mask 82A having fine through holes 82a is formed in the photoresist 82 (see Figure 22(b)). Then, the through holes 82a are etched using an appropriate etching method to form recesses 260a on the surface of the conductive material 260 (see Figure 22(c)). After etching, metal 83 is deposited on the bottom of the recesses 260a in the conductive material 260 formed in the through holes 82a and on top of the photoresist 82 (see Figure 22(d)). At this time, as the amount of metal 83 deposited on top of the photoresist 82 increases, the through holes 82a are closed and the width of the through holes 82a narrows. As a result, the metal 83 deposited at the bottom of the recess 260a in the conductive material 260 is deposited widely at the bottom, and the deposition area gradually narrows as it moves upwards, ultimately forming a cone-shaped needle electrode 262 (see Figure 22(e)).

[0195] Subsequently, by removing the mask 82A (see Figure 22(f)), a portion of the conical needle electrode 262, including its tip, is slightly protruding from the conductive material 260.

[0196] By cutting out the conductive material 260 around the needle-shaped electrode 262, a conductive probe 26B is formed, as shown in Figure 20, which has a needle-shaped electrode 262 formed in a cone shape in the recess 261a of the substrate 261.

[0197] According to this manufacturing method, the needle-shaped electrode 262 is formed within the recess 261a and surrounded by the substrate 261, thereby reducing the risk of damage to the needle-shaped electrode 262 when the conductive probe 26B accidentally comes into contact with the ferroelectric recording medium 10. Furthermore, vibration and deformation of the needle-shaped electrode 262 caused by airflow generated by the rotation of the ferroelectric recording medium 10 can be suppressed.

[0198] Furthermore, by making the shape of the through-hole 82a triangular or square, the conductive probe 26B can be formed in the shape of a triangular pyramid or a square pyramid.

[0199] ((Third aspect)) Another example of the configuration of the conductive probe 26 is shown in Figure 23. As shown in Figure 23, the conductive probe 26C comprises an insulating layer 263 formed by heating and oxidizing a conductive material 260 on a substrate 261 made of a conductive material, and preferably a needle-shaped electrode 262 is provided on the substrate 261 inside a through-hole 263a of the insulating layer 263, with a part of the needle-shaped electrode 262 protruding from the surface (main surface) 263b of the insulating layer 263. Note that the surface 263b of the insulating layer 263 refers to the main surface of the insulating layer 263, excluding the through-hole 263a.

[0200] The needle-shaped electrode 262 is formed on the surface of the substrate 261 within the through-hole 263a and is covered with an insulating layer 263, thereby reducing the risk of damage to the needle-shaped electrode 262 if it accidentally comes into contact with the ferroelectric recording medium 10. Furthermore, vibration and deformation of the needle-shaped electrode 262 caused by airflow generated by the rotation of the ferroelectric recording medium 10 can be suppressed. In addition, by providing an insulating layer 263 around the needle-shaped electrode 262, the needle-shaped electrode 262 is shielded, thereby suppressing the influence of surrounding charges and preventing the leakage of charges applied to the needle-shaped electrode 262 to the outside.

[0201] A method for manufacturing the conductive probe 26C will now be described. The method for manufacturing the conductive probe 26C includes the steps of forming an insulating layer on a conductive material 260 by oxidizing the conductive material 260, and forming a separation layer on the insulating layer, with a portion of the needle-shaped electrode protruding from the insulating layer. This yields the conductive probe 26C.

[0202] Figure 24 is an explanatory diagram showing an example of a manufacturing method for conductive probe 26C. As shown in Figure 24, an insulating layer 263 is formed by heating and oxidizing the conductive material 260 (see Figures 24(a) and (b)). Then, a separation layer 29 is formed on the insulating layer 263, and a photoresist 82 is applied to the surface of the separation layer 29 and patterned to form circular fine through-holes 82a in the photoresist 82 (see Figure 24(c)). Then, the insulating layer 263 and separation layer 29 of the through-holes 82a are etched using an appropriate etching method (see Figure 24(d)). In Figure 24(d), the photoresist 82 is removed after etching the insulating layer 263 and separation layer 29, but at least some of the photoresist 82 may be left on.

[0203] After etching, metal 83 is deposited on the surface of the conductive material 260 within the through-holes 263a and 29a of the insulating layer 263 and the separation layer 29, and on top of the separation layer 29 (see Figure 24(e)). At this time, as the amount of metal 83 deposited on the photoresist 82 increases, the through-holes 29a become blocked and their width narrows. As a result, the metal deposited on the surface of the conductive material 260 within the through-holes 29a is deposited widely at the surface, and the deposition area gradually narrows as it goes upwards, eventually depositing in a conical shape. This forms a sharpened needle-shaped electrode 262.

[0204] Subsequently, the separation layer 29 is etched, causing a portion of the conical needle-shaped electrode 262, including its top, to protrude slightly from the main surface of the insulating layer 263 (see Figure 24(f)).

[0205] By cutting out the conductive material 260 around the needle-shaped electrode 262, a conductive probe 26C is formed, as shown in Figure 23, which has a needle-shaped electrode 262 formed in a cone shape in a through-hole 263a of the insulating layer 263 on the substrate 261.

[0206] According to this manufacturing method, the needle-shaped electrode 262 is formed on the surface of the substrate 261 within the through-hole 263a and is covered with an insulating layer 263, thereby reducing the risk of damage to the needle-shaped electrode 262 when the conductive probe 26C accidentally comes into contact with the ferroelectric recording medium 10. Furthermore, vibration and deformation of the needle-shaped electrode 262 due to airflow generated by the rotation of the ferroelectric recording medium 10 can be suppressed. In addition, by providing an insulating layer 263 around the needle-shaped electrode 262, the conductive probe 26C can shield the needle-shaped electrode 262, suppressing the influence of surrounding charges and charge leakage from the needle-shaped electrode 262.

[0207] (First piezoelectric element and second piezoelectric element) As shown in Figure 11, in the probe slider 23 with the configuration shown in Figure 11, the first piezoelectric element 27A is provided at the tip of the probe slider 23, between the probe slider 23 and the conductive probe 26, sandwiched between electrodes 28A-1 and 28A-2. The second piezoelectric element 27B is provided between the tip of the probe slider 23 and electrode 28A-2, and electrodes 28B-1 and 28B-2 are provided on the side surface on the +Y axis side and the side surface on the -Y axis side of the second piezoelectric element 27B, respectively. The first piezoelectric element 27A is used to adjust the dynamic fly height (DFH) of the conductive probe 26, and the second piezoelectric element 27B has the function of moving the conductive probe 26 in the track direction of the ferroelectric recording medium 10.

[0208] In this embodiment, electrostrictive elements may be used instead of the first piezoelectric element 27A and the second piezoelectric element 27B. Both the first piezoelectric element 27A and the second piezoelectric element 27B and the electrostrictive element have the common function of being displaced when an electric field is applied, but the direction of displacement of the first piezoelectric element 27A and the second piezoelectric element 27B changes depending on the direction of the electric field, whereas the electrostrictive element only stretches and does not contract. In addition, the first piezoelectric element 27A and the second piezoelectric element 27B generate an electric charge in accordance with the stress, whereas the electrostrictive element does not generate an electric charge even when stress is applied. Since both the first piezoelectric element 27A and the second piezoelectric element 27B and the electrostrictive element have the common function of being displaced when an electric field is applied, they can be used similarly in ferroelectric memory devices.

[0209] As shown in Figure 11, the first piezoelectric element 27A is provided on the lower end surface of the tip of the probe slider 23 and can be used to adjust the dynamic fly height (DFH) of the conductive probe 26. That is, the first piezoelectric element 27A is provided between the probe slider 23 and the conductive probe 26, and by adjusting the distance between the ferroelectric recording medium 10 and the conductive probe 26, the probe slider 23 can be made to levitate and travel on the surface of the ferroelectric recording medium 10 at a nanometer-level distance. The displacement of this first piezoelectric element 27A can be controlled at the nanometer level, and its response time is 10 microseconds or less. Therefore, the distance between the conductive probe 26 and the ferroelectric recording medium 10 can be controlled with high precision and at high speed.

[0210] For the first piezoelectric element 27A and the second piezoelectric element 27B, for example, quartz, lithium niobate (LiNbO3), barium titanate (BaTiO3), lead zirconate titanate (PZT), zinc oxide (ZnO), aluminum nitride (AlN), lithium tantalate (LiTaO3), lead titanate (PT), etc., can be used. The same materials can also be used when electrostrictive elements are used instead of the first piezoelectric element 27A and the second piezoelectric element 27B.

[0211] By equipping the probe slider 23 with a first piezoelectric element 27A at its tip, the DFH control of the conductive probe 26 can be performed with higher precision and speed, and the distance between the conductive probe 26 and the ferroelectric recording medium 10 can be controlled with high precision and speed. Therefore, the ferroelectric storage device 100 can further improve the recording and playback sensitivity of the conductive probe 26.

[0212] Furthermore, by equipping the probe slider 23 with a first piezoelectric element 27A at its tip, it can have an AGC (automatic gain control) function between tracks and sectors of the ferroelectric recording medium 10.

[0213] As described later, a voice coil motor can be used as the probe drive unit 40 (see Figure 8) for moving the conductive probe 26 in the direction of the track on the ferroelectric recording medium 10. However, when a voice coil motor is used, the positioning accuracy of the conductive probe 26 is only about 10 nm. In addition, the time required to move to another track (seek time) is several milliseconds, which hinders the increase in capacity and speed of the ferroelectric memory device 100.

[0214] In this embodiment, a second piezoelectric element 27B is used to move the conductive probe 26 in the track direction of the ferroelectric recording medium 10. This makes it possible to achieve a positioning accuracy of 1 nm or less in the track direction of the ferroelectric recording medium 10, and to perform correction operations within the same track and movement operations to another track in a few microseconds or less. This enables higher capacity and higher speed of the ferroelectric storage device 100.

[0215] As shown in Figures 12 and 13, a first piezoelectric element 27A and a second piezoelectric element 27B are provided between the probe slider 23 and the conductive probe 26. The first piezoelectric element 27A and the second piezoelectric element 27B are preferably formed as columnar shapes such as cubes, rectangular parallelepipeds, or cylinders, as this allows for stable placement between the probe slider 23 and the conductive probe 26. A pair of electrodes 28A-1 and 28A-2 are provided on the +Z axis direction and -Z axis direction faces of the first piezoelectric element 27A. Electrode 28A-1 is provided between the conductive probe 26 and the first piezoelectric element 27A. Electrode 28A-2 is provided between the first piezoelectric element 27A and the second piezoelectric element 27B.

[0216] As shown in Figure 14, the second piezoelectric element 27B is provided with a pair of electrodes 28B-1 and 28B-2. Electrode 28B-1 is provided on the -Y axis side of the second piezoelectric element 27B. Electrode 28B-2 is provided on the +Y axis side of the second piezoelectric element 27B.

[0217] Electrodes 28A-1 and 28A-2 are used to apply voltage to adjust the distance between the conductive probe 26 and the ferroelectric recording medium 10 by extending and retracting the first piezoelectric element 27A in the +Z axis direction and the -Z axis direction. This will be explained later.

[0218] Electrodes 28B-1 and 28B-2 are used to expand and contract the second piezoelectric element 27B in the +Y axis direction and the -Y axis direction, and to move the conductive probe 26 provided on the probe slider 23 in the track direction of the ferroelectric recording medium 10. The second piezoelectric element 27B has its -Z axis direction face fixed to the probe slider 23, but its +Z axis direction face is not fixed. Therefore, when a voltage is applied to electrodes 28B-1 and 28B-2, the +Z axis direction face 271 of the second piezoelectric element 27B expands and contracts in the +Y axis direction and the -Y axis direction. Here, since the +Z axis direction face 271 of the second piezoelectric element 27B expands and contracts equally in the +Y axis direction and the -Y axis direction, the center of the face does not displace in the Y axis direction. However, since the conductive probe 26 is positioned offset from the center of the +Z axis direction face 271 of the second piezoelectric element 27B, it will be displaced in the +Y axis direction or the -Y axis direction, which is the track direction of the ferroelectric recording medium 10. For example, as shown in Figure 25, when the second piezoelectric element 27B is extended in the +Y axis direction and the -Y axis direction, the conductive probe 26 is displaced in the +Y axis direction as indicated by the arrow.

