ScAlN laminate and method for manufacturing the same

Abstract

To provide an ScAlN laminate with improved piezoelectric performance.SOLUTION: An ScAlN laminate according to an embodiment of the present invention has the improved piezoelectric performance, and includes a substrate, an underlayer formed on the substrate, and an ScAlN thin film formed on the underlayer, and the closest interatomic distance, which is the distance between the closest atoms in the lattice plane parallel to the surface of the underlayer, is shorter than the a-axis length of the ScAlN thin film.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a ScAlN laminate and a method for manufacturing the same.

Background Art

[0002] Since AlN thin films exhibit excellent piezoelectric properties, they are used in various devices (for example, FBAR high-frequency filters for mobile communication, piezoelectric sensors, energy harvesters, MEMS microphones, fingerprint authentication sensors).

[0003] In recent years, diverse and highly functional sensors have been demanded, and since the high-frequencyization of communication has been progressing, further improvement in the piezoelectric properties of AlN thin films has been required.

[0004] By the way, as disclosed in Patent Document 1 for example, it is known that an AlN thin film added with Sc, that is, a ScAlN thin film, exhibits higher piezoelectric properties than an AlN thin film.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] It has been shown by theoretical calculations (first-principles calculations) that the higher the addition amount of Sc, the higher the piezoelectric properties (the higher the piezoelectric constant) exhibited by the ScAlN thin film. For this reason, in Patent Document 1, Sc is added to the ScAlN thin film at a concentration exceeding 43 mol%.

[0007] However, when the present inventor examined the piezoelectric properties of the ScAlN thin film disclosed in Patent Document 1, the piezoelectric properties expected from the theoretical calculations were not shown.

[0008] Therefore, the present invention has been made in view of the above problems, and an object of the present invention is to provide a ScAlN laminate with improved piezoelectric performance.

Means for Solving the Problems

[0009] In order to solve the above problems, according to an aspect of the present invention, there is provided a ScAlN laminate including a substrate, an underlayer formed on the substrate, and a ScAlN thin film formed on the underlayer, wherein the nearest-neighbor interatomic distance, which is the distance between the closest atoms in the lattice plane parallel to the surface of the underlayer, is shorter than the a-axis length of the ScAlN thin film.

[0010] Here, the ScAlN thin film may contain more than 43 mol% of Sc with respect to the total number of atoms of Al and Sc.

[0011] Further, the ScAlN thin film may contain 59 mol% or more of Sc with respect to the total number of atoms of Al and Sc.

[0012] Further, the ScAlN thin film may contain 81.5 mol% or less of Sc with respect to the total number of atoms of Al and Sc.

[0013] Further, the ScAlN thin film may contain 67 mol% or less of Sc with respect to the total number of atoms of Al and Sc.

[0014] Further, the nearest-neighbor interatomic distance of the underlayer may be 3.34 Å or more and 3.71 Å or less.

[0015] Further, the underlayer may include a <111>-oriented film of YN.

[0016] Further, the thickness of the ScAlN thin film may be 100 nm or less.

[0017] Further, it may be used in any one or more selected from the group consisting of a transistor, an inverter, a ferroelectric memory, and a MEMS device.

[0018] According to another aspect of the present invention, there is provided a method for manufacturing a ScAlN laminate, including a step of forming an underlayer on a substrate and a step of forming a ScAlN thin film on the underlayer, wherein the nearest-neighbor interatomic distance, which is the distance between the closest atoms in the lattice plane parallel to the surface of the underlayer, is shorter than the a-axis length of the ScAlN thin film.

Advantages of the Invention

[0019] According to the above aspect of the present invention, it is possible to provide a ScAlN laminate with improved piezoelectric performance and a method for manufacturing the same.

Brief Description of the Drawings

[0020]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Figure 12

Figure 13

Mode for Carrying Out the Invention

[0021] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. Note that the numerical limitation range represented by "~" means a range including the numerical values described before and after "~" as the lower limit value and the upper limit value. A numerical value indicated by "more than" or "less than" is not included in the numerical range.

