ferroelectric memory device

Abstract

To provide a ferroelectric storage device capable of controlling a distance between a probe and a recording medium with high accuracy and high speed.SOLUTION: A ferroelectric storage device according to the present invention includes: a ferroelectric recording medium; a conductive probe; a probe slider; a piezoelectric element or an electrostrictive element which is provided on the probe slider and in which the conductive probe is connected so as to face the ferroelectric recording medium; and a control unit that controls a voltage applied to the piezoelectric element or the electrostrictive element to expand and contract the piezoelectric element or the electrostrictive element and adjusts a distance between the ferroelectric recording medium and the conductive probe. The control unit controls the voltage applied to the piezoelectric element or the electrostrictive element based on a read signal from the conductive probe to expand and contract the piezoelectric element or the electrostrictive element.SELECTED DRAWING: Figure 9

Description

[Technical Field]

[0001] The present invention relates to a ferroelectric memory device. [Background technology]

[0002] Ferroelectric recording media are used in hard disk drives, various recording media, and the like, and are capable of repeatedly recording information by changing the polarization of the ferroelectric material. Ferroelectric recording media are ultra-high-density recording media that have a ferroelectric layer and achieve high storage capacity by utilizing the spontaneous polarization of the ferroelectric material that occurs when an external electric field is applied. Because ferroelectric recording media can achieve high storage capacities, the development of ferroelectric memory devices that include ferroelectric recording media is being considered.

[0003] Patent Document 1 discloses a dielectric recording and reproducing device that applies an AC electric field to the dielectric material that constitutes the dielectric recording medium and reproduces information recorded on the dielectric recording medium by utilizing the nonlinear dielectric characteristics 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 lifted a predetermined distance above the surface of the recording medium, thereby recording and reproducing information on the recording medium, and also discloses the use of a semiconductor sensor for reading information. Furthermore, as materials for the ferroelectric layer, perovskite-based 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 / reading head that has a circular guard surrounding the vicinity of the tip of the probe to prevent dust from contacting or colliding with the probe, and uses a piezoelectric material as a moving means to move the probe in a direction approximately perpendicular to the recording surface.

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

[0007] Patent Document 5 discloses a memory device having a dielectric laminate formed by laminating a ferroelectric material and a paraelectric material.

[0008] Patent Document 6 discloses a dielectric recording and reproducing head that records multi-valued information by applying a voltage corresponding to data between the probe and each bias electrode, causing the probe and each bias electrode to form polarization domains with a polarization direction parallel to the surface of the dielectric recording medium, and recording four types of data at predetermined locations on the dielectric recording medium. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Application Laid-Open No. 2004-14016 [Patent Document 2] Japanese Patent Application Laid-Open No. 2007-272961 [Patent Document 3] JP 2004-171622 A [Patent Document 4] Japanese Patent Application Laid-Open No. 2005-158117 [Patent Document 5] Japanese Patent Application Publication No. 9-307073 [Patent Document 6] Japanese Patent Application Laid-Open No. 2004-178750 Summary of the Invention [Problem to be solved by the invention]

[0010] Storage services that utilize 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 types of information are shared. These communications generally use optical fiber, with communication speeds exceeding 10 Gbps (gigabits per second).

[0011] Furthermore, in environments where laying optical fiber is difficult or for mobile devices, there is a shift to wireless and mobile communications of 10 Gbps or more, known as 5G, and even the application of 6G of 100 Gbps or more is being considered.

[0012] These storage services primarily use recording media such as HDDs and flash memory (SSDs). HDDs typically have a transfer speed of around 1 Gbps, while SSDs typically have a transfer speed of around 3 Gbps. This makes it difficult for a single storage device to meet the strict input / output requirements of ultra-high-speed communications. Furthermore, the capacity required for storage is also steadily increasing.

[0013] Furthermore, with global warming becoming a major social issue, there are concerns about the increase in power consumption due to the expansion of storage services, which has led to a demand for efficient storage that consumes less energy per unit of storage capacity and reduces the environmental impact.

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

[0015] In order to increase the recording density on the recording medium, it is necessary to bring the probe close to the surface of the recording medium at the nanometer level, and to control the distance at high speed with high precision in order to detect weak signals from the probe.

[0016] An object of one aspect of the present invention is to provide a ferroelectric memory device that can control the distance between a probe and a recording medium with high precision and at high speed. [Means for solving the problem]

[0017] One aspect of the ferroelectric memory device according to the present invention comprises a ferroelectric recording medium, a conductive probe for writing and reading information to and from the ferroelectric recording medium, a probe slider for causing the conductive probe to float and run on the surface of the ferroelectric recording medium, a piezoelectric element or electrostrictive element provided on the probe slider and connected so that the conductive probe faces the ferroelectric recording medium, and a control unit for controlling the voltage applied to the piezoelectric element or electrostrictive element to expand and contract the piezoelectric element or electrostrictive element and adjust the distance between the ferroelectric recording medium and the conductive probe, wherein the control unit controls the voltage applied to the piezoelectric element or electrostrictive element based on a read signal from the conductive probe to expand and contract the piezoelectric element or electrostrictive element. [Effects of the Invention]

[0018] One aspect of the ferroelectric memory device according to the present invention is capable of controlling the distance between the probe and the recording medium with high precision and at high speed. [Brief explanation of the drawings]

[0019] [Figure 1] FIG. 1 is a partial cross-sectional view showing the structure of a ferroelectric recording medium. [Figure 2] FIG. 1 is a perspective view of a ferroelectric recording medium. [Figure 3] 1 is a cross-sectional view showing an example of a state in which a ferroelectric recording medium is inserted into a spindle shaft of an information recording device. [Figure 4] FIG. 10 is a diagram illustrating an example of the relationship between the radius of curvature of a conductive probe and the electric field intensity. [Figure 5] 10A and 10B are explanatory diagrams schematically illustrating a process in which polarization inversion of a ferroelectric layer spreads from the central portion directly under a conductive probe to the peripheral portion. [Figure 6] 1 is a diagram showing a data area and a servo information area of ​​a ferroelectric recording medium. [Figure 7]1 is a cross-sectional view showing an example of a state in which a conventional ferroelectric recording medium is inserted into a spindle shaft of an information recording device. [Figure 8] FIG. 1 is a perspective view showing a ferroelectric memory device according to an embodiment. [Figure 9] FIG. 2 is a perspective view showing the configuration of the head assembly when viewed from below. [Figure 10] FIG. 2 is a cross-sectional view showing an example of the configuration of a probe slider. [Figure 11] FIG. 11 is a partially enlarged cross-sectional view of FIG. [Figure 12] 11 is a partially enlarged cross-sectional view of FIG. 10 as viewed from another direction. [Figure 13] FIG. 11 is a partially enlarged view of FIG. 10 as viewed from below. [Figure 14] FIG. 2 is a cross-sectional view showing the configuration of a conductive probe. [Figure 15] 1A to 1C are diagrams illustrating an example of a method for manufacturing a conductive probe. [Figure 16] 10A to 10C are diagrams illustrating another example of a method for manufacturing a conductive probe. [Figure 17] FIG. 10 is a perspective view showing another configuration of the conductive probe. [Figure 18] FIG. 10 is a perspective view showing another configuration of the conductive probe. [Figure 19] FIG. 10 is a cross-sectional view showing another configuration of the conductive probe. [Figure 20] FIG. 2 is a cross-sectional view showing an example of the configuration of a probe slider. [Figure 21] 10A to 10C are explanatory views showing an example of another method for manufacturing a conductive probe. [Figure 22] FIG. 10 is a cross-sectional view showing another configuration of the conductive probe. [Figure 23] 10A to 10C are explanatory views showing an example of another method for manufacturing a conductive probe. [Figure 24] FIG. 10 is an explanatory diagram showing the displacement of a conductive probe. [Figure 25] FIG. 2 is a cross-sectional view showing the configuration of a ferroelectric recording medium driving unit. [Figure 26] FIG. 2 is a diagram showing a voltage waveform. [Figure 27]FIG. 10 is a cross-sectional view showing an example of another configuration of the probe slider. [Figure 28] FIG. 1 is an explanatory diagram showing an example of the configuration of a conventional magnetic head slider. [Figure 29] FIG. 1 is a diagram illustrating a configuration of a data management system. [Figure 30] FIG. 2 is a diagram illustrating an example of the configuration of an external storage device. [Figure 31] FIG. 2 is an explanatory diagram showing an example of a connection relationship of data stored in a ferroelectric recording medium. DETAILED DESCRIPTION OF THE INVENTION

[0020] Hereinafter, embodiments of the present invention will be described in detail. To facilitate understanding of the description, the same components in each drawing are denoted by the same reference numerals, and duplicate descriptions will be omitted. The scale of each component in the drawings may differ from the actual scale. In this specification, unless otherwise specified, "to" indicating a range of values ​​means that the values ​​before and after it are included as the lower and upper limits.

[0021] Before describing the ferroelectric memory device according to the embodiment of the present invention, a ferroelectric recording medium provided in the ferroelectric memory device will be described.

[0022] <Ferroelectric recording media> A ferroelectric recording medium will now be described. FIG. 1 is a partial cross-sectional view showing the configuration of the ferroelectric recording medium, FIG. 2 is a perspective view of the ferroelectric recording medium, and FIG. 3 is a cross-sectional view showing an example of a state in which the ferroelectric recording medium is inserted into a spindle shaft of an information recording device. As shown in FIG. 1, a ferroelectric recording medium 10 includes a substrate 11, an electrode layer 12, a ferroelectric recording layer 13, a protective layer 14, and a lubricant layer 15, and the electrode layer 12, the ferroelectric recording layer 13, the protective layer 14, and the lubricant layer 15 are stacked in this order on the substrate 11. A conductive probe 17 is disposed near a surface (main surface) 101 of the ferroelectric recording medium 10. The conductive probe 17 is attached to the surface of a probe slider 16 facing the ferroelectric recording medium 10. Information is recorded (written) and reproduced (read) on a ferroelectric layer 131 included in the ferroelectric recording layer 13.

[0023] As shown in Fig. 2, the ferroelectric recording medium 10 is formed in a disk shape having an opening 10a in the center of the main surface. As shown in Fig. 3, a spindle shaft 18 of a ferroelectric recording medium drive unit that 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 FIG. 3, the ferroelectric recording medium 10 has ferroelectric recording layers 13 (ferroelectric layer 131, paraelectric layer 132) on both the top and bottom surfaces of the substrate 11, and information can be recorded on both the top and bottom surfaces of the substrate 11 (double-sided recording). However, the ferroelectric recording medium 10 may have a ferroelectric recording layer 13 on only one of the top and bottom surfaces of the substrate 11, and information can be recorded on only one surface of the substrate 11 (single-sided recording).

[0025] [substrate] The substrate 11 has the function of supporting 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 it may be either an insulator or a conductor.

[0027] The insulator may be, for example, glass, silicon, magnesium oxide (MgO), sapphire, or the like.

[0028] Examples of conductors that can be used include metal materials such as aluminum and its alloys, chromium, platinum, gold, silver, and iron, and oxides such as indium oxide (InO2).Also, silicon that has been made conductive by doping can be used.

[0029] When the substrate 11 is a conductor, the substrate 11 can function as the electrode layer 12. Therefore, when the electrode layer 12 is not disposed on the substrate 11, the material constituting the substrate 11 is preferably a conductor.

[0030] It is preferable that the substrate 11 has little waviness, is highly smooth, and exhibits little 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 is preferably, for example, 100 μm to 1 mm.

[0032] In consideration of lattice matching with the ferroelectric layer 131, the material constituting the substrate 11 preferably has a lattice constant that matches within a range of ±10% between the lattice constant of the material constituting the ferroelectric layer 131 and the lattice constant of the material constituting the substrate 11. This improves the crystallinity of the ferroelectric layer 131, enabling an increase in the recording density of the ferroelectric recording medium.

[0033] It is also preferable that the crystal system of the material constituting the substrate 11 is the same as the crystal system of the material constituting the ferroelectric layer 131, and it is also preferable that the crystal type of the material constituting the substrate 11 is the same as the crystal type of the material constituting the ferroelectric layer 131. While it is sufficient if either the crystal system or the crystal type is the same, it is most preferable that both the crystal system and the crystal type are the same. This increases the crystallinity of the ferroelectric layer 131, thereby enabling the recording density of the ferroelectric recording medium to be increased.

[0034] Here, crystal systems are classified into triclinic, monoclinic, orthorhombic (orthorhombic), tetragonal, hexagonal, cubic, etc. according to the classification that defines the symmetry of the crystal, and therefore it is preferable that the material constituting the substrate 11 and the material constituting the ferroelectric layer 131 are the same.

[0035] Crystal structures are classified based on the closest packing of the crystals, and include a simple cubic lattice structure, a face-centered cubic lattice structure, a body-centered cubic lattice structure, a hexagonal close-packed structure, a diamond structure, a white tin structure, a graphite structure, an A15 structure, a sodium chloride structure, a cesium chloride structure, a zinc blende structure, a wurtzite structure, a nickel arsenide structure, a lead monoxide structure, a fluorite structure, a pyrite structure, a cuprite structure, a rutile structure, a cadmium iodide structure, a bismuth fluoride structure, a rhenium oxide structure, a Ni4Mo structure, an Al4Ba structure, a calcium boride structure, a CaCu5 structure, a corundum structure, a perovskite structure, an ullmanite structure, a spinel structure, a silver phosphate structure, a CuAuI structure, a K4 crystal structure, etc. It is preferable that the material constituting the substrate 11 and the material constituting the ferroelectric layer 131 have the same crystal structure.

[0036] For example, when hafnium oxide is selected for the ferroelectric layer 131, hafnium oxide has an orthorhombic fluorite 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 can change from the stable monoclinic system to the tetragonal system and then to the cubic system. Since these crystal systems do not exhibit ferroelectric properties, it is important to convert this cubic hafnium oxide to the orthorhombic system by heat treatment or the like to make it a ferroelectric.

[0037] Therefore, the material constituting the substrate 11 has a lattice constant of 4.6 Å to 5.8 Å so as to achieve lattice matching within a range of ±10%, and the crystal system is preferably one of orthorhombic, monoclinic, tetragonal, and cubic, and more preferably orthorhombic or cubic. It is also preferable to use a material whose crystal structure is a fluorite 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 a cubic crystal system. Ge has a lattice constant of 5.7 Å and is a cubic crystal system. Pd has a lattice constant of 5.0 Å and is a cubic crystal system. CeO2 has a lattice constant of 5.4 Å and is a cubic crystal system.

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

[0039] In consideration of lattice matching with the ferroelectric layer 131 constituting the ferroelectric recording layer 13, the material constituting the electrode layer 12 is preferably of the same crystal system and / or crystal type as the ferroelectric layer 131. This increases the crystallinity of the ferroelectric layer 131, enabling the recording density of the ferroelectric recording medium to be increased.

[0040] The lattice constant of the material constituting the electrode layer 12 is preferably lattice-matched to the lattice constant of the material constituting the ferroelectric layer 131 within a range of ±10%, thereby improving the crystallinity of the ferroelectric layer 131 and increasing the recording density of the ferroelectric recording medium.

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

[0042] The material for the electrode layer 12 can be appropriately selected depending on the material for the ferroelectric layer 131, and for example, metal materials such as aluminum, chromium, platinum, gold, silver, iron, oxides such as InO2, etc. can be used.

[0043] For example, when 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 so as to achieve lattice matching within a range of ±10%, and the crystal system should preferably be orthorhombic, monoclinic, tetragonal, or cubic, more preferably orthorhombic or cubic. It is also preferable to use a material whose crystal structure is a fluorite structure. Examples of such materials include Ge and Pd. Ge has a lattice constant of 5.7 Å and is a cubic crystal system. Pd has a lattice constant of 5.0 Å and is a cubic crystal system.

[0044] Here, since a metal material can more easily relax lattice strain than an oxide, it can easily relax the resulting lattice strain even if the metal material has a different crystal system, crystal type, or lattice constant from the ferroelectric layer 131. From this perspective, the substrate 11 has a greater effect of increasing the crystallinity of the ferroelectric layer 131 than the electrode layer 12.

[0045] The electrode layer 12 can be produced by forming a conductive thin film on the substrate 11 using a material for forming the electrode layer 12 by any 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 is preferably, for example, 10 nm to 500 nm.

[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 a ferroelectric layer 131 and may include other layers.

[0048] The ferroelectric layer 131 has a function of recording information. The ferroelectric layer 131 is not particularly limited as long as it is a ferroelectric that exhibits ferroelectricity, but an oxide ferroelectric is preferable in terms of electrical properties.

[0049] Examples of oxide ferroelectrics include lead titanate (PbTiO), lead zirconate (PbZrO), barium titanate (BaTiO), lithium niobate (LiNbO), lithium tantalate (LiTaO), and hafnium oxide (HfO). 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 high film formation temperatures. On the other hand, hafnium oxide has an orthorhombic fluorite crystal structure, which is a binary system with a simpler structure than the perovskite crystal structure, allowing film formation at lower temperatures.

[0050] When the ferroelectric layer 131 contains hafnium oxide, it may contain an additive or may be a mixed crystal of hafnium oxide and zirconium dioxide (ZrO2) (Hf x Zr 1-x O2).

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

[0052] The content of the additive is preferably in the range of 1 atomic % to 20 atomic %, more preferably in the range of 3 atomic % to 17 atomic %, and further preferably in the range of 5 atomic % to 15 atomic %. If the content of the additive is within the above-mentioned preferable range, the film formation temperature when forming the ferroelectric layer 131 can be lowered, and the amount of additive used can be reduced.

[0053] The method for adding the additive to hafnium oxide is not particularly limited, and any appropriate method can be used to add the additive.