[0219] When moving the conductive probe 26 in the track direction of the ferroelectric recording medium 10, it is preferable to use a probe drive unit 40 such as a voice coil motor or pulse motor for coarse movements where the travel distance of the conductive probe 26 is 10 nm or more, and to use a second piezoelectric element 27B provided on the probe slider 23 to extend and retract the second piezoelectric element 27B and move the conductive probe 26 for fine movements where the travel distance of the conductive probe 26 is less than 10 nm.

[0220] [Ferroelectric recording medium drive unit] As shown in Figure 8, the ferroelectric recording medium drive unit 30 drives and rotates the ferroelectric recording medium 10. Figure 26 is a cross-sectional view showing the configuration of the ferroelectric recording medium drive unit 30. As shown in Figure 26, the ferroelectric recording medium drive unit 30 has a housing (bearing cylinder) 31, a bearing sleeve 32, a shaft member (spindle shaft) 33, a housing bottom 34, a permanent magnet 35, a stator 36, and lubricating oil O. Within the housing 31, the ferroelectric recording medium drive unit 30 supports the spindle shaft 33 in a non-contact manner in the radial direction (direction perpendicular to the spindle shaft 33) by the pressure generated by the dynamic pressure action of the lubricating oil O.

[0221] The housing 31 is a container that houses a portion of the spindle shaft 33 and is formed to allow the spindle shaft 33 to be inserted and removed.

[0222] The bearing sleeve 32 is located inside the housing 31.

[0223] The spindle shaft 33 is a rod-shaped member inserted into the inner circumferential surface of the bearing sleeve 32 and is conductive. The spindle shaft 33 can be made of a conductive material such as metal. The spindle shaft 33 is inserted into the opening 10a (see Figure 3) of the ferroelectric recording medium 10 and connected to the ferroelectric recording layer 13 via the substrate 11 and the electrode layer 12 (see Figure 3). Because the spindle shaft 33 is conductive, information can be read from and written to the ferroelectric recording layer 13 by the conductive probe 26.

[0224] The shaft end 331 of the spindle shaft 33 preferably has a convex curved surface. The radius of curvature R of the curved surface of the shaft end 331 is preferably 2 mm or more, and more preferably 5 mm or more. The curved surface of the shaft end 331 may also be concave.

[0225] The spindle shaft 33 may have a V-shaped groove 332 on its outer circumference. The presence of the groove 332 allows the lubricating oil O to flow as the spindle shaft 33 rotates, making it easier to collect the lubricating oil O at the apex of the V-shape of the groove 332. This generates pressure, supporting the spindle shaft 33.

[0226] The housing bottom 34 is provided inside the housing 31 so as to face the axial end 331 of the spindle shaft 33, and is electrically conductive.

[0227] The housing bottom 34, like the shaft end 331, preferably has a curved surface formed in a convex or concave shape. The radius of curvature R of the curved surface of the housing bottom 34 is preferably 2 mm or more, and more preferably 5 mm or more.

[0228] Multiple permanent magnets 35 are provided on the inner circumferential surface of the cover 37, along the circumferential direction.

[0229] The stator 36 is located between the housing 31 and the permanent magnet 35.

[0230] Lubricating oil O fills the gap between the inner circumferential surface of the bearing sleeve 32 and the outer circumferential surface of the spindle shaft 33. The lubricating oil O generates pressure through dynamic pressure action, supporting the spindle shaft 33 in a non-contact manner in its radial direction.

[0231] The lubricating oil O preferably contains inorganic conductive powder. By configuring the lubricating oil O to contain inorganic conductive powder, electrical connection can be made between the housing 31 and the spindle shaft 33.

[0232] Preferred inorganic conductive powders include metal-based powders such as silver, copper, nickel, tin, silver-plated copper, stainless steel, aluminum, brass, iron, and zinc; carbon-based powders such as carbon, carbon black, graphite, and carbon nanotubes; metal oxide-based powders such as tin oxide, indium oxide, and zinc oxide; and metal-plated materials with a coating layer formed on the surface of glass, mica powder, glass fiber, carbon fiber, etc.

[0233] The metal powder can be in the form of powder, sphere, fiber, or foil flakes.

[0234] The carbon-based powder is preferably spherical or fibrous in shape.

[0235] The metal oxide powder is preferably in the form of a powder or a sphere.

[0236] The metal-plated powder is preferably in the form of a powder or a sphere.

[0237] These materials have high heat resistance, good volatility resistance, and do not increase the viscosity of lubricating oil O when incorporated into it.

[0238] The particle size of the conductive powder is preferably 0.1 μm to 10 μm if it is in particulate form. If the conductive powder is in the form of foil, the side length is preferably 0.1 μm to 100 μm. If the conductive powder is in the form of fibers, the fiber length is preferably 0.1 μm to 100 μm. When the conductive powder is in the form of parts, foil, or fibers, as long as the particle size of the conductive powder is within the above preferred range, it can be dispersed in the lubricating oil O without increasing the viscosity of the lubricating oil O.

[0239] In the ferroelectric recording medium drive unit 30, the spindle shaft 33 is rotated by the coupling of the permanent magnet 35 with the electromagnet of the stator 36. The spindle shaft 33 is subjected to a downward thrust as shown in Figure 26 by magnetic coupling with the electromagnet of the stator 36, its own weight, and other methods, and this thrust is supported by the housing bottom 34.

[0240] [Probe drive unit] As shown in Figure 8, the probe drive unit 40 drives the probe slider 23.

[0241] [Control Unit] As shown in Figure 11, the ferroelectric memory device 100 may include a control unit (not shown) if the probe slider 23 has a first piezoelectric element 27A and a second piezoelectric element 27B between the probe slider 23 and the conductive probe 26 at its tip.

[0242] A control unit (not shown) is mounted as a printed circuit board (PCB) on the back side of the housing 60. The control unit (not shown) is electrically connected to the first piezoelectric element 27A and the second piezoelectric element 27B and the conductive probe 26. The control unit (not shown) has the function of adjusting the distance and relative position between the ferroelectric recording medium 10 and the conductive probe 26 by controlling the voltage applied to the first piezoelectric element 27A and the second piezoelectric element 27B to expand and contract the first piezoelectric element 27A and the second piezoelectric element 27B. Preferably, the control unit (not shown) controls the voltage applied to the first piezoelectric element 27A and the second piezoelectric element 27B based on a reading signal from the conductive probe 26 to expand and contract the first piezoelectric element 27A and the second piezoelectric element 27B.

[0243] Furthermore, when the first piezoelectric element 27A is expanded or contracted in the +Z axis direction or the -Z axis direction, the first piezoelectric element 27A will also expand or contract in the +Y axis direction or the -Y axis direction. Preferably, the control unit (not shown) has a function to compensate for this expansion or contraction of the first piezoelectric element 27A in the +Y axis direction or the -Y axis direction by expanding or contracting the second piezoelectric element 27B in the +Y axis direction or the -Y axis direction.

[0244] The control unit (not shown) increases the voltage applied to the first piezoelectric element 27A when the read signal level from the conductive probe 26 is low, thereby reducing the distance between the conductive probe 26 and the ferroelectric recording medium 10 and increasing the signal level. On the other hand, the control unit (not shown) decreases the voltage applied to the first piezoelectric element 27A when the read signal level from the conductive probe 26 is high, thereby increasing the distance between the conductive probe 26 and the ferroelectric recording medium 10 and decreasing the signal level. This enables highly accurate and responsive control of the distance between the ferroelectric recording medium 10 and the conductive probe 26A. Therefore, the control unit (not shown) can provide the ferroelectric storage device 100 with an AGC (automatic gain control) function between tracks and sectors of the ferroelectric recording medium 10.

[0245] [Recording and Playback Signal Processing Unit] The recording and playback signal processing unit 50 shown in Figure 8 has the function of processing signals for writing and reading information with the conductive probe 26.

[0246] The recording / replaying signal processing unit 50 writes information to the ferroelectric recording layer 13 constituting the ferroelectric recording medium 10 by applying a positive or negative voltage using the conductive probe 26. It also reads information by reading the positive or negative charge written to the ferroelectric recording layer 13 using the conductive probe 26. When writing, the recording / replaying signal processing unit 50 generates a positive or negative voltage corresponding to the writing information to be applied to the conductive probe 26, and when reading, it processes the electrical signal from the conductive probe 26 and converts it into the written information.

[0247] The recording and playback signal processing unit 50 is equipped with a bipolar power supply (not shown) internally. During writing, for example, if the information to be written is a binary number of 1 or 0, a positive voltage is generated for 1 and a negative voltage for 0 using the bipolar power supply (not shown).

[0248] The recording and playback signal processing unit 50 preferably records (writes) multi-valued information to the ferroelectric layer 131 included in the ferroelectric recording layer 13 of the ferroelectric recording medium 10 and plays back (reads) the recorded multi-valued information. Since the content regarding the recording and playback of multi-valued information is as described above, the details are omitted.

[0249] As described above, position information (servo information) for detecting the relative position between the conductive probe 26 of the probe slider 23 and the track on the ferroelectric recording medium 10 may be recorded in the ferroelectric layer 131. Then, a servo information area in which servo information is recorded and a data area for recording and playing back data may be alternately arranged at regular intervals in the circumferential direction of the ferroelectric recording medium 10.

[0250] The servo information may be written to the ferroelectric recording medium 10 using a servo writer (not shown) before the ferroelectric recording medium 10 is incorporated into the ferroelectric memory device 100, or may be written using the recording and playback signal processing unit 50 after the ferroelectric recording medium 10 is incorporated into the ferroelectric memory device 100. In the latter case, after mechanically fixing the actuator arm 21 or the suspension arm 22 using a lever (not shown) from outside the ferroelectric memory device 100, the servo information may be written to the ferroelectric recording medium 10 while finely moving the lever (not shown) to position the conductive probe 26 on the surface of the ferroelectric recording medium 10.

[0251] In this case, it is preferable that the servo information and the data area for recording and playing back data are alternately arranged at regular intervals in the circumferential direction of the track on the ferroelectric recording medium 10. Thereby, during the playback of the recorded data, the probe slider 23 can more accurately detect the position of the conductive probe 26 based on the servo information.

[0252] As described above, it is preferable that the recording / reproducing signal processing unit 50 records multi-valued information in the smallest one recording area by the simplest one writing operation in the ferroelectric memory device 100, and reproduces the information multi-valued recorded in the ferroelectric recording medium 10 by the simplest one reading operation in the ferroelectric memory device 100.

[0253] As described above, the burst information area 131B-1, the address information area 131B-2, and the preamble information area 131B-3 may be included in the servo information area 131B of the ferroelectric layer 131. In this case, in the ferroelectric recording medium 10, the conductive probe 26 moving on the surface in the circumferential direction reads the preamble information in the preamble information area 131B-3 to prepare for reading the address information. Then, the conductive probe 26 reads the address information of the data area in the address information area 131B-2. Then, the probe slider 23 reads the burst information in the burst information area 131B-1 to finely adjust the track position (radial position). After that, the conductive probe 26 can record information in the data area 131A.

[0254] As described above, the reference signal information 131B-4 for multi-valued recording may be included in the servo information area 131B of the ferroelectric layer 131. In this case, by reading the reference signal information in the servo information area 131B with the conductive probe 26, the recording / reproducing signal processing unit 50 can grasp the signal level of the multi-valued recording, and reproduce the multi-valued information recorded in the data area 131A using the grasped signal level.

[0255] When writing information to the ferroelectric recording medium 10, the recording and reproducing signal processing unit 50 preferably adjusts the voltage waveform generated from a bipolar power supply (not shown) and applied to the conductive probe 26 to any one of a triangular wave, a sawtooth wave, and a trapezoidal wave, as shown in FIG. 27. When the voltage waveform generated from the bipolar power supply (not shown) is a square wave, a large amount of charge flows at the moment when a positive or negative voltage is applied, so the sharp tip of the conductive probe 26 may become dull due to thermal melting or rapid field evaporation. In the present embodiment, by making the voltage waveform any one of a triangular wave, a sawtooth wave, and a trapezoidal wave, the potential can be gradually increased from the zero potential, so that damage to the conductive probe 26 can be reduced.

[0256] At the time of reading, for example, while applying an alternating current electric field smaller than the coercive electric field of the ferroelectric layer 131 to the conductive probe 26, by measuring the capacitance change of the ferroelectric layer 131 between the conductive probe 26 and the electrode layer 12, it is known that the information written in the ferroelectric recording medium 10 can be read as a capacitance change. This principle is as follows.