[0022] <1. Consideration by the Inventor of the Present Invention> First, the consideration by the inventor of the present invention will be described. As phase morphologies of the ScAlN thin film, the wurtzite phase and the rock salt phase are known. Among these, it is considered that more of the more stable phase exists in the ScAlN thin film. Since the wurtzite phase exhibits piezoelectricity, it can be said that when the wurtzite phase stably exists in the ScAlN thin film, the ScAlN thin film exhibits high piezoelectric performance. Therefore, first, the inventor confirmed which phase becomes stable by adding Sc.

[0023] It can be said that each phase is more stable as the formation enthalpy is lower. The formation enthalpy ΔH is represented by the following mathematical formula (1). Here, it is assumed that the ScAlN thin film is directly formed on some substrate.

[0024] [Number]

[0025] Incidentally, the total energy in formula (1) is automatically calculated by first-principles calculation (code: VASP). That is, in this embodiment, theoretical calculations such as first-principles calculations are automatically performed by an electronic computer capable of executing the theoretical calculations. The results are shown in FIG. 2. FIG. 2 shows the correlation between the molar concentration of Sc dissolved in the AlN thin film (molar concentration with respect to the total number of atoms of Sc and Al) and the formation enthalpy of the wurtzite phase or the rock-salt phase. Specifically, the horizontal axis in FIG. 2 represents the molar concentration of Sc in the ScAlN thin film, and the vertical axis represents the formation enthalpy (kJ / mol). Point P10 represents the formation enthalpy of the wurtzite phase at each composition (each molar concentration of Sc) of the ScAlN thin film, and graph L10 is obtained by linearly connecting point P10. Point P20 represents the formation enthalpy of the rock-salt phase at each composition (each molar concentration of Sc) of the ScAlN thin film, and graph L20 is obtained by linearly connecting point P20.

[0026] As is clear from FIG. 2, it can be said that the wurtzite phase is thermodynamically stable in the region where the molar concentration of Sc is smaller than the broken line A, and the rock-salt phase is thermodynamically stable in the region where the molar concentration of Sc is larger than the broken line A. The broken line A indicates the intersection of graph L10 and graph L20. Hereinafter, the value of the molar concentration of Sc indicated by the broken line A is also referred to as the Sc solid solubility limit of the wurtzite phase. That is, in the region where the molar concentration of Sc is smaller than the broken line A, many wurtzite phases exist in the ScAlN thin film, and in the region where the molar concentration of Sc is larger than the broken line A, many rock-salt phases exist in the ScAlN thin film.

[0027] Therefore, the inventor considered that if the position of the dashed line A could be shifted to the higher Sc concentration side, as shown in FIG. 3 for example, Sc could be dissolved in the AlN thin film at a high concentration while the wurtzite phase could be stably present. As a result, it would be possible to improve the piezoelectric performance of the ScAlN thin film. The definitions of the horizontal and vertical axes in FIG. 3 are the same as those in FIG. 2. Point P11 indicates the formation enthalpy of the wurtzite phase at each composition (each molar concentration of Sc) of the ScAlN thin film, and graph L11 is obtained by linearly connecting point P11. Point P21 indicates the formation enthalpy of the rock salt phase at each composition (each molar concentration of Sc) of the ScAlN thin film, and graph L21 is obtained by linearly connecting point P21.

[0028] In order to shift the position of the dashed line A to the higher Sc concentration side, the inventor focused on the underlayer. That is, an underlayer is interposed between the substrate and the ScAlN thin film. Then, strain energy is introduced into the ScAlN thin film by this underlayer. As a result, if the rock salt phase can be relatively destabilized (the formation enthalpy is increased) compared to the wurtzite phase, the position of the dashed line A can be shifted to the higher Sc concentration side. The formation enthalpy ΔH considering strain energy thin film is represented by the following formula (2). Note that the method for obtaining the formation enthalpy ΔH in formula (2) is the same as that in formula (1).