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

[0055] The ferroelectric layer 131 is preferably composed of a single crystal, but may also include an amorphous structure with short-range order. The ferroelectric layer 131 may be composed solely of an amorphous structure with short-range order, or may include a single-crystal region. Short-range order refers to the short-range order between atoms constituting the amorphous structure, specifically, the orderly nature of the number of nearest neighbor atoms (the number of nearest 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 that are distant from each other among the atoms constituting the crystal, specifically, the orderly number of nearest neighbor atoms, the bond distance, the bond angle, etc., over a range far exceeding the interatomic distance. A single crystal is a general term for materials having such a structure. When the ferroelectric layer 131 includes an amorphous structure with short-range order, polarization reversal due to crystal lattice distortion occurs in the region containing the amorphous structure, enabling information to be recorded. Furthermore, since the region containing the amorphous structure does not contain crystal grain boundaries or lattice defects, the wider the region forming the amorphous structure, the wider the recording region 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 refers to the length of a region having short-range order between atoms, and refers to the vertical and horizontal lengths between atoms relative to the surface of the ferroelectric recording medium. The length of the short-range order is also referred to as the vertical and horizontal 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 surface of the magnetic recording medium, and this region is composed of only a few magnetic particles. In order for ferroelectric recording media to have a higher recording density than magnetic recording media, the bit size of the ferroelectric recording media must be smaller than the bit size of the magnetic recording media. In order to include several recording regions 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] An amorphous structure with short-range order of 2 nm or less in length can be confirmed by X-ray diffraction, electron microscope observation, electron beam diffraction, etc. That is, in such a structure, clear crystals cannot be confirmed by electron microscope observation, and a blurred intensity distribution called a halo pattern is obtained by electron beam diffraction. Furthermore, a halo pattern is also obtained by X-ray diffraction at the position of the crystal planes with short-range order. Then, when the ferroelectric layer 131 is heated to a temperature above the film-forming temperature (approximately +200°C from the film-forming temperature), the halo pattern changes to a signal with a sharp peak. This is because the short-range order generated in the ferroelectric layer 131 changes to long-range order by heating the substrate.

[0058] It is preferable that the lattice constant of the short-range order of the amorphous structure and the lattice constant of the material constituting the substrate 11 are lattice-matched within a range of ±10%, thereby enhancing the short-range order of the ferroelectric layer 131 and increasing the recording density of the ferroelectric recording medium 10.

[0059] The total thickness of the ferroelectric layer 131 is appropriately selected within the range of 1 nm to 1000 nm. If the thickness of the ferroelectric layer 131 is within the above range, polarization inversion can be caused in the ferroelectric layer 131, and the voltage required to invert the polarization of the ferroelectric layer 131 can be reduced.

[0060] Furthermore, from the viewpoint of causing polarization inversion in the ferroelectric layer 131 while minimizing the voltage required for polarization inversion of the ferroelectric layer 131, the film 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.

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

[0062] From the viewpoint of the recording density of the ferroelectric recording medium, when the bit length in the track direction is set to, for example, 10 nm, the film thickness of the ferroelectric layer 131 is preferably set to 1 to 5 times the bit length, that is, 10 to 50 nm, and when the bit length is set to 1 nm, the film thickness is preferably set to 1 to 5 nm. This is because, since one bit in the ferroelectric layer 131 is composed of a solid ferroelectric material, the ratio of its length to width and height is empirically stable within the range of 1 to 5.

[0063] From the perspective of read sensitivity in the conductive probe, when a tunnel current is used for reading, if you want to increase the read sensitivity, you increase the film thickness of the ferroelectric layer, and if you want to decrease the read sensitivity, you decrease the film thickness of the ferroelectric layer 131. That is, because ferroelectrics are insulators with a large band gap, the thicker the film thickness of the ferroelectric layer 131, the larger the energy band barrier becomes, making it difficult for the tunnel current to flow, and the sensitivity of the conductive probe that detects this must also be increased. On the other hand, if the film thickness of the ferroelectric layer 131 is decreased, the tunnel current flows more easily, which can decrease the sensitivity of the conductive probe.

[0064] Furthermore, when atomic force is used for reading, the thicker the ferroelectric layer 131, the greater the amount of charge in the ferroelectric layer 131, so the reading sensitivity of the conductive probe that detects this can be reduced. On the other hand, when the thickness of the ferroelectric layer 131 is reduced, the amount of charge in the ferroelectric layer 131 also decreases, so it is necessary to increase the sensitivity of the conductive probe.

[0065] From the viewpoint of leakage current of the ferroelectric layer 131, the thicker the ferroelectric layer 131, the greater the amount of charge in the ferroelectric layer 131, and therefore the effect of leakage current can be reduced, thereby reducing the frequency of refresh (rewriting). On the other hand, as the thickness of the ferroelectric layer 131 becomes thinner, the amount of charge also decreases, and therefore the effect of leakage current also increases, and the frequency of refresh increases.

[0066] In terms of the crystallinity of the ferroelectric layer 131, the thicker the film thickness of the ferroelectric layer 131, the higher the crystallinity tends to be.

[0067] In terms of smoothness of the growth surface of the ferroelectric layer 131, the thinner the film thickness of the ferroelectric layer 131, the higher the smoothness tends to be.

[0068] The ferroelectric layer 131 can be formed by a known method, such as a sputtering method, a CVD method, a sol-gel method, a laser ablation method, etc. Among these, the sputtering method and the CVD method are preferred.

[0069] In order to improve the crystallinity of the ferroelectric recording layer 13, it is preferable to increase the film formation temperature to about 500°C. However, if the film formation temperature exceeds 500°C, the ferroelectric layer 131 is likely to become polycrystalline and the growth surface is likely to become rough. This also limits the types of substrates 11 that can be used. If sputtering and CVD are used as the film formation method, the film formation temperature can be reduced, and the ferroelectric layer 131 can be prevented from becoming polycrystalline and the growth surface can be prevented from becoming rough.

[0070] When a sputtering method is used to form the ferroelectric layer 131, it is preferable to use a plasma-assisted high-frequency sputtering method or a reactive sputtering method in order to increase the electron temperature while lowering the gas temperature in the film formation region. When a CVD method is used to form the ferroelectric layer 131, it is particularly preferable to use a plasma CVD method, an EACVD method, or a metal organic CVD method (MOCVD method). This makes it easier to change cubic hafnium oxide into an orthorhombic system.

[0071] As described above, information is recorded in the ferroelectric layer 131. The principle of recording and retaining information in the ferroelectric layer 131 is as follows. Specifically, the ferroelectric material constituting the ferroelectric layer 131 has the property that its polarization direction changes when an electric field exceeding its coercive field is applied. Furthermore, once the polarization direction of a ferroelectric material is changed by the application of an electric field, the ferroelectric material maintains the 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 aligned in advance in one direction perpendicular to the surface of the ferroelectric recording medium 10. Then, an electric field exceeding the coercive 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 portion to which the electric field is 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" corresponds to a downward polarization direction, and the bit state "1" corresponds to an upward polarization direction. In this case, an electric field is applied to the ferroelectric layer 131 only when recording the bit state "1." In this way, information can be recorded and retained in the ferroelectric layer 131.

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

[0074] The ferroelectric recording layer 13 may be a multi-layer structure in which a plurality of ferroelectric layers 131 are stacked.

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

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

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

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

[0079] 5A and 5B are explanatory diagrams schematically illustrating the process of polarization reversal in a ferroelectric recording layer. As shown in FIG. 5A, a conductive probe 17 is placed in a non-contact state facing the surface of a ferroelectric layer 131 that is spontaneously polarized to a positive polarity (see FIG. 5A). When a positive voltage is applied to the conductive probe 17 and the electric field strength generated in the space at the tip by the applied voltage exceeds the polarization reversal potential of the ferroelectric layer 131, the polarization is first reversed to negative directly below the conductive probe 17 (see FIG. 5B). Then, as the applied voltage is further increased, the polarization reversal to negative progresses from directly below the conductive probe 17 to the peripheral area (see FIGS. 5C and 5D).

[0080] Here, the five consecutive charged positions shown in Figure 5 are considered to be the smallest single recording area. In this case, the number of positive charges in this single recording area is five in Figure 5(a), four in Figure 5(b), two in Figure 5(c), and zero in Figure 5(d). Depending on the number of positive charges, four multi-level information is recorded. Writing multi-level information to this smallest single recording area is performed by a single write operation, which is the simplest operation in a ferroelectric memory device.

[0081] The information (multi-valued information) recorded in the ferroelectric layer 131 as multi-values ​​is read and reproduced by the ferroelectric memory device. The reproduction (reading) of this multi-valued information is performed by a single read operation, which is the simplest operation in the ferroelectric memory device.

[0082] Methods for reproducing information recorded in the ferroelectric layer 131 include, for example, a method utilizing the fact that the dielectric constant of the ferroelectric layer 131 varies depending on the polarization direction of the ferroelectric layer 131, a method detecting a weak tunnel current flowing between the conductive probe 17 and the electrode layer 12, and a method detecting the atomic force between the conductive probe 17 and the ferroelectric layer 131. All of these methods make it possible to reproduce multi-value recorded information from the smallest amount of charge in one recording area of ​​the ferroelectric layer 131.

[0083] That is, when a method is adopted 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 in one recording area, the greater the difference in dielectric constant.

[0084] When a method of detecting a weak tunneling current flowing between the conductive probe 17 and the electrode layer 12 is adopted, the tunneling barrier of the ferroelectric layer 131 changes depending on the polarization direction and amount. The tunneling current injected from the electrode layer 12 also changes, and by detecting the amount of this change, it is possible to reproduce the multi-value information recorded in the ferroelectric recording medium 10.

[0085] When a method of detecting the atomic force between the conductive probe 17 and the ferroelectric layer 131 is employed, the electric force (Maxwell stress) between the ferroelectric layer 131 and the conductive probe 17 varies depending on the polarization direction and amount of polarization of the ferroelectric layer 131. By detecting this variation together with the atomic force, it is possible to reproduce the information recorded in the ferroelectric recording medium 10 as multi-values.

[0086] In the ferroelectric memory device, position information (also referred to as "servo information") for detecting the relative positions of the conductive probe 17 and the ferroelectric recording medium 10 in the track direction on the ferroelectric recording medium 10 is preferably recorded in the ferroelectric layer 131. In this case, it is preferable that the ferroelectric layer 131 have servo information areas in which servo information is recorded and data areas in which data is recorded and reproduced, arranged alternately at regular intervals in the circumferential direction of the track. This allows the conductive probe 17 to detect its own position using the servo information while reproducing the recorded data.

[0087] 6A and 6B are diagrams showing the data region and servo information region of the ferroelectric layer 131, with FIG. 6A being a plan view of the ferroelectric layer 131 and FIG. 6B being an enlarged view of the rectangular region A in FIG. 6A. As shown in FIGS. 6A and 6B, the ferroelectric layer 131 may include a data region 131A and a servo information region 131B on one surface of the disk-shaped electrode layer 12. In FIG. 6A, 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 FIG. 6B, the data region 131A has a regular circular ring shape.

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

[0089] The burst information area 131B-1 records burst information for positioning the conductive probe 17 at 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) indicating the address of the data area 131A.

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

[0092] 6(a), conductive probe 17 moving circumferentially on the surface of ferroelectric layer 131 reads preamble information in preamble information region 131B-3 to prepare for reading address information. Then, conductive probe 17 reads address information in data region 131A in address information region 131B-2, and reads burst information in burst information region 131B-1 to fine-tune the track position (radial position). Thereafter, conductive probe 17 can record and reproduce information in data region 131A.

[0093] The servo information area 131B preferably includes reference signal information 131B-4, which indicates a reference for the signal level of the multi-value recorded information. For example, in the four-value recording of Fig. 5, the reference signal information 131B-4 may record four types of charge amounts shown in Fig. 5(a), Fig. 5(b), Fig. 5(c), and Fig. 5(d). Alternatively, the reference signal information 131B-4 may record two types of charge amounts shown in Fig. 5(a) and Fig. 5(d), with Fig. 5(b) and Fig. 5(c) being 1 / 3 or 2 / 3 of the difference in signal level between Fig. 5(a) and Fig. 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-value recording, and the determined signal level is used to reproduce the multi-value information recorded in the data area 131A.

[0095] As shown in FIG. 1, the ferroelectric recording layer 13 preferably 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] When the ferroelectric recording layer 13 is composed of a single layer of only the ferroelectric layer 131, the ferroelectric layer 131 is provided in contact with the electrode layer 12. In this case, the charge of the polarized ferroelectric layer 131 may leak out to the electrode layer 12, possibly causing the information recorded in the ferroelectric layer 131 to be lost.

[0098] In particular, the crystallinity of the ferroelectric layer 131 in the early stage of growth, i.e., the region near the interface with the electrode layer 12, is likely to deteriorate during film formation, and the region is likely to have a polycrystalline or amorphous structure, for example. Therefore, charges are likely to leak from this polycrystalline or amorphous structure to the electrode layer 12. That is, the amorphous structure portion formed near the interface of the ferroelectric layer 131 is completely amorphous, lacking both long-range order and short-range order, and therefore causes charge leakage in the same way as the grain boundary portion of a polycrystalline material.

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

[0100] Furthermore, the provision of the 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] Ferroelectrics are insulators with a large band gap, making it difficult for tunneling current to flow. However, by making the ferroelectric thin film, the tunneling barrier can be reduced, and by forming a junction structure with a conductive electrode layer, a weak tunneling current can be made to flow due to the electronic state at the junction. Furthermore, if a paraelectric layer is added to this junction, 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 due to electric charges, lowering the tunneling barrier, making it even easier for tunneling current to flow.

[0102] Known materials can be used as the paraelectric material for the paraelectric layer 132. For example, oxides, nitrides, carbides, borides, and silicides are preferably used as the paraelectric material. These may be used alone or in combination of two or more.

[0103] Examples of oxides include alumina, zirconia, yttrium-stabilized zirconia, silicon oxide, titanium oxide, cerium oxide, titanium oxide, 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 the borides include titanium boride, iron boride, and neodymium boride.

[0107] The silicide may include molybdenum silicide.

[0108] Furthermore, similarly to the substrate 11 and the electrode layer 12, it is preferable that the paraelectric material constituting the paraelectric layer 132 has the same crystal system and / or the same crystal structure as the ferroelectric layer 131, in consideration of lattice matching with the ferroelectric layer 131.

[0109] The lattice constant of the paraelectric material constituting the paraelectric layer 132 is preferably lattice-matched within a range of ±10% with the lattice constant of the ferroelectric material constituting the ferroelectric layer 131, similarly to the substrate 11 and the electrode layer 12. This makes it easier for the ferroelectric layer 131 to grow on the paraelectric layer 132, and the crystallinity of the ferroelectric layer 131 can be improved.

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

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

[0112] The effect of the paraelectric layer 132 in enhancing the crystallinity of the ferroelectric layer 131 due to 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. As mentioned above, band bending in a direction that lowers the tunnel barrier occurs at the interface between the ferroelectric layer 131 and the paraelectric layer 132, but this band bending is strongly affected by lattice strain at the interface. That is, lattice strain increases the energy at the interface, and this increased energy may obscure the energy state due to the band bending. Therefore, in order to detect a weak tunnel current flowing between the conductive probe 26 and the ferroelectric layer 131, it is important to enhance the lattice matching between the paraelectric layer 132 and the ferroelectric layer 131 and reduce the increase in energy caused by lattice strain.

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

[0114] If the film thickness of the paraelectric layer 132 is within the above-mentioned preferred range, polarization inversion can be caused in the ferroelectric layer 131, and leakage of charges from the ferroelectric layer 131 can be suppressed, thereby suppressing leakage of charges to the outside from the ferroelectric layer 131. Furthermore, if the film thickness of the paraelectric layer 132 is within the above-mentioned preferred range, tunnel current can be made to flow easily, and therefore the tunnel current can be increased.

[0115] Furthermore, since the increase in the distance between the ferroelectric layer 131 and the electrode layer 12 is suppressed, it is possible to suppress the voltage required to polarize the ferroelectric layer 131. Therefore, it is possible to suppress an increase in the voltage required to polarize the ferroelectric layer 131 while suppressing an 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 approximately the same as the film thickness of the ferroelectric layer 131. In this case, the total film thickness of the ferroelectric layer 131 is also preferably 1 nm to 30 nm. The film thickness of the paraelectric layer 132 is preferably equal to the total film thickness of the ferroelectric layer 131. When there is a difference between the film thickness of the paraelectric layer 132 and the total film thickness of the ferroelectric layer 131, the paraelectric layer 132 is preferably 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 the outside, and can reduce damage to the ferroelectric recording layer 13 even if the ferroelectric recording medium 10 comes into contact with a conductive probe 17, etc.

[0118] The protective layer 14 is preferably made of an insulating material with a low dielectric constant so as not to impair the information recording and reproducing functions of the ferroelectric recording layer 13 .

[0119] Usable 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 film, and polymer insulating materials. From the viewpoint of protecting the ferroelectric recording layer and preventing a decrease in its 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, taking into consideration the voltage required for the ferroelectric of the ferroelectric layer 131 to undergo polarization inversion, 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 FIG. 3, the ferroelectric recording medium 10 preferably does not have the ferroelectric recording layer 13 (ferroelectric layer 131, paraelectric layer 132) and the protective layer 14 around the opening 10a. That is, the ferroelectric recording layer 13 and the protective layer 14 are preferably provided in an area other than the central opening 10a of the ferroelectric recording medium 10 and its periphery. Note that the area around the opening 10a refers to a range of approximately 10 mm in length from the inner periphery 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 an area other than the opening 10a and its periphery, the ferroelectric recording layer 13 and the protective layer 14 are not in contact with the spindle shaft 18, and leakage of charge in 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 a substrate 11 and an electrode layer 12. The spindle shaft 18 to which this ferroelectric recording medium 10 is attached is cylindrical and has a stepped portion with a reduced diameter at the top. The ferroelectric recording medium 10 is placed on this stepped portion, and a mounting bracket 19 is placed on top of the ferroelectric recording medium 10 and screwed to the spindle shaft 18, thereby fixing the ferroelectric recording medium 10 to the spindle shaft 18. The ferroelectric recording layer 13, protective layer 14, and lubricant layer 15 are not provided at the location where the ferroelectric recording medium 10 is fixed to the spindle shaft 18, and the substrate 11 and spindle shaft 18 are in direct contact. The spindle shaft 18 is connected to the ferroelectric layer 131 via the substrate 11, and forms part of a circuit that reads and writes information from and to the ferroelectric layer 131.