[0257] Assuming that the voltage applied to the conductive probe 26 is E, the electric flux density derived from the charge of the ferroelectric is D, the dielectric constant is ε, and the polarization voltage is P, the electric flux density D derived from the charge of the ferroelectric is expressed by the following formula (1).

[0258]

Equation

[0259] Here, when the voltage E is an alternating current, the following formula (2) is obtained. Substituting this into the above formula (1), the dielectric constants ε3, ε5,... with odd subscripts become non-linear and the sign changes depending on the direction of the spontaneous polarization of the ferroelectric. Therefore, by measuring the variation of this dielectric constant, the direction of the spontaneous polarization of the ferroelectric can be known. E = E p cos ωt ···(2) (In the formula, E p is the peak voltage of the alternating current.)

[0260] On the other hand, this method is limited by the frequency of the AC electric field, meaning that information cannot be read at a bit rate higher than the frequency of the AC electric field. For example, to achieve a read speed of 1 Gbit / s or more, an AC electric field of 1 GHz or higher is required. Here, the dielectric constant of the ferroelectric material depends on the frequency of the AC electric field, and the loss increases as the frequency increases. Therefore, the types of ferroelectric materials that can be used in ferroelectric rotating media that can achieve high-speed operation are limited.

[0261] In this embodiment, instead of using an alternating electric field to read information from the ferroelectric recording medium, the charge of the ferroelectric layer 131 is detected using a weak tunnel current flowing between the conductive probe 26 and the electrode layer 12.

[0262] That is, since the conductive probe 26 is positioned very close to the surface of the ferroelectric layer 131, the capacitance C between the conductive probe 26 and the electrode layer 12, originating from the ferroelectric layer 131, is given by the following equation (3). C = ε·ε0A / d ···(3) (In the formula, A is the relative area of ​​the conductive probe, d is the thickness of the ferroelectric layer, ε is the relative permittivity of the ferroelectric layer, and ε0 is the permittivity of vacuum.)

[0263] Then, if Q is the charge stored in the ferroelectric layer 131, the voltage V generated at the conductive probe 26 is given by equation (4) below. By detecting this voltage V, the information written to the ferroelectric layer 131 can be read. V = Q / C ... (4)

[0264] Here, since the ferroelectric material is an insulator with a large band gap, tunnel current is difficult to flow, and it is difficult to detect the voltage V in equation (4) above. However, by making the ferroelectric material a thin film, the tunnel barrier is reduced, and by forming a junction structure with the conductive electrode layer, a weak tunnel current can be made to flow from the electronic state at the junction. Furthermore, by adding a paraelectric layer to this junction, and creating a junction structure in which the ferroelectric, paraelectric, and conductor are joined in this order, band bending occurs at the interface between the ferroelectric and paraelectric materials in a direction that lowers the tunnel barrier due to the charge, making it even easier for tunnel current to flow.

[0265] Furthermore, since the tunnel barrier of the ferroelectric layer 131 changes depending on the polarization direction, the polarization direction of the ferroelectric layer 131 can be determined by measuring the tunnel current between the ferroelectric layer 131 and the conductive probe 26. For example, in a certain ferroelectric material, positively charging the surface side increases the tunnel barrier, and the tunnel current from the ferroelectric layer 131 towards the conductive probe 26 decreases. On the other hand, negatively charging the surface side increases the tunnel current from the ferroelectric layer 131 towards the conductive probe 26.

[0266] The recording and playback signal processing unit 50 may apply a bias voltage to the conductive probe 26 when detecting and reading the charge of the ferroelectric layer 131.

[0267] The bias voltage applied to the conductive probe 26 is used to facilitate the detection of tunnel current between the ferroelectric layer 131 and the conductive probe 26, and also to reduce fluctuations in the amount of charge stored in the ferroelectric layer 131 when reading out information.

[0268] The bias voltage can be positive, negative, or both. When the bias applied to the conductive probe 26 is a constant positive or negative voltage, the polarization direction of the ferroelectric layer 131 is detected by the magnitude of the tunnel current generated by the bias application.

[0269] For example, in a ferroelectric material where the tunnel barrier increases when the surface side is positively charged and decreases when the surface side is negatively charged, when a bias voltage is applied such that a tunnel current flows from the ferroelectric layer 131 side (electrode layer 12 side) in the direction of the conductive probe 26, the magnitude of the tunnel barrier and the magnitude of the tunnel current are inversely related, and from this, the charge direction of the ferroelectric layer 131 can be detected.

[0270] Also, when the bias voltage applied to the conductive probe 26 is both positive and negative, the polarization direction of the ferroelectric layer 131 can be detected by comparing the tunnel current when a positive bias voltage is applied with the tunnel current when a negative bias voltage is applied. In this case, when the reading speed of information from the ferroelectric layer 131 is N bits / second (N is a number greater than or equal to 1), the bias voltage is preferably a sine wave or a rectangular wave with a frequency of N Hz or higher (N is a number greater than or equal to 1). By doing so, the polarization direction of the ferroelectric layer 131 can be detected with higher accuracy. The reason is as follows. That is, since the tunnel current varies depending on the distance between the ferroelectric layer 131 and the conductive probe 26, in order to detect the polarization direction of the ferroelectric layer 131, it is necessary to distinguish this variation from the variation of the tunnel current due to the polarization direction of the ferroelectric layer 131. On the other hand, when positive and negative bias voltages are used, the polarization direction of the ferroelectric layer 131 is the relative comparison of the tunnel currents at the time of positive bias and negative bias. As a result, the variation of the tunnel current due to the distance between the ferroelectric layer 131 and the conductive probe 26 is canceled, and thus it is less affected.

[0271] Furthermore, when the recording / replaying signal processing unit 50 detects the charge of the ferroelectric layer 131 in the ferroelectric memory device 100 and reads information, if the amount of charge stored in the ferroelectric layer 131 decreases and the resulting tunnel current decreases, the unit may rewrite (refresh) the same information that was written to the ferroelectric recording medium 10 to the location where the same information was read from the ferroelectric recording medium 10 in order to compensate for the charge that was reduced by reading information from the ferroelectric recording medium 10. Note that the amount of charge stored in the ferroelectric layer 131 can also be caused by charge being trapped in defects contained in the ferroelectric layer 131.

[0272] Furthermore, the recording and playback signal processing unit 50 may perform rewriting after each reading of information from the ferroelectric recording medium 10, or after a predetermined number of readings.

[0273] The information read from the ferroelectric layer 131 described above is obtained using a non-destructive method, but a destructive method can also be used to read information from the ferroelectric layer 131.

[0274] When a non-destructive method is used to read the information, the electric field generated by the bias applied to the conductive probe 26 does not exceed the coelectric field of the ferroelectric material constituting the ferroelectric layer 131, and therefore the polarization direction of the ferroelectric layer 131 does not change when reading the information.

[0275] On the other hand, when using the destructive method, a bias voltage exceeding the coelectric field of the ferroelectric material is applied to the conductive probe 26, and information is read by detecting the tunnel current when the polarization direction of the ferroelectric layer 131 changes. For example, if the conductive probe 26 side of the ferroelectric layer 131 has a positive charge, and a negative bias voltage is applied to the conductive probe 26, the charge of the ferroelectric layer 131 does not reverse, so the tunnel current flowing from the ferroelectric layer 131 to the conductive probe 26 is small. On the other hand, if the ferroelectric layer 131 has a negative charge, the negative bias applied to the conductive probe 26 reverses the charge of the ferroelectric layer 131 to a positive value, so the tunnel current increases. By observing this fluctuation in tunnel current, the information recorded in the ferroelectric layer 131 can be read. Note that when using the destructive method to read information, it is necessary to rewrite the read information to the ferroelectric layer 131.

[0276] The interatomic force between the conductive probe 26 and the ferroelectric layer 131 may be used to read information from the ferroelectric recording medium 10. A method using the interatomic force between the conductive probe 26 and the ferroelectric layer 131 to read information from the ferroelectric recording medium 10 will be described. Since the interatomic force between the conductive probe 26 and the ferroelectric layer 131 is affected by the charge of the ferroelectric layer 131, the information recorded on the ferroelectric recording medium 10 can be read by measuring the interatomic force between the two. In this case, unlike when tunnel current is used to read the information, the amount of charge stored in the ferroelectric layer 131 does not decrease, so refreshing is unnecessary. In addition, it becomes unnecessary to consider the band gap of the ferroelectric material used in the ferroelectric layer 131 and the interfacial electronic state between it and the electrode layer 12.

[0277] Here, an atomic force microscope (AFM) is known as a device that detects and maps the interatomic interactions between the surface of a sample and a probe. In an AFM, an optical lever method is used, where a laser beam is shone onto a cantilever equipped with a probe while the sample is moved along the XY axis, and interatomic interactions are detected from the shift in the reflected light. Because this method relies on detecting the physical fluctuations of the cantilever, it is difficult to use for reading information in the GHz band. Also, because laser light is used for detection, it may induce the internal photoelectric effect or temperature rise of the ferroelectric material used in ferroelectric recording media.

[0278] Therefore, in this embodiment, as shown in Figure 28, a third piezoelectric element 27C can be provided between the probe slider 23 and the conductive probe 26. This allows the third piezoelectric element 27C to detect the interatomic force between the conductive probe 26 and the ferroelectric layer 131, and to convert this detected interatomic force into an electrical signal. In this case, since an optical lever is not used to detect the interatomic force, it becomes possible to read information in the GHz band.

[0279] For example, the third piezoelectric element 27C can be made of quartz, lithium niobate (LiNbO3), barium titanate (BaTiO3), lead zirconate titanate (PZT), zinc oxide (ZnO), aluminum nitride (AlN), lithium tantalate (LiTaO3), lead titanate (PT), etc. The third piezoelectric element 27C may be made of the same material as the first piezoelectric element 27A and the second piezoelectric element 27B.

[0280] To enhance the ability to detect interatomic forces, the third piezoelectric element 27C is preferably positioned close to the conductive probe 26. Therefore, as shown in Figure 27, the third piezoelectric element 27C is preferably placed between the conductive probe 26 and the fourth piezoelectric element 27D. The fourth piezoelectric element 27D, like the first piezoelectric element 27A, is used to drive the conductive probe 26 in the +Z axis direction or the -Z axis direction, bringing the conductive probe 26 closer to the surface of the ferroelectric recording medium 10. The fourth piezoelectric element 27D may be made of the same material as the third piezoelectric element 27C.

[0281] [Cabinet] As shown in Figure 8, the housing 60 is formed in a substantially rectangular shape and houses a ferroelectric recording medium 10, a conductive probe 26, a probe slider 23, a ferroelectric recording medium drive unit 30, and a recording / reproduction signal processing unit 50.

[0282] In the ferroelectric memory device 100, it is preferable to fill the housing 60 with at least one of argon gas, nitrogen gas, and helium gas. Inside the housing 60, friction between the ferroelectric recording medium 10, the ferroelectric recording medium drive unit 30, and the conductive probe 26 as they move, and friction between these objects and the air, generates so-called triboelectric charging. This charge can combine with the charge recorded on the ferroelectric recording medium 10, causing the written information to be lost, and may also adversely affect the reading and writing of information by the conductive probe 26. In this embodiment, triboelectric charging can be mitigated by filling the housing 60 with these gases.

[0283] Furthermore, the triboelectric charge within the housing 60 can be evaluated by the number of triboelectric charges using a known measurement method.

[0284] Thus, the ferroelectric memory device 100 comprises a ferroelectric recording medium 10, a conductive probe 26, a probe slider 23, a ferroelectric recording medium drive unit 30, and a recording / reproduction signal processing unit 50. The recording / reproduction signal processing unit 50 can record multi-level information onto the ferroelectric recording medium 10 and reproduce the recorded multi-level information. The multi-level information can be recorded in the smallest single recording area of ​​the ferroelectric layer 131 of the ferroelectric recording layer 13 provided on the ferroelectric recording medium 10.

[0285] The ferroelectric memory device 100 can record multi-level information on the ferroelectric recording medium 10 using the recording and playback signal processing unit 50, thereby increasing the recording density of the ferroelectric recording medium 10. By increasing the recording density of the ferroelectric recording medium 10, the ferroelectric memory device 100 can be miniaturized in terms of unit storage capacity, and the speed required for recording information on the ferroelectric layer 131 and for retrieving recorded information (read / write speed) can be increased to, for example, 10 Gbps or more.