[0029]

Equation

[0030] The strain energy E in formula (2) strain is represented by the following formulas (3) and (4). In formulas (3) and (4), V is the molar volume (the volume occupied by atoms per mole), which is automatically calculated by first-principles calculations. h is the film thickness of the ScAlN thin film, G is the stiffness ratio of the ScAlN thin film, ν is the Poisson's ratio, ε is the strain, and h c is the critical film thickness (the film thickness at which misfit dislocations are introduced). Here, misfit dislocations are dislocations introduced into the ScAlN thin film due to the lattice misfit between the underlayer and the ScAlN thin film.

[0031]

Number

[0032] The stiffness ratio is expressed by the following mathematical formulas (5) to (10). In mathematical formula (6), B V is the bulk modulus (Voigt notation) of the ScAlN thin film. Also, C ij (i, j: natural numbers in mathematical formulas (6) to (10)) is the elastic coefficient tensor and is automatically calculated by first-principles calculation.

[0033]

Number

[0034] The Poisson's ratio is expressed by the following mathematical formulas (11) to (13). B in mathematical formula (11) is the bulk modulus, and B R in mathematical formula (12) is the bulk modulus (Reuss notation) of the ScAlN thin film. G, B V , C 2 and M in mathematical formulas (11) to (13) are expressed by mathematical formulas (5) to (10).

[0035]

Number

[0036] The strain ε is the lattice misfit between the underlying layer and the ScAlN thin film and is expressed by the following mathematical formula (14). In mathematical formula (14), a is the lattice constant (a-axis length) in the a-axis direction of the ScAlN thin film and is automatically calculated by first-principles calculation. a0 is the nearest-neighbor atomic distance of the underlying layer (described later) and takes different values depending on the substance constituting the underlying layer. For example, when the underlying layer is a <111>-oriented film of YN, a0 is 3.46 Å.

[0037]

Number

[0038] The critical film thickness is expressed by the following mathematical formula (15). In the mathematical formula (15), ε is strain, ν is the Poisson's ratio, which is obtained by the above-described mathematical formula. b is the Burgers vector. The Burgers vector of the zinc blende phase is expressed by the following mathematical formula (16), and the Burgers vector of the rock salt phase is expressed by the following mathematical formula (17). In the mathematical formulas (16) and (17), a is the lattice constant (a-axis length) in the a-axis direction, which is automatically calculated by first-principles calculation. In the mathematical formula (16), c is the lattice constant in the c-axis direction, which is automatically calculated by first-principles calculation.

[0039]

Number

[0040] According to the above-described mathematical formulas (2) to (17), the formation enthalpy ΔH considering the strain energy thin film depends on the interatomic distance of the underlying layer. Therefore, if the interatomic distance of the underlying layer is appropriately determined, the rock salt phase can be made relatively more unstable than the zinc blende phase, and it is considered that the position of the broken line A in FIG. 2 can be shifted to the high Sc concentration side. Therefore, the present inventor has intensively studied the interatomic distance of the underlying layer. As a result, it has been found that when the nearest-neighbor interatomic distance, which is the distance between the closest atoms in the lattice plane parallel to the surface of the underlying layer, is shorter than the a-axis length of the ScAlN thin film, the position of the broken line A can be shifted to the high Sc concentration side compared to the case where there is no underlying layer (the case of FIG. 2). The nearest-neighbor interatomic distance is a value after laminating the ScAlN thin film on the surface of the underlying layer and is a value obtained by first-principles calculation. That is, by making the nearest-neighbor interatomic distance of the underlying layer shorter than the a-axis length of the ScAlN thin film, a wurtzite-phase ScAlN thin film containing Sc at a high concentration can be produced. An example of a substance that satisfies such a requirement for the nearest-neighbor interatomic distance is a <111>-oriented film of YN.