[0123] 7, when a ferroelectric recording medium having an electrode layer 12, a ferroelectric layer 131, and a protective layer 14 provided on the entire surface of a substrate 11 is attached to a spindle shaft 18, the ferroelectric layer 131 has insulating properties, so even if the ferroelectric layer 131 comes into contact with the spindle shaft 18, it is not possible to read or write information from or 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 continuity between the substrate 11 and the spindle shaft 18 by having the convex portions 181a on the attachment surface 181 of the spindle shaft 18 penetrate the ferroelectric layer 131 and the protective layer 14 and bite into the electrode layer 12, thereby establishing electrical continuity between the substrate 11 and the spindle shaft 18.

[0124] When the ferroelectric recording medium 10 is installed in a ferroelectric storage device and used for a long period of time, the protective layer 14 deteriorates due to moisture and dirt inside the ferroelectric storage device, which reduces the insulating properties of the protective layer 14 and may cause charge leakage from the ferroelectric layer 131 that constitutes the ferroelectric recording layer 13. Furthermore, although the ferroelectric recording layer 13 is covered with the protective layer 14 and therefore does not deteriorate to a large extent, the ferroelectric recording layer 13 may also deteriorate in the same manner as the protective layer 14, causing charge leakage from the ferroelectric layer 131. In this embodiment, as shown in FIG. 3, the ferroelectric recording layer 13 and the protective layer 14 are formed in areas other than the opening 10a of the ferroelectric recording medium 10 and its surrounding area, thereby preventing charge leakage from the ferroelectric recording layer 13 from the protective layer 14 to the spindle shaft 18.

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

[0126] Lubricants used in the lubricant layer 15 include saturated fatty acids such as stearic acid, dyes such as phthalocyanine, fluorine-based resins such as perfluoropolyether (PFPE), and in particular fluorine-based resins such as PFPE, which have good lubricity and are therefore preferred.

[0127] The lubricant layer 15 is preferably made of an insulating material with a low dielectric constant so as not to impair the information recording and reproducing functions of the ferroelectric recording layer 13 .

[0128] As described above, the ferroelectric recording medium 10 according to this embodiment 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 can improve the crystallinity of the ferroelectric material constituting the ferroelectric layer 131. The ferroelectric material contained in the ferroelectric layer 131 becomes single crystal, eliminating grain boundaries and thereby reducing the influence of the grain boundaries. Since polarization reversal of the ferroelectric material contained in the ferroelectric layer 131 occurs due to crystal lattice distortion, the ferroelectric material contained in the ferroelectric layer 131 becoming single crystal can increase the area in the ferroelectric layer 131 where polarization reversal of the ferroelectric material occurs. Therefore, the ferroelectric recording medium 10 can achieve a high recording density.

[0129] Furthermore, in the ferroelectric recording medium 10, when the substrate 11 is a conductor, the substrate 11 can also serve as an electrode because of its conductivity. In this case, the substrate 11 can function as the electrode layer 12, making the electrode layer 12 unnecessary. Therefore, 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 substrate 11 within a range of ±10% without providing the electrode layer 12 on the substrate 11. Therefore, as described above, the ferroelectric recording medium 10 can increase the region in the ferroelectric layer 131 where polarization reversal of the ferroelectric occurs, thereby achieving high recording density.

[0130] The ferroelectric recording medium 10 can also have the ferroelectric layer 131 as a single crystal film. This can eliminate crystal grain boundaries in 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 2 nm or less, and the lattice constant of the amorphous structure can be lattice-matched to the lattice constant of the material constituting the substrate 11 within a range of ±10%. The ferroelectric layer 131 has an amorphous structure with short-range order, which allows polarization reversal due to crystal lattice distortion in the region with short-range order and reduces grain boundaries. Therefore, the ferroelectric recording medium 10 can use a wide region of the ferroelectric layer 131 as a recording region. Furthermore, by setting the length of the short-range order of the ferroelectric layer 131 to 2 nm or less, the storage region can be increased, thereby increasing the recording density. Therefore, the ferroelectric recording medium 10 can have a high recording density.

[0132] Furthermore, when 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 of a single crystal film. This makes the growth surface of the ferroelectric layer 131 smooth, thereby making it possible to smooth the surface of the ferroelectric recording medium 10. Therefore, the ferroelectric recording medium 10 can reduce spacing loss with the conductive probe 17, thereby enabling a higher recording density.

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

[0134] In the ferroelectric recording medium 10, the ferroelectric layer 131 includes an amorphous structure having short-range order, which makes it easy to thin the ferroelectric layer 131, and therefore makes it easy to form a smooth surface for the ferroelectric layer 131. Therefore, the smoothness of the ferroelectric recording medium 10 can be improved.

[0135] Furthermore, since the ferroelectric recording medium 10 has a ferroelectric layer 131 that includes an amorphous structure with short-range order, the crystal grain boundaries of the ferroelectric layer 131 can be reduced, thereby increasing the recording capacity per ferroelectric recording medium 10.

[0136] In the ferroelectric recording medium 10, the substrate 11 can contain silicon and the ferroelectric layer 131 can contain hafnium oxide. This improves the lattice matching between the substrate 11 and the ferroelectric layer 131, thereby improving 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 O2) (x is 0.3 to 0.6). As a result, the ferroelectric layer 131 is formed from the above mixture or mixed crystal containing hafnium oxide, and thus the film formation temperature can be lowered when forming the ferroelectric layer 131 on the substrate 11, and the ferroelectric layer 131 can be formed at a low temperature on the substrate 11. Generally, the higher the temperature when forming the ferroelectric layer 131, the easier it is to form polycrystals. In this embodiment, the ferroelectric can increase the single-crystal region, and therefore 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 have a smooth surface over a wider area, thereby further improving the smoothness of the ferroelectric layer 131.

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

[0140] The ferroelectric recording medium 10 can have a film thickness of 5 nm to 1000 nm for the ferroelectric layer 131. This allows the ferroelectric recording medium 10 to cause polarization reversal in the ferroelectric contained in the ferroelectric layer 131, and also reduces the voltage required for polarization reversal of the ferroelectric, thereby reducing the burden 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. When the ferroelectric layer 131 is formed, the initial growth region, i.e., the region near the interface with the electrode layer 12, is polycrystalline or amorphous. The crystallinity increases toward the growth direction of the film thickness, eventually resulting in a single crystal at the surface. Therefore, charge in the ferroelectric layer 131 is likely to leak from the polycrystalline or amorphous region near the interface toward the substrate 11. The paraelectric layer 132 can prevent charge in the ferroelectric layer 131 from leaking to the substrate 11. Therefore, the ferroelectric recording medium 10 can prevent charge from leaking from the ferroelectric layer 131, thereby preventing loss of information recorded in the ferroelectric layer 131.

[0142] Furthermore, in the ferroelectric recording medium 10, the lattice constant of the material constituting the paraelectric layer 132 can be lattice-matched within a range of ±10% of the lattice constant of the material constituting the ferroelectric layer 131. This further improves the crystallinity of the ferroelectric layer 131, and further suppresses charge leakage from the ferroelectric layer 131.

[0143] Furthermore, the ferroelectric recording medium 10 can have the film thickness of the paraelectric layer 132 set to 1 nm to 100 nm. This allows the ferroelectric recording medium 10 to more effectively suppress charge leakage from the ferroelectric layer 131 while causing polarization inversion in the ferroelectric layer 131.

[0144] In the ferroelectric recording medium 10, the film thickness of the paraelectric layer 132 is 1 nm to 30 nm, the film thickness of the ferroelectric layer 131 is 1 nm to 30 nm, the film thickness of the paraelectric layer 132 and the ferroelectric layer 131 are approximately equal or the film thickness of the paraelectric layer 132 is thinner than the total film thickness of the ferroelectric layer 131, and the difference in film thickness between the paraelectric layer 132 and the ferroelectric layer 131 can be 10 nm or less. As a result, when the ferroelectric recording medium 10 is applied to a ferroelectric memory device, it is possible to 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 areas other than the opening 10a and its periphery. Because 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 prevent charges in the ferroelectric layer 131 from leaking to the spindle shaft 18 through the protective layer 14. Therefore, the ferroelectric recording medium 10 can prevent charges from leaking from the ferroelectric layer 131, thereby preventing loss of information recorded in the ferroelectric layer 131 and suppressing adverse effects caused by deterioration of the protective layer 14 over time.

[0146] Furthermore, the ferroelectric recording medium 10 can be provided with the ferroelectric recording layer 13 in an area other than the opening 10a and its periphery. This prevents the ferroelectric recording layer 13 from coming into contact with the spindle shaft 18, so that even if the ferroelectric recording layer 13 and the protective layer 14 deteriorate over time, the ferroelectric recording medium 10 can prevent the charge in the ferroelectric layer 131 from leaking to the spindle shaft 18 through the ferroelectric recording layer 13 and the protective layer 14. Therefore, the ferroelectric recording medium 10 can more reliably prevent the charge from leaking from the ferroelectric layer 131, and can more stably prevent the loss of information recorded in the ferroelectric layer 131, and can more effectively prevent the adverse effects of deterioration of the ferroelectric recording layer 13 and the protective layer 14 over time.

[0147] <Ferroelectric memory device> A ferroelectric memory device including a ferroelectric recording medium 10 according to this embodiment will be described. The ferroelectric memory device includes the above-described ferroelectric recording medium 10 as the ferroelectric recording medium. FIG. 8 is a perspective view showing a ferroelectric memory device. As shown in FIG. 8, the ferroelectric memory device 100 includes the ferroelectric recording medium 10, a head assembly 20, a ferroelectric recording medium driver 30, a probe driver 40, a control unit (not shown), and a recording / reproducing signal processor 50, all of which are housed within a housing 60. The ferroelectric memory device 100 may include a plurality of ferroelectric recording media 10. Since the ferroelectric recording medium according to this embodiment described above is used as the ferroelectric recording medium 10, details will be omitted.

[0148] [Head assembly] As shown in FIG. 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 .

[0149] 9 is a perspective view showing the configuration of the head assembly 20 as seen from below. As shown in Fig. 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 FIG. 8, the actuator arm 21 has a hole at one end for being fixed to the fixed shaft of the ferroelectric memory device 100, and a suspension arm 22 is connected to the tip end.

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

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

[0153] FIG. 10 is a cross-sectional view showing an example of the configuration of the probe slider 23. Among the arrows in FIG. 10, the +X-axis and −X-axis directions are sector directions of the ferroelectric recording medium 10, the +Y-axis and −Y-axis directions are track directions of the ferroelectric recording medium 10, and the Z-axis direction is a direction facing the recording surface of the ferroelectric recording medium 10. As shown in FIG. 10, the probe slider 23 includes 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 includes 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 and −Z-axis directions, and the second piezoelectric element 27B drives the conductive probe 26 in the +Y-axis and −Y-axis directions. In addition to the first piezoelectric element 27A and the second piezoelectric element 27B, the piezoelectric element 27 may also include a third piezoelectric element 27C and a fourth piezoelectric element 27D, which will be described later.

[0154] Fig. 11 is a partially enlarged cross-sectional view of Fig. 10, Fig. 12 is a partially enlarged cross-sectional view of Fig. 10 as viewed from another direction, and Fig. 13 is a partially enlarged view of Fig. 10 as viewed from below. As shown in Figs. 11 to 13, an electrode 28A-1 is provided between the electrostrictive element 29 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. The electrodes 28A-1 and 28A-2 are paired on the upper and lower surfaces of the first piezoelectric element 27A in the Z-axis direction, sandwiching the first piezoelectric element 27A therebetween.

[0155] Furthermore, a second piezoelectric element 27B is provided between the probe slider 23 and the electrode 28A-2, and electrodes 28B-1 and 28B-2 are provided on the side surfaces of the second piezoelectric element 27B in the Y-axis direction. The electrode 28B-1 is provided on the +Y-axis direction surface of the second piezoelectric element 27B, and the electrode 28B-2 is provided on the -Y-axis direction surface of the second piezoelectric element 27B, and the electrodes 28B-1 and 28B-2 form a pair with the second piezoelectric element 27B sandwiched between them.

[0156] 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 pad 28C, which is provided on the lower surface of probe slider 23 and further outward in the Y-axis direction than electrode 28A-2.

[0157] Here, electrode 28A-2 is used to polarize first piezoelectric element 27A, but since it is also in contact with second piezoelectric element 27B, it may polarize 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 second piezoelectric element 27B, and the dielectric constant in this case is preferably 1 / 100 or less of the dielectric constant of second piezoelectric element 27B.

[0158] Furthermore, the piezoelectric effect includes the piezoelectric longitudinal effect, the piezoelectric transverse effect, and the piezoelectric thickness shear effect. The electrode positions shown in Figures 11 and 12 are for using the piezoelectric longitudinal effect together with the first piezoelectric element 27A and the second piezoelectric element 27B, but if other effects are to be used, each electrode can be provided on a different surface of the piezoelectric element.

[0159] Furthermore, when the first piezoelectric element 27A is driven in the +Z-axis direction or the -Z-axis direction, the first piezoelectric element 27A is also displaced in the +Y-axis direction or the -Y-axis direction due to the transverse piezoelectric effect. This displacement of the first piezoelectric element 27A in the +Y-axis direction or the -Y-axis direction is preferably compensated for by driving the second piezoelectric element 27B in the +Y-axis direction or the -Y-axis direction.

[0160] (Conductive probe) The conductive probe 26 has a function of recording and reproducing information to and from 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 a scanning tunneling microscope (STM) and an atomic force microscope (AFM).

[0161] As shown in Fig. 10, 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 shifted from the center of the lower surface of the second piezoelectric element 27B toward the track direction (+Y axis direction or -Y axis direction) of the ferroelectric recording medium 10. For example, as shown in Fig. 13, the conductive probe 26 is preferably provided on the side of an electrode 28B-2 located on the +Y axis direction side of the center of the lower surface of the second piezoelectric element 27B. Alternatively, different from Fig. 13, the conductive probe 26 may be provided on the side of an electrode 28B-1 located on the -Y axis direction side of the center of the lower surface of the second piezoelectric element 27B.

[0162] The conductive probe 26 is caused to float and run on the surface of the ferroelectric recording medium 10. It is preferable to record and reproduce information on the ferroelectric recording medium 10 by rotating the recording medium at high speed like an HDD, causing the probe slider 23 to float with the airflow generated on the surface of the recording medium, and then recording and reproducing information using the conductive probe 26 attached to the probe slider 23. That is, by attaching a needle-shaped conductive probe 26 to the probe slider 23 and causing the probe slider 23 to float and run on the surface of the ferroelectric recording medium 10 at the nano level, the needle-shaped conductive probe 26 can be brought very close to the surface of the ferroelectric recording medium 10 to record and reproduce information.

[0163] The conductive probe 26 may be made of a metal such as tungsten, molybdenum, or platinum.

[0164] Fig. 14 is a cross-sectional view showing the configuration of conductive probe 26. As shown in Fig. 14, conductive probe 26 has, for example, a base 261 provided on piezoelectric element 27 and a sharpened needle electrode 262 formed on base 261. Note that base 261 and needle electrode 262 may be integrally formed from the same material, or may be formed from different materials.

[0165] It should be noted that an electrostrictive element may be used instead of the piezoelectric element 27. Both piezoelectric elements and electrostrictive elements have a common function in that they are displaced when an electric field is applied, but whereas the direction of displacement of a piezoelectric element changes depending on the direction of the electric field, an electrostrictive element only expands and does not contract. Furthermore, whereas a piezoelectric element generates an electric charge according to stress, an electrostrictive element does not generate an electric charge even when stress is applied. Because both piezoelectric elements and electrostrictive elements have a common function in that they are displaced when an electric field is applied, they can be used in the same way in ferroelectric memory devices.

[0166] The needle-like electrode 262 is an electrode formed in a cone shape, and the width and height of the needle-like electrode 262 are, for example, several nm to several mm.

[0167] The smaller the curvature of the tip of the needle electrode 262, the stronger the electric field strength at the tip and the lower the voltage applied to the needle electrode 262, which is advantageous when recording information to the ferroelectric recording medium 10. It is also advantageous when reading information because it becomes easier to bring the tip of the needle electrode 262 close to the ferroelectric recording medium 10. The radius of curvature of the tip of the needle electrode 262 is preferably several nm or less.

[0168] The conductive probe 26 is scanned over the surface (recording surface) of the ferroelectric recording medium 10. The conductive probe 26 is brought close to a position very close to the surface (recording surface) of the ferroelectric recording medium 10. Then, an electric field exceeding the coercive electric field of the ferroelectric layer 131 is applied from the conductive probe 26, reversing the polarization direction of the ferroelectric layer 131 located directly below the conductive probe 26. This applied voltage is a pulse signal whose level changes according to the information to be recorded, and while the voltage is being applied 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. This allows information to be recorded as the polarization state of the ferroelectric substance in the ferroelectric layer 131 provided in the ferroelectric recording medium 10.

[0169] A method for reproducing information recorded on the ferroelectric recording medium 10 will be described later.

[0170] The conductive probe 26 can be manufactured using any manufacturing method. The manufacturing method for the conductive probe 26 can include, for example, a step of forming a dot-shaped mask on the surface of a conductive material, a step of etching the conductive material to obtain a conical needle electrode, and a step of removing the mask. As a result, the conductive probe 26 is formed on the base 261 made of the etched conductive material, and has a conical, sharpened needle electrode 262.

[0171] An example of a method for manufacturing the conductive probe 26 will be described. FIG. 15 is a diagram showing an example of a method for manufacturing the conductive probe 26. As shown in FIG. 15, after a dot-shaped mask 71 is formed on the surface of a conductive material 260 (see FIG. 15(a)), the conductive material 260 is etched (see FIG. 15(b)). Since etching of the portion of the conductive material 260 where the mask 71 is provided is delayed, a substantially conical needle electrode 262 is formed below the mask 71 by lifting off the mask 71 (see FIG. 15(c)). Thereafter, the conductive material 260 around the needle electrode 262 is cut out, thereby forming the conductive probe 26 having the needle electrode 262 formed in a conical shape on the base 261 (see FIG. 15(d)).

[0172] Note that if the step of cutting out the needle-like electrode 262 is performed by machining, it may damage the needle-like electrode 262. In that case, it is preferable to etch the conductive material 260 after cutting out the area where the needle-like electrode is to be formed.

[0173] Furthermore, when the conductive material 260 used in the above process is an insulator or semiconductor material, the surface of the manufactured needle-like electrode may be coated with Au (gold) or the like by a sputtering method to form a conductive film, thereby making the needle-like electrode conductive.

