Multilayer structure and ferroelectric memory

Abstract

Provided is a multilayer structure having a novel and simple structure, provided with a ferroelectric epitaxial film having high crystallinity. Also provided is a ferroelectric memory provided with the multilayer structure. This multilayer structure comprises, in the stated order: a substrate having a first main surface; and a lattice-matched layer and a dielectric layer provided on the first main surface side of the substrate. The substrate is an Si substrate or an SOI substrate having a resistivity of 0.5 Ωcm or less in at least a surface layer portion on the first main surface side. The lattice-matched layer is an electrically conductive epitaxial film having a fluorite structure with a resistivity of 1 x 106 Ωcm or less. The dielectric layer is an epitaxial ferroelectric film.

Description

Stacked structures and ferroelectric memory

[0001] This invention relates to a stacked structure and a ferroelectric memory.

[0002] Ferroelectric memory (FeRAM) is a non-volatile memory that uses the positive and negative residual polarization (spontaneous polarization) based on the hysteresis of a ferroelectric material to correspond to the 1s and 0s of digital data. When an electric field is applied to a ferroelectric material, polarization is generated inside, and this polarization remains even when the electric field is removed. Therefore, it is possible to construct a memory that utilizes this polarization state. Ferroelectric memory does not require battery backup for data storage. It also features high processing speed, low power consumption, and can withstand many rewrites.

[0003] Ferroelectric memory is manufactured by forming a ferroelectric film sandwiched between a pair of electrode layers on a substrate. Regarding ferroelectric memory having such a structure, Patent Document 1 discloses a method for manufacturing a ferroelectric device, which includes the steps of forming a buffer layer on a substrate, forming an electrode film on the buffer layer, forming a ferroelectric thin film on the electrode film, and forming an electrode film on the ferroelectric thin film (Claim 1 of Patent Document 1).

[0004] Furthermore, various materials have been developed for use in memory. For example, Patent Document 2 discloses a memory element having a resistor whose electrical resistance changes when a voltage is applied, and an electrode for applying a predetermined voltage to the resistor, wherein the resistor is made of a compound having a layered triangular lattice structure containing rare earth elements (Claim 1 of Patent Document 2). Also, the resistor is (RMbO ３－δ ) ｎ (MaO) ｍ It is described as a compound having a layered triangular lattice structure represented as, or a compound in which a part of R of the same compound is substituted with an element with a positive divalent or lower valency (Claim 2 of Patent Document 2).

[0005] Japanese Patent Publication No. 2004-96050, International Publication No. 2010 / 038786

[0006] Conventional ferroelectric memory consists of a stacked structure comprising a substrate, a lower electrode, a ferroelectric film, and an upper electrode. Since ferroelectric memory stores information by utilizing the polarization state of the ferroelectric film, the greater the dielectric polarization (spontaneous polarization), the better the memory characteristics it exhibits. Furthermore, to obtain large dielectric polarization, it is effective to epitaxially grow the ferroelectric film and highly orient it in a uniaxial direction.

[0007] However, with conventional techniques, it has sometimes been difficult to realize epitaxial ferroelectric films with high crystallinity in a simple multilayer structure. For example, the present inventors have found that compounds having a layered triangular lattice structure, such as those proposed in Patent Document 2, are difficult to epitaxially grow directly on a Si substrate or lower electrode while maintaining high crystallinity.

[0008] In light of these problems, the inventors conducted diligent research. As a result, they succeeded in developing a novel and simple laminated structure comprising a conductive substrate having a predetermined resistivity, a lattice-matching layer made of a conductive epitaxial film, and a dielectric layer made of a ferroelectric epitaxial film. Furthermore, they found that this laminated structure can realize a ferroelectric film with high crystallinity.

[0009] This invention was completed based on such findings, and aims to provide a novel and simple stacked structure comprising a ferroelectric epitaxial film with high crystallinity, and a ferroelectric memory comprising the said stacked structure.

[0010] The present invention encompasses the following embodiments (1) to (9). In this specification, the expression "~" includes the numerical values ​​at both ends. That is, "X~Y" is synonymous with "X or more and Y or less".

[0011] (1) A laminated structure comprising a substrate having a first main surface, and a lattice matching layer and a dielectric layer provided on the first main surface side of the substrate, wherein the substrate is a Si substrate or SOI substrate with a resistivity of 0.5 Ωcm or less in at least the surface layer on the first main surface side, and the lattice matching layer has a resistivity of 1 × 10 ６A laminated structure comprising a conductive epitaxial film having a fluorite structure with a resistivity of less than Ω cm, wherein the dielectric layer is an epitaxial ferroelectric film.

[0012] (2) The laminated structure according to (1) above, wherein the substrate is a Si(111) substrate or a SOI(111) substrate, and the conductive epitaxial film is (111)-oriented in the direction perpendicular to the substrate surface.

[0013] (3) The laminated structure according to (1) or (2) above, wherein the conductive epitaxial film contains conductive yttrium-stabilized zirconia (YSZ) as a main component.

[0014] (4) The laminated structure according to any one of (1) to (3) above, wherein the conductive epitaxial film has a composition that is inclined along the direction perpendicular to the substrate surface.

[0015] (5) The epitaxial ferroelectric film is an electronic ferroelectric material having a layered triangular lattice structure represented by the general formula: (RMbO<> ３－δ )<> ｎ (MaO)<> ｍ or a compound in which a part of R is substituted with an element having a valence of +2 or less (where R is at least one element selected from Sc, Y, Dy, Ho, Er, Tm, Yb, Lu, Ca, Ce; Ma and Mb are at least one element selected from Ti, Mn, Fe, Co, Cu, Ga, Zn, Al, Mg, Cd with repetition allowed; n is an integer of 1 or more; m is an integer of 0 or more; δ is a real number of 0 or more and 0.2 or less). The laminated structure according to any one of (1) to (4) above, which contains the above as a main component.