[0041] <2. Regarding the nearest-neighbor interatomic distance> The present inventor further examined the nearest-neighbor atomic distance. The horizontal axis in Fig. 5 is the molar concentration of Sc (molar concentration relative to the total number of atoms of Al and Sc), and the vertical axis indicates the formation enthalpy. Graph L30 schematically shows the correlation between the molar concentration of Sc and the formation enthalpy of the ScAlN bulk. Here, the ScAlN bulk is almost synonymous with the ScAlN thin film without an underlying layer. Graph L31 schematically shows the correlation between the molar concentration of Sc and the formation enthalpy of the ScAlN thin film. Note that both the ScAlN thin film and the ScAlN bulk are in the wurtzite phase, and the underlying layer of the ScAlN thin film is a substance with a nearest-neighbor atomic distance of 3.34 Å. The film thickness of the ScAlN thin film was 5 nm. As shown in Fig. 5, graphs L30 and L31 intersect at point P30. The molar concentration of Sc corresponding to point P30 is 43 mol%. That is, when the molar concentration of Sc is 43 mol%, the formation enthalpy of the ScAlN bulk and the formation enthalpy of the ScAlN thin film coincide. In other words, the formation enthalpy becomes invariant. In this case, no strain energy is generated in the ScAlN thin film. This means that the a-axis length of the ScAlN thin film coincides with the nearest-neighbor atomic distance of the underlying layer. When the molar concentration of Sc is different from 43 mol%, the a-axis length of the ScAlN thin film does not coincide with the nearest-neighbor atomic distance of the underlying layer, so strain energy is generated in the ScAlN thin film and the formation enthalpy increases. Therefore, the formation enthalpy of the ScAlN bulk and the formation enthalpy of the ScAlN thin film do not coincide. Hereinafter, the point at which the formation enthalpy of the ScAlN bulk and the formation enthalpy of the ScAlN thin film coincide is also referred to as a "fixed point". Point P30 is an example of a fixed point.

[0042] The horizontal axis of FIG. 6 is the molar concentration of Sc (molar concentration relative to the total number of atoms of Al and Sc), and the vertical axis indicates the formation enthalpy. Graph L40 schematically shows the correlation between the molar concentration of Sc and the formation enthalpy of the ScAlN bulk. Graph L41 schematically shows the correlation between the molar concentration of Sc and the formation enthalpy of the ScAlN thin film. Note that both the ScAlN thin film and the ScAlN bulk are in the rock salt phase, and the underlying layer of the ScAlN thin film is a substance with a nearest-neighbor atomic distance of 3.34 Å. The film thickness of the ScAlN thin film was 5 nm. As shown in FIG. 6, graphs L40 and L41 do not intersect. That is, at any Sc concentration, the a-axis length of the ScAlN thin film does not match the nearest-neighbor atomic distance of the underlying layer. For this reason, strain energy is generated in the ScAlN thin film regardless of the Sc concentration.

[0043] FIG. 7 is a superimposition of FIGS. 5 and 6. When compared with the ScAlN bulk, the wurtzite phase is stabilized when the Sc concentration is lower than that at point P51 (Sc solid solubility limit). Note that the molar concentration of Sc indicated by point P51 is 59 mol%. On the other hand, when compared with the ScAlN thin film, the wurtzite phase is stabilized when the Sc concentration is lower than that at point P52 (Sc solid solubility limit). The molar concentration of Sc indicated by point P52 is larger than that of point P51 and is 67 mol%. Therefore, a wurtzite-phase ScAlN thin film containing Sc at a higher concentration can be fabricated.

[0044] The above-described phenomenon is considered to occur when the nearest-neighbor atomic distance of the underlying layer is shorter than the a-axis length of the ScAlN thin film. That is, when the nearest-neighbor atomic distance of the underlying layer is shorter than the a-axis length of the ScAlN thin film, a fixed point exists, and it is considered that the Sc solid solubility limit of the ScAlN thin film can be shifted to the higher concentration side than the Sc solid solubility limit of the bulk. And it is considered that the shift width of the Sc solid solubility limit becomes particularly large when the molar concentration of Sc exceeds 43 mol%.

[0045] The horizontal axis of FIG. 8 indicates the molar concentration (mol%) of Sc, and the vertical axis indicates the a-axis length (Å) of the ScAlN thin film. Point P60 shows the correlation between the molar concentration of Sc and the a-axis length of the ScAlN thin film obtained by first-principles calculation, and graph L60 is obtained by linearly connecting point P60. Point P61 shows the correlation between the molar concentration of Sc and the a-axis length (experimental value) of the ScAlN thin film obtained by experiment. Here, the experimental value is calculated from the electron diffraction pattern of the thin film fabricated by magnetron sputtering. As shown in FIG. 8, the first-principles calculation value and the experimental value show a good correlation.