[0174] The conductive probe 26 can also be manufactured using other manufacturing methods. Figure 16 is a diagram showing one example of another manufacturing method for the conductive probe 26. As shown in Figure 16, a photoresist 72 is applied to the surface of a conductive material 260 (photoresist application step (see Figure 16(a))).

[0175] Next, the photoresist 72 is etched into a circular shape to form a mask 72A having minute circular through holes 72a in the photoresist 72 (mask formation step (see FIG. 16(b))).

[0176] Next, metal 73 is vapor-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 formed in an approximately conical shape (needle-shaped electrode formation process (see Figures 16(c) and (d))).

[0177] At this time, as the amount of metal 73 deposited on the mask 72A increases, the through-holes 72a are blocked and the width of the through-holes 72a narrows. As a result, the metal 73 deposited on the bottom of the through-holes 72a of the mask 72A is deposited widely at the bottom and the deposition range gradually narrows toward the top, ultimately forming a needle-like electrode 262 deposited in a substantially conical shape (see FIG. 16(d)).

[0178] Next, the mask 72A is removed to obtain the conductive probe 26 having the needle-like electrode 262 (conductive probe manufacturing process (see FIG. 16(e))). The mask 72A is removed and the conductive material 260 around the needle-like electrode 262 is cut out, thereby obtaining the conductive probe 26 having the needle-like electrode 262 formed in a conical shape on the base 261.

[0179] The conductive probe 26 can be used in various forms. The following describes the conductive probe 26.

[0180] ((First Aspect)) 17 and 18, the conductive probe 26A is preferably formed into a triangular or quadrangular pyramid. If anisotropic etching is performed using a single crystal as the conductive material, it is easy to form a tip formed by any crystal plane (textured plane).

[0181] For example, when the conductive material 260 uses the (100) plane of a single crystal having a diamond structure, such as Si, by processing the (100) plane as the crystal plane, the conductive probe 26A can be easily formed into a square pyramid with a sharp apex, which is made up of four (111) equivalent planes. Therefore, when the conductive probe 26A is an approximately triangular pyramid or an approximately square pyramid, it can be made into a conductive probe with a sharper apex than when it is an approximately cone.

[0182] Furthermore, when the conductive probe 26A is a substantially triangular pyramid or a substantially square pyramid, it can have a sharper top than when it is a substantially cone like the conductive probe 26.

[0183] Furthermore, in order to uniformize 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.

[0184] The conductive probe 26A can be manufactured using any manufacturing method. For example, in the manufacturing method of the conductive probe shown in FIG. 15, the shape of the mask 71 formed on the surface of the conductive material 260 is made triangular or rectangular. This makes it possible to form the conductive probe 26A having a triangular or quadrangular pyramidal needle electrode 262A on the base 261, as shown in FIGS. 17 and 18.

[0185] It is also preferable to use a method of forming a top formed by any crystal plane (textured plane) by anisotropically etching a single crystal for the conductive material 260. For example, when the conductive material 260 uses the (100) plane of a single crystal having a diamond structure such as Si, the conductive probe 26A can be formed with good reproducibility into a square pyramid having a sharp top made of four (111) equivalent planes.

[0186] In this case, the anisotropic etching is preferably reactive ion etching (RIE) using an etching gas such as SF6, or wet etching using KOH as an etchant.

[0187] The conductive probe 26A can also be manufactured using other manufacturing methods. As another manufacturing method for the conductive probe 26A, for example, in the mask formation step of the manufacturing method of the conductive probe shown in FIG. 16 above, the shape of the through-holes 72a formed in the photoresist 72 is made triangular or rectangular, and a mask 72A having triangular or rectangular through-holes 72a is formed in the photoresist 72. By making the shape of the through-holes 72a triangular or rectangular, a needle-like electrode 262 having an approximately triangular or rectangular pyramid shape is formed on the conductive material 260. As a result, the conductive probe 26A having a triangular or rectangular pyramidal needle-like electrode 262A on the base 261 is obtained, as shown in FIGS. 17 and 18.

[0188] ((Second Aspect)) A cross-sectional view of another configuration of conductive probe 26 is shown in Fig. 19. As shown in Fig. 19, conductive probe 26B has base 261 made of a conductive material, recess 261a formed in base 261, and needle electrode 262 formed in a conical shape in recess 261a, and it is preferable that a part of needle electrode 262 protrudes from surface (main surface) 261b of base 261. Note that surface 261b of base 261 refers to the main surface of base 261, excluding recess 261a.

[0189] In the conductive probe 26B, most of the needle-like electrode 262 is covered with the base 261, which reduces damage to the needle-like electrode 262 when the conductive probe 26B accidentally comes into contact with the ferroelectric recording medium 10. In addition, vibration and deformation of the needle-like electrode 262 caused by air currents generated by the rotation of the ferroelectric recording medium 10 can be suppressed.

[0190] 20 is a cross-sectional view showing an example of the configuration of probe slider 23 on which conductive probe 26B is mounted. Note that second piezoelectric element 27B may be used in place of first piezoelectric element 27A.

[0191] 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 with fine through-holes, etching the conductive material in the holes to form recesses in the surface of the conductive material, depositing a metal film on the bottom of the recesses in the conductive material formed in the through-holes, and removing the photoresist to obtain a conical needle electrode at the bottom of the recess in the conductive material, with a portion of the needle electrode 262 protruding from the conductive material. This results in the conductive probe 26B.

[0192] FIG. 21 is an explanatory diagram showing an example of a method for manufacturing the conductive probe 26B. As shown in FIG. 21, a photoresist 82 is applied to the surface of a conductive material 260 (see FIG. 21(a)). After patterning, a mask 82A having minute through-holes 82a is formed in the photoresist 82 (see FIG. 21(b)). The through-holes 82a are then etched using an appropriate etching method to form recesses 260a in the surface of the conductive material 260 (see FIG. 21(c)). After etching, a metal 83 is vapor-deposited on the bottoms of the recesses 260a of the conductive material 260 formed in the through-holes 82a and on the photoresist 82 (see FIG. 21(d)). At this time, as the amount of metal 83 vapor-deposited on the photoresist 82 increases, the through-holes 82a are blocked and the width of the through-holes 82a becomes narrower. As a result, the metal 83 deposited on the bottom of the recess 260a of the conductive material 260 is deposited widely at the bottom, and the deposition area gradually narrows as it goes up, ultimately forming a needle-shaped electrode 262 deposited in a cone shape (see Figure 21(e)).

[0193] Thereafter, the mask 82A is removed (see FIG. 21(f)), so that a part of the conical needle electrode 262, including the top thereof, protrudes slightly from the conductive material 260.

[0194] By cutting out the conductive material 260 around the needle-like electrode 262, a conductive probe 26B having a conical needle-like electrode 262 formed in a recess 261a of a base 261 is formed as shown in FIG.

[0195] According to this manufacturing method, the needle electrode 262 is formed in the recess 261a and is surrounded by the base 261, which reduces damage to the needle electrode 262 when the conductive probe 26B accidentally comes into contact with the ferroelectric recording medium 10. In addition, it is possible to prevent vibration and deformation of the needle electrode 262 caused by airflow generated by the rotation of the ferroelectric recording medium 10.

[0196] By forming the through-holes 82a in a triangular or rectangular shape, the conductive probes 26B can be formed in a triangular or quadrangular pyramid shape.

[0197] ((Third Aspect)) Another example of the configuration of conductive probe 26 is shown in Fig. 22. As shown in Fig. 22, conductive probe 26C includes an insulating layer 263 formed by heating and oxidizing conductive material 260 on base 261 made of a conductive material, and preferably includes a needle-like electrode 262 provided on base 261 inside through-hole 263a of insulating layer 263, with a portion of needle-like electrode 262 protruding from surface (main surface) 263b of insulating layer 263. Note that surface 263b of insulating layer 263 refers to the main surface of insulating layer 263, excluding the surface that faces through-hole 263a.

[0198] The needle-like electrode 262 is formed on the surface of the base 261 inside the through-hole 263a and is surrounded by an insulating layer 263, which reduces damage to the needle-like electrode 262 when the needle-like electrode 262 accidentally comes into contact with the ferroelectric recording medium 10. It also reduces vibration and deformation of the needle-like electrode 262 caused by air currents generated by the rotation of the ferroelectric recording medium 10. Furthermore, providing the insulating layer 263 around the needle-like electrode 262 shields the needle-like electrode 262, which reduces the influence of surrounding charges and the leakage of charges applied to the needle-like electrode 262 to the outside.

[0199] A method for manufacturing the conductive probe 26C will be described. The method for manufacturing the conductive probe 26C includes a step of forming an insulating layer on the conductive material 260 by oxidizing the conductive material 260, and a step of forming a separation layer on the insulating layer, so that a part of the needle-like electrode protrudes from the insulating layer. In this way, the conductive probe 26C is obtained.

[0200] FIG. 23 is an explanatory diagram showing an example of a manufacturing method of the conductive probe 26C. As shown in FIG. 23, an insulating layer 263 is formed by heating and oxidizing a conductive material 260 (see FIGS. 23(a) and 23(b)). Thereafter, an isolation layer 29 is formed on the insulating layer 263, and a photoresist 82 is applied to the surface of the isolation layer 29 and patterned to form a circular, minute through-hole 82a in the photoresist 82 (see FIG. 23(c)). Thereafter, the insulating layer 263 and isolation layer 29 at the through-hole 82a are etched using any appropriate etching method (see FIG. 23(d)). Note that in FIG. 23(d), the photoresist 82 is removed after etching the insulating layer 263 and isolation layer 29, but at least a portion of the photoresist 82 may remain.

[0201] After etching, metal 83 is vapor-deposited on the insulating layer 263 and the surface of the conductive material 260 in the through-holes 263a and 29a of the separation layer 29, and on the separation layer 29 (see FIG. 23(e)). At this time, as the amount of metal 83 vapor-deposited on the photoresist 82 increases, the through-hole 29a is closed and its width narrows. As a result, the metal vapor-deposited on the surface of the conductive material 260 in the through-hole 29a is vapor-deposited widely on the surface, and the vapor-deposited area gradually narrows as it goes up, eventually depositing in a cone shape. This forms a sharpened needle-like electrode 262.

[0202] Thereafter, the separation layer 29 is etched to cause a portion of the conical needle electrode 262, including the top thereof, to slightly protrude from the main surface of the insulating layer 263 (see FIG. 23(f)).

[0203] By cutting out the conductive material 260 around the needle-shaped electrode 262, a conductive probe 26C is formed, as shown in Figure 22, having a conical needle-shaped electrode 262 formed in a through hole 263a of an insulating layer 263 on a base 261.

[0204] According to this manufacturing method, the needle-like electrode 262 is formed on the surface of the base 261 inside the through-hole 263a and is surrounded by the insulating layer 263, which reduces damage to the needle-like electrode 262 when the conductive probe 26C accidentally comes into contact with the ferroelectric recording medium 10. Also, vibration and deformation of the needle-like electrode 262 caused by airflow generated by the rotation of the ferroelectric recording medium 10 can be suppressed. Furthermore, by providing the insulating layer 263 around the needle-like electrode 262, the conductive probe 26C can shield the needle-like electrode 262 and suppress the influence of surrounding charges and charge leakage from the needle-like electrode 262.

[0205] (First Piezoelectric Element and Second Piezoelectric Element) 10, in the probe slider 23 having the configuration shown in FIG. 10, a first piezoelectric element 27A is provided at the tip of the probe slider 23, between the probe slider 23 and the conductive probe 26, and is sandwiched between electrodes 28A-1 and 28A-2. Further, a second piezoelectric element 27B is provided between the tip of the probe slider 23 and the electrode 28A-2, and electrodes 28B-1 and 28B-2 are provided on the side surface of the second piezoelectric element 27B on the +Y-axis direction side and the -Y-axis direction side, respectively. The first piezoelectric element 27A is used to adjust the head flying 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.

[0206] In this embodiment, electrostrictive elements may be used instead of the first piezoelectric element 27A and the second piezoelectric element 27B. The first piezoelectric element 27A, the second piezoelectric element 27B, and the electrostrictive element share a common function in that they are displaced when an electric field is applied. However, 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 expands and does not contract. Furthermore, the first piezoelectric element 27A and the second piezoelectric element 27B generate electric charges according to stress, whereas the electrostrictive element does not generate electric charges even when stress is applied. Because the first piezoelectric element 27A, the second piezoelectric element 27B, and the electrostrictive element share a common function in that they are displaced when an electric field is applied, they can be used in a similar manner in a ferroelectric memory device.

[0207] As shown in FIG. 10 , the first piezoelectric element 27A is provided on the lower 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 caused to float and run above 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 μsec or less. Therefore, the distance between the conductive probe 26 and the ferroelectric recording medium 10 can be controlled quickly and with high precision.

[0208] The first piezoelectric element 27A and the second piezoelectric element 27B can be made of, for example, quartz crystal, lithium niobate (LiNbO3), barium titanate (BaTiO3), lead zirconate titanate (PZT), zinc oxide (ZnO), aluminum nitride (AlN), lithium tantalate (LiTaO3), lead titanate (PT), etc. Furthermore, similar materials can be used when electrostrictive elements are used instead of the first piezoelectric element 27A and the second piezoelectric element 27B.

[0209] The probe slider 23 is provided with the first piezoelectric element 27A at its tip, which allows for more accurate and faster DFH control of the conductive probe 26, and allows for more accurate and faster control of the distance between the conductive probe 26 and the ferroelectric recording medium 10. Therefore, the ferroelectric memory device 100 can further improve the recording and reproducing sensitivity of the conductive probe 26.

[0210] Furthermore, the probe slider 23 can have an AGC (automatic gain control) function between tracks and between sectors of the ferroelectric recording medium 10 by providing the first piezoelectric element 27A at its tip.

[0211] As will be described later, for example, a voice coil motor can be used as the probe driver 40 (see FIG. 8) that moves the conductive probe 26 in the track direction of the ferroelectric recording medium 10. However, when a voice coil motor is used, the positioning accuracy of the conductive probe 26 is about 10 nm. In addition, the time required to move to another track (seek time) is about several milliseconds, which hinders the ferroelectric memory device 100 from achieving higher capacity and higher speed.

[0212] In this embodiment, the second piezoelectric element 27B is used to move the conductive probe 26 in the track direction of the ferroelectric recording medium 10, so that the positioning accuracy in the track direction of the ferroelectric recording medium 10 can be reduced to 1 nm or less, and correction operations within the same track and movement operations to another track can be performed in several microseconds or less, thereby achieving higher capacity and higher speed of the ferroelectric memory device 100.

[0213] 11 and 12, 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 in a columnar shape such as a cube, rectangular parallelepiped, or cylinder so that they can be stably disposed 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 and −Z-axis direction surfaces of the first piezoelectric element 27A. The electrode 28A-1 is provided between the conductive probe 26 and the first piezoelectric element 27A. The electrode 28A-2 is provided between the first piezoelectric element 27A and the second piezoelectric element 27B.

[0214] 13, the second piezoelectric element 27B is provided with a pair of electrodes 28B-1 and 28B-2. The electrode 28B-1 is provided on the -Y-axis direction side of the second piezoelectric element 27B. The electrode 28B-2 is provided on the +Y-axis direction side of the second piezoelectric element 27B.

[0215] The electrodes 28A-1 and 28A-2 are used to apply a voltage that expands and contracts the first piezoelectric element 27A in the +Z-axis and −Z-axis directions and adjusts the distance between the conductive probe 26 and the ferroelectric recording medium 10. This will be described later.

[0216] The electrodes 28B-1 and 28B-2 are used to expand and contract the second piezoelectric element 27B in the +Y-axis and −Y-axis directions, thereby moving the conductive probe 26 provided on the probe slider 23 in the track direction of the ferroelectric recording medium 10. The −Z-axis direction surface of the second piezoelectric element 27B is fixed to the probe slider 23, but the +Z-axis direction surface is not fixed. Therefore, when a voltage is applied to the electrodes 28B-1 and 28B-2, the +Z-axis direction surface 271 of the second piezoelectric element 27B expands and contracts in the +Y-axis and −Y-axis directions. Here, the +Z-axis direction surface 271 of the second piezoelectric element 27B expands and contracts evenly in the +Y-axis and −Y-axis directions, so the center of the surface 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 surface 271 of the second piezoelectric element 27B, it displaces in the +Y-axis or −Y-axis direction, which is the track direction of the ferroelectric recording medium 10. For example, as shown in FIG. 24, when second piezoelectric element 27B is stretched in the +Y-axis direction and the −Y-axis direction, conductive probe 26 is displaced in the +Y-axis direction indicated by the arrow.

[0217] When moving the conductive probe 26 in the track direction of the ferroelectric recording medium 10, a probe driving unit 40 such as a voice coil motor or a pulse motor is used for coarse movements where the conductive probe 26 moves a distance of 10 nm or more, and for fine movements where the conductive probe 26 moves a distance of less than 10 nm, it is preferable to use the second piezoelectric element 27B provided on the probe slider 23 to expand and contract the second piezoelectric element 27B and move the conductive probe 26.

[0218] [Ferroelectric recording medium drive unit] As shown in FIG. 8, the ferroelectric recording medium drive unit 30 drives and rotates the ferroelectric recording medium 10. FIG. 25 is a cross-sectional view showing the configuration of the ferroelectric recording medium drive unit 30. As shown in FIG. 25, the ferroelectric recording medium drive unit 30 has a housing (bearing sleeve) 31, a bearing sleeve 32, an axial member (spindle shaft) 33, a housing bottom 34, a permanent magnet 35, a stator 36, and lubricating oil O. The ferroelectric recording medium drive unit 30 supports the spindle shaft 33 in a non-contact manner in the radial direction of the spindle shaft 33 (a direction perpendicular to the spindle shaft 33) within the housing 31 by pressure generated by the dynamic pressure action of the lubricating oil O.

[0219] The housing 31 is a container that accommodates a part of the spindle shaft 33, and is formed so that the spindle shaft 33 can be inserted and removed.

[0220] The bearing sleeve 32 is provided inside the housing 31 .