[0016] (6) The laminated structure according to (5) above, wherein the epitaxial ferroelectric film is c-axis-oriented in the direction perpendicular to the substrate surface.

[0017] (7) The laminated structure according to (5) or (6) above, wherein the epitaxial ferroelectric film contains LuFe<> ２ O<> ４ as a main component and has an electrical resistance derived from charge order.

[0018] (8) The laminated structure according to any one of (1) to (7) above, further comprising a first electrode layer electrically connected to the first main surface through the substrate and a second electrode layer provided on the dielectric layer.

[0019] (9) A ferroelectric memory comprising any of the stacked structures described in (1) to (8) above.

[0020] The present invention provides a novel and simple stacked structure comprising a ferroelectric epitaxial film having high crystallinity, and a ferroelectric memory comprising the stacked structure.

[0021] This is a schematic cross-sectional diagram showing an example of a laminated structure. This is a schematic cross-sectional diagram showing another example of a laminated structure. This is a schematic cross-sectional diagram showing yet another example of a laminated structure. This is a schematic diagram illustrating the arrangement of each element in a plan view of a compound having a layered triangular lattice structure. This is a schematic diagram illustrating the arrangement of each element in a side view of a compound having a layered triangular lattice structure. This shows the XRD spectrum of the laminated structure (Reference Example 1). This shows the RHEED image of the laminated structure (Reference Example 1). This shows the XRD spectrum of the laminated structure (Example 1). This shows the RHEED image of the laminated structure (Example 1). Substrate (YSZ substrate) and dielectric layer (LuFe ２ O ４ AFM image of the film is shown (Reference Example 4). Substrate (YSZ substrate) and dielectric layer (LuFe ２ O ４ AFM image of the film is shown (Comparative Example 4). Dielectric layer (LuFe ２ O ４ The XRD spectrum of the film is shown (Reference Example 4). Dielectric layer (LuFe ２ O ４ The XRD spectrum of the film is shown (Comparative Example 4). Dielectric layer (LuFe ２ O ４ The resistivity temperature characteristics of the film are shown (Reference Example 4).

[0022] Specific embodiments of the present invention (hereinafter referred to as "these embodiments") are described below. However, the present invention is not limited to the following embodiments, and various modifications are possible as long as they do not alter the essence of the invention. Furthermore, in this specification, any combination of preferred embodiments can be adopted as long as technical consistency can be maintained. For example, one of the preferred numerical ranges can be arbitrarily combined with the other.

[0023] <<1. Laminated Structure>> The laminated structure of this embodiment comprises a substrate having a first main surface, and a lattice matching layer and a dielectric layer provided on the first main surface side of the substrate, in this order. The substrate is a Si substrate or SOI substrate with a resistivity of 0.5 Ωcm or less in at least the surface layer on the first main surface side, and the lattice matching layer has a resistivity of 1 × 10 ６ This is a conductive epitaxial film having a fluorite structure of less than Ωcm. The dielectric layer is an epitaxial ferroelectric film.

[0024] Figure 1 shows an example of a schematic cross-sectional view of a laminated structure. The laminated structure (100) comprises a substrate (2). This substrate (2) has a pair of opposing main surfaces, namely a first main surface (top surface in the figure) and a second main surface (bottom surface in the figure). A lattice matching layer (4) and a dielectric layer (6) are provided in this order on the first main surface side of the substrate (2). The laminated structure of this embodiment does not exclude configurations in which other layers are provided between the substrate (2), the lattice matching layer (4), and the dielectric layer (6). However, it is preferable that no other layers (such as electrode layers) other than the lattice matching layer (4) are interposed between the substrate (2) and the dielectric layer (6). That is, it is preferable that the lattice matching layer (4) is in direct contact with the substrate (2). It is also preferable that the dielectric layer (6) is in direct contact with the lattice matching layer (4).

[0025] <Substrate> The substrate is either a Si substrate or an SOI substrate. An SOI substrate consists of a Si substrate portion, a surface Si portion, and SiO2 placed between them. ２ The substrate comprises an insulating film, such as a film. Si substrates and SOI substrates may have an insulating film, such as a thermal oxide film, on the second main surface side. By using a Si substrate or an SOI substrate, it becomes possible to perform microfabrication using semiconductor processes on the laminated structure. Preferably, at least the surface layer on the first main surface side of the substrate is composed of a single crystal. That is, a single-crystal Si substrate or an SOI substrate having a single-crystal surface Si portion is preferred.

[0026] The substrate (Si substrate or SOI substrate) has a resistivity (electrical resistivity) of 0.5 Ωcm or less in the surface layer, at least on the first main surface side. That is, at least the surface layer of the substrate is conductive. This makes it possible to make the substrate itself function as an electrode. The resistivity is preferably 0.3 Ωcm or less, more preferably 0.1 Ωcm or less. The thickness of the surface layer may be 0.1 μm or more, preferably 0.2 μm or more, and preferably 1.0 μm or more. The entire substrate may be conductive, or only the surface layer may be conductive. As long as the surface layer is a conductive Si substrate or SOI substrate, the type of substrate is not limited. For example, it may be an N-type Si substrate doped with phosphorus (P), arsenic (As), antimony (Sb), etc. It may be a P-type Si substrate doped with boron (B), etc. It may also be an SOI substrate having a surface Si portion having dopant components such as P or B. Furthermore, if the substrate is an SOI substrate, it is preferable to provide a lattice matching layer or a dielectric layer on the surface Si portion of the SOI substrate.