[0286] Furthermore, the ferroelectric memory device 100 can suppress the increase in power consumption per unit storage capacity required for recording and retrieving information on the ferroelectric recording medium 10 by increasing the recording density of the ferroelectric recording medium 10.

[0287] Therefore, the ferroelectric memory device 100 can improve recording density, reduce the size of the device per unit storage capacity, and increase the read / write speed, while also suppressing an increase in energy consumption.

[0288] In typical magnetic recording media such as HDDs, information is recorded (written) and read (reproduced) by moving the read / write head in the track direction (radial direction) while the magnetic recording media rotates at 5000rpm to 10000rpm (83 revolutions / second to 167 revolutions / second). The size of one information bit is approximately 5nm in the sector direction (circumferential direction) and 50nm in the track direction, and this one bit contains approximately 10 magnetic particles. The read / write speed is approximately 1Gbps on average. The ferroelectric recording media 10 records information by polarization reversal caused by lattice distortion of the ferroelectric crystal contained in the ferroelectric layer 131. Therefore, compared to magnetic recording media that record information by magnetizing the magnetic layer at the magnetic particle level, the recording density and read / write speed can be significantly increased. Furthermore, in the ferroelectric storage device 100, since the ferroelectric recording media 10 has a recording area in which the ferroelectric layer 131 is recorded at multiple levels, it is possible to improve recording density, miniaturize the device, and increase processing speed, while suppressing the increase in energy consumption.

[0289] The ferroelectric memory device 100 can achieve storage for wireless and mobile communications of, for example, 10 Gbps or more by improving recording density, increasing read / write speed, and miniaturizing the device. Furthermore, the ferroelectric memory device 100 can reduce the environmental impact by suppressing the increase in power consumption and conserving energy, thereby reducing resource consumption.

[0290] In the ferroelectric memory device 100, the recording and playback signal processing unit 50 can read position information (servo information) from the ferroelectric layer 131 to detect the relative position of the conductive probe 26 and the ferroelectric recording medium 10 in the track direction of the ferroelectric recording medium 10. As a result, the ferroelectric memory device 100 can accurately detect the position of the conductive probe 26 using the servo information during playback of data recorded in the data area of ​​the ferroelectric layer 131 of the ferroelectric recording medium 10, thereby enabling recording and playback to the ferroelectric layer 131 with high accuracy. Therefore, the ferroelectric memory device 100 can increase the processing speed during information recording and playback of recorded information.

[0291] The ferroelectric memory device 100 can alternately arrange the servo information area, where servo information is recorded, and the data area, where information is written and read, in the circumferential direction of the track on the ferroelectric recording medium 10. As a result, the ferroelectric memory device 100, in a control unit (not shown), can accurately detect the position of the conductive probe 26 for each data area using the servo information while reproducing the data recorded in the data area. Therefore, the ferroelectric memory device 100 can improve the accuracy of writing information to the ferroelectric layer 131 and reading recorded information, thereby further increasing the processing speed during information recording and reproduction.

[0292] The ferroelectric memory device 100 can include reference signal information 131B-4 in the servo information area 131B of the ferroelectric layer 131 contained in the ferroelectric recording medium 10. As a result, the ferroelectric memory device 100 can regenerate the multi-level information recorded in the data area 131A by using the signal level of the multi-level recording, which has been read and grasped by the conductive probe 26, based on the reference signal information 131B-4. Therefore, the ferroelectric memory device 100 can further reduce the energy consumption per unit storage capacity.

[0293] The method for writing and reading information in the ferroelectric memory device 100 having the above configuration allows for the writing of multi-level information to the ferroelectric recording medium 10 and the writing of recorded multi-level information to the smallest single recording area in the simplest single operation. By using the above method for writing and reading information, the recording and playback signal processing unit 50 can improve the recording density stored in the ferroelectric layer 131, reduce the size of the device per unit storage capacity, increase the read and write speed, and suppress the increase in energy consumption.

[0294] The ferroelectric memory device 100 can be equipped with a first piezoelectric element 27A and a second piezoelectric element 27B on the probe slider 23. The ferroelectric memory device 100 can extend and retract the first piezoelectric element 27A in the height direction using a pair of electrodes 28A-1 and 28A-2, and extend and retract the second piezoelectric element 27B in the track direction of the ferroelectric recording medium 10 using electrodes 28B-1 and 28B-2. As a result, the ferroelectric memory device 100 can control the distance between the ferroelectric recording medium 10 and the conductive probe 26 at the nanoscale, and move the conductive probe 26 in the track direction of the ferroelectric recording medium 10 at the nanoscale. Furthermore, the movement time within the same track and the movement time to another track (seek time) can be reduced to a few microseconds or less. Therefore, the ferroelectric memory device 100 can achieve high-precision positioning of the conductive probe 26 in the data plane direction and track direction of the ferroelectric recording medium 10, with accuracy of 1 nm or less, and can perform correction operations within the same track and movement operations to another track in a few microseconds or less. Thus, the ferroelectric memory device 100 can improve recording capacity and increase the speed required for recording and retrieving information.

[0295] The ferroelectric memory device 100 can be configured to have a second piezoelectric element 27B between the probe slider 23 and the conductive probe 26, with the conductive probe 26 offset from the center of the mounting surface of the second piezoelectric element 27B in the track direction of the ferroelectric recording medium 10. This allows the conductive probe 26 to move in the track direction due to the expansion and contraction of the second piezoelectric element 27B. As a result, the ferroelectric memory device 100 can improve the positioning accuracy of the conductive probe 26 in the track direction of the ferroelectric recording medium 10, and can perform correction operations within the same track and movement operations to other tracks more reliably in a few microseconds or less. Thus, the ferroelectric memory device 100 can achieve further improvements in recording capacity and further increases in the speed required for recording and retrieving information.

[0296] The ferroelectric memory device 100 uses the probe drive unit 40 for coarse movements of the conductive probe 26 in the track direction of the ferroelectric recording medium 10 when the movement distance is 10 nm or more, and the second piezoelectric element 27B for fine movements of less than 10 nm. This allows the ferroelectric memory device 100 to appropriately move the conductive probe 26 according to the movement distance of the conductive probe 26 in the track direction of the ferroelectric recording medium 10. Thus, the ferroelectric memory device 100 can further improve the recording density and further increase the speed required for recording and retrieving information.

[0297] In the ferroelectric memory device 100, the recording and playback signal processing unit 50 detects the charge of the ferroelectric layer 131 by applying positive and negative bias voltages to the conductive probe 26 and measuring the weak tunnel current flowing between the conductive probe 26 and the electrode layer 12. As a result, the ferroelectric memory device 100 can read information without using an AC electric field in the recording and playback signal processing unit 50, and the reading speed is no longer limited by the frequency of the AC electric field. Therefore, information stored in the ferroelectric recording medium 100 can be read at high speed.

[0298] The ferroelectric memory device 100 has a recording / reproduction signal processing unit 50 that can apply a positive or negative bias voltage to the conductive probe 26 when detecting the charge in the ferroelectric layer 131. Since the ferroelectric memory device 100 can detect the charge stored in the ferroelectric layer 131 by applying a voltage to the conductive probe 26, it can read the information stored in the ferroelectric layer 131 at high speed.

[0299] The ferroelectric memory device 100's recording and playback signal processing unit 50 can apply positive and negative bias voltages to the conductive probe when detecting the charge in the ferroelectric layer 131. This allows the ferroelectric memory device 100 to detect the polarization direction of the ferroelectric layer 131 by comparing the tunnel current when a positive bias voltage is applied with the tunnel current when a negative bias voltage is applied. Therefore, by applying a voltage to the conductive probe 26, the ferroelectric memory device 100 can more easily detect the charge stored in the ferroelectric layer 131, and thus more reliably read the information stored in the ferroelectric layer 131.

[0300] The ferroelectric memory device 100 can apply positive and negative bias voltages as sine waves or square waves. This makes it easier for the ferroelectric memory device 100 to detect the polarization direction of the ferroelectric layer 131. Therefore, by applying a voltage to the conductive probe 26, the ferroelectric memory device 100 can more easily detect the charge stored in the ferroelectric layer 131, thereby enabling more reliable reading of the information stored in the ferroelectric layer 131.

[0301] The method for reading information in the ferroelectric memory device 100 having the above configuration involves comparing the tunnel current flowing between the conductive probe 26 and the ferroelectric layer 131 when a positive bias voltage is applied to the conductive probe 26 with the tunnel current flowing between the conductive probe 26 and the ferroelectric layer 131 when a negative bias voltage is applied to the conductive probe 26 to detect the charge of the ferroelectric layer 131. By using the above information reading method, the recording and playback signal processing unit 50 can more easily detect the charge stored in the ferroelectric layer 131 and more reliably read the information stored in the ferroelectric layer 131.

[0302] In the ferroelectric memory device 100 having the above configuration, when the information reading speed is N bits / second (where N is a number of 1 or more), the frequency of the applied bias voltage can be set to NHz (where N is a number of 1 or more). As a result, by using the above information reading method, the recording and playback signal processing unit 50 can more easily detect the charge stored in the ferroelectric layer 131, and thus the information stored in the ferroelectric layer 131 can be read more reliably.

[0303] The method for writing and reading information in the ferroelectric memory device 100 having the above configuration involves rewriting the same information that was written to the ferroelectric recording medium 10 to the location where the same information was read from the ferroelectric recording medium 10. By using the above method for writing and reading information, the charge lost due to reading information from the ferroelectric recording medium 10 can be compensated for. Therefore, when the recording / reproduction signal processing unit 50 measures the weak tunnel current flowing between the conductive probe 26 and the electrode layer 12, the charge of the ferroelectric layer 131 can be stably detected.

[0304] The method for writing and reading information in the ferroelectric memory device 100 having the above configuration allows rewriting to be performed after each reading of information from the ferroelectric recording medium 10, or after a predetermined number of information readings. This makes it possible to replenish the charge lost due to reading information from the ferroelectric recording medium 10 after each reading, and thus allows for appropriate rewriting at the necessary time according to the amount of charge loss in the ferroelectric layer 131.

[0305] In the ferroelectric memory device 100, the radius of curvature R of at least one of the axial end 331 of the spindle shaft 33 of the ferroelectric recording medium drive unit 30 and the housing bottom 34 can be made into a convex or concave spherical surface of 2 mm or more. As a result, the ferroelectric memory device 100 can more stabilize the torque even when the ferroelectric recording medium drive unit 30 is used for a long time, and can more effectively suppress the decrease in conductivity between the spindle shaft 231 and the housing 31.

[0306] The ferroelectric memory device 100 allows the conductive probe 26 to be conical in shape. This gives the conductive probe 26 a sharp tip, allowing for increased field strength and a reduction in the voltage applied to the needle-shaped electrode 262. Therefore, the ferroelectric memory device 100 can advantageously record information onto the ferroelectric recording medium 10.

[0307] The ferroelectric memory device 100 can have a conductive probe 26A shaped like a triangular pyramid or a square pyramid. This allows the ferroelectric memory device 100 to be equipped with a conductive probe 26 having a sharp tip.

[0308] The ferroelectric memory device 100 can have a conductive probe 26A with a rotationally symmetric shape around an axis passing through its tip. This allows the conductive probe 26A to uniformize the electric field distribution at its tip during writing and stabilize the tunnel current flowing between the conductive probe 26A and the ferroelectric layer 131 during reading. As a result, the ferroelectric memory device 100 can stably write to and read from the ferroelectric recording medium 10.

[0309] A method for manufacturing the conductive probe 26A may include the steps of forming a triangular or square-shaped mask on the surface of a conductive material 260, and etching the conductive material 260 to obtain a needle-shaped electrode 262 formed in the shape of a triangular pyramid or a square pyramid. This allows for the reproducible manufacture of a conductive probe 26A having a needle-shaped electrode 262 formed in the shape of a triangular pyramid or a square pyramid on a substrate 261.

[0310] Furthermore, the method for manufacturing the conductive probe 26A may include the steps of forming a mask 72A having triangular or square-shaped through holes 72a on the surface of a conductive material 260, and depositing a conductive material onto the surface of the conductive material 260 within the through holes 72a to obtain a needle-shaped electrode 262A formed in the shape of a triangular pyramid or a square pyramid. This makes it possible to manufacture a conductive probe 26A having a needle-shaped electrode 262A formed in the shape of a triangular pyramid or a square pyramid on a substrate 261 with good reproducibility.