[0221] The spindle shaft 33 is a rod-shaped member inserted into the inner circumferential surface of the bearing sleeve 32, and is electrically conductive. The spindle shaft 33 can be made of an electrically conductive material such as metal. The spindle shaft 33 is inserted into an opening 10a (see FIG. 3) of the ferroelectric recording medium 10, and is connected to the ferroelectric recording layer 13 via the substrate 11 and the electrode layer 12 (see FIG. 3). The electrical conductivity of the spindle shaft 33 allows information to be read from and written to the ferroelectric recording layer 13 using the electrically conductive probe 26.

[0222] It is preferable that the shaft end 331 of the spindle shaft 33 has a curved surface formed in a convex shape. 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 formed in a concave shape.

[0223] The spindle shaft 33 can have a V-shaped groove 332 on its outer periphery. When the spindle shaft 33 has the groove 332, a flow is generated in the lubricating oil O when the spindle shaft 33 rotates, making it easier for the lubricating oil O to gather at the apex of the V-shape of the groove 332. This generates pressure and supports the spindle shaft 33.

[0224] The housing bottom 34 is disposed in the housing 31 so as to face the shaft end 331 of the spindle shaft 33, and is electrically conductive.

[0225] The housing bottom 34 also preferably has a curved surface formed in a convex or concave shape, similar to the shaft end portion 331. 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.

[0226] A plurality of permanent magnets 35 are provided on the inner peripheral surface of the cover 37 along the circumferential direction.

[0227] The stator 36 is provided between the housing 31 and the permanent magnet 35 .

[0228] The lubricating oil O is filled in the gap between the inner peripheral surface of the bearing sleeve 32 and the outer peripheral surface of the spindle shaft 33. The lubricating oil O generates pressure by dynamic pressure action, thereby supporting the spindle shaft 33 in a non-contact state in the radial direction.

[0229] The lubricating oil O preferably contains inorganic conductive powder. By configuring the lubricating oil O to contain inorganic conductive powder, the housing 31 and the spindle shaft 33 can be electrically connected.

[0230] Preferred inorganic conductive powders include metal powders such as silver, copper, nickel, tin, silver-plated copper, stainless steel, aluminum, brass, iron, and zinc; carbon powders such as carbon, carbon black, graphite, and carbon nanotubes; metal oxide powders such as tin oxide, indium oxide, and zinc oxide; and metal-plated products having a coating layer formed on the surface of glass, mica powder, glass fiber, carbon fiber, and the like.

[0231] The metal powder is preferably in the form of a powder, sphere, fiber, or foil.

[0232] The carbonaceous powder preferably has a spherical or fibrous shape.

[0233] The metal oxide powder is preferably in the form of a powder or a sphere.

[0234] The metal-plated powder preferably has a powdery or spherical shape.

[0235] These materials have high heat resistance and good volatility resistance, and do not increase the viscosity of the lubricating oil O when contained in the lubricating oil O.

[0236] The particle size of the conductive powder is preferably 0.1 μm to 10 μm when it is in particulate form. When the conductive powder is in the form of foil, the side preferably has a length of 0.1 μm to 100 μm. When the conductive powder is in the form of fibers, the length of the fibers preferably has a length of 0.1 μm to 100 μm. When the conductive powder is in the form of particles, 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.

[0237] In the ferroelectric recording medium drive unit 30, the spindle shaft 33 is rotated by coupling between the permanent magnet 35 and the electromagnet of the stator 36. A downward thrust in FIG. 25 is applied to the spindle shaft 33 by magnetic coupling with the electromagnet of the stator 36, its own weight, or other methods, and this thrust is supported by the housing bottom 34.

[0238] [Probe drive unit] As shown in FIG. 8, the probe driver 40 drives the probe slider 23.

[0239] [Control Unit] The ferroelectric memory device 100 may include a control unit (not shown) when the probe slider 23 has a first piezoelectric element 27A and a second piezoelectric element 27B at its tip between the probe slider 23 and the conductive probe 26, as shown in FIG.

[0240] The control unit (not shown) is attached as a printed circuit board (PCB) to the back side of the housing 60. The control unit (not shown) is electrically connected to the first piezoelectric element 27A, the second piezoelectric element 27B, and the conductive probe 26. The control unit (not shown) has the function of 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, thereby adjusting the distance and relative position between the ferroelectric recording medium 10 and the conductive probe 26. It is preferable that the control unit (not shown) controls the voltage applied to the first piezoelectric element 27A and the second piezoelectric element 27B based on a read signal from the conductive probe 26 to expand and contract the first piezoelectric element 27A and the second piezoelectric element 27B.

[0241] Furthermore, when first piezoelectric element 27A expands or contracts in the +Z-axis or −Z-axis direction, first piezoelectric element 27A also expands or contracts in the +Y-axis or −Y-axis direction. It is preferable that a control unit (not shown) has a function to compensate for the expansion or contraction of first piezoelectric element 27A in the +Y-axis or −Y-axis direction by expanding or contracting second piezoelectric element 27B in the +Y-axis or −Y-axis direction.

[0242] When the read signal level from the conductive probe 26 is low, the control unit (not shown) increases the voltage applied to the first piezoelectric element 27A, thereby shortening the distance between the conductive probe 26 and the ferroelectric recording medium 10 and increasing the signal level. On the other hand, when the read signal level from the conductive probe 26 is high, the control unit (not shown) decreases the voltage applied to the first piezoelectric element 27A, thereby increasing the distance between the conductive probe 26 and the ferroelectric recording medium 10 and decreasing the signal level. This makes it possible to control the distance between the ferroelectric recording medium 10 and the conductive probe 26A with high accuracy and response. Therefore, the control unit (not shown) can provide the ferroelectric memory device 100 with an AGC (automatic gain control) function between tracks and sectors of the ferroelectric recording medium 10.

[0243] [Recording / playback signal processing section] The recording / reproducing signal processing unit 50 shown in FIG. 8 has a function of processing signals for writing information to and reading information from the conductive probe 26.

[0244] The recording / reproducing 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 / reproducing signal processing unit 50 generates a positive or negative voltage corresponding to the write 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.

[0245] The recording / playback signal processing unit 50 has a bipolar power supply (not shown) inside, and when 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 is generated for 0 using the bipolar power supply (not shown).

[0246] The recording / reproducing 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 reproduces (reads) the recorded multi-valued information. The details regarding the recording and reproduction of multi-valued information are as described above, and therefore will not be described in detail.

[0247] As described above, the ferroelectric layer 131 may have recorded thereon 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. The ferroelectric layer 131 may have servo information areas in which servo information is recorded and data areas in which data is recorded and reproduced, arranged alternately at regular intervals in the circumferential direction of the ferroelectric recording medium 10.

[0248] 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 / reproducing signal processing unit 50 after the ferroelectric recording medium 10 is incorporated into the ferroelectric memory device 100. In the latter case, the actuator arm 21 or the suspension arm 22 may be mechanically fixed using a lever (not shown) from outside the ferroelectric memory device 100, and then the servo information may be written to the ferroelectric recording medium 10 while positioning the conductive probe 26 on the surface of the ferroelectric recording medium 10 by slightly moving the lever (not shown).

[0249] In this case, it is preferable that the servo information and the data areas for recording and reproducing data are alternately arranged at regular intervals in the circumferential direction of the track on the ferroelectric recording medium 10. This allows the probe slider 23 to more accurately detect the position of the conductive probe 26 using the servo information while reproducing the recorded data.

[0250] As described above, it is preferable that the recording / playback signal processing unit 50 records multi-value information in the smallest single recording area by a single write operation, which is the simplest operation in the ferroelectric memory device 100, and plays back the information recorded in multi-value form in the ferroelectric recording medium 10 by a single read operation, which is the simplest operation in the ferroelectric memory device 100.

[0251] As described above, the servo information region 131B of the ferroelectric layer 131 may include a burst information region 131B-1, an address information region 131B-2, and a preamble information region 131B-3. In this case, in the ferroelectric recording medium 10, the conductive probe 26 moving in the circumferential direction on the surface reads preamble information in the preamble information region 131B-3 to prepare for reading address information. Then, the conductive probe 26 reads address information in the data region in the address information region 131B-2. Then, the probe slider 23 reads burst information in the burst information region 131B-1 to fine-tune the track position (radial position). Thereafter, the conductive probe 26 can record information in the data region 131A.

[0252] As described above, the reference signal information 131B-4 for multi-value 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-value recording and reproduce the multi-value information recorded in the data area 131A using the grasped signal level.

[0253] When writing information to the ferroelectric recording medium 10, the recording / reproducing signal processing unit 50 preferably adjusts the voltage waveform generated by a bipolar power supply (not shown) and applied to the conductive probe 26 to one of a triangular wave, a sawtooth wave, and a trapezoidal wave, as shown in FIG. 26 . If the voltage waveform generated by the bipolar power supply (not shown) is a square wave, a large amount of charge flows the instant a positive or negative voltage is applied, which may dull the sharp tip of the conductive probe 26 due to thermal melting or rapid field evaporation. In this embodiment, by using one of a triangular wave, a sawtooth wave, and a trapezoidal wave for the voltage waveform, the potential can be gradually increased from zero potential, thereby reducing damage to the conductive probe 26.

[0254] During reading, for example, it is known that information written in the ferroelectric recording medium 10 can be read as a change in capacitance by measuring the change in capacitance of the ferroelectric layer 131 between the conductive probe 26 and the electrode layer 12 while applying an AC electric field smaller than the coercive electric field of the ferroelectric layer 131 to the conductive probe 26. The principle is as follows.

[0255] When the voltage applied to the conductive probe 26 is E, the electric flux density resulting from the charge of the ferroelectric is D, the dielectric constant is ε, and the polarization voltage is P, the electric flux density D resulting from the charge of the ferroelectric is expressed by the following formula (1).

[0256]

number

[0257] Here, when the voltage E is AC, the following equation (2) is obtained. Substituting this into the above equation (1), the odd-numbered dielectric constants ε3, ε5, ... become nonlinear, and their signs change depending on the direction of spontaneous polarization of the ferroelectric. Therefore, by measuring the fluctuations in this dielectric constant, the direction of spontaneous polarization of the ferroelectric can be determined. E=E p cosωt (2) (In the formula, E p is the peak voltage of the AC.)

[0258] However, this method cannot read information at a bit rate higher than the frequency of the AC field because the data read speed is limited by the frequency of the AC field. For example, to achieve a read speed of 1 Gbps or higher, an AC field of 1 GHz or higher must be applied. The dielectric constant of the ferroelectric material depends on the frequency of the AC field, and the higher the frequency, the greater the loss. Therefore, the ferroelectric materials that can be used in ferroelectric rotating media that can achieve high-speed operation are limited.

[0259] In this embodiment, an AC electric field is not used to read information from a ferroelectric recording medium, but a weak tunnel current flowing between the conductive probe 26 and the electrode layer 12 is used to detect the charge in the ferroelectric layer 131.

[0260] That is, since the conductive probe 26 is brought very close to the surface of the ferroelectric layer 131, the capacitance C between the conductive probe 26 and the electrode layer 12, which is derived from the ferroelectric layer 131, is given by the following equation (3). C=ε·ε0A / d (3) where 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 ε is the permittivity of vacuum.

[0261] If the charge stored in the ferroelectric layer 131 is Q, the voltage V generated in the conductive probe 26 is given by the following equation (4). By detecting this voltage V, the information written in the ferroelectric layer 131 can be read. V=Q / C (4)

[0262] Here, since ferroelectrics are insulators with a large band gap, tunneling current does not easily flow, making it difficult to detect the voltage V in the above equation (4). However, by making the ferroelectric thin film, the tunneling barrier can be reduced, and by forming a junction structure with a conductive electrode layer, a weak tunneling current can be made to flow due to the electronic state at the junction. Furthermore, if a paraelectric layer is added to this junction, 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 due to electric charges, in a direction that lowers the tunneling barrier, making it even easier for tunneling current to flow.

[0263] Since the tunnel barrier of the ferroelectric layer 131 changes depending on the polarization direction, it is possible to know the polarization direction of the ferroelectric layer 131 by measuring the tunnel current between the ferroelectric layer 131 and the conductive probe 26. For example, in a certain ferroelectric, when the surface side is positively charged, the tunnel barrier increases and the tunnel current from the ferroelectric layer 131 side toward the conductive probe 26 decreases, whereas when the surface side is negatively charged, the tunnel current from the ferroelectric layer 131 side toward the conductive probe 26 increases.

[0264] When detecting and reading the charge in the ferroelectric layer 131, the recording / reproducing signal processing unit 50 may apply a bias voltage to the conductive probe 26.

[0265] The bias voltage applied to the conductive probe 26 is used to make it easier to detect the tunneling 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 information.

[0266] 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 can be detected by the magnitude of the tunneling current generated by the bias application.

[0267] For example, in a ferroelectric material in which the tunnel barrier increases when the surface layer side is positively charged and decreases when the surface layer side is negatively charged, when a bias voltage is applied so that a tunnel current flows from the ferroelectric layer 131 side (electrode layer 12 side) toward the conductive probe 26, the magnitude of the tunnel barrier and the magnitude of the tunnel current have an inverse relationship, and the charge direction of the ferroelectric layer 131 can be detected from this.

[0268] Furthermore, when the bias voltages applied to the conductive probe 26 are both positive and negative, the polarization direction of the ferroelectric layer 131 can be detected by comparing the tunneling current when a positive bias voltage is applied with the tunneling current when a negative bias voltage is applied. In this case, if the speed at which information is read from the ferroelectric layer 131 is N bits / second (N is a number equal to or greater than 1), the bias voltage is preferably a sine wave or square wave with a frequency of N Hz (N is a number equal to or greater than 1) or more. By doing so, the polarization direction of the ferroelectric layer 131 can be detected with higher accuracy. The reason for this is as follows: The tunneling current varies depending on the distance between the ferroelectric layer 131 and the conductive probe 26. Therefore, in order to detect the polarization direction of the ferroelectric layer 131, it is necessary to distinguish this variation from the variation in the tunneling 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 a relative comparison of the tunnel current at the time of positive bias and that at the time of negative bias, and as a result, fluctuations in the tunnel current due to the distance between the ferroelectric layer 131 and the conductive probe 26 are canceled out, making it less susceptible to the influence thereof.

[0269] Furthermore, when the recording / reproducing signal processing unit 50 detects the charge in the ferroelectric layer 131 in the ferroelectric memory device 100 to read information, if the amount of charge stored in the ferroelectric layer 131 decreases and the tunneling current obtained from this decreases, the recording / reproducing signal processing unit 50 may rewrite (refresh) the same information as that written in the ferroelectric recording medium 10 to the location on the ferroelectric recording medium 10 from which the same information was read, in order to compensate for the charge decreased by reading the information from the ferroelectric recording medium 10. Note that a decrease in the amount of charge stored in the ferroelectric layer 131 can also occur when charge is trapped by defects contained in the ferroelectric layer 131.

[0270] Furthermore, the recording / reproducing signal processing unit 50 may perform rewriting each time information is read from the ferroelectric recording medium 10, or may perform rewriting after a predetermined number of reads.

[0271] Although the above-described method of reading information from the ferroelectric layer 131 is a non-destructive method, a destructive method can also be adopted for reading information from the ferroelectric layer 131.

[0272] When a non-destructive method is used to read information, the electric field generated by the bias applied to the conductive probe 26 does not exceed the coercive electric field of the ferroelectric material that constitutes the ferroelectric layer 131, and therefore the polarization direction of the ferroelectric layer 131 does not change when reading information.

[0273] On the other hand, when using the destructive method, a bias voltage exceeding the coercive electric field of the ferroelectric material is applied to the conductive probe 26, and information is read by detecting a tunneling current generated 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, and therefore a small tunneling current flows from the ferroelectric layer 131 to the conductive probe 26. On the other hand, if the ferroelectric layer 131 has a negative charge, the charge of the ferroelectric layer 131 reverses to a positive state due to the negative bias applied to the conductive probe 26, and therefore the tunneling current increases. Information recorded in the ferroelectric layer 131 can be read by this variation in the tunneling current. Note that when using the destructive method to read information, the read information must be rewritten to the ferroelectric layer 131.

[0274] The atomic 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 of using the atomic force between the conductive probe 26 and the ferroelectric layer 131 to read information from the ferroelectric recording medium 10 will be described. The atomic force between the conductive probe 26 and the ferroelectric layer 131 is affected by the charge of the ferroelectric layer 131, so by measuring the atomic force between them, information recorded in the ferroelectric recording medium 10 can be read. In this case, unlike when a tunnel current is used to read information, the amount of charge stored in the ferroelectric layer 131 does not decrease, so refreshing is not required. In addition, it is no longer necessary to consider the band gap of the ferroelectric material used in the ferroelectric layer 131 and the electronic state at the interface with the electrode layer 12.

[0275] Here, the atomic force microscope (AFM) is known as a device for detecting and mapping the atomic forces acting between the sample surface and the probe. The AFM uses an optical lever method, shining a laser beam onto a cantilever equipped with a probe while moving the sample in the X and Y axes, and detecting the atomic forces from the change in the reflected light. Because this method works by detecting the physical movement of the cantilever, it is difficult to use it to read information in the GHz range. Furthermore, because laser light is used for detection, it can induce an internal photoelectric effect or temperature rise in the ferroelectric material used in ferroelectric recording media.

[0276] 27, in this embodiment, 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 atomic force between the conductive probe 26 and the ferroelectric layer 131 and convert this detected atomic force into an electric signal. In this case, since no optical lever is used to detect the atomic force, it becomes possible to read information in the GHz band.

[0277] The third piezoelectric element 27C may be made of, for example, quartz crystal, lithium niobate (LiNbO), barium titanate (BaTiO), lead zirconate titanate (PZT), zinc oxide (ZnO), aluminum nitride (AlN), lithium tantalate (LiTaO), 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.

[0278] In order to enhance the ability to detect atomic forces, it is preferable to place the third piezoelectric element 27C close to the conductive probe 26. Therefore, as shown in Fig. 27, it is preferable to provide the third piezoelectric element 27C between the conductive probe 26 and the fourth piezoelectric element 27D. Like the first piezoelectric element 27A, the fourth piezoelectric element 27D is used to drive the conductive probe 26 in the +Z-axis direction or the -Z-axis direction, thereby bringing the conductive probe 26 closer to the surface of the ferroelectric recording medium 10. Note that the fourth piezoelectric element 27D may be made of the same material as the third piezoelectric element 27C.