[0046] Referring to this graph, when the molar concentration of Sc is 43 mol%, the a-axis length of the ScAlN thin film is 3.34 Å, and when the molar concentration of Sc is 100 mol%, the a-axis length of the ScAlN thin film (ScN thin film) is 3.71 Å. Therefore, when the molar concentration of Sc in the ScAlN thin film exceeds 43 mol%, the material of the underlayer may be determined so that the nearest-neighbor atomic distance of the underlayer is between 3.34 Å and 3.71 Å and is shorter than the a-axis length of the ScAlN thin film.

[0047] The horizontal axis of FIG. 9 indicates the molar concentration (mol%) of Sc, and the vertical axis indicates the formation enthalpy (kJ / mol) of the wurtzite phase or the rock-salt phase. Graph L70 shows the correlation between the molar concentration of Sc and the formation enthalpy of the ScAlN bulk. Graph L71 shows the correlation between the molar concentration of Sc and the formation enthalpy of the ScAlN thin film. Note that both the ScAlN thin film and the ScAlN bulk in graphs L70 and L71 are in the wurtzite phase, and the nearest-neighbor atomic distance of the underlayer of the ScAlN thin film is 3.71 Å. Also, the film thickness of the ScAlN thin film is 5 nm.

[0048] Graph L80 shows the correlation between the molar concentration of Sc and the formation enthalpy of the ScAlN bulk. Graph L81 shows the correlation between the molar concentration of Sc and the formation enthalpy of the ScAlN thin film. Note that both the ScAlN thin film and the ScAlN bulk in graphs L80 and L81 are in the rock-salt phase, and the nearest-neighbor atomic distance of the underlayer of the ScAlN thin film is 3.71 Å. Also, the film thickness of the ScAlN thin film is 5 nm.

[0049] As is clear from FIG. 9, regardless of the molar concentration of Sc, the formation enthalpy of the wurtzite-phase ScAlN thin film is lower than that of the rock-salt-phase ScAlN thin film. On the other hand, focusing on the bulk, the Sc solid solubility limit for Sc in the wurtzite-phase ScAlN bulk is 59 mol%. That is, the wurtzite-phase ScAlN bulk cannot exist stably unless the molar concentration of Sc is 59 mol% or less. Therefore, by forming an underlayer with a nearest-neighbor atomic distance of 3.71 Å, Sc can be dissolved in the ScAlN thin film at a high concentration while the wurtzite phase exists stably.

[0050] The horizontal axis of FIG. 10 indicates the molar concentration (mol%) of Sc, and the vertical axis indicates the formation enthalpy (kJ / mol) of the wurtzite phase or the rock-salt phase. Graph L70 shows the correlation between the molar concentration of Sc and the formation enthalpy of the ScAlN bulk. Graph L91 shows the correlation between the molar concentration of Sc and the formation enthalpy of the ScAlN thin film. Note that both the ScAlN thin film and the ScAlN bulk in graphs L70 and L91 are in the wurtzite phase, and the nearest-neighbor atomic distance of the underlayer of the ScAlN thin film is 3.34 Å. Also, the film thickness of the ScAlN thin film is 5 nm.

[0051] Graph L80 shows the correlation between the molar concentration of Sc and the formation enthalpy of the ScAlN bulk. Graph L101 shows the correlation between the molar concentration of Sc and the formation enthalpy of the ScAlN thin film. Note that both the ScAlN thin film and the ScAlN bulk in graphs L80 and L101 are in the rock-salt phase, and the nearest-neighbor atomic distance of the underlayer of the ScAlN thin film is 3.34 Å. Also, the film thickness of the ScAlN thin film is 5 nm.

[0052] As is clear from Fig. 10, when the nearest-neighbor atomic distance of the underlying layer is 3.34 Å, the Sc solid solubility limit for the wurtzite-phase ScAlN thin film is 67 mol% (point P91). In contrast, focusing on the bulk material, the Sc solid solubility limit for the wurtzite-phase ScAlN bulk material is 59 mol%. That is, the ScAlN bulk material cannot stably exist unless the molar concentration of Sc is 59 mol% (point P92) or less. Therefore, by forming an underlying layer with a nearest-neighbor atomic distance of 3.34 Å, it is possible to stably form the wurtzite phase while dissolving Sc in the ScAlN thin film at a high concentration.