[0027] The plane orientation of the substrate is not limited as long as an epitaxially oriented lattice-matched layer and dielectric layer can be obtained. The substrate may be a (111) substrate or a (100) substrate. However, preferably the substrate is a Si(111) substrate or an SOI(111) substrate. In this case, it is preferable that the conductive epitaxial film is oriented (111) in the direction perpendicular to the substrate surface. In this specification, the direction perpendicular to the substrate surface means the direction perpendicular to the first main surface of the substrate.

[0028] <Lattice-matching layer> A lattice-matching layer is provided on the first main surface side of the substrate. The lattice-matching layer is a conductive epitaxial film having a fluorite structure. In other words, the lattice-matching layer is a conductive film having a fluorite-type crystal structure, and a heteroepitaxial interface is formed between the lattice-matching layer and the substrate.

[0029] A lattice-matching layer (conductive epitaxial film) works to alleviate lattice mismatch between the substrate and the dielectric layer, thereby promoting the epitaxial growth of the dielectric layer laid on top of the lattice-matching layer. In other words, if lattice mismatch exists between the materials constituting the substrate and the materials constituting the dielectric layer due to differences in lattice constants or crystal structures, even if the dielectric layer is laid directly on the substrate, the growth direction of the dielectric layer will be non-uniform due to the lattice mismatch. As a result, the dielectric layer may become an unoriented polycrystalline film, or in some cases, problems such as delamination may occur, making film deposition impossible. For example, if the dielectric layer is made of LuFe, as described later... ２ O ４ In the case of a film, in conventional structures where a dielectric layer is provided on a metal or oxide electrode, LuFe ２ O ４ The film cannot be epitaxially grown. By providing an epitaxial film (lattice-matching layer) having a fluorite structure, highly epitaxially oriented LuFe ２ O ４ A dielectric layer can be obtained.

[0030] The laminated structure of this embodiment is also characterized in that the lattice-matching layer, along with the substrate, exhibits conductivity. Specifically, the electrical resistivity of the lattice-matching layer is 1 × 10⁻⁶. ６ The resistivity is less than or equal to Ωcm. By making the substrate and lattice matching layer conductive, it becomes possible to make the substrate and lattice matching layer themselves function as lower electrodes for the dielectric layer. Since there is no need to separately provide an electrode layer between the substrate and the dielectric layer that may inhibit the epitaxial growth of the dielectric layer, it becomes possible to further improve the crystallinity (epitaxial orientation) of the dielectric layer. From the viewpoint of suppressing resistive loss, a low resistivity of the lattice matching layer is preferable.

[0031] The material for the lattice-matching layer (conductive epitaxial film) is not particularly limited as long as it has a fluorite-type crystal structure. For example, zirconia (ZrO ２ ), hafnia (HfO ２ ), Celia (CeO ２Examples include zirconia (ZrO2), and their solid solutions. Among these, zirconia is preferred, and it is particularly preferred that the lattice matching layer (conductive epitaxial film) mainly contains conductive yttrium-stabilized zirconia (YSZ). YSZ (yttrium-stabilized zirconia) has high temperature stability of its crystal structure. Therefore, a stable laminated structure can be obtained over a wide temperature range. The amount of Y solid solution in YSZ is not particularly limited. For example, zirconia (ZrO2) ２ Yttria (Y ２ O ３ The amount of Y when converted to ) may be 1 mol% or more and 10 mol% or less. In this specification, the main component is the component with the largest mass percentage.

[0032] As described above, the lattice matching layer of this embodiment has an electrical resistivity of 1 × 10⁻⁶. ６ The conductivity is low, below Ωcm. Materials with a fluorite-type crystal structure, such as YSZ, are usually insulating materials. Therefore, it is preferable to impart conductivity by controlling the composition of the material. One method of imparting conductivity is to incorporate dopant elements having different valencies than the materials constituting the lattice-matching layer into the material. Another method is to incorporate crystal defects such as oxygen vacancies into the materials constituting the lattice-matching layer.

[0033] Preferably, the lattice-matched layer (conductive epitaxial film) has a composition that is graded along the direction perpendicular to the substrate surface. By providing a graded composition, a lattice-matched layer with high crystallinity while possessing conductivity can be easily obtained. A lattice-matched layer with a graded composition is, for example, a layer in which the dopant element concentration and the amount of oxygen vacancies are varied along the direction perpendicular to the substrate surface (thickness direction). Preferably, the lattice-matched layer has a graded composition in which the amount of oxygen vacancies decreases in the direction away from the first main surface of the substrate. In a graded composition, the composition may change continuously or discontinuously. For example, a lattice-matched layer may be composed of multiple layers with different compositions (dopant elements, amount of oxygen vacancies, etc.). In this case, a graded composition layer (lattice-matched layer) with discontinuously changing composition is realized.

[0034] The thickness of the lattice-matching layer is not particularly limited. However, by making the thickness moderately large, it becomes possible to more significantly utilize the effects based on the lattice-matching layer. Conversely, by making the thickness moderately small, it becomes possible to make the laminated structure thinner and smaller. The thickness of the lattice-matching layer is preferably 5 nm to 100 nm, and more preferably 15 nm to 50 nm.