[0279] [Case] As shown in FIG. 8, the housing 60 is formed in a substantially rectangular shape and contains the ferroelectric recording medium 10, the conductive probe 26, the probe slider 23, the ferroelectric recording medium driving unit 30, and the recording / reproducing signal processing unit 50 inside.

[0280] 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 objects caused by the movement of the ferroelectric recording medium 10, the ferroelectric recording medium drive unit 30, and the conductive probe 26, as well as friction between these objects and the air, generates electric charges, so-called triboelectric charging, which may combine with the charges recorded on the ferroelectric recording medium 10, causing the loss of written information and adversely affecting 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.

[0281] The frictional charge inside the housing 60 can be evaluated by a known measurement method for the number of frictional charges.

[0282] As described above, the ferroelectric memory device 100 includes a ferroelectric recording medium 10, a conductive probe 26, a probe slider 23, a ferroelectric recording medium drive unit 30, and a recording / playback signal processing unit 50. The ferroelectric memory device 100 also includes a piezoelectric element 27 or an electrostrictive element, and a control unit (not shown). The ferroelectric memory device 100 uses the control unit (not shown) to control the voltage applied to the piezoelectric element 27 or the electrostrictive element based on a read signal from the conductive probe 26, thereby expanding or contracting the piezoelectric element 27 or the electrostrictive element. This allows the ferroelectric memory device 100 to adjust the distance between the ferroelectric recording medium 10 and the conductive probe 26, thereby enabling high-precision, high-speed control of the distance between the conductive probe 26 and the ferroelectric recording medium 10.

[0283] FIG. 28 shows a cross-sectional view of an example of the configuration of a conventional magnetic head slider. As shown in FIG. 28, a magnetic head slider 110 used in an HDD includes a heating element 111 provided inside the magnetic head slider 110 and a magnetic head 112 provided below the heating element 111 so as to face a magnetic recording medium 120. The magnetic head slider 110 uses a technology in which the heating element 111 is energized to generate heat, causing the magnetic head slider 110 to thermally expand, thereby adjusting the distance between the magnetic head 112 and the magnetic recording medium 120, i.e., the DFH (Definition of Head-Holding) (see, for example, Japanese Patent Application Laid-Open 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 DFH adjustment technology narrows the distance from the surface of the magnetic recording medium 120 to the magnetic head 112 to the sub-nano level while maintaining the flying height of the magnetic head slider 110 above the surface of the magnetic recording medium 120 at the nanometer level. However, when the heating element 111 is used, the heating range covers a wide area including the magnetic head 112 and the magnetic head slider 110 on which it is mounted, so it takes time for the heating element 111 to heat the magnetic head 112, and the response and precision of the DFH control are not high. Also, since the leakage electric field in the ferroelectric recording medium 10 is smaller than the leakage magnetic field used to read magnetic information from the magnetic recording medium 120, the ferroelectric memory device 100 that detects it requires a technique for adjusting the DFH with even higher precision.

[0284] 28 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 include the heating element 111 used in the conventional DFH technology, and therefore can prevent thermal fluctuations in the dielectric constant of the ferroelectric layer 131.

[0285] The ferroelectric memory device 100 can expand and contract the piezoelectric element 27 or the electrostrictive element by controlling the voltage applied to the piezoelectric element 27 or the electrostrictive element based on the read signal from the conductive probe 26 using a control unit (not shown). This allows the ferroelectric memory device 100 to more easily control the distance between the ferroelectric recording medium 10 and the conductive probe 26 at high speed with high accuracy.

[0286] The ferroelectric memory device 100 can record multi-level information to the ferroelectric recording medium 10 and reproduce the recorded multi-level information using the recording / reproducing signal processing unit 50. 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 in the ferroelectric recording medium 10.

[0287] The ferroelectric memory device 100 can record multi-value information on the ferroelectric recording medium 10 using the recording / reproducing 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 reduce the size of the ferroelectric memory device 100 per unit storage capacity, and can increase the speed (read / write speed) required to record information on the ferroelectric layer 131 and reproduce the recorded information to, for example, 10 Gbps or more.

[0288] Furthermore, by increasing the recording density of the ferroelectric recording medium , the ferroelectric memory device 100 can suppress an increase in the amount of power consumption per unit storage capacity required for recording and reproducing information on the ferroelectric recording medium .

[0289] Therefore, the ferroelectric memory device 100 can improve the recording density, reduce the size of the device per unit storage capacity, and increase the read / write speed, while suppressing an increase in energy consumption.

[0290] In a typical magnetic recording medium such as a hard disk drive (HDD), information is recorded (written) and reproduced (read) by moving a read / write head in the track direction (radial direction) while the magnetic recording medium rotates at 5,000 to 10,000 rpm (83 to 167 rpm). The size of one information bit is approximately 5 nm in the sector direction (circumferential direction) and 50 nm in the track direction, and each bit contains approximately 10 magnetic particles. The average read / write speed is approximately 1 Gbps. The ferroelectric recording medium 10 records information by polarization reversal caused by lattice distortion of the ferroelectric crystal contained in the ferroelectric layer 131. This allows for significantly higher recording density and read / write speed compared to magnetic recording media that record information by magnetizing the magnetic layer on a magnetic particle-by-magnetic particle basis. Furthermore, in the ferroelectric memory device 100, the ferroelectric recording medium 10 has a recording region in the ferroelectric layer 131 where multi-value data is recorded, thereby enabling improved recording density, miniaturization, and faster processing speeds while suppressing increases in energy consumption.

[0291] The ferroelectric memory device 100 can realize storage for wireless and mobile communications at speeds of, for example, 10 Gbps or more by improving recording density, increasing read / write speed, and reducing the size of the device. Furthermore, the ferroelectric memory device 100 can suppress increases in power consumption and achieve energy conservation, thereby reducing resource consumption and thereby reducing the environmental impact.

[0292] In the ferroelectric memory device 100, the recording / reproducing signal processing unit 50 can read position information (servo information) for detecting the relative position between the conductive probe 26 and the ferroelectric recording medium 10 in the track direction of the ferroelectric recording medium 10 from the ferroelectric layer 131. As a result, the ferroelectric memory device 100 can accurately detect the position of the conductive probe 26 using the servo information while reproducing data recorded in the data area of ​​the ferroelectric layer 131 provided in the ferroelectric recording medium 10, and can therefore perform recording and reproduction on the ferroelectric layer 131 with high accuracy. Therefore, the ferroelectric memory device 100 can increase the processing speed when recording information and reproducing recorded information.

[0293] In the ferroelectric memory device 100, servo information areas in which servo information is recorded and data areas in which information is written and read can be arranged alternately in the circumferential direction of the tracks on the ferroelectric recording medium 10. This allows the ferroelectric memory device 100 to accurately detect the position of the conductive probe 26 for each data area using the servo information while a control unit (not shown) is reproducing 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, and can therefore further increase the processing speed when recording information and reproducing recorded information.

[0294] The ferroelectric memory device 100 can include reference signal information 131B-4 in the servo information region 131B of the ferroelectric layer 131 included in the ferroelectric recording medium 10. This allows the ferroelectric memory device 100 to reproduce the multi-valued information recorded in the data region 131A by using the signal level of multi-valued recording that is grasped by reading the reference signal information 131B-4 using the conductive probe 26. This allows the ferroelectric memory device 100 to further reduce energy consumption per unit storage capacity.

[0295] The method for writing and reading information in the ferroelectric memory device 100 having the above configuration allows multi-level information to be written to the ferroelectric recording medium 10 and multi-level information to be written back to the ferroelectric recording medium 10 in a single, simple operation for the smallest single recording area. By using the above method for writing and reading information, the recording and reproduction 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, and increase the read / write speed, while suppressing an increase in energy consumption.

[0296] The ferroelectric memory device 100 can provide a first piezoelectric element 27A and a second piezoelectric element 27B on the probe slider 23. The ferroelectric memory device 100 can expand and contract the first piezoelectric element 27A in the height direction of the first piezoelectric element 27A using a pair of electrodes 28A-1 and 28A-2, and can expand and contract the second piezoelectric element 27B in the track direction of the ferroelectric recording medium 10 using electrodes 28B-1 and 28B-2. This allows the ferroelectric memory device 100 to control the distance between the ferroelectric recording medium 10 and the conductive probe 26 at the nano level and to move the conductive probe 26 in the track direction of the ferroelectric recording medium 10 at the nano level. Furthermore, the movement time within the same track and the movement time to another track (seek time) can be reduced to several microseconds or less. Therefore, the ferroelectric memory device 100 can perform highly accurate positioning of the conductive probe 26 in the data surface direction and track direction of the ferroelectric recording medium 10 at 1 nm or less, and can perform correction operations within the same track and movement operations to another track within several microseconds or less. Therefore, the ferroelectric memory device 100 can improve the recording capacity and increase the speed required for recording and reproducing information.

[0297] In the ferroelectric memory device 100, the second piezoelectric element 27B is provided between the probe slider 23 and the conductive probe 26, and the conductive probe 26 can be provided offset in the track direction of the ferroelectric recording medium 10 from the center of the mounting surface of the second piezoelectric element 27B. This allows the conductive probe 26 to move in the track direction due to expansion and contraction of the second piezoelectric element 27B. Therefore, 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 more reliably perform correction operations within the same track and movement operations to another track within several microseconds or less. Therefore, the ferroelectric memory device 100 can further improve the recording capacity and further increase the speed required for recording and reproducing information.

[0298] The ferroelectric memory device 100 can use the probe driver 40 for coarse movement of the conductive probe 26 in the track direction of the ferroelectric recording medium 10, where the movement distance is 10 nm or more, and can use the second piezoelectric element 27B for fine movement of the conductive probe 26 in the track direction of the ferroelectric recording medium 10. This allows the ferroelectric memory device 100 to appropriately move the conductive probe 26 in accordance with the movement distance of the conductive probe 26 in the track direction of the ferroelectric recording medium 10. Therefore, the ferroelectric memory device 100 can further improve the recording density and further increase the speed required for recording and reproducing information.

[0299] In the ferroelectric memory device 100, when detecting the charge in the ferroelectric layer 131, the recording / reproducing signal processing unit 50 applies positive and negative bias voltages to the conductive probe 26 and measures a weak tunneling current flowing between the conductive probe 26 and the electrode layer 12 to detect the charge in the ferroelectric layer 131. This allows the ferroelectric memory device 100 to read information without using an AC electric field using the recording / reproducing signal processing unit 50, so the reading speed is not limited by the frequency of the AC electric field. Therefore, information stored in the ferroelectric recording medium 10 can be read at high speed.

[0300] In the ferroelectric memory device 100, the recording / reproducing signal processing unit 50 can apply a positive or negative bias voltage to the conductive probe 26 when detecting the charge in the ferroelectric layer 131. The ferroelectric memory device 100 can detect the charge stored in the ferroelectric layer 131 by applying a voltage to the conductive probe 26, and therefore can read information stored in the ferroelectric layer 131 at high speed.

[0301] In the ferroelectric memory device 100, the recording / reproducing 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, thereby more reliably reading out the information stored in the ferroelectric layer 131.

[0302] The ferroelectric memory device 100 can make the positive and negative bias voltages sine waves or square waves. This allows the ferroelectric memory device 100 to more easily 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 more reliably reading the information stored in the ferroelectric layer 131.

[0303] The method for reading information in the ferroelectric memory device 100 having the above configuration detects the charge in the ferroelectric layer 131 by comparing the tunnel current that flows 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 that flows between the conductive probe 26 and the ferroelectric layer 131 when a negative bias voltage is applied to the conductive probe 26. By using the above information reading method, the charge stored in the ferroelectric layer 131 can be more easily detected in the recording / reproducing signal processing unit 50, and the information stored in the ferroelectric layer 131 can be read more reliably.

[0304] In the method for reading information in the ferroelectric memory device 100 having the above configuration, when the information reading speed is N bits / second (N is a number equal to or greater than 1), the frequency of the applied bias voltage can be set to N Hz (N is a number equal to or greater than 1). As a result, by using the above information reading method, the charge stored in the ferroelectric layer 131 can be more easily detected in the recording / reproducing signal processing unit 50, and the information stored in the ferroelectric layer 131 can be read more reliably.

[0305] The method of writing and reading information in the ferroelectric memory device 100 having the above configuration rewrites the same information as that written in the ferroelectric recording medium 10 to the location of the ferroelectric recording medium 10 from which the same information was read. By using the above method of writing and reading information, it is possible to compensate for the charge lost by reading information from the ferroelectric recording medium 10, and therefore, when the recording / reproducing signal processing unit 50 measures a weak tunnel current flowing between the conductive probe 26 and the electrode layer 12, it is possible to stably detect the charge in the ferroelectric layer 131.

[0306] The method of writing and reading information in the ferroelectric memory device 100 having the above configuration allows rewriting to be performed each time information is read from the ferroelectric recording medium 10, or after a predetermined number of times of information reading. This allows the charge lost by reading information from the ferroelectric recording medium 10 to be replenished each time it is read, so that rewriting can be performed appropriately when necessary depending on the amount of charge lost in the ferroelectric layer 131.

[0307] In the ferroelectric memory device 100, the conductive probe 26 can be made conical in shape. This allows the conductive probe 26 to have a sharp tip and be sharpened, thereby increasing the electric field strength and reducing the voltage applied to the needle electrode 262. Therefore, the ferroelectric memory device 100 can advantageously record information on the ferroelectric recording medium 10.

[0308] The conductive probes 26A of the ferroelectric memory device 100 can be shaped like a triangular pyramid or a square pyramid, thereby enabling the ferroelectric memory device 100 to include conductive probes 26 with sharp tips.

[0309] In the ferroelectric memory device 100, the shape of the conductive probe 26A can be made rotationally symmetrical about an axis passing through the tip of the conductive probe 26A. This allows the conductive probe 26A to homogenize 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. Therefore, the ferroelectric memory device 100 can stably write to and read from the ferroelectric recording medium 10.

[0310] The manufacturing method of the conductive probe 26A can include a step of forming a triangular or quadrangular mask on the surface of the conductive material 260, and a step of etching the conductive material 260 to obtain a triangular or quadrangular pyramidal needle electrode 262. This allows the conductive probe 26A having the triangular or quadrangular pyramidal needle electrode 262 on the base 261 to be manufactured with good reproducibility.

[0311] The method for manufacturing the conductive probe 26A can include the steps of forming a mask 72A having triangular or quadrangular through-holes 72a on the surface of the conductive material 260, and depositing a conductive material on the surface of the conductive material 260 in the through-holes 72a to obtain triangular or quadrangular pyramidal needle electrodes 262A. This allows the conductive probe 26A having the triangular or quadrangular pyramidal needle electrodes 262A on the base 261 to be manufactured with good reproducibility.

[0312] In the ferroelectric memory device 100, the conductive probe 26B has a base 261 made of a conductive material 260, a recess 212, and a sharpened needle electrode 262, and a part of the needle electrode 262 can protrude from the surface of the conductive material 260. This makes it possible for the ferroelectric memory device 100 to suppress damage to the sharpened needle electrode 262. Furthermore, the ferroelectric memory device 100 can suppress vibration and deformation of the needle electrode 262 due to airflow generated by rotation of the ferroelectric recording medium 10. Furthermore, the ferroelectric memory device 100 shields the needle electrode 262, thereby suppressing the influence of surrounding charges and charge leakage from the needle electrode 262.

[0313] In the ferroelectric memory device 100, the conductive probe 26C has a base 261 made of a conductive material 260, an insulating layer 263 having a through-hole 263a provided on the base 261, and a conical needle electrode 262 on the base 261 inside the through-hole 263a, and a part 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 rotation of the ferroelectric recording medium 10. Furthermore, the ferroelectric memory device 100 shields the needle electrode 262, thereby suppressing the influence of surrounding charges and charge leakage from the needle electrode 262.

[0314] The method for manufacturing the conductive probe 26B includes the steps of applying photoresist to the surface of the conductive material 260, forming minute through-holes in the photoresist, etching the surface of the conductive material 260 in the holes through the holes to form concave depressions (recesses), depositing metal on the photoresist having the holes, and removing the photoresist to obtain a conical needle electrode 262, and a part of the needle electrode 262 can protrude from the conductive material 260. This allows the conductive probe 26B to be manufactured with good reproducibility.

[0315] 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 a through-hole 82a in the photoresist 82; etching the through-hole 82a down to the surface of the conductive material 260; depositing a metal on the surface of the conductive material 260 inside the through-hole 82a to obtain a needle-like electrode 262; and removing the photoresist 82, so that a part of the needle-like electrode 262 can protrude from the insulating layer 263. Even in this case, the conductive probe 26C can be manufactured with good reproducibility.

[0316] The method for manufacturing the conductive probe 26C can include a step of forming a triangular or quadrangular mask on the surface of the conductive material 260, and a step of etching the conductive material 260 to obtain the triangular or quadrangular pyramidal needle electrode 262. This allows the conductive probe 26C having the triangular or quadrangular pyramidal needle electrode 262 on the base 261 to be manufactured with good reproducibility.

[0317] The method for manufacturing the conductive probe 26C can include the steps of forming a mask having triangular or quadrangular through-holes 72a on the surface of the conductive material 260, and depositing a conductive material on the surface of the conductive material 260 in the through-holes 72a to obtain triangular or quadrangular pyramidal needle electrodes 262. This allows the conductive probe 26C having the triangular or quadrangular pyramidal needle electrodes 262 on the base 261 to be manufactured with good reproducibility.

[0318] That is, in the method for manufacturing the conductive probe 26, when the conductive probe 26 is manufactured using photoresist, the holes in the photoresist can be made circular, triangular, or rectangular. As a result, by depositing metal on the photoresist and then removing the photoresist, the conductive probe 26 having the needle electrode 262 formed in a conical, triangular, or quadrangular pyramid shape on the base 261 can be manufactured with good reproducibility.

[0319] The ferroelectric memory device 100 can detect a signal generated by the atomic force between the conductive probe 26 and the ferroelectric recording medium 10 using the recording / reproducing signal processing unit 50. As a result, the ferroelectric memory device 100 can read information without using an AC electric field using the recording / reproducing 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. Furthermore, since the amount of charge stored in the ferroelectric layer 131 does not decrease, rereading can be omitted.

[0320] The ferroelectric memory device 100 can detect atomic forces by using a piezoelectric element 28 provided between the probe slider 23 and the conductive probe 26. This allows the ferroelectric memory device 100 to improve its ability to detect atomic forces, thereby enabling it to read information stored in the ferroelectric recording medium 10 at higher speeds.