[0053] From the above points, when the film thickness of the ScAlN thin film is 5 nm, it can be seen that when the molar concentration of Sc exceeds 43 mol%, the nearest-neighbor atomic distance of the underlying layer is preferably 3.34 Å or more and 3.71 Å or less.

[0054] The nearest-neighbor atomic distance of the <111>-oriented YN film is 3.46 Å, which satisfies the condition of 3.34 Å or more and 3.71 Å or less. Fig. 11 shows the correlation between the molar concentration of Sc and the formation enthalpy of the wurtzite-phase or rock-salt-phase ScAlN thin film when the <111>-oriented YN film is used as the underlying layer.

[0055] More specifically, the horizontal axis in Fig. 11 indicates the molar concentration of Sc (mol%), and the vertical axis indicates the formation enthalpy of the wurtzite-phase or rock-salt-phase ScAlN thin film (kJ / mol). Point P110 shows the correlation between the molar concentration of Sc and the formation enthalpy of the wurtzite-phase ScAlN thin film. Graph L110 is obtained by linearly connecting point P110. Point P111 shows the correlation between the molar concentration of Sc and the formation enthalpy of the rock-salt-phase ScAlN thin film. Graph L111 is obtained by linearly connecting point P111. As is clear from Fig. 11, by using the <111>-oriented YN film as the underlying layer, the Sc solid solubility limit for the ScAlN thin film can be increased to 81.5 mol%. Based on such findings, the present inventor conceived the ScAlN laminate according to the present embodiment. Hereinafter, the ScAlN laminate according to the present embodiment will be described.

[0056] <3.Structure of the ScAlN laminate> Next, based on FIG. 1, the structure of the ScAlN laminate 1 according to the present embodiment will be described. The ScAlN laminate 1 includes a substrate 10, an underlayer 20, and a ScAlN thin film (piezoelectric film) 30.

[0057] The type of the substrate 10 is not particularly limited, and any substrate may be used as long as the underlayer 20 and the ScAlN thin film 30 described later can be laminated thereon. The substrate 10 may be made of, for example, silicon, a conductive metal, sapphire, SiC, glass, or an organic material.

[0058] The underlayer 20 introduces strain energy into the ScAlN thin film 30 as described above, thereby relatively destabilizing the rock salt phase of the ScAlN thin film with respect to the wurtzite phase. The nearest-neighbor atomic distance of the underlayer 20 is shorter than the a-axis length of the ScAlN thin film. For example, when the molar concentration of Sc in the ScAlN thin film exceeds 43 mol%, the nearest-neighbor atomic distance of the underlayer 20 is 3.34 Å or more and 3.71 Å or less, and is shorter than the a-axis length of the ScAlN thin film. Note that the a-axis length of the ScAlN thin film varies depending on the molar concentration of Sc in the ScAlN thin film. Therefore, the a-axis length corresponding to the molar concentration of the ScAlN thin film may be obtained by first-principles calculation, and an underlayer (material) having a nearest-neighbor atomic distance shorter than the a-axis length may be selected. The underlayer 20 is formed on the substrate 10 by, for example, sputtering or the like. The underlayer 20 may be, for example, a <111>-oriented film of YN. Further, the underlayer 20 may have a multilayer structure. In this case, at least the nearest-neighbor atomic distance of the layer in contact with the ScAlN thin film 30 is shorter than the a-axis length of the ScAlN thin film. For example, the underlayer 20 may have a multilayer structure, and the layer in contact with the ScAlN thin film 30 may be composed of a <111>-oriented film of YN.