[0035] <Dielectric Layer> A dielectric layer is provided on top of the lattice matching layer. The dielectric layer is an epitaxial ferroelectric film and is the main layer responsible for the dielectric properties of the multilayer structure. By providing a dielectric layer, it becomes possible to use the multilayer structure as a dielectric element.

[0036] The dielectric layer (ferroelectric film) is an epitaxial film. That is, a heteroepitaxial interface is formed between the dielectric layer and the lattice-matching layer. By providing such a dielectric layer, it becomes possible to impart excellent dielectric properties to the multilayer structure. In other words, the ferroelectric film has electric dipoles and dielectric polarization (spontaneous polarization) based on them. The direction of spontaneous polarization can be changed by applying an external electric field, and this property can be used to make it function as a dielectric element. If the ferroelectric film is an epitaxial film, the orientation of the crystal lattice is aligned within the film, so the direction of spontaneous polarization can be aligned. Therefore, the dielectric properties of the dielectric layer are improved. In contrast, if the dielectric film is a non-epitaxial film, for example, an unoriented polycrystalline film, the dielectric properties will be lower.

[0037] Known ferroelectric materials can be used as the material for the dielectric layer (epitaxial ferroelectric film). For example, hafnium silicate (HfSiO ｘ ), lead titanate (PbTiO ３ ), lead zirconate titanate (Pb(Ti,Zr)O ３ ), barium titanate (BaTiO ３ ), strontium titanate (SrTiO ３ ), barium strontium titanate ((Ba,Sr)TiO ３ ), strontium bismastantalate (SrBi ２ Ta ２ O ９ ), bismuth titanate (Bi４ Ti ３ O １２ ), calcium manganite (CaMnO ３ ), bismuth ferrite (BiFeO ３ Examples include scandium aluminum nitride (AlScN), gallium aluminum nitride (AlGaN), and yttrium aluminum nitride (AlYN).

[0038] Preferably, the dielectric layer (epitaxial ferroelectric film) has the general formula: (RMbO ３－δ ) ｎ (MaO) ｍ The main component is an electron ferroelectric material having a layered triangular lattice structure represented by , or a compound in which a portion of R is substituted with an element of positive divalent or less (where R is at least one element selected from Sc, Y, Dy, Ho, Er, Tm, Yb, Lu, Ca, Ce; Ma and Mb are at least one element selected from Ti, Mn, Fe, Co, Cu, Ga, Zn, Al, Mg, Cd with overlapping allowed; n is an integer of 1 or more; m is an integer of 0 or more; and δ is a real number between 0 and 0.2). In electron ferroelectric materials, polarization is reversed by electron transfer. By using electron ferroelectric materials, dielectric elements with low power consumption that can perform high-speed polarization reversal with minute currents can be realized.

[0039] Elements with a valency of +2 or less are elements whose valency is greater than 0 and less than or equal to +2. Examples of elements with a valency of +2 or less include Mg, Ca, Sr, Ba, Na, K, Rb, Cs, Sn, or Zn. Preferably, the elements with a valency of +2 or less are Ca, Sr, and Na. Ca is more preferably the element with a valency of +2 or less because it facilitates the synthesis of electron ferroelectrics.

[0040] Preferably, the epitaxial ferroelectric film is made of LuFe having electrical resistance derived from charge order. ２ O ４ It contains as its main component.

[0041] In the following, R is Lu, and Ma and Mb are Fe, so LuFe ２ O ４ Using this as a representative example, we will explain compounds having a layered triangular lattice structure.

[0042] LuFe ２ O ４ The crystal structure of LuFe will be explained using Figures 4 and 5. For the sake of explanation, LuFe ２ O ４ The crystal structure is such that Fe ions in the crystal ３＋ and Fe ２＋ This is shown in the state before the so-called charge ordering, where the regular structure has not yet appeared.

[0043] Figure 4 is a schematic diagram illustrating the arrangement of each element in a plan view. Figure 4 shows the positional relationship between the triangular lattices of element A, element B, and element C. Hereafter, the position of the lattice points in the triangular lattice of element A will be referred to as "position A," the position of the lattice points in the triangular lattice of element B will be referred to as "position B," and the position of the lattice points in the triangular lattice of element C will be referred to as "position C."

[0044] Figure 5 is a schematic diagram illustrating the arrangement of each element in a side view. In Figure 5, each element is positioned in the following order along the c-axis from the top layer downwards.

[0045] Lu-B position O -C position Fe-C position O -B position O -C position Fe-B position O -B position Lu-C position O -A position Fe-A position ○ O -C position ○ O -A position ○ Fe-C position ○ O -C position Lu-A position O -B position Fe-B position O -A position O -B position Fe-A position O -A position Lu-B position

[0046] Of these, the portion consisting of the four layers marked with a circle is called the W layer (W-Layer). Having this W layer is characteristic of LuFe. ２ O ４ This is a characteristic of LuFe. ２ O ４ It is known that a W layer is similarly formed in compounds having a layered triangular lattice structure other than LuFe. The W layer is a stacked triangular lattice structure, and LuFe ２ O ４ In the same number of Fe ２＋ and Fe ３＋By allowing it to exist, it creates charge frustration.

[0047] This will allow LuFe ２ O ４ So, in the W layer, Fe ３＋ Regions with a high concentration of positive charge play the role of Fe ２＋ Regions with a high concentration of Fe play the role of negative charge. As a result, Fe is present in the W layer. ２＋ Fe from regions with high concentrations ３＋ Electric dipoles (electric polarization) that are oriented towards regions with a high concentration of LuFe appear. ２ O ４ It exhibits ferroelectric polarization due to the order and bias of iron ions with different valencies. The polarization axis or polarity axis of the W layer is parallel to the c-axis of the crystal. LuFe ２ O ４ Therefore, the state of the electric dipole can be controlled by applying an electric current from an external source. Depending on the state of this electric dipole, LuFe ２ O ４ These exhibit different nonlinear voltages.