[0321] In the ferroelectric storage device 100, the ferroelectric recording medium drive unit 30 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. At least one of the shaft end 331 of the spindle shaft 33 and the housing bottom 34 has a convex or concave spherical surface with a radius of curvature R of 2 mm or more, and the lubricating oil O contains inorganic conductive powder. This allows the ferroelectric recording medium drive unit 30 to function as a fluid dynamic bearing, a type of sliding bearing. The lubricating oil O filled between the spindle shaft 231 and the housing 31 allows the spindle shaft 231 to rotate stably without contact due to the dynamic pressure generated during rotation of the spindle shaft 231. This allows the ferroelectric recording medium 10 to be rotated with reduced axial runout and low vibration. Furthermore, the inorganic conductive powder has high heat resistance and good volatility resistance, and even when contained in the lubricating oil O, it can suppress an increase in the viscosity of the lubricating oil O. Therefore, even when used for a long period of time, the ferroelectric memory device 100 can stabilize the torque of the ferroelectric recording medium drive unit 30 and can suppress a decrease in electrical continuity between the spindle shaft 231 and the housing 31. Therefore, the ferroelectric memory device 100 can stabilize the reading and writing of information from and to the ferroelectric recording medium 10.

[0322] Conventionally, the spindle shaft 33 does not come into contact with the housing 31 due to the presence of a non-conductive lubricating fluid film, so the spindle shaft 33 is in an electrically floating state 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 a circuit that writes information to the ferroelectric layer 131. Therefore, when the spindle shaft 33 is in an electrically floating state relative to the housing 31, it is difficult to apply a voltage between the ferroelectric recording medium 10 and the conductive probe 26 to write information.

[0323] There is also a method of incorporating a conductive material into the lubricating fluid O (see, for example, Japanese Patent Application Laid-Open No. 2001-208069), but the addition of the conductive material increases the viscosity of the lubricating fluid O, resulting in an increase in bearing torque. As the duration of use increases, the conductive material deteriorates and the conductivity decreases, so even if this method is applied to the ferroelectric recording medium 10, there is a high possibility that the error rate will increase when writing information to the ferroelectric recording medium 10.

[0324] The ferroelectric memory device 100 has the above-described configuration, which stabilizes torque even when the ferroelectric recording medium drive unit 30 is used for a long period of time, and prevents a decrease in conductivity between the spindle shaft 231 and the housing 31, thereby enabling stable reading and writing of information to the ferroelectric recording medium 10.

[0325] In the ferroelectric memory device 100, the radius of curvature R of at least one of the shaft end 331 of the spindle shaft 33 provided in 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. This makes it possible for the ferroelectric memory device 100 to further stabilize torque even when the ferroelectric recording medium drive unit 30 is used for a long period of time, and to further prevent a decrease in conductivity between the spindle shaft 231 and the housing 31.

[0326] The ferroelectric memory device 100 can have a V-shaped groove 231B on the outer periphery of the spindle shaft 33. As a result, when the spindle shaft 33 rotates, the lubricating oil O can be easily collected at the apex of the V-shape of the groove 231B, which makes it easier for the lubricating oil O to flow. This makes it easier for pressure to be generated by the lubricating oil O, which makes it easier to support the spindle shaft 33.

[0327] In the ferroelectric memory device 100, a permanent magnet 35 and a stator 36 can be provided facing each other inside a cover 37. By coupling the permanent magnet 35 with the electromagnet of the stator 36, the spindle shaft 33 can be rotated and a downward thrust can be generated in the spindle shaft 33, so that the spindle shaft 33 can be reliably supported by the housing bottom 34.

[0328] In the ferroelectric memory device 100, the recording / reproducing signal processing unit 50 can adjust the voltage waveform applied to the conductive probe 26 when writing information to the ferroelectric recording layer 13 to any one of a triangular wave, a sawtooth wave, and a trapezoidal wave. This allows the ferroelectric memory device 100 to reduce damage to the conductive probe 26.

[0329] The ferroelectric memory device 100 includes a housing 60, which can be filled with at least one of argon gas, nitrogen gas, and helium gas. The housing 60 contains the ferroelectric recording medium 10, the conductive probe 26, the probe slider 23, the ferroelectric recording medium drive unit 30, and the recording / reproducing signal processing unit 50. By filling the housing 60 with these gases, the ferroelectric memory device 100 can mitigate frictional electrification that occurs within the housing 60. Therefore, the ferroelectric memory device 100 can prevent charges from combining with charges recorded in the ferroelectric recording medium 10, which could result in the loss of written information, and can prevent adverse effects on writing by the conductive probe 26.

[0330] <Data Management System> A data management system including the above-mentioned ferroelectric memory device 100 as an external storage device will be described. Fig. 29 is a diagram showing the configuration of the data management system. As shown in Fig. 29, 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.

[0331] As shown in FIG. 29, the data management unit 310 includes an internal storage device 311.

[0332] The data management unit 310 stores large amounts of data flowing at high speed over the high-speed communication network in the external storage device 320 as is, and also stores metadata used to read the stored data in the internal storage device 311. This allows the data management unit 310 to store large amounts of data flowing at high speed over the high-speed communication network in the external storage device 320. The external storage device 320 is easier to replace than the internal storage device 311, which increases the scalability of the data management system. Furthermore, the data management unit 310 can easily read the stored data from the external storage device 320 by using the metadata stored in the internal storage device 311.

[0333] Metadata is data stored in the internal storage device 311 that describes additional information about data stored in the external storage device 320. Specifically, the metadata includes the type, size, attributes, format, title, author name, publisher name, related keywords, time and place where the data was generated, etc. The metadata also includes the location in the external storage device 320 where the data is stored, such as the drive number, track number, and sector number.

[0334] As shown in FIG. 29, the external storage device 320 is connected to the data management unit 310 by wire or wirelessly so as to be able to send and receive data, and stores data sent from the data management unit 310 over a high-speed communication network.

[0335] A plurality of external storage devices 320 may be provided. Preferably, the plurality of external storage devices 320 are arranged in parallel. The plurality of external storage devices 320 can store a large amount of data at high speed, and can reduce the load required for storing data on each external storage device 320. Furthermore, by arranging the plurality of external storage devices 320 in parallel, the plurality of external storage devices 320 have high expandability, and therefore the amount of data that can be stored can be increased.

[0336] An example of the configuration of the external storage device 320 is shown in Fig. 30. As shown in Fig. 30, the external storage device 320 includes a ferroelectric recording medium 321, and preferably further includes a storage element 322, a reading element 323, and a driving unit 324.

[0337] The ferroelectric recording medium 321 is similar to the above-mentioned ferroelectric recording medium 10 (see FIG. 8), and therefore a detailed description thereof will be omitted.

[0338] The memory element 322 is an element for storing data in the ferroelectric recording medium 321 .

[0339] The read element 323 is an element for reading data from the ferroelectric recording medium 321 .

[0340] The storage element 322 and the reading element 323 are similar to the conductive probe 26 (see FIG. 14) described above, and so details thereof will be omitted.

[0341] The driving unit 324 has a first driving unit 324A that drives the memory element 322 on the memory surface 321a of the ferroelectric recording medium 321, and a second driving unit 324B that drives the reading element 323 on the memory surface 321a. The first driving unit 324A and the second driving unit 324B are similar to the above-mentioned probe driving unit 40 (see FIG. 8), and therefore details thereof will be omitted.

[0342] The storage element 322 and the reading element 323 may be driven independently by a driving unit 324 to drive the same storage surface 321 a of the ferroelectric recording medium 321 .

[0343] The external storage device 320 may delete old data stored in the past without updating it and store new data on the high-speed communication network, or may overwrite the old data with the new data.

[0344] In other words, it is preferable that the data stored in the external storage device 320 is not an update of old data stored in the past, but rather that the old data stored in the past is deleted or overwritten and new data is stored on the high-speed communication network.

[0345] The ferroelectric recording medium is a rotating medium, and while the ferroelectric recording medium 321 is rotating at high speed, the read / write elements (storage element 322 and read element 323) are moved in the sector direction (circumferential direction) and track direction (radial direction) to read and write information. This method is most efficient when reading and writing information in consecutive sectors and consecutive tracks. Reading and writing information in different, non-consecutive tracks or sectors incurs a time loss as the read / write element must be moved to a different track and sector. Therefore, to update old data stored in the past, the write element must be moved to the track and sector where that data is stored, resulting in a time loss.

[0346] The external storage device 320 deletes old data stored in the ferroelectric recording medium 321 without updating it and stores new data on a high-speed communication network, or overwrites old data with new data. This allows information to be written in consecutive sectors and consecutive tracks, enabling data to be stored at high speed.

[0347] An example of the connection relationship of data stored in the ferroelectric recording medium 321 is shown in Figure 31. As shown in Figure 31(a), data stored in the external storage device 320 is not stored in a storage area with a hierarchical structure, but rather in a unified storage area without a hierarchical structure, as shown in Figure 31(b). This allows the external storage device 320 to reduce the time required to move the storage element to a different hierarchical position in the ferroelectric recording medium 321 when writing information, thereby allowing data to be stored in the ferroelectric recording medium 321 at high speed.

[0348] The hierarchical structure is a structure in which a plurality of data located at lower levels of the ferroelectric recording medium 321 are arranged in a branched state from one piece of data belonging to a certain level of the ferroelectric recording medium 321.

[0349] As described above, the data management system 300 includes a data management unit 310 and at least one external storage device 320. The data management unit 310 stores data on the high-speed communication network in the external storage device 320 and metadata in the internal storage device 311. The system may include multiple external storage devices 320. The data management system 300 can store large amounts of data flowing at high speed over the high-speed communication network from the data management unit 310 to the external storage device 320, and can easily read data stored in the external storage device 320 by using the metadata stored in the internal storage device 311. Furthermore, the external storage device 320 is easily replaceable or addable, and multiple external storage devices 320 can be provided, thereby increasing the storable data capacity and reducing the burden on the external storage device 320 for storing data. Therefore, the data management system 300 can write data at high speed with high recording density and has high scalability. Therefore, the data management system 300 can efficiently store data on the high-speed communication network and improve convenience.

[0350] 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 store data on the high-speed communication network in the external storage device 320 via the data management unit 310, and also to reliably store metadata used to read the metadata stored in the external storage device 320 in the internal storage device 311. Therefore, the data management system 300 can efficiently store data on the high-speed communication network, and can also read out data stored in the external storage device 320 more quickly.

[0351] The data management system 300 can arrange multiple external storage devices 320 in parallel. This allows the data management system 300 to distribute and evenly write data over a high-speed communication network to each external storage device 320. Therefore, the data management system 300 can store large amounts of data in multiple external storage devices 320 at high speed and more efficiently, and can further improve scalability. Therefore, the data management system 300 can increase the data storage capacity and further improve convenience.

[0352] In the data management system 300, the external storage device 320 can delete old data recorded in the ferroelectric recording medium 321 without updating it and store new data on the high-speed communication network, or can overwrite the old data with the new data. This allows the data management system 300 to easily write data to the ferroelectric recording medium 321 of the external storage device 320, and also makes it easy to read data stored in the external storage device 320. Therefore, the data management system 300 can reliably store and read data in the external storage device 320 while maintaining high speed.

[0353] In the data management system 300, the external storage device 320 can include a storage element 322, a reading element 323, and a driving unit 324. This allows the data management system 300 to simultaneously write and read information to and from consecutive sectors and consecutive tracks on the same storage surface 321a of the ferroelectric recording medium 321. It also makes it possible to process sporadic requests to read information while writing information to consecutive sectors and consecutive tracks. This allows the data management system 300 to store data in the ferroelectric recording medium 321 at high speed, and to read the stored data. [Example]

[0354] Hereinafter, the embodiments will be specifically described with reference to examples and comparative examples, but the embodiments are not limited to these examples and comparative examples.

[0355] Example 1 [Preparation of target for forming ferroelectric layer] (Hf 0.5 Zr 0.5 (O2 target preparation) A 1:1 mixture of HfO2 powder and ZrO2 powder was made into a slurry using water as a solvent, and then spray-dried to produce a mixed powder. This mixed powder was pressed to form a compact, which was then sintered in an inert atmosphere to produce a target. The density of the target was 1:1. 0.5 Zr 0.5 The O2 concentration was approximately 96% of the theoretical value.

[0356] (Preparation of 4(Y2O3)-96(HfO2) target) The same procedure was used as above except that the Y2O3 powder and hafnium oxide (HfO2) powder were mixed in a ratio of 4:96. 0.5 Zr 0.5 The density of the target was approximately 95% of the theoretical value for 4(Y2O3)-96(HfO2).

[0357] [Fabrication of ferroelectric recording media] A ferroelectric recording medium was fabricated using the following method. Non-doped single-crystal silicon with a (001) surface orientation was used as the substrate. The substrate was disk-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 disk-shaped substrate was placed in a film-forming apparatus (manufactured by Canon Anelva Corporation), and a 30-nm gold (Au) electrode layer was formed on the substrate surface using RF sputtering at a substrate temperature of 200°C, Ar gas at a pressure of 1 Pa, and a 30-nm CeO2 paraelectric layer was then formed using RF sputtering at a substrate temperature of 350°C, Ar and O2 gas at a 3:1 ratio, and a 1-Pa pressure. Next, a 30-nm CeO2 paraelectric layer was then formed using RF sputtering at a substrate temperature of 400°C, Ar and O2 gas at a 3:1 ratio, and a 1-Pa pressure. 0.5 Zr 0.5A 30 nm O2 film was formed on top of this. A 5 nm hard carbon film (DLC film) was formed on top of this as a protective layer using an ion beam method at a substrate temperature of 150°C. The electrode layer was formed on the entire surface of the substrate, but the paraelectric layer, ferroelectric layer, and protective layer were not formed around the central opening by masking a 10 mm wide area of ​​the inner periphery. Finally, a 1.5 nm thick perfluoropolyether lubricant was applied to the protective layer using a dipping method to form a lubricant layer, resulting in a ferroelectric recording medium.

[0358] Tables 1 to 4 show the configuration of each layer that constitutes the ferroelectric recording medium.

[0359] [Evaluation of ferroelectric layer properties] The characteristics of the ferroelectric layer of the ferroelectric recording medium were evaluated by XRD, including the diffraction intensity of the (111) plane, smoothness, leak current density, and leak current density due to deterioration over time.

[0360] (Evaluation of diffraction intensity of (111) plane of ferroelectric layer by XRD) After the ferroelectric layer was formed in [Preparation of Ferroelectric Recording Medium], the substrate was removed from the film-forming apparatus. The substrate was analyzed for Hf using an X-ray diffraction device (XRD, incident X-ray: θ, detection angle: 2θ). 0.5 Zr 0.5 The diffraction intensity of the (111) plane of O2 was measured and found to be 1200 cps. The measurement results are shown in Table 5.

[0361] (Evaluation of smoothness of ferroelectric layer) After the ferroelectric layer was formed in [Preparation of Ferroelectric Recording Medium], the substrate on which the ferroelectric layer had been formed was removed from the film-forming 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 following evaluation criteria. An atomic force microscope (manufactured by BRUKER) was used for the measurement. ((Evaluation Criteria)) A: The surface roughness of the substrate was less than 0.5 nm. B: The surface roughness of the substrate was 0.5 nm to less than 1.0 nm. C: The surface roughness of the substrate was 1.0 nm or more.

[0362] (Evaluation of leakage current density of ferroelectric layer) As in the above (Evaluation of the crystal structure of the ferroelectric layer), after the ferroelectric layer was formed in the above (Preparation of ferroelectric recording medium), the substrate was removed from the film-forming device. A 0.5 mm square Au electrode pad (200 nm thick) was formed on the surface of the ferroelectric layer of the removed substrate, and an evaluation sample was prepared. 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 when 5 V was applied. -6 A / cm 2 The measurement results are shown in Table 5.

[0363] (Evaluation of leakage current density due to degradation of ferroelectric layer over time) The ferroelectric recording medium prepared in the above-mentioned [Preparation of Ferroelectric Recording Medium] was kept in an environment of 80°C temperature and 80% humidity for two weeks. After the removed ferroelectric recording medium was dried, a 0.5 mm square Au electrode pad (200 nm thick) was formed at a position with a radius of 40 mm to prepare 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 of the evaluation sample and the Au electrode pad was measured. When 5 V was applied, the leakage current density was approximately 5 × 10 -6 A / cm 2 The measurement results are shown in Table 5.

[0364] The leakage current density due to deterioration over time of the ferroelectric recording medium of Example 13 described later was approximately 1×10 when 5 V was applied. -5 A / cm 2 The measurement results are shown in Table 5 (see Example 13).

[0365] Table 5 shows the evaluation results of the above-mentioned characteristics of the ferroelectric layer.

[0366] [Manufacturing of ferroelectric memory devices] (Manufacture of the first conductive probe) The first conductive probe was fabricated as follows. A 1 μm thick molybdenum film was formed on a 0.2 mm thick (0001) quartz crystal substrate by sputtering. A photoresist pattern with a 0.3 μm equilateral triangular opening was then formed on the molybdenum surface by photoresist processing. Next, the molybdenum 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. A 1 μm thick molybdenum film was then formed by sputtering. The quartz crystal substrate containing the probe-forming area was then cut into 0.5 mm square pieces. The photoresist was then stripped off to form a first conductive probe tip with a molybdenum needle electrode on the quartz crystal substrate. The needle electrode of this tip was surrounded by a molybdenum layer, with the tip slightly protruding from the molybdenum layer.

[0367] (Manufacture of the first probe slider) A first probe slider was fabricated from Al2O3-TiC (AlTiC). The outer dimensions of the first probe slider were a 2mm x 1.5mm floating surface, a 0.5mm thickness, and a 0.2mm width at the leading end, which is the airflow inlet end. A 0.2mm recess was provided at the outflow end for attaching the tip of the first conductive probe. The outflow end also contained gold wiring to the conductive probe, and gold electrodes and wiring for applying voltage to the piezoelectric element (quartz crystal).