[0311] The ferroelectric memory device 100 has a conductive probe 26B which comprises a substrate 261 made of a conductive material 260, a recess 212, and a sharpened needle-shaped electrode 262, with a portion of the needle-shaped electrode 262 protruding from the surface of the conductive material 260. This allows the ferroelectric memory device 100 to suppress damage to the sharpened needle-shaped electrode 262. Furthermore, the ferroelectric memory device 100 can suppress vibration and deformation of the needle-shaped electrode 262 due to airflow generated by the rotation of the ferroelectric recording medium 10. In addition, the ferroelectric memory device 100 can shield the needle-shaped electrode 262, suppressing the influence of surrounding charges and charge leakage from the needle-shaped electrode 262.

[0312] The ferroelectric memory device 100 has a conductive probe 26C which comprises a substrate 261 made of a conductive material 260, an insulating layer 263 having through holes 263a provided on the substrate 261, and a cone-shaped needle electrode 262 formed on the substrate 261 inside the through holes 263a, and a portion of the needle electrode can protrude from the surface of the insulating layer 263. Even in this case, the ferroelectric memory device 100 can suppress damage to the needle electrode 262. Furthermore, the ferroelectric memory device 100 can suppress vibration and deformation of the needle electrode 262 due to airflow generated by the rotation of the ferroelectric recording medium 10. In addition, the ferroelectric memory device 100 can shield the needle electrode 262 and suppress the influence of surrounding charges and charge leakage from the needle electrode 262.

[0313] The method for manufacturing the conductive probe 26B includes the steps of: applying a photoresist to the surface of a conductive material 260; forming fine through holes in the photoresist; etching the surface of the conductive material 260 within the holes to form concave depressions; depositing a metal onto the photoresist having holes; and removing the photoresist to obtain a cone-shaped needle electrode 262, wherein a portion of the needle electrode 262 can be made to protrude from the conductive material 260. This allows for the reproducible manufacturing of the conductive probe 26B.

[0314] The method for manufacturing the conductive probe 26C includes the steps of: forming an insulating layer 263 on a conductive material 260 by oxidizing the conductive material 260; forming a separation layer 29 on the insulating layer 263; applying a photoresist 82 to the surface of the separation layer 29; forming through holes 82a in the photoresist 82; etching the through holes 82a to the surface of the conductive material 260; depositing a metal onto the surface of the conductive material 260 within the through holes 82a to obtain a needle-shaped electrode 262; and removing the photoresist 82. In this method, a portion of the needle-shaped electrode 262 can be made to protrude from the insulating layer 263. Even in this case, the conductive probe 26C can be manufactured with good reproducibility.

[0315] Furthermore, the manufacturing method for the conductive probe 26C may include the steps of forming a triangular or square-shaped mask on the surface of the conductive material 260 and etching the conductive material 260 to obtain a needle-shaped electrode 262 formed in the shape of a triangular pyramid or a square pyramid. This makes it possible to manufacture a conductive probe 26C having a needle-shaped electrode 262 formed in the shape of a triangular pyramid or a square pyramid on a substrate 261 with good reproducibility.

[0316] Furthermore, the method for manufacturing the conductive probe 26C may include the steps of forming a mask having triangular or square-shaped through holes 72a on the surface of a conductive material 260, and depositing a conductive material onto the surface of the conductive material 260 within the through holes 72a to obtain a needle-shaped electrode 262 formed in the shape of a triangular pyramid or a square pyramid. This makes it possible to manufacture a conductive probe 26C having a needle-shaped electrode 262 formed in the shape of a triangular pyramid or a square pyramid on a substrate 261 with good reproducibility.

[0317] In other words, the method for manufacturing the conductive probe 26 allows the holes in the photoresist to be circular, triangular, or square when manufacturing the conductive probe 26 using photoresist. This makes it possible to reproducibly manufacture a conductive probe 26 having needle-shaped electrodes 262 formed in the shape of a cone, triangular pyramid, or square pyramid on a substrate 261 by depositing metal onto the photoresist and then removing the photoresist.

[0318] The ferroelectric memory device 100 can detect signals generated by interatomic forces between the conductive probe 26 and the ferroelectric recording medium 10 in its recording and playback signal processing unit 50. As a result, the ferroelectric memory device 100 can read information without using an AC electric field in its recording and playback signal processing unit 50, and the reading speed is no longer limited by the frequency of the AC electric field. Therefore, information stored in the ferroelectric recording medium 10 can be read at high speed. In addition, since the amount of charge stored in the ferroelectric layer 131 does not decrease, re-reading can be omitted.

[0319] The ferroelectric memory device 100 can detect interatomic forces using a piezoelectric element 28 placed between the probe slider 23 and the conductive probe 26. This enhances the ferroelectric memory device 100's ability to detect interatomic forces, allowing it to read information stored in the ferroelectric recording medium 10 at a faster speed.

[0320] The ferroelectric memory device 100 may include a piezoelectric element 27 or an electrostrictive element and a control unit (not shown). The ferroelectric memory device 100 controls the voltage applied to the piezoelectric element 27 or electrostrictive element based on a reading signal from the conductive probe 26 using the control unit (not shown), thereby causing the piezoelectric element 27 or electrostrictive element to expand or contract. As a result, the ferroelectric memory device 100 can adjust the distance between the ferroelectric recording medium 10 and the conductive probe 26, enabling high-precision and high-speed control of the distance between the conductive probe 26 and the ferroelectric recording medium 10.

[0321] Here, Figure 29 shows a cross-sectional view illustrating an example of a conventional magnetic head slider configuration. As shown in Figure 29, the magnetic head slider 110 used in an HDD comprises a heating element 111 located inside the magnetic head slider 110 and a magnetic head 112 located below the heating element 111, facing the magnetic recording medium 120. The magnetic head slider 110 employs a technique to adjust the distance between the magnetic head 112 and the magnetic recording medium 120, i.e., DFH, by energizing the heating element 111 to generate heat and causing thermal expansion of the magnetic head slider 110 (see, for example, Japanese Patent Application Publication No. 2003-168274). The heating element 111 is called a DFH heater, and the power applied to the heating element 111 is called DFH power. This technique for adjusting DFH allows the amount of levitation of the magnetic head slider 110 on the surface of the magnetic recording medium 120 to be kept at the nanometer level, while the distance from the surface of the magnetic recording medium 120 to the magnetic head 112 is narrowed to the sub-nanometer level. However, when using the heating element 111, the heating range extends over a wide area including the magnetic head 112 and the magnetic head slider 110 on which it is mounted. Therefore, heating the magnetic head 112 with the heating element 111 takes time, and the response and accuracy of the DFH control are not high. Furthermore, since the leakage electric field in the ferroelectric recording medium 10 is less than the leakage magnetic field used to read magnetic information in the magnetic recording medium 120, the ferroelectric memory device 100 that detects it requires even higher precision DFH adjustment technology.

[0322] Furthermore, conventional DFH technology, as shown in Figure 29, uses a heating element 111, which may cause thermal fluctuations in the dielectric constant of the ferroelectric layer 131. On the other hand, the ferroelectric memory device 100 does not have a heating element 111 like conventional DFH technology, so it can prevent thermal fluctuations in the dielectric constant of the ferroelectric layer 131.

[0323] The ferroelectric memory device 100 can expand or contract by controlling the voltage applied to the piezoelectric element 27 or electrostrictive element based on a reading signal from the conductive probe 26 using a control unit (not shown). This allows the ferroelectric memory device 100 to control the distance between the ferroelectric recording medium 10 and the conductive probe 26 more easily, with higher precision, and at higher speed.

[0324] The ferroelectric memory device 100 has a ferroelectric recording medium drive unit 30 which includes a housing 31, a bearing sleeve 32, a spindle shaft 33, a housing bottom 34, a permanent magnet 35, a stator 36, and lubricating oil O. The radius of curvature R of at least one of the axial end 331 of the spindle shaft 33 and the housing bottom 34 is a convex or concave spherical surface of 2 mm or more, and the lubricating oil O contains inorganic conductive powder. As a result, the ferroelectric recording medium drive unit 30 is a fluid bearing, which is a type of sliding bearing, and the spindle shaft 231 can be rotated stably without contact by the dynamic pressure generated when the spindle shaft 231 rotates due to the lubricating oil O filled between the spindle shaft 231 and the housing 31, so that the ferroelectric recording medium 10 can be rotated with reduced axial runout and low vibration. Furthermore, inorganic conductive powder has high heat resistance and good volatility resistance, and even when included in the lubricating oil O, it can suppress an increase in the viscosity of the lubricating oil O. Therefore, the ferroelectric memory device 100 can stabilize the torque of the ferroelectric recording medium drive unit 30 even after prolonged use, and can suppress the decrease in conductivity between the spindle shaft 231 and the housing 31. Consequently, the ferroelectric memory device 100 can stabilize the reading and writing of information to the ferroelectric recording medium 10.

[0325] Conventionally, the spindle shaft 33 does not come into contact with the housing 31 due to the interposition of a fluid film of non-conductive lubricating fluid, so the spindle shaft 33 is electrically floating relative to the housing 31 of the ferroelectric recording medium drive unit. Here, the spindle shaft 33 is connected to the ferroelectric layer 131 via the ferroelectric recording medium and constitutes part of the circuit that writes information to the ferroelectric layer 131. Therefore, the fact that the spindle shaft 33 is electrically floating relative to the housing 31 hinders the application of voltage between the ferroelectric recording medium 10 and the conductive probe 26 to write information.

[0326] Another method involves incorporating a conductive material into the lubricating fluid O (see, for example, Japanese Patent Publication No. 2001-208069). However, the addition of the conductive material increases the viscosity of the lubricating oil O, leading to an increase in bearing torque. As the usage time increases, the conductive material deteriorates and its conductivity decreases. Therefore, even when applied to a ferroelectric recording medium 10, there is a high possibility that the error rate when writing information to the ferroelectric recording medium 10 will increase.

[0327] Because the ferroelectric memory device 100 has the above configuration, the torque remains stable even during long-term use of the ferroelectric recording medium drive unit 30, and the decrease in conductivity between the spindle shaft 231 and the housing 31 can be suppressed, thereby enabling stable reading and writing of information to the ferroelectric recording medium 10.

[0328] In the ferroelectric memory device 100, the radius of curvature R of at least one of the axial end 331 of the spindle shaft 33 of the ferroelectric recording medium drive unit 30 and the housing bottom 34 can be made into a convex or concave spherical surface of 2 mm or more. As a result, the ferroelectric memory device 100 can more stabilize the torque even when the ferroelectric recording medium drive unit 30 is used for a long time, and can more effectively suppress the decrease in conductivity between the spindle shaft 231 and the housing 31.

[0329] The ferroelectric memory device 100 may have a V-shaped groove 231B on the outer circumference of the spindle shaft 33. This makes it easier for lubricating oil O to collect at the apex of the V-shape of the groove 231B when the spindle shaft 33 rotates, thus facilitating the generation of flow in the lubricating oil O. As a result, it becomes easier to generate pressure from the lubricating oil O, making it easier to support the spindle shaft 33.

[0330] The ferroelectric memory device 100 can be configured with a permanent magnet 35 and a stator 36 facing each other within the cover 37. The coupling between the permanent magnet 35 and the electromagnet of the stator 36 allows the spindle shaft 33 to rotate and generates a downward thrust on the spindle shaft 33, thereby ensuring that the spindle shaft 33 is securely supported by the housing bottom 34.

[0331] The ferroelectric memory device 100 allows the recording and playback signal processing unit 50 to adjust the voltage waveform applied to the conductive probe 26 when writing information to the ferroelectric recording layer 13 to either a triangular wave, a sawtooth wave, or a trapezoidal wave. This reduces damage to the conductive probe 26.

[0332] The ferroelectric memory device 100 includes a housing 60, the housing 60 of which can be filled with at least one of argon gas, nitrogen gas, and helium gas. Since the housing 60 houses the ferroelectric recording medium 10, conductive probe 26, probe slider 23, ferroelectric recording medium drive unit 30, and recording / reproduction signal processing unit 50, filling the housing 60 with these gases allows the ferroelectric memory device 100 to mitigate triboelectric charging that occurs within the housing 60. Therefore, the ferroelectric memory device 100 can suppress the loss of written information due to charge coupling with charges recorded on the ferroelectric recording medium 10, and prevent adverse effects on writing by the conductive probe 26.