[0048] Thus, compounds containing rare earth elements and having a layered triangular lattice structure possess a W layer, and by controlling the charge order structure within the W layer with an externally applied current, different nonlinear voltage states can be generated. In other words, this compound exhibits different nonlinear voltage values ​​depending on the direction of the polar order, which can be controlled by the current. The polar order retains the same direction even after the current is turned off, and is non-volatile. Therefore, a non-volatile memory (non-volatile binary memory) can be constructed using this compound.

[0049] Preferably, the dielectric layer (epitaxial ferroelectric film) mainly contains an electron ferroelectric material, and the epitaxial ferroelectric film is c-oriented perpendicular to the substrate surface. The electron ferroelectric material that satisfies the above general formula has a hexagonal crystal structure, and its spontaneous polarization (dielectric polarization) is oriented in the c-axis direction. Therefore, when the dielectric layer is c-oriented, the dielectric polarization is maximized, and excellent dielectric properties can be obtained.

[0050] The thickness of the dielectric layer is not particularly limited. However, by making the thickness moderately large, it becomes possible to effectively utilize the dielectric properties of the dielectric layer. On the other hand, excessively thick dielectric layers are difficult to epitaxially grow while maintaining high crystallinity. From the viewpoint of utilizing excellent dielectric properties while maintaining high crystallinity, the thickness of the dielectric layer is preferably 50 nm to 1000 nm, and more preferably 100 nm to 500 nm.

[0051] <Electrodes> The laminated structure may further include a first electrode layer electrically connected to the first main surface via a substrate, and a second electrode layer provided on a dielectric layer.

[0052] As mentioned above, in the laminated structure of this embodiment, the substrate and the lattice matching layer themselves can function as lower electrodes for the dielectric layer. However, it is sometimes difficult to directly provide wiring such as wires in the substrate and the lattice matching layer. In such cases, it is preferable to separately provide a first electrode layer that acts as an extraction electrode for the lower electrode. The first electrode layer is electrically connected to the lattice matching layer provided on the first main surface side via the substrate. The second electrode layer functions as an upper electrode provided on top of the dielectric layer. Therefore, when a voltage is applied between the first electrode layer and the second electrode layer, polarization occurs in the dielectric layer, and charge accumulates on both the upper and lower surfaces. Furthermore, unlike conventional structures, there is no electrode provided between the substrate and the dielectric layer, so there is no risk of inhibiting the epitaxial growth of the dielectric layer.

[0053] The arrangement of the first electrode layer is not limited as long as it is electrically connected to the first main surface via the substrate. As shown in Figure 2, the first electrode layer (8) may be provided on the side of the substrate (2) opposite to the first main surface (the second main surface). Alternatively, as shown in Figure 3, the first electrode layer (8) may be provided on the side of the substrate (2) that is the first main surface. In this case, it is preferable to provide the first electrode layer (8) in a region separate from the lattice matching layer (4) on the first main surface side, but it is also acceptable for it to be in contact with the lattice matching layer (4). Furthermore, if insulation from the second electrode layer (10) can be ensured by the steps in the laminated structure, the first electrode layer (8) may be formed over the entire surface.

[0054] The electrode layers (first electrode layer and second electrode layer) may be composed of known electrode materials. Examples of electrode materials include gold (Au), silver (Ag), platinum (Pt), palladium (Pd), copper (Cu), aluminum (Al), nickel (Ni), molybdenum (Mo), tungsten (W), ruthenium (Ru), osmium (Os), rhodium (Rh), and iridium (Ir). The electrode material may be a single metal or an alloy. The electrode material may be strontium ruthenate (SrRuO). ３ The electrode layer may also be an oxide such as . The electrode layer may also include an adhesion layer made of titanium (Ti) or the like. The electrode material of the first electrode layer and the electrode material of the second electrode layer may be the same or different. If the electrode material of the first electrode layer and the electrode material of the second electrode layer are the same, it is desirable that at least ohmic contact is made with the substrate.

[0055] The laminated structure of this embodiment comprises a substrate (Si substrate, SOI substrate), a lattice matching layer, and a dielectric layer (ferroelectric film). Because it includes a Si substrate or an SOI substrate, microfabrication using semiconductor processes can be applied to the laminated structure. Therefore, dielectric elements such as ferroelectric memories can be mass-produced with high productivity.

[0056] Furthermore, since the dielectric layer is provided on a predetermined lattice-matching layer, a dielectric layer with high crystallinity (epitaxial orientation) is realized. In particular, LuFe ２ O ４ Electron ferroelectric materials having a layered triangular lattice structure, such as those described above, are difficult to epitaxially grow directly on Si substrates or electrodes. Conventionally, only film deposition on oxide single crystal substrates has been reported, which has the problem of poor compatibility with semiconductor processes. In contrast, according to this embodiment, a dielectric layer of an electron ferroelectric material with high crystallinity (epitaxial orientation) can be obtained, making it possible to fabricate dielectric devices exhibiting excellent dielectric properties. However, the laminated structure of this embodiment is not limited to those containing an electron ferroelectric material having a layered triangular lattice structure.