[0368] (Manufacture of the second conductive probe and the second probe slider) A 0.2 mm thick single-crystal silicon substrate (with a (001) surface orientation) was heated to 550°C, and the following layers were deposited on its surface using RF sputtering: 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). For each electrode layer, a circuit pattern connecting to the electrode layer was created on the outside of the laminated structure using photolithography. A molybdenum needle electrode was then formed on top of this using the same method as for the first conductive probe, to produce the tip of the second conductive probe.

[0369] Using the tip of this second conductive probe, a second probe slider was manufactured using the same method as for the first probe slider. The second probe slider was provided with wiring connecting the first, second, and third electrodes. 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 a write signal to the ferroelectric recording medium.

[0370] (Manufacturing of ferroelectric recording medium drive units) A spindle motor with the structure shown in Figure 25 was manufactured to rotate the ferroelectric recording medium. The housing was made of aluminum alloy, the shaft member was made of S45C hardened steel with a diameter of 3 mm, and the shaft end was convex with a radius of curvature R of 6 mm. The bearing sleeve was made of cylindrical 50Cu-47Fe-3Sn sintered metal, and the housing bottom was made of flat 50Cu-47Fe-3Sn sintered metal. 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.

[0371] (Manufacturing of probe drive units) The probe driver was manufactured using a general-purpose HDD driver, with the structure shown in Figure 8.

[0372] (Manufacturing of control units) The control unit was manufactured using a general-purpose power supply and control equipment, with the structure shown in Figure 8.

[0373] (Manufacturing of recording / playback signal processing section) A recording / playback signal processing unit was manufactured. For write signals, it consisted of a bipolar power supply that generated positive or negative voltages corresponding to the write information, and for read signals, it consisted of an amplifier that amplifies the weak tunnel current that flows between the conductive probe and the ferroelectric recording layer, and an A / D converter that converts this into digital data. A triangular wave was used as the waveform generated by the bipolar power supply. In addition, a DC power supply was installed to expand and contract the piezoelectric element placed between the conductive probe and the head slider.

[0374] (Housing) A housing having the structure shown in FIG. 8 was manufactured.

[0375] (Manufacture of Ferroelectric Memory Device 1) Using the manufactured ferroelectric recording medium, first conductive probe, first probe slider, ferroelectric recording medium drive unit, probe drive unit, control unit, recording / playback signal processing unit, and housing, a ferroelectric memory device 1 having the structure shown in Figure 8 was manufactured. The 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. 1 g of silica gel was also sealed inside the housing as a desiccant.

[0376] (Manufacture of Ferroelectric Memory Device 2) A 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 changed to a second conductive probe and second probe slider.

[0377] [Performance of Ferroelectric Memory Device 1] As the performance of the ferroelectric memory device 1, a recording / reproducing test 1 of the ferroelectric memory device 1, a tunnel current difference during reading, and a frictional charge amount were measured.

[0378] (Recording / playback test) A read / write test was performed on the manufactured ferroelectric memory device 1. The ferroelectric recording medium was rotated at 5,400 rpm, and the probe slider was levitated and moved over the surface of the ferroelectric recording medium. The probe slider was then fixed at a track position with a radius of 40 mm on the ferroelectric recording medium. The conductive probe was then switched to the information read circuit, a bias voltage of +500 mV was applied between the ferroelectric recording medium and the conductive probe, and the tunneling current from the conductive probe was monitored. A DC voltage was then 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 a position where the average tunneling current reached 2 pA.

[0379] The conductive probe was then switched to the information writing circuit, and information was written in 255 sectors at that 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 2.3GHz. Information was written once, and the writing time was approximately 10ms. This corresponds to a 10nm circumferential bit length on the surface of the ferroelectric recording medium.

[0380] After writing data to the ferroelectric recording medium, the conductive probe was switched to the data read circuit and the tunneling current was monitored. The voltage applied to the piezoelectric element was adjusted to achieve an SNR of 3 dB or higher. The amplitude of the tunneling current was approximately 3 pA, and the time from the completion of writing data to the ferroelectric recording medium to the time of reading data was approximately 0.1 ms.

[0381] It was confirmed that information could be written to the ferroelectric memory device 1 and the written information could be read using the above method.

[0382] After repeating the above reading of information 10 times, the SNR during reading of information fell below 3 dB, so the same data was rewritten at the same track position, and as a result, the SNR during reading of information recovered to 3 dB or more.

[0383] (Measurement of tunnel current difference during reading 1) When reading information from a ferroelectric recording medium, the difference in tunneling current between a bit with a positively charged ferroelectric layer and a bit with a negatively charged ferroelectric layer was measured. The bias voltage between the ferroelectric recording medium and the conductive probe was +500 mV. Specifically, the tunneling current of a bit with a positively charged surface layer of the ferroelectric layer decreased, while the tunneling current of a bit with a negatively charged surface layer increased. The distance between the conductive probe and the ferroelectric recording medium was adjusted so that the tunneling current of the bit with a positively charged surface layer was 1 pA. The tunneling current from the negatively charged bit was measured. The tunneling current from the negatively charged bit was approximately 3 pA, and the difference between the two tunneling currents was approximately 2 pA. The results are shown in Table 6.

[0384] (Measurement of tunnel current difference during reading 2) When reading information from the ferroelectric recording medium, a bias voltage of ±500 mV was applied between the ferroelectric recording medium and the conductive probe. The bias voltage was a square wave of 1 GHz, and the reading speed was 500 Mbit / s.

[0385] The distance between the conductive probe and the ferroelectric recording medium was adjusted so that the tunneling current of a bit with a positively charged surface of the ferroelectric layer averaged 5 pA when the bias voltage applied to the conductive probe was -500 mV. When the bias voltage applied to the conductive probe was increased to +500 mV, the average tunneling current dropped to 1 pA. This is believed to be due to a rectification effect at the junction between the ferroelectric layer and the paraelectric layer. Furthermore, when a similar evaluation was performed on a bit with a negatively charged surface of the ferroelectric layer, the forward tunneling current averaged 25 pA and the reverse tunneling current averaged 5 pA. From these measurements, the difference in tunneling current between a positively charged bit and a negatively charged bit when the bias voltage applied to the conductive probe was changed between positive and negative was a maximum of 20 pA. The results are shown in Table 6.

[0386] (amount of triboelectric charge) The ferroelectric memory device fabricated in the above-mentioned [Fabrication of Ferroelectric Memory Device] (i.e., a ferroelectric memory device in which the interior is filled with argon gas at atmospheric pressure, the housing is sealed, and a desiccant is enclosed) was neutralized. The ferroelectric recording medium was then rotated at 5600 rpm, and the probe slider was subjected to a seek operation at 4 Hz (one cycle is defined as a movement from the innermost circumference to the outermost circumference, and then a return movement to the innermost circumference) for one hour. The seek operation was then stopped, and the amount of charge between the housing and the conductive probe was measured one second later. The result was a triboelectric charge of 0.1 nC.

[0387] As reference ferroelectric memory devices, a first reference ferroelectric memory device (Example 11) was manufactured in which the inside was filled with nitrogen gas at atmospheric pressure, the housing was sealed, and a desiccant was enclosed, and a second reference ferroelectric memory device (Example 12) was manufactured in which a desiccant was placed inside and the inside was made atmospheric 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, and the triboelectric charge amount was 3 nC in the first reference ferroelectric memory device and 4 nC in the second reference ferroelectric memory device.

[0388] Therefore, it was confirmed that the ferroelectric memory device of Example 1 can mitigate frictional charging by sealing argon gas inside the housing.

[0389] [Performance of Ferroelectric Memory Device 2] As a measure of the performance of the ferroelectric memory device 2, reading using atomic force was performed when reading information from the ferroelectric recording medium. Reading using atomic force was performed by measuring the amplitude of the signal of read information obtained from the piezoelectric element during a recording / reproducing test of the ferroelectric memory device 2.

[0390] (Measurement of the amplitude of the read information signal obtained from the piezoelectric element when read using atomic force 3) A read / write test was performed on the manufactured ferroelectric memory device. The ferroelectric recording medium was rotated at 5,400 rpm, and a probe slider was levitated and moved over the surface of the ferroelectric recording medium. The probe slider was then fixed at a track position with a radius of 40 mm on the ferroelectric recording medium. The DC voltage applied to the electrostrictive element between the first and second electrodes was gradually increased while monitoring the output voltage from the piezoelectric element between the second and third electrodes, bringing 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.

[0391] In this state, an information write signal was applied to the conductive probe from the third electrode, and information for 255 sectors was written at that 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 write frequency was 2.3GHz. Information was written once, and the write time was approximately 10ms. This corresponds to a 10nm circumferential bit length on the surface of the ferroelectric recording medium.

[0392] 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 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 read from the piezoelectric element was approximately 3 μV, and the time from the completion of writing the information to the ferroelectric recording medium to reading it was approximately 0.1 ms. The measurement results of the amplitude of the signal read from the piezoelectric element are shown in Table 6.

[0393] <Examples 2 to 6> The procedure of Example 1 was repeated except that the materials contained in the electrode layer in [Preparation of ferroelectric recording medium] were changed to Ge, Pb, Al, Cu, and Cr as shown in Table 2. 0.5 Zr 0.5 The diffraction intensity of the (111) plane of O2 and the recording and reproduction tests of the ferroelectric memory device were carried out. The configuration of each layer constituting the ferroelectric recording medium is shown in Tables 1 to 4, the formation conditions of the ferroelectric layer and the evaluation results of the characteristics are shown in Table 5, and the test results of the dielectric memory device are shown in Table 6.

[0394] <Examples 7 to 9> In Example 1, the procedure for [Preparation of Ferroelectric Recording Media] was the same as in Example 1, except that the materials contained in the paraelectric layer were changed to 10(Y2O3)-90(ZrO2), Al2O3, and TiO2 as shown in Table 3. 0.5 Zr 0.5 The diffraction intensity of the (111) plane of O2 and the recording and reproduction tests of the ferroelectric memory device were carried out. The configuration of each layer constituting the ferroelectric recording medium is shown in Tables 1 to 4, the formation conditions of the ferroelectric layer and the evaluation results of the characteristics are shown in Table 5, and the test results of the dielectric memory device are shown in Table 6.

[0395] Example 10 The procedure of Example 1 was repeated except that no paraelectric layer was provided in [Preparation of Ferroelectric Recording Medium]. 0.5 Zr 0.5The diffraction intensity of the (111) plane of O2, a recording / reproducing test of the ferroelectric memory device, and the amount of triboelectric charge were measured. In this example, the leakage current density between the electrode layer of the substrate on which the ferroelectric layer was formed and the Au electrode pad was approximately 1 × 10 when 5 V was applied. -5 A / cm 2 The configuration of each layer constituting the ferroelectric recording medium is shown in Tables 1 to 4, the forming conditions of the ferroelectric layer and the evaluation results of the characteristics are shown in Table 5, and the test results of the dielectric memory device are shown in Table 6.

[0396] <Examples 11 and 12> The same procedures as in Example 1 were carried out except that the interior was filled with nitrogen gas or air at atmospheric pressure, the housing was sealed, and a desiccant was enclosed in the housing. The configurations of the layers constituting the ferroelectric recording medium are shown in Tables 1 to 4, the formation conditions of the ferroelectric layer and the evaluation results of the characteristics are shown in Table 5, and the test results of the dielectric memory device are shown in Table 6.

[0397] Example 13 In Example 1, the procedure was the same as in Example 1 except that in [Preparation of ferroelectric recording medium], the 10 mm wide inner peripheral portion of the substrate was not masked during film formation, and a ferroelectric recording medium was prepared in which an electrode layer, a paraelectric layer, a ferroelectric layer, and a protective layer were formed 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 characteristic 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 14, Comparative Examples 1 to 4> In Example 1, in the "Preparation of ferroelectric recording media", the materials used for the substrates were changed to a-plane sapphire substrate (Example 14), NiP electroless plated aluminum 5000 series alloy substrate (Comparative Example 1), amorphous glass substrate (Comparative Example 2), MgO (100) plane substrate (Comparative Example 3), and c-plane sapphire substrate (Comparative Example 4), as shown in Table 1, and the electrode layer was not provided in Comparative Example 1. 0.5 Zr 0.5The diffraction intensity of the (111) plane of O2 and the recording and reproduction tests of the ferroelectric memory device were carried out. The configuration of each layer constituting the ferroelectric recording medium is shown in Tables 1 to 4, the formation conditions of the ferroelectric layer and the evaluation results of the characteristics are shown in Table 5, and the test results of the dielectric memory device are shown in Table 6.

[0399] Example 15 The same procedure as in Example 1 was carried out except that the substrate temperature during deposition 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 of the ferroelectric layer and the evaluation results of the characteristics are shown in Table 5, and the test results of the dielectric memory device are shown in Table 6.

[0400] Example 16 Example 5 was carried out in the same manner as Example 1, except that the substrate temperature during deposition of the ferroelectric layer was lowered by 80° C. to 320° C. The configurations of the layers constituting the ferroelectric recording medium are shown in Tables 1 to 4, the formation conditions of the ferroelectric layer and the evaluation results of the characteristics are shown in Table 5, and the test results of the dielectric memory device are shown in Table 6.

[0401] Example 17 Example 6 was carried out in the same manner as Example 1, except that the substrate temperature during deposition of the ferroelectric layer was lowered by 80° C. to 320° C. The configurations of the layers constituting the ferroelectric recording medium are shown in Tables 1 to 4, the formation conditions of the ferroelectric layer and the evaluation results of the characteristics are shown in Table 5, and the test results of the dielectric memory device are shown in Table 6.

[0402] In Examples 15 to 17, the XRD (111) diffraction patterns all showed halos and reduced diffraction intensities. However, when the substrate was heated to 520°C (200°C higher than the deposition temperature), the halo patterns changed to signals with sharp peaks, and the diffraction intensities were 1800 (Example 15), 1600 (Example 16), and 1600 (Example 17), respectively. Consequently, based on the results of electron microscope observation and electron beam diffraction, it is inferred that the ferroelectric layers of Examples 15 to 17 had amorphous structures with short-range order and length, width, and height, of 2 nm or less. Therefore, in Examples 15 to 17, there was no change in leakage current density compared to Examples 1, 5, and 6, respectively, but the smoothness of the growth surface of the ferroelectric layer was improved.

[0403] <Examples 18 to 23> The same procedure as in Example 1 was carried out except that in [Preparation of ferroelectric recording medium], the film thicknesses of the paraelectric layer and ferroelectric layer were changed to the values ​​shown in Tables 3 and 4.

[0404] [Table 1]

[0405] [Table 2]

[0406] [Table 3]

[0407] [Table 4]

[0408] [Table 5]

[0409] [Table 6]

[0410] From Table 6, it was confirmed that in Examples 1, 2-1 to 2-3, and 3 to 23, it was possible to read information, and that there was a correlation between the atomic force obtained from the ferroelectric recording medium during reading and the tunnel current.

[0411] Therefore, by applying a voltage to the electrostrictive element, it is possible to write information to the ferroelectric memory device and read the written information.

[0412] <Examples 2-1 to 2-3> In Example 2, except that the composition and film-forming method of the ferroelectric layer in [Fabrication of Ferroelectric Recording Medium] were changed as shown in Table 7, the ferroelectric layer was formed in the same manner as in Example 2, the substrate was removed from the film-forming apparatus, and the diffraction intensity of the (111) plane of hafnium oxide of the ferroelectric layer in each Example was measured. In addition, using the fabricated ferroelectric layer, a recording medium ferroelectric memory device was fabricated in the same manner as in Example 1, and a recording / reproducing test of the fabricated ferroelectric memory device and measurement of the tunnel current difference during reading were performed. These test results are shown in Table 7.

[0413] The substrate temperature during film formation was 400°C in all cases. For the microwave plasma MOCVD method, the source gases used were tetrakis(ethylmethylamido)hafnium as the hafnium source and tetrakis(ethylmethylamido)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). For the MOCVD method, only substrate heating was performed without applying microwaves. XRD θ-2θ scans were performed on the removed substrates to measure the (111) plane diffraction intensity of hafnium oxide. The measurement results are shown in Table 7.

[0414] [Table 7]

[0415] From Table 7, it was confirmed that the crystallinity of the ferroelectric layer was improved by using plasma to assist the reaction space during film formation.

[0416] Furthermore, Table 7 confirms that in Examples 2-1 to 2-3, information can be read in the same way as in Example 1, and that there is a correlation between the atomic force and tunnel current obtained from the ferroelectric recording medium during reading. Therefore, in Examples 2-1 to 2-3, information can be written to the ferroelectric memory device and the written information can be read by applying a voltage to the electrostrictive element.

[0417] Although the embodiments have been described above, they are presented as examples and the present invention is not limited to the above embodiments. The above embodiments can be implemented in various other forms, and various combinations, omissions, substitutions, modifications, etc. can be made without departing from the spirit of the invention. These embodiments and their modifications are included within the scope and spirit of the invention, and are also included in the inventions and their equivalents as set forth in the claims. [Explanation of symbols]

[0418] 10 Ferroelectric recording media 11 Circuit Board 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 device 23 Probe Slider 26, 26A, 26B, 26C Conductive Probes 261 Substrate 261a Recess 261b, 263b surface (principal surface) 262 Needle electrode 263 Insulating Layer 263a Through hole 27A First Piezoelectric Element 27B Second piezoelectric element 30 Ferroelectric recording medium drive unit 31 Housing (bearing sleeve) 33 Shaft member (spindle shaft) 332 Groove 34 Bottom of housing 40 Probe drive unit 50 Recording / playback signal processing section 60 cabinets 83 metal 320 External storage device 321a Memory surface 322 Memory Element 323 Reading element 324 Drive Unit 324A First Drive Unit 324B Second Drive Unit

Claims

[Claim 1] a ferroelectric recording medium; a conductive probe for writing and reading information to and from the ferroelectric recording medium; a probe slider that causes the conductive probe to float and travel on the surface of the ferroelectric recording medium; a piezoelectric element or an electrostrictive element provided on the probe slider and connected between the probe slider and the conductive probe so that the conductive probe faces the ferroelectric recording medium; a control unit that controls a voltage applied to the piezoelectric element or the electrostrictive element to expand and contract the piezoelectric element or the electrostrictive element, and adjusts a distance between the ferroelectric recording medium and the conductive probe; The control unit controls a voltage applied to the piezoelectric element or the electrostrictive element based on a read signal from the conductive probe, thereby expanding and contracting the piezoelectric element or the electrostrictive element.

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