[0333] <Data Management System> A data management system according to this embodiment will now be described. The data management system according to this embodiment includes the ferroelectric storage device 100 described above as an external storage device. Figure 30 is a diagram showing the configuration of the data management system according to this embodiment. As shown in Figure 30, the data management system 300 is a data management system that manages data on a high-speed communication network, and includes a data management unit 310 and at least one external storage device 320.

[0334] As shown in Figure 30, the data management unit 310 includes an internal storage device 311.

[0335] The data management unit 310 stores high-speed, large amounts of data flowing over the high-speed communication network directly into the external storage device 320, and simultaneously stores metadata used to read the stored data in the internal storage device 311. This allows the data management unit 310 to store high-speed, large amounts of data flowing over the high-speed communication network into the external storage device 320. Since the external storage device 320 is easier to replace than the internal storage device 311, the expandability of the data management system can be increased. Furthermore, the data management unit 310 can easily read the data stored from the external storage device 320 by using the metadata stored in the internal storage device 311.

[0336] Metadata is data stored in the internal storage device 311 that contains supplementary information about the data stored in the external storage device 320. Specifically, metadata includes the data type, size, attributes, format, title, author name, publisher name, related keywords, time and location where the data was generated, etc. Metadata also includes the location within the external storage device 320 where the data is stored, such as drive number, track number, and sector number.

[0337] As shown in Figure 30, the external storage device 320 is connected to the data management unit 310 by wired or wireless connection, enabling data transmission and reception, and stores data sent from the data management unit 310 over a high-speed communication network.

[0338] Multiple external storage devices 320 can be provided. Preferably, the multiple external storage devices 320 are arranged in parallel. Multiple external storage devices 320 can store large amounts of data at high speed and reduce the load required for data storage on each external storage device 320. In addition, by arranging multiple external storage devices 320 in parallel, the multiple external storage devices 320 have high expandability, so the amount of data that can be stored can be increased.

[0339] An example of the configuration of the external storage device 320 is shown in Figure 31. As shown in Figure 31, the external storage device 320 preferably further comprises a ferroelectric recording medium 321, a memory element 322, a reading element 323, and a drive unit 324.

[0340] Since the ferroelectric recording medium 321 is the same as the ferroelectric recording medium 10 (see Figure 8) described above, details are omitted.

[0341] The memory element 322 is an element for storing data in the ferroelectric recording medium 321.

[0342] The reading element 323 is an element for reading data from the ferroelectric recording medium 321.

[0343] Since the memory element 322 and the reading element 323 are the same as the conductive probe 26 (see Figure 15) and the like described above, their details are omitted.

[0344] The drive unit 324 includes a first drive unit 324A that drives the memory element 322 on the memory surface 321a of the ferroelectric recording medium 321, and a second drive unit 324B that drives the reading element 323 on the memory surface 321a. The first drive unit 324A and the second drive unit 324B are the same as the probe drive unit 40 (see Figure 8) described above, so their details are omitted.

[0345] The memory element 322 and the reading element 323 may be driven independently by the drive unit 324 on the same memory surface 321a of the ferroelectric recording medium 321.

[0346] The external storage device 320 may delete previously saved old data without updating it and save new data on the high-speed communication network, or it may overwrite the old data with the new data and save it.

[0347] In other words, it is preferable that the data stored in the external storage device 320 is not an update of previously stored old data, but rather that previously stored old data is deleted or overwritten and new data stored on the high-speed communication network is saved.

[0348] The ferroelectric recording medium is a rotating medium, and information is read and written by moving the read / write elements (storage element 322 and read element 323) in the sector direction (circumferential direction) and track direction (radial direction) while the ferroelectric recording medium 321 is rotated at high speed. The most efficient use of this method is reading and writing information on consecutive sectors and consecutive tracks. Reading and writing information to a different, non-contiguous track or sector results in a time loss as the read / write elements need to be moved to the other track or sector. Therefore, updating old data that has been stored in the past results in a time loss because the write element needs to be moved to the track and sector where the data is stored.

[0349] The external storage device 320 can write information to the ferroelectric recording medium 321 in a high-speed communication network by deleting old data that has been previously stored without updating it and saving new data, or by overwriting old data with new data. This allows for high-speed data storage by writing information to consecutive sectors and consecutive tracks.

[0350] Figure 32 shows an example of the data connection relationships stored in the ferroelectric recording medium 321. As shown in Figure 32(a), the data stored in the external storage device 320 is stored in a unified storage area without a hierarchical structure, as shown in Figure 32(b), rather than in a storage area with a hierarchical structure. This allows the external storage device 320 to reduce the time required to move the storage elements to a different hierarchical location in the ferroelectric recording medium 321 when writing information, thus enabling high-speed data storage in the ferroelectric recording medium 321.

[0351] The hierarchical structure refers to a structure in which multiple data located in lower hierarchies of the ferroelectric recording medium 321 are arranged in a branched manner from a single data belonging to a certain hierarchical level of the ferroelectric recording medium 321.

[0352] As described above, the data management system 300 according to this embodiment comprises a data management unit 310 and at least one external storage device 320. The data management unit 310 stores data on a high-speed communication network in the external storage device 320 and stores metadata in an internal storage device 311. Multiple external storage devices 320 can be provided. The data management system 300 can store a large amount of high-speed data flowing on a high-speed communication network from the data management unit 310 to the external storage device 320, and can easily read the data stored in the external storage device 320 by using the metadata stored in the internal storage device 311. Furthermore, since the external storage devices 320 are easy to replace or add and multiple devices can be provided, the amount of data that can be stored can be increased and the burden on the external storage device 320 for storing data can be reduced. Therefore, the data management system 300 can write at high speed with high recording density and has high expandability. Thus, the data management system 300 can efficiently store data on a high-speed communication network and improve convenience.

[0353] The data management system 300 can be configured by connecting the data management unit 310 to a high-speed communication network and connecting the external storage device 320 to the data management unit 310. This allows the data management system 300 to save data on the high-speed communication network to the external storage device 320 via the data management unit 310, and to reliably save metadata used to read metadata stored in the external storage device 320 to the internal storage device 311. Therefore, the data management system 300 can efficiently save data on the high-speed communication network and read data stored in the external storage device 320 more quickly.

[0354] The data management system 300 can arrange multiple external storage devices 320 in parallel. This allows the data management system 300 to distribute and write data from the high-speed communication network evenly to each external storage device 320. Therefore, the data management system 300 can store large amounts of data on multiple external storage devices 320 at high speed and more efficiently, while also improving scalability. Thus, the data management system 300 can increase the amount of data that can be stored while also improving convenience.

[0355] In the data management system 300, the external storage device 320 can either delete old data recorded on the ferroelectric recording medium 321 without updating it and save new data on the high-speed communication network, or overwrite the old data with new data and save it. This makes it easier for the data management system 300 to write data to the ferroelectric recording medium 321 of the external storage device 320, and also makes it easier to read data stored in the external storage device 320. Therefore, the data management system 300 can reliably save and read data from the external storage device 320 while maintaining high speed.

[0356] In the data management system 300, the external storage device 320 can include a memory element 322, a reading element 323, and a drive unit 324. This allows the data management system 300 to simultaneously write and read information in consecutive sectors and consecutive tracks on the same memory surface 321a of the ferroelectric recording medium 321. Furthermore, it can process sporadic requests for reading information while writing information in consecutive sectors and consecutive tracks. Thus, the data management system 300 can store data in the ferroelectric recording medium 321 at high speed and read the stored data. [Examples]

[0357] The embodiments will be described in detail below with reference to examples and comparative examples, but the embodiments are not limited to these examples and comparative examples.

[0358] <Example 1> [Fabrication of targets for ferroelectric layer formation] (Hf 0.5 Zr 0.5 O2 target fabrication) A mixture of HfO2 powder and ZrO2 powder in a 1:1 ratio was slurryed using water as a solvent, and then spray-dried to produce a mixed powder. A molded body formed from this mixed powder by pressing was sintered in an inert atmosphere to produce a target. The density of the produced target was Hf 0.5 Zr 0.5 The O2 level was approximately 96% of the theoretical value.

[0359] (Fabrication of 4(Y2O3)-96(HfO2) targets) Aside from mixing Y2O3 powder and hafnium oxide (HfO2) powder in a 4:96 ratio, the above (Hf 0.5 Zr 0.5 A 4(Y2O3)-96(HfO2) target was fabricated using the same procedure as for the O2 target. The density of the fabricated target was approximately 95% of the theoretical value for 4(Y2O3)-96(HfO2).

[0360] [Fabrication of ferroelectric recording media] A ferroelectric recording medium was fabricated using the following method. Undoped single-crystal silicon with a (001) crystal orientation was used as the substrate. The substrate was disc-shaped with an opening in the center, with an outer diameter of 65 mm, an inner diameter of 20 mm, and a thickness of 0.8 mm. The disc-shaped substrate was placed in a deposition apparatus (manufactured by Canon Anelva Corporation), and a 30 nm layer of gold (Au) was deposited on the substrate surface using RF sputtering at a substrate temperature of 200°C, with Ar as the sputtering gas and a pressure of 1 Pa, as the electrode layer. Next, a 30 nm layer of CeO2 was deposited using RF sputtering at a substrate temperature of 350°C, with Ar and O2 (mixing ratio 3:1) as the sputtering gas and a pressure of 1 Pa, as the paraelectric layer. Next, Hf was deposited using RF sputtering at a substrate temperature of 400°C, with Ar and O2 (mixing ratio 3:1) as the sputtering gas and a pressure of 1 Pa, as the ferroelectric layer. 0.5 Zr 0.5 A 30 nm O2 film was deposited. On top of this, a 5 nm DLC film was deposited as a protective layer using an ion beam method at a substrate temperature of 150°C. The electrode layer was deposited on the entire surface of the substrate, but the paraelectric layer, ferroelectric layer, and protective layer were not deposited around the central opening by masking a 10 mm wide area on the inner circumference. Finally, a 1.5 nm thick perfluoropolyether-based lubricant was applied to the protective layer using a dip method to form a lubricant layer and obtain a ferroelectric recording medium.

[0361] Tables 1 to 4 show the configuration of each layer that makes up the ferroelectric recording medium.

[0362] [Evaluation of the properties of ferroelectric layers] As characteristics of the ferroelectric layer in the ferroelectric recording medium, the diffraction intensity of the (111) plane by XRD, smoothness, leakage current density, and leakage current density due to degradation over time were evaluated.

[0363] (Evaluation of the diffraction intensity of the (111) plane in the ferroelectric layer by XRD) In the fabrication of ferroelectric recording media, after depositing the ferroelectric layer, the substrate was removed from the deposition apparatus. The removed substrate was then subjected to an X-ray diffractometer (XRD, incident X-ray: θ, detection angle: 2θ) to determine the Hf 0.5Zr 0.5 The diffraction intensity of the (111) plane of O2 was measured, and the intensity was 1200 cps. The measurement results are shown in Table 5.

[0364] (Evaluation of the smoothness of the ferroelectric layer) In the fabrication of ferroelectric recording media, after depositing the ferroelectric layer, the substrate with the deposited ferroelectric layer was removed from the deposition apparatus. The surface roughness (Ra) of the ferroelectric layer side of the removed substrate was measured, and the surface roughness of the ferroelectric layer was evaluated based on the evaluation criteria below. An atomic force microscope (BRUKER) was used for the measurements. ((Evaluation Criteria)) A: The surface roughness of the substrate was less than 0.5 nm. B: The surface roughness of the substrate was between 0.5 nm and less than 1.0 nm. C: The surface roughness of the substrate was 1.0 nm or greater.

[0365] (Evaluation of leakage current density in ferroelectric layers) Similar to the evaluation of the crystal structure of the ferroelectric layer described above, after depositing the ferroelectric layer in the fabrication of the ferroelectric recording medium described above, the substrate was removed from the deposition apparatus. A 0.5 mm square Au electrode pad (film thickness 200 nm) was formed on the surface of the ferroelectric layer of the removed substrate to prepare an evaluation sample. The leakage current density between the electrode layer and the Au electrode pad of this evaluation sample was measured, and it was found to be approximately 5 × 10⁻¹⁶ when 5V was applied. -6 A / cm 2 The results are shown in Table 5.