[0057] Furthermore, because the substrate and lattice-matching layer are conductive, the substrate and lattice-matching layer themselves can function as electrodes. This simplifies the layer structure of the laminated structure, making it possible to further enhance the crystallinity of the dielectric layer. In contrast, conventional techniques require an electrode layer to be placed between the substrate and the dielectric layer. Such an electrode layer may inhibit the epitaxial growth of the dielectric layer, making it difficult to improve its dielectric properties.

[0058] <<2. Ferroelectric Memory>> The ferroelectric memory of this embodiment includes the stacked structure described above. The ferroelectric memory may also include elements other than the stacked structure described above, such as a current source and a voltage detection unit. The current source may include a pulse current source for applying a pulse current for data writing and an AC current source for applying an AC current for data reading.

[0059] <<3. Manufacturing of Laminated Structures>> The manufacturing method of the laminated structure of this embodiment is not limited as long as the above requirements are satisfied. However, it is preferable to include the following steps: a step of preparing a substrate having at least a first main surface (substrate preparation step), a step of epitaxially depositing a lattice matching layer on the first main surface of the prepared substrate (lattice matching layer deposition step), and a step of epitaxially depositing a dielectric layer on the deposited lattice layer (dielectric layer deposition step). Details of each step are described below.

[0060] <Substrate Preparation Process> In the substrate preparation process, a substrate having at least a first main surface is prepared. The details of the substrate are as described above. That is, the substrate is a Si substrate or SOI substrate with a resistivity of 0.5 Ωcm or less in the surface layer on the first main surface side. A commercially available conductive Si substrate or SOI substrate having a surface layer with low resistivity may be used as the substrate. Alternatively, a commercially available Si substrate or SOI substrate may be manufactured by ion implanting dopant elements (P, B, etc.) into it.

[0061] The prepared substrate may be treated with a solution such as hydrofluoric acid aqueous solution to remove unwanted components such as the surface native oxide film on the substrate surface.

[0062] <Lattice-Matching Layer Deposition Process> In the lattice-matching layer deposition process, a lattice-matching layer having a fluorite structure is epitaxially deposited on the first main surface of the prepared substrate. The deposition method is not limited as long as the desired lattice-matching layer can be obtained. Examples include pulsed laser deposition (PLD), chemical vapor deposition (CVD), sputtering, and molecular beam epitaxy.

[0063] However, the resulting lattice-matched layer has a resistivity of 1 × 10⁻⁶. ６ The film must be a conductive epitaxial film with a fluorite structure of Ωcm or less. To achieve this, it is important to use a deposition source (target material, etc.) that can produce a lattice-matched layer with the desired composition. Adjusting the deposition conditions is also important. For example, by controlling the vacuum level of the deposition chamber, a conductive lattice-matched layer with oxygen vacancies can be obtained. In this case, by changing the vacuum level during deposition, a lattice-matched layer with a gradient composition (amount of oxygen vacancies) can be obtained.

[0064] <Dielectric Layer Deposition Process> In the dielectric layer deposition process, a dielectric layer is epitaxially deposited on the lattice deposition layer. The deposition method is not limited as long as the desired dielectric layer can be obtained. Examples include pulsed laser deposition (PLD), chemical vapor deposition (CVD), sputtering, and molecular beam epitaxy.

[0065] In this way, a laminated structure can be obtained. The obtained laminated structure may be processed to have the desired dimensions.

[0066] The present invention will be described in more detail using the following examples and comparative examples. However, the present invention is not limited to the following examples.

[0067] <<Experimental Example 1>> (1) Fabrication of the laminated structure [Reference Example 1] In Reference Example 1, a YSZ film (lattice-matched film) was formed on a Si substrate by pulsed laser deposition (PLD) method to fabricate a laminated structure.

[0068] First, a P-type Si (111) wafer with a resistivity of 0.1 Ωcm or less was prepared, and this wafer was immersed in a 1% hydrofluoric acid aqueous solution to remove the surface native oxide film. The wafer after removing the native oxide film was used as the Si substrate.

[0069] Next, the Si substrate (Si (111) wafer with the oxide film removed) was set in the chamber of the PLD film-forming apparatus, and a YSZ film (lattice-matching layer) was formed on the surface of the Si substrate. At this time, the substrate temperature was set to 750 °C, and in the initial stage of film formation, a YSZ film with a thickness of 5 nm was formed in a reduced-pressure atmosphere of 10 －６ Torr or less, and then, a YSZ film with a thickness of 50 nm was formed in an oxygen atmosphere of a pressure of 4 × 10 －４ Torr. In addition, other film-forming conditions were as follows.

[0070] - Film-forming apparatus: PLD apparatus manufactured by Pascal Co., Ltd. - Laser source: COMPex pro 110 excimer laser (oscillation wavelength 248 nm) manufactured by COHERENT - Laser oscillation intensity: 200 mJ - Laser irradiation area: 0.1 cm ２ - Target: Zirconia (ZrO ２ ) added with 8 mol% of yttria (Y ３ O ２ )

[0071] [Reference Example 2] In Reference Example 2, with the substrate temperature set to 750 °C, a YSZ film with a thickness of 55 nm was formed in a reduced-pressure atmosphere of 10 －６ Torr or less. Otherwise, a laminated structure was fabricated in the same procedure as in Reference Example 1.

[0072] [Reference Example 3] In Reference Example 3, in the initial stage of film formation, a YSZ film with a thickness of 5 nm was formed in a reduced-pressure atmosphere of 10 －６ Torr or less, and then, a YSZ film with a thickness of 50 nm was formed in an oxygen atmosphere of a pressure of 1 × 10 －２ Torr. Otherwise, a laminated structure was fabricated in the same procedure as in Reference Example 1.