[0059] The ScAlN thin film 30 is an AlN thin film in which Sc is solid-solved. The ScAlN thin film 30 may be a single layer or may be a plurality of layers of ScAlN thin films having different polarization directions. Here, the ScAlN thin film solid-solves Sc below the wurtzite-phase Sc solid solubility limit. And, due to the underlayer 20, the wurtzite-phase Sc solid solubility limit is higher than that in the case where the underlayer 20 does not exist. Therefore, the ScAlN thin film 30 exhibits high piezoelectric performance. The molar concentration of Sc in the ScAlN thin film is not particularly limited, but the lower limit may be more than 43 mol%, or may be 59 mol% or more. 59 mol% is the Sc solid solubility limit in the ScAlN bulk, but in this embodiment, more Sc can be solid-solved in the AlN thin film. The upper limit may be 81.5 mol% or less, or may be 67 mol% or less. When a <111>-oriented film of YN is selected as the underlayer, the Sc solid solubility limit is 81.5 mol%, so that up to 81.5 mol% of Sc can be solid-solved in the AlN thin film. 67 mol% is the solid solubility limit when the nearest-neighbor atomic distance of the underlayer 20 is 3.34 Å.

[0060] Here, as shown in Equations (3), (4), etc., the strain energy introduced into the ScAlN thin film 30 is also affected by the film thickness (h) of the ScAlN thin film 30. When the present inventor examined the film thickness, it was found that when the thickness of the ScAlN thin film is 100 nm or less, the wurtzite-phase Sc solid solubility limit becomes particularly high. The film thickness is more preferably 10 nm or less, and even more preferably 5 nm or less. There is no particular limitation on the lower limit of the film thickness, and it may be more than 0 nm. The variation of the strain energy due to the film thickness will be described with reference to FIG. 4.

[0061] Figures 4(a) to 4(c) show the correlation between the molar concentration of Sc dissolved in the ScAlN thin film (molar concentration with respect to the total number of atoms of Sc and Al) and the formation enthalpy of the wurtzite phase or the rock salt phase when the film thicknesses are 100 nm, 50 nm, and 10 nm, respectively. Note that an example when the film thickness of the underlayer 20 is 5 nm is shown in FIG. 11. The underlayer 20 was a <111>-oriented film of YN (nearest-neighbor atomic distance a0 = 3.46 Å). The definitions of the vertical axis and the horizontal axis are the same as those in FIG. 2. Point P12 indicates the formation enthalpy of the wurtzite phase for each composition (each molar concentration of Sc) of the ScAlN thin film with a thickness of 100 nm, and graph L12 is obtained by linearly connecting point P12. Point P22 indicates the formation enthalpy of the rock salt phase for each composition (each molar concentration of Sc) of the ScAlN thin film with a thickness of 100 nm, and graph L22 is obtained by linearly connecting point P22. As shown in FIG. 4(a), the Sc solid solubility limit of the wurtzite phase when the film thickness is 100 nm is 61 mol%.

[0062] Point P13 indicates the formation enthalpy of the wurtzite phase for each composition (each molar concentration of Sc) of the ScAlN thin film with a thickness of 50 nm, and graph L13 is obtained by linearly connecting point P13. Point P23 indicates the formation enthalpy of the rock salt phase for each composition (each molar concentration of Sc) of the ScAlN thin film with a thickness of 50 nm, and graph L23 is obtained by linearly connecting point P23. As shown in FIG. 4(b), the Sc solid solubility limit of the wurtzite phase when the film thickness is 50 nm is 62.5 mol%.

[0063] Point P14 indicates the formation enthalpy of the wurtzite phase for each composition (each molar concentration of Sc) of the ScAlN thin film with a thickness of 10 nm, and graph L14 is obtained by linearly connecting point P14. Point P24 indicates the formation enthalpy of the rock salt phase for each composition (each molar concentration of Sc) of the ScAlN thin film with a thickness of 10 nm, and graph L24 is obtained by linearly connecting point P24. As shown in FIG. 4(c), the Sc solid solubility limit of the wurtzite phase when the film thickness is 10 nm is 75.5 mol%.

[0064] Here, according to first-principles calculations, when the molar concentration of Sc is about 70%, if the wurtzite phase is stable, the piezoelectric constant of the ScAlN thin film becomes maximum (about 100 pC / N). Therefore, when the molar concentration of Sc is, for example, 59 to 81.5 mol%, it is preferable that the wurtzite phase is stable. Thus, the film thickness is preferably 10 nm or less. This is because when the film thickness is 10 nm or less, the solid solubility limit of Sc in the wurtzite phase exceeds 70 mol%.