[0366] (Evaluation of leakage current density due to degradation of the ferroelectric layer over time) The ferroelectric recording medium fabricated in the above-mentioned [Fabrication of Ferroelectric Recording Medium] was kept in an environment of 80°C and 80% humidity for two weeks. After drying the removed ferroelectric recording medium, a 0.5 mm square Au electrode pad (film thickness 200 nm) was formed at a radius of 40 mm to create an evaluation sample. This evaluation sample was attached to the spindle shaft of a ferroelectric memory device, and the leakage current density between the spindle shaft and the Au electrode pad of the evaluation sample was measured. The result showed that when 5V was applied, the leakage current density was approximately 5 × 10⁻¹⁶.-6 A / cm 2 The results are shown in Table 5.

[0367] Furthermore, the leakage current density due to degradation over time in the ferroelectric recording medium of Example 13, described later, is approximately 1 × 10⁻⁶ when 5V is applied. -5 A / cm 2 The measurement results are shown in Table 5 (see Example 13).

[0368] Table 5 shows the evaluation results for each of the above properties of the ferroelectric layer.

[0369] [Manufacturing of ferroelectric memory devices] (Manufacturing of the first conductive probe) The first conductive probe was manufactured by the following method. A 1 μm thick molybdenum film was deposited on a 0.2 mm thick quartz substrate with a (0001) surface orientation by sputtering, and a photoresist pattern with equilateral triangular openings with sides of 0.3 μm was formed on the molybdenum surface by photoresist. Next, the molybdenum in the areas not covered by the photoresist pattern was etched to a depth of approximately 0.3 μm by wet etching. A mixture of phosphoric acid (H3PO4), nitric acid (HNO3), acetic acid (CH3COOH), and water was used as the etching solution. Subsequently, a 1 μm thick molybdenum film was deposited by sputtering. Then, the quartz substrate including the probe formation area was cut into 0.5 mm squares, and the photoresist was then peeled off to form a tip of the first conductive probe having a molybdenum needle-shaped electrode on the quartz substrate. The needle-shaped electrode of this tip was surrounded by a molybdenum layer, and the tip of the needle protruded slightly from this molybdenum layer.

[0370] (Manufacturing of the first probe slider) A first probe slider made of Al2O3-TiC (AlTiC) was manufactured. The external dimensions of the first probe slider were a floating surface of 2 mm x 1.5 mm, a thickness of 0.5 mm, a leading end face which is the airflow inlet end face with a width of 0.2 mm, and a 0.2 mm recess provided on the outlet end face for attaching the tip of the first conductive probe. In addition, the outlet end face was provided with Au wiring to the conductive probe, and Au electrodes and wiring for applying voltage to a piezoelectric element (quartz).

[0371] (Manufacturing of the second conductive probe and the second probe slider) A 0.2 mm thick single-crystal silicon substrate (plane orientation (001)) was heated to 550°C, and a 200 nm Au electrode layer (first electrode), a 500 nm PZT layer (electrostrictive element), a 200 nm Au electrode layer (second electrode), a 500 nm PZT layer (piezoelectric element), and a 200 nm Au electrode layer (third electrode) were deposited on its surface by RF sputtering. For each electrode layer, a circuit pattern communicating with the electrode layer was created on the outside of the stacked structure by photolithography. A molybdenum needle-shaped electrode was formed on top of this using the same method as for the first conductive probe, and the tip of the second conductive probe was manufactured.

[0372] Using the tip of this second conductive probe, a second probe slider was manufactured in the same manner as the first probe slider. The second probe slider was provided with wiring connecting to the first electrode, the second electrode, and the third electrode. Here, the first and second electrodes are used to apply voltage to the electrostrictive element, the second and third electrodes are used to detect the output signal from the piezoelectric element, and the third electrode is used to apply the writing signal to the ferroelectric recording medium.

[0373] (Manufacturing of ferroelectric recording medium drive unit) A spindle motor having the structure shown in Figure 26 was manufactured for the rotational drive of a ferroelectric recording medium. An aluminum alloy was used for the housing, and the shaft member was made of S45C hardened steel with a diameter of 3 mm and a convex shape with a radius of curvature R of 6 mm at the shaft end. A cylindrical 50Cu-47Fe-3Sn sintered metal was used for the bearing sleeve, and a flat plate of 50Cu-47Fe-3Sn sintered metal was used for the bottom of the housing. ISO VG100 was used as the lubricant, to which 2 mass% of conductive carbon fiber (VGCF-H, manufactured by Showa Denko, fiber diameter 150 nm) was added.

[0374] (Manufacturing of probe drive unit) A probe drive unit having the structure shown in Figure 8 was manufactured. It was manufactured using a general-purpose HDD drive unit.

[0375] (Manufacturing of the control unit) A control unit with the structure shown in Figure 8 was manufactured. It was manufactured using general-purpose power supplies and control equipment.

[0376] (Manufacturing of recording and playback signal processing unit) A recording and playback signal processing unit was manufactured. For the writing signal, a bipolar power supply was used to generate a positive or negative voltage corresponding to the written information. For reading, an amplifier was used to amplify the weak tunnel current flowing between the conductive probe and the ferroelectric recording layer, and an A / D converter was used to convert this into digital data. A triangular wave was used to generate the waveform of the bipolar power supply. In addition, a DC power supply was provided to expand and contract a piezoelectric element placed between the conductive probe and the head slider.

[0377] (Enclosure) As the enclosure, we manufactured an enclosure having the structure shown in Figure 8.

[0378] (Manufacturing of ferroelectric memory device 1) A ferroelectric memory device 1 having the structure shown in Figure 8 was manufactured using the fabricated ferroelectric recording medium, first conductive probe, first probe slider, ferroelectric recording medium drive unit, probe drive unit, control unit, recording / reproduction signal processing unit, and housing. A conductive probe was mounted on the underside of the tip of the probe slider, and this probe slider was attached to a suspension arm, which was then driven by a voice coil motor to move the surface of the ferroelectric recording medium. The housing of the ferroelectric memory device was sealed and filled with argon gas at atmospheric pressure. In addition, 1 g of silica gel was sealed inside the housing as a desiccant.

[0379] (Manufacturing of ferroelectric memory device 2) The ferroelectric memory device 2 was manufactured in the same manner as the ferroelectric memory device 1, except that the conductive probe and probe slider of the ferroelectric memory device 1 were replaced with a second conductive probe and a second probe slider.

[0380] [Performance of Ferroelectric Memory Device 1] To assess the performance of the ferroelectric memory device 1, a recording and playback test 1 was conducted, along with measurements of the tunnel current difference during reading and the amount of triboelectric charge.

[0381] (Recording and playback test) Recording and playback tests were performed on the manufactured ferroelectric memory device 1. The ferroelectric recording medium was rotated at 5400 rpm, and the probe slider was made to levitate and move along the surface of the ferroelectric recording medium. The position of the probe slider was fixed at a track position with a radius of 40 mm on the ferroelectric recording medium. Then, the conductive probe was switched to the information reading circuit, and a bias voltage of +500 mV was applied between the ferroelectric recording medium and the conductive probe, and the tunnel current from the conductive probe was monitored. Next, a DC voltage was applied to the piezoelectric element to gradually bring the conductive probe closer to the ferroelectric recording medium, and the voltage applied to the piezoelectric element was fixed at the position where the average value of the tunnel current was 2 pA.

[0382] Subsequently, the conductive probe was switched to the information writing circuit, and information for 255 sectors was written at that track position. Each sector consisted of a data area and a servo information area, with the servo information area consisting of a burst information area and an address information area. Information was written using a triangular wave with a peak voltage of ±5V, and the writing frequency was set to 2.3GHz. Information was written in one pass, and the writing time was approximately 10ms. This corresponds to a bit length of 10nm in the circumferential direction on the surface of the ferroelectric recording medium.

[0383] After writing information to the ferroelectric recording medium, the conductive probe was switched to the information reading circuit, and the tunnel current was monitored. At this time, the voltage applied to the piezoelectric element was adjusted so that the signal-to-noise ratio (SNR) was 3 dB or higher. The amplitude of the tunnel current was approximately 3 pA, and the time from the completion of writing to the ferroelectric recording medium to reading was approximately 0.1 ms.

[0384] Using the method described above, we confirmed that information can be written to the ferroelectric memory device 1 and read back from it.

[0385] After repeating the above information reading process 10 times, the signal-to-noise ratio (SNR) during information reading fell below 3dB. Therefore, the same data was rewritten to the same track location. As a result, the SNR during information reading recovered to above 3dB.

[0386] (Measurement of tunnel current difference during reading 1) When reading information from a ferroelectric recording medium, the difference in tunnel current between bits with a positively charged ferroelectric layer and bits with a negatively charged ferroelectric layer was measured. The bias voltage between the ferroelectric recording medium and the conductive probe was set to +500mV. Specifically, the tunnel current of bits with a positively charged surface layer of the ferroelectric layer decreased, while the tunnel current of bits with a negatively charged surface layer increased. However, the tunnel current from the negatively charged bits was measured when the distance between the conductive probe and the ferroelectric recording medium was adjusted so that the tunnel current of the bits with a positively charged surface layer was 1pA. As a result, the tunnel current from the negatively charged bits was approximately 3pA, and the difference in tunnel current between the two was approximately 2pA. The results are shown in Table 6.

[0387] (Measurement of tunnel current difference during reading 2) When reading information from a ferroelectric recording medium, a bias voltage of ±500mV was applied between the ferroelectric recording medium and a conductive probe. The bias voltage was a 1GHz square wave, and the reading speed was 500Mbit / second.

[0388] The distance between the conductive probe and the ferroelectric recording medium was adjusted so that the average tunnel current of a bit with a positively charged surface on the ferroelectric layer was 5 pA when the bias voltage to the conductive probe was -500 mV. When the bias voltage to the conductive probe was increased to +500 mV, the average tunnel current decreased to 1 pA. This is thought to be due to a rectifying effect at the junction between the ferroelectric layer and the paraelectric layer. When the same evaluation was performed on a bit with a negatively charged surface on the ferroelectric layer, the average forward tunnel current was 25 pA and the reverse tunnel current was 5 pA. From these measurements, the maximum difference in tunnel current between positively charged and negatively charged bits when the bias voltage to the conductive probe was changed to positive and negative was 20 pA. The results are shown in Table 6.

[0389] (Friction charge) The ferroelectric memory device fabricated in the above-mentioned [manufacturing of ferroelectric memory device] (i.e., a ferroelectric memory device with the interior filled with argon gas at atmospheric pressure, a sealed enclosure, and a desiccant sealed inside) was discharged. Then, the ferroelectric recording medium was rotated at 5600 rpm, and the probe slider was subjected to a 1-hour seek operation at 4 Hz (one cycle being the movement from the innermost to the outermost circumference, and then returning to the innermost circumference). After stopping the seek operation, the amount of charge between the enclosure and the conductive probe was measured 1 second later, and the amount of triboelectric charge was found to be 0.1 nC.

[0390] As reference ferroelectric memory devices, a first reference ferroelectric memory device (Example 11) was manufactured by filling the interior with nitrogen gas at atmospheric pressure, sealing the casing, and enclosing a desiccant. A second reference ferroelectric memory device (Example 12) was also manufactured by placing a desiccant inside and allowing the interior to be exposed to the atmosphere through a filter. The charge amounts of these reference ferroelectric memory devices were measured in the same manner as the ferroelectric memory device of Example 1. The result showed that the triboelectric charge amount of the first reference ferroelectric memory device was 3 nC, and the triboelectric charge amount of the second reference ferroelectric memory device was 4 nC.

[0391] Therefore, it was confirmed that the ferroelectric memory device of Example 1 can mitigate triboelectric charging by sealing argon gas inside the casing.

[0392] [Performance of ferroelectric memory device 2] As a performance measure for ferroelectric memory device 2, information was read from the ferroelectric recording medium using interatomic forces. This interatomic force reading was performed by measuring the amplitude of the signal obtained from the piezoelectric element during the recording and playback test of ferroelectric memory device 2.

[0393] (Measurement of the amplitude of the signal obtained from the piezoelectric element when reading using interatomic force 3) Recording and playback tests were performed on the manufactured ferroelectric memory device. The ferroelectric recording medium was rotated at 5400 rpm, and the probe slider was made to levitate and move along the surface of the ferroelectric recording medium. The position of the probe slider was fixed at a track position with a radius of 40 mm on the ferroelectric recording medium. Then, while monitoring the output voltage from the piezoelectric element between the second and third electrodes, the DC voltage applied to the electrostrictive element between the first and second electrodes was gradually increased to bring the conductive probe closer to the ferroelectric recording medium. The voltage applied to the electrostrictive element was controlled so that the average output voltage from the piezoelectric element was approximately +5 μV.