[0073] [Example 1] In Example 1, a YSZ film (lattice-matching layer) was formed on a Si substrate by the PLD method, and further LuFe ２ O ４ film (dielectric layer) was formed on the YSZ film to fabricate a laminated structure.

[0074] First, a Si substrate was prepared in the same procedure as in Reference Example 1, and a YSZ film was formed on the prepared Si substrate. At this time, the substrate temperature was set to 750 °C, and a YSZ film with a thickness of 5 nm was formed in a reduced-pressure atmosphere with a vacuum degree of 10 －６ Torr or less at the initial stage of film formation. Then, a YSZ film with a thickness of 15 nm was formed in an oxygen atmosphere with a pressure of 4 × 10 －４ Torr.

[0075] Next, using a LuFe ２ O ４ target, a LuFe ２ O ４ film was formed on the YSZ film by PLD film formation. At this time, the substrate temperature was set to 900 °C, and a LuFe －６ O film with a thickness of 150 nm was formed in a reduced-pressure atmosphere with a vacuum degree of 10 ２ Torr or less. ４ film was formed.

[0076] (2) Fabrication of dielectric element A ferroelectric element was fabricated by providing a first electrode (lower electrode) and a second electrode (upper electrode) above and below the laminated structure obtained in Example 1.

[0077] Specifically, Ti and Au were sputter-deposited in this order on the LuFe ２ O ４ film (ferroelectric film) of the laminated structure to form an upper electrode. At this time, using a Ti metal target and an Au metal target, film formation was performed in an Ar atmosphere. Also, the substrate temperature was set to room temperature. The obtained Ti film had a thickness of 10 nm, and the Au film had a thickness of 50 nm.

[0078] Next, the laminated structure was processed by a dry etching method using photolithography to obtain a columnar structure with a diameter of 400 μmφ. Finally, an Al film with a thickness of 100 nm was formed on the back surface of the Si substrate of the obtained columnar structure by vapor deposition to form a lower electrode. In this way, Au / Ti / LuFe ２ O ４A dielectric element having the structure / YSZ / Si / Al was fabricated.

[0079] (3) The multilayer structures and dielectric elements obtained were evaluated for various properties using the following procedure.

[0080] <XRD> YSZ film (lattice matching layer) and LuFe of a multilayer structure ２ O ４ The film (dielectric layer) was analyzed by X-ray diffraction (XRD) to determine its crystallinity. The analysis was performed under the following conditions.

[0081] - Equipment: SmartLab manufactured by Rigaku Co., Ltd. - X-rays used: Cu-Kα rays - Method: 2θ-ω reflection method

[0082] <RHEED> YSZ film and LuFe ２ O ４ The films were analyzed using reflection high-energy electron diffraction (RHEED), and the electron diffraction patterns of each film were observed. The analysis was performed under the following conditions.

[0083] - Equipment: RHEED unit attached to the PLD device - Acceleration voltage: 20kV (maximum 30kV) - Filament: Tungsten

[0084] <Resistivity> The film resistance of the YSZ film (lattice matching layer) was measured using the two-terminal method. For the measurement, dot-shaped Au / Ti electrodes were formed on the surface of the YSZ film. Specifically, a 10 nm thick Ti film and a 50 nm thick Au film were deposited on the YSZ film at room temperature using DC sputtering. Next, the deposited Au / Ti film was processed by dry etching using photolithography to create dot-shaped Au / Ti electrodes with a diameter of 400 μmφ. The film resistance of the YSZ film was then evaluated by measuring the resistance between the Si substrate located below the YSZ film and the Au / Ti electrodes.

[0085] (4) Evaluation results [Reference Example 1] Figures 6 and 7 show the XRD spectrum and RHEED image obtained for the sample (layered structure) of Reference Example 1, respectively.

[0086] From the XRD spectrum, a strong diffraction peak based on the (111) plane of YSZ (YSZ(111)) was observed (Figure 6). This confirmed that the YSZ film was uniaxially oriented in the

[111] direction. In addition, a single-crystal spot pattern with streaks was observed in the RHEED image (Figure 7). These results indicate that the YSZ film (lattice-matched layer) was epitaxially grown on the Si(111) substrate.

[0087] Furthermore, the resistivity of the YSZ film measured using the two-terminal method is 10 ４ ~10 ６ The conductivity of this YSZ film was confirmed to be low, at Ωcm.

[0088] [Reference Example 2] In Reference Example 2, the YSZ film (lattice matching layer) peeled off after film formation. Because the YSZ film was destroyed, sample evaluation was not possible.

[0089] [Reference Example 3] In Reference Example 3, as in Reference Example 1, it was confirmed that the YSZ film (lattice matching layer) was epitaxially grown on the Si(111) substrate. However, the resistivity of the YSZ film was 10 １０～１２ It was high at Ωcm.

[0090] [Example 1] The XRD spectrum and RHEED image obtained for the sample (layered structure) of Example 1 are shown in Figures 8 and 9, respectively.

[0091] The XRD spectrum contains LuFe ２ O ４ A strong diffraction peak based on the (00l) plane of the film (LFO film; dielectric layer) was observed, indicating LuFe ２ O ４ It was confirmed that the film was uniaxially oriented in the

[001] direction (Figure 8). In addition, a single-crystal spot pattern with streaks was observed in the RHEED image (Figure 9). From these results, it was found that LuFe ２ O ４ It was found that the film was epitaxially grown in the

[001] direction via a YSZ film (lattice-matching layer) that was epitaxially grown in the

[111] direction.