[0394] In this state, an information writing signal was applied to the conductive probe from the third electrode, and information for 255 sectors was written to the track position. Each sector consisted of a data area and a servo information area, and the servo information area consisted of a burst information area and an address information area. Information was written using a triangular wave with a peak voltage of ±5V, and the writing frequency was set to 2.3GHz. Information was written in one pass, and the writing time was approximately 10ms. This corresponds to a bit length of 10nm in the circumferential direction on the surface of the ferroelectric recording medium.

[0395] After writing information to the ferroelectric recording medium, the voltage applied to the electrostrictive element was adjusted while monitoring the output voltage from the piezoelectric element, so that the signal-to-noise ratio (SNR) of the output voltage from the piezoelectric element was 3 dB or higher. As a result, the same 255 sectors of information that had been written were read from the ferroelectric recording medium. The amplitude of the signal of the read information from the piezoelectric element was approximately 3 μV, and the time from the completion of writing to the ferroelectric recording medium to reading was approximately 0.1 m seconds. The measurement results of the amplitude of the signal of the read information obtained from the piezoelectric element are shown in Table 6.

[0396] <Examples 2-6> In Example 1, the procedure was the same as in Example 1, except that the materials included in the electrode layer were changed to Ge, Pb, Al, Cu, and Cr, as shown in Table 2, in the [Fabrication of Ferroelectric Recording Medium]. Hf of the ferroelectric layer in each example 0.5 Zr 0.5The diffraction intensity of the (111) plane of O2 and recording / recovery tests of ferroelectric memory devices were performed. The composition of each layer constituting the ferroelectric recording medium is shown in Tables 1 to 4, the formation conditions and evaluation results of the ferroelectric layer are shown in Table 5, and the test results of the dielectric memory device are shown in Table 6.

[0397] <Examples 7-9> In Example 1, the procedure was the same as in Example 1, except that the materials included in the paraelectric layer were changed to 10(Y2O3)-90(ZrO2), Al2O3, and TiO2, as shown in Table 3, in the [Fabrication of Ferroelectric Recording Medium]. 0.5 Zr 0.5 The diffraction intensity of the (111) plane of O2 and recording / recovery tests of ferroelectric memory devices were performed. The composition of each layer constituting the ferroelectric recording medium is shown in Tables 1 to 4, the formation conditions and evaluation results of the ferroelectric layer are shown in Table 5, and the test results of the dielectric memory device are shown in Table 6.

[0398] <Example 10> In Example 1, the procedure was the same as in Example 1, except that a paraelectric layer was not provided in the [Fabrication of Ferroelectric Recording Medium]. The Hf of the ferroelectric layer in this example 0.5 Zr 0.5 The diffraction intensity of the (111) plane of O2, recording and playback tests of the ferroelectric memory device, and measurement of the amount of triboelectric charge were performed. In this example, the leakage current density between the electrode layer of the substrate with the ferroelectric layer deposited and the Au electrode pad was approximately 1 × 10⁻¹⁶ when 5V was applied. -5 A / cm 2 The configuration of each layer constituting the ferroelectric recording medium is shown in Tables 1 to 4, the formation conditions and evaluation results of the ferroelectric layer are shown in Table 5, and the test results of the dielectric memory device are shown in Table 6.

[0399] <Examples 11 and 12> In Example 1, the procedure was the same as in Example 1, except that the interior was filled with nitrogen gas or air at atmospheric pressure, the enclosure was sealed, and a desiccant was sealed inside. The configuration of each layer constituting the ferroelectric recording medium is shown in Tables 1 to 4, the formation conditions and evaluation results of the ferroelectric layer are shown in Table 5, and the test results of the dielectric memory device are shown in Table 6.

[0400] <Example 13> In Example 1, the procedure was the same as in Example 1, except that the inner circumference of the substrate was not masked during the [Fabrication of Ferroelectric Recording Medium] step. A ferroelectric recording medium was fabricated by depositing an electrode layer, a paraelectric layer, a ferroelectric layer, and a protective layer on the entire surface of the substrate. The configuration of each layer constituting the ferroelectric recording medium is shown in Tables 1 to 4, the formation conditions and characteristics evaluation results of the ferroelectric layer are shown in Table 5, and the test results of the dielectric memory device are shown in Table 6.

[0401] <Example 14, Comparative Examples 1-4> In Example 1, the material used for the substrate in [Fabrication of Ferroelectric Recording Medium] was changed to sapphire substrate a-side (Example 14), NiP electroless plated aluminum 5000 series alloy substrate (Comparative Example 1), amorphous glass substrate (Comparative Example 2), MgO(100)-side substrate (Comparative Example 3), and sapphire substrate c-side (Comparative Example 4), as shown in Table 1. The procedure was the same as in Example 1, except that no electrode layer was provided in Comparative Example 1. The Hf of the ferroelectric layer in each example and comparative example 0.5 Zr 0.5 The diffraction intensity of the (111) plane of O2 and recording / recovery tests of ferroelectric memory devices were performed. The composition of each layer constituting the ferroelectric recording medium is shown in Tables 1 to 4, the formation conditions and evaluation results of the ferroelectric layer are shown in Table 5, and the test results of the dielectric memory device are shown in Table 6.

[0402] <Example 15> In Example 1, the procedure was the same as in Example 1, except that the substrate temperature during film formation of the ferroelectric layer was lowered by 80°C to 320°C. The configuration of each layer constituting the ferroelectric recording medium is shown in Tables 1 to 4, the formation conditions and evaluation results of the ferroelectric layer are shown in Table 5, and the test results of the dielectric memory device are shown in Table 6.

[0403] <Example 16> In Example 5, the procedure was the same as in Example 1, except that the substrate temperature during film formation of the ferroelectric layer was lowered by 80°C to 320°C. The configuration of each layer constituting the ferroelectric recording medium is shown in Tables 1 to 4, the formation conditions and evaluation results of the ferroelectric layer are shown in Table 5, and the test results of the dielectric memory device are shown in Table 6.

[0404] <Example 17> In Example 6, the procedure was the same as in Example 1, except that the substrate temperature during film formation of the ferroelectric layer was lowered by 80°C to 320°C. The configuration of each layer constituting the ferroelectric recording medium is shown in Tables 1 to 4, the formation conditions and evaluation results of the ferroelectric layer are shown in Table 5, and the test results of the dielectric memory device are shown in Table 6.

[0405] In Examples 15-17, the XRD(111) diffraction patterns were all halos and the diffraction intensity decreased. However, when the substrate was heated to 520°C (200°C above the deposition temperature), the halo patterns changed to signals with sharp peaks, and the diffraction intensities became 1800 (Example 15), 1600 (Example 16), and 1600 (Example 17), respectively. Based on these results, electron microscope observation and electron diffraction results suggest that the ferroelectric layers in Examples 15-17 had an amorphous structure with short-range order of 2 nm or less in length, width, and height. Therefore, compared to Examples 1, 5, and 6, there was no change in leakage current density in Examples 15-17, but the smoothness of the growth surface of the ferroelectric layer was improved in all cases.

[0406] <Examples 18-23> In Example 1, the procedure was carried out in the same manner as in Example 1, except that the film thicknesses of the paraelectric layer and the ferroelectric layer were changed to the values ​​shown in Tables 3 and 4 in the [Fabrication of Ferroelectric Recording Medium] step.

[0407] [Table 1]

[0408] [Table 2]

[0409] [Table 3]

[0410] [Table 4]

[0411] [Table 5]

[0412] [Table 6]

[0413] Table 6 shows that in Examples 1, 2-1 to 2-3, and 3 to 23, it was confirmed that information could be read, and that there was a correlation between the interatomic force obtained from the ferroelectric recording medium and the tunneling current during reading.

[0414] Therefore, by applying a voltage to an electrostrictive element, information can be written to a ferroelectric memory device, and the written information can also be read.

[0415] <Examples 2-1 to 2-3> In Example 2, the ferroelectric layer was deposited in the same manner as in Example 2, except that the composition and deposition method of the ferroelectric layer in [Fabrication of Ferroelectric Recording Medium] were changed as shown in Table 7. The substrate was then removed from the deposition apparatus, and the diffraction intensity of the (111) plane of hafnium oxide in the ferroelectric layer in each example was measured. Furthermore, a ferroelectric recording medium was manufactured using the fabricated ferroelectric layer in the same manner as in Example 1, and recording and playback tests and the tunnel current difference during reading were performed on the manufactured ferroelectric recording medium. The results of these tests are shown in Table 7.

[0416] The substrate temperature during film deposition was set to 400°C in all cases. In the microwave plasma MOCVD method, tetrakis(ethylmethylamide)hafnium was used as the hafnium source and tetrakis(ethylmethylamide)zirconium as the zirconium source, with oxygen and argon added in a 1:1 ratio. The reaction pressure was 100 Pa, and the input power was 800 W (2.45 GHz). In the MOCVD method, only substrate heating was performed without applying microwaves. XRD θ-2θ scans were performed on the extracted substrates to measure the (111) plane diffraction intensity of hafnium oxide. The measurement results are shown in Table 7.

[0417] [Table 7]

[0418] Table 7 confirms that assisting the reaction space during film deposition with plasma improves the crystallinity of the ferroelectric layer.

[0419] Furthermore, as shown in Table 7, in Examples 2-1 to 2-3, information can be read in the same way as in Example 1, and it was confirmed that there is a correlation between the interatomic force obtained from the ferroelectric recording medium and the tunneling current during reading. Therefore, in Examples 2-1 to 2-3, information can be written to the ferroelectric memory device and read by applying a voltage to the electrostrictive element.

[0420] As described above, embodiments have been explained, but these embodiments are presented as examples only, and the present invention is not limited by these embodiments. The above embodiments can be implemented in various other forms, and various combinations, omissions, substitutions, and modifications are possible without departing from the spirit of the invention. These embodiments and their variations are included in the scope and spirit of the invention, as well as in the claims of the invention and its equivalents. [Explanation of Symbols]

[0421] 10. Ferroelectric recording media 11 circuit boards 12 Electrode layer 13 Ferroelectric recording layer 131 Ferroelectric layer 131A Data Area 131B Servo Information Area 131B-4 Reference signal information 132 Paraelectric layer 14 Protective layer 15 Lubricant layer 100 Ferroelectric memory devices 23 Probe Slider 26, 26A, 26B, 26C conductive probes 261 Base 261a Recess 261b, 263b surface (principal surface) 262 Needle electrode 263 Insulating layer 263a Through hole 27 Piezoelectric element 27A First piezoelectric element 27B Second piezoelectric element 30 Ferroelectric recording medium drive unit 31 Housing (bearing cylinder) 33. Shaft component (spindle shaft) 332 Groove 34 Housing bottom 40 Probe drive unit 50 Recording and Playback Signal Processing Unit 60 cabinets 83 metal 320 External storage device 321a Memory surface 322 Memory mechanism 323 Reading element 324 Drive Unit 324A First drive unit 324B Second drive unit

Claims

1. A data management system for storing data on a high-speed communication network having a communication speed of 10 Gbps or more, A data management unit including internal storage, It comprises at least one external storage device having a ferroelectric recording medium that supports read / write speeds of 10 Gbps or more, The ferroelectric recording medium is a rotating medium having a disc shape, The external storage device rotates the ferroelectric recording medium at high speed while moving the read / write elements in the sector direction and track direction of the ferroelectric recording medium to read and write information. The data management unit stores data on the high-speed communication network in the external storage device and stores metadata used to read the stored data in the internal storage device. The data management unit is a data management system that stores data on the high-speed communication network in the ferroelectric recording medium of the external storage device in consecutive sectors and consecutive tracks.

2. The data management unit is connected to the high-speed communication network. The data management system according to claim 1, wherein the external storage device is connected to the data management unit.

3. The data management system according to claim 1 or 2, wherein a plurality of the external storage devices are arranged in parallel.

4. The data management system according to any one of claims 1 to 3, wherein the external storage device deletes old data previously stored without updating it and stores new data on the high-speed communication network, or overwrites the old data with the new data and stores it.

5. The aforementioned external storage device is A memory element for storing data in the ferroelectric recording medium, A reading element for reading the data from the ferroelectric recording medium, On the same memory surface of the ferroelectric recording medium, a first drive unit for driving the memory element and a second drive unit for driving the reading element are provided. A data management system according to any one of claims 1 to 4.

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