[0092] <<Experimental Example 2>> (1) Fabrication of a laminated structure [Reference Example 4] In Reference Example 4, LuFe was laid on the (111) surface of a planarized YSZ substrate. ２ O ４ A membrane was fabricated.

[0093] (Planarization treatment) The YSZ substrate was roughly polished by chemical mechanical polishing, and then heat-treated at 1200°C for 2 hours in an air atmosphere to form a step-terrace structure on its surface, consisting of atomic planar terraces and step portions with a height of 0.3 nm.

[0094] (LuFe ２ O ４ (Film) A 50 nm thick layer of LuFe is deposited on the (111) plane of a planarized YSZ by pulsed laser deposition. ２ O ４ A film was fabricated. The fabrication process involved a substrate temperature of 950°C and an oxygen partial pressure of 4.0 × 10⁻⁶. －５ It was done under the conditions of Pa.

[0095] [Comparative Example 4] In Comparative Example 4, LuFe was applied to the (111) surface of a YSZ substrate that had not undergone planarization treatment. ２ O ４ A film was fabricated. The fabrication conditions, other than the substrate, were the same as in Reference Example 4.

[0096] <LuFe ２ O ４ Evaluation of film flatness > The flatness of the structures of Reference Example 4 and Comparative Example 4 was evaluated using an AFM (Atomic Force Microscope, Hitachi High-Tech Corporation, AFM5300E). The obtained surface topography images are shown in Figure 10 (Reference Example 4) and Figure 11 (Comparative Example 4), respectively.

[0097] As shown in Figure 10, when a planarized YSZ substrate is used, LuFe ２ O ４ The promotion of membrane growth was confirmed.

[0098] <LuFe ２ O ４Evaluation of film crystallinity > The crystallinity of the structures of Reference Example 4 and Comparative Example 4 was evaluated using XRD (Brker D8 Discoverer). The obtained XRD patterns are shown in Figure 12 (Reference Example 4) and Figure 13 (Comparative Example 4), respectively.

[0099] As shown in Figure 13, when a planarized YSZ substrate is used, (0001) oriented LuFe ２ O ４ The promotion of membrane growth was confirmed.

[0100] <LuFe ２ O ４ Evaluation of the temperature characteristics of film resistivity > For the structure in Reference Example 4, the temperature dependence of electrical resistivity was measured to evaluate the formation of charge order. Specifically, LuFe ２ O ４ A sample was prepared by applying Ag paste as an electrode to a film. Next, the electrical resistivity of the sample was measured using the DC four-terminal method while varying the temperature of the sample stage to determine the resistivity temperature characteristics. The obtained resistivity temperature characteristics are shown in Figure 14.

[0101] As shown in Figure 14, LuFe on a periodic surface substrate ２ O ４ A deviation from the proportional relationship (dashed line) of resistivity values ​​near room temperature was observed for the film. This resistivity temperature characteristic suggests the formation of charge order. This indicates that charge order is easily formed near room temperature depending on the surface state of the YSZ substrate.

[0102] From the results above, it is understood that this embodiment provides a stacked structure having a novel structure comprising a ferroelectric epitaxial film with high crystallinity, and a ferroelectric memory comprising the stacked structure.

[0103] 2 Substrate 4 Lattice matching layer 6 Dielectric layer 8 First electrode layer 10 Second electrode layer 100 Multilayer structure

Claims

1. A laminated structure comprising, in this order, a substrate having a first main surface, and a lattice matching layer and a dielectric layer provided on the first main surface side of the substrate, wherein the substrate is a Si substrate or SOI substrate with a resistivity of 0.5 Ωcm or less in at least the surface layer on the first main surface side, and the lattice matching layer has a resistivity of 1 × 10 ６ A laminated structure comprising a conductive epitaxial film having a fluorite structure of Ω cm or less, wherein the dielectric layer is an epitaxial ferroelectric film.

2. The laminated structure according to claim 1, wherein the substrate is a Si(111) substrate or an SOI(111) substrate, and the conductive epitaxial film is oriented (111) in the direction perpendicular to the substrate surface.

3. The laminated structure according to claim 1, wherein the conductive epitaxial film mainly comprises conductive yttrium-stabilized zirconia (YSZ).

4. The laminated structure according to claim 1, wherein the conductive epitaxial film has a composition that is inclined along a direction perpendicular to the substrate surface.

5. The epitaxial ferroelectric film is defined by the general formula: (RMbO ３－δ ) ｎ (MaO) ｍ A laminated structure according to claim 1, comprising as a main component a compound having a layered triangular lattice structure represented by , or a compound in which a portion of R is substituted with an element of positive divalent value or less (wherein R is at least one element selected from Sc, Y, Dy, Ho, Er, Tm, Yb, Lu, Ca, Ce, Ma and Mb are at least one element selected from Ti, Mn, Fe, Co, Cu, Ga, Zn, Al, Mg, Cd with overlapping allowed, n is an integer of 1 or more, m is an integer of 0 or more, and δ is a real number of 0 to 0.2).

6. The laminated structure according to claim 5, wherein the epitaxial ferroelectric film is c-oriented in a direction perpendicular to the substrate surface.

7. The epitaxial ferroelectric film is made of LuFe having electrical resistance derived from charge order. ２ O ４ A laminated structure according to claim 5 or 6, comprising as a main component.

8. The laminated structure according to claim 1, further comprising a first electrode layer electrically connected to the first main surface via the substrate, and a second electrode layer provided on the dielectric layer.

9. A ferroelectric memory comprising the stacked structure described in any one of claims 1 to 8.

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