[0065] Therefore, an example of a preferred embodiment of the ScAlN laminate 1 is as follows. First, the underlying layer 20 is composed of a <111>-oriented film of YN. The film thickness of the ScAlN thin film 30 is 10 nm or less, and Sc is solid-solved at more than 43 mol%, preferably 59 to 81.5 mol%, and more preferably 70 mol%. In this case, the ScAlN thin film 30 exhibits very high piezoelectric performance. Of course, this is merely an example, and as long as the requirements of this embodiment are satisfied, the ScAlN thin film 30 exhibits high piezoelectric performance. As another example of the underlying layer, films using Y, La, Ce, Pr, Nd, Gd, Tb, Dy, Ho, Er, Tm, Lu, and Tl can be mentioned.

[0066] Furthermore, with reference to FIGS. 12 and 13, the variation in strain energy with film thickness when the nearest-neighbor atomic distance of the underlying layer 20 is changed will be described.

[0067] Figures 12(a) to (c) show the correlation between the molar concentration of Sc dissolved in the ScAlN thin film (molar concentration relative to the total number of Sc and Al atoms) and the formation enthalpy of the wurtzite phase or rock salt phase when the nearest-neighbor atomic distance a0 of the underlying layer 20 is 3.50 Å and the film thicknesses are 100 nm, 50 nm, and 10 nm, respectively. The definitions of the vertical and horizontal axes are the same as those in Fig. 2. Point P120 indicates the formation enthalpy of the wurtzite phase for each composition (each molar concentration of Sc) of the ScAlN thin film with a thickness of 100 nm, and graph L120 is obtained by linearly connecting points P120. Point P220 indicates the formation enthalpy of the rock salt phase for each composition (each molar concentration of Sc) of the ScAlN thin film with a thickness of 100 nm, and graph L220 is obtained by linearly connecting points P220. As shown in Fig. 12(a), the Sc solid solubility limit of the wurtzite phase when the film thickness is 100 nm is 63.5 mol%.

[0068] Point P121 indicates the formation enthalpy of the wurtzite phase for each composition (each molar concentration of Sc) of the ScAlN thin film with a thickness of 50 nm, and graph L121 is obtained by linearly connecting points P121. Point P221 indicates the formation enthalpy of the rock salt phase for each composition (each molar concentration of Sc) of the ScAlN thin film with a thickness of 50 nm, and graph L221 is obtained by linearly connecting points P221. As shown in Fig. 12(b), the Sc solid solubility limit of the wurtzite phase when the film thickness is 50 nm is 68.5 mol%.

[0069] Point P122 indicates the formation enthalpy of the wurtzite phase for each composition (each molar concentration of Sc) of the ScAlN thin film with a thickness of 10 nm, and graph L122 is obtained by linearly connecting points P122. Point P222 indicates the formation enthalpy of the rock salt phase for each composition (each molar concentration of Sc) of the ScAlN thin film with a thickness of 10 nm, and graph L222 is obtained by linearly connecting points P222. As shown in Fig. 12(c), the Sc solid solubility limit of the wurtzite phase when the film thickness is 10 nm is 86.5 mol%.

[0070] Figs. 13(a) to (c) show the correlation between the molar concentration of Sc dissolved in the ScAlN thin film (molar concentration relative to the total number of Sc and Al atoms) and the formation enthalpy of the wurtzite phase or rock-salt phase when the nearest-neighbor interatomic distance a0 of the underlayer 20 is 3.60 Å and the film thicknesses are 100 nm, 50 nm, and 10 nm, respectively. The definitions of the vertical and horizontal axes are the same as those in Fig. 2. Point P123 indicates the formation enthalpy of the wurtzite phase for each composition (each molar concentration of Sc) of the ScAlN thin film with a thickness of 100 nm, and graph L123 is obtained by linearly connecting points P123. Point P223 indicates the formation enthalpy of the rock-salt phase for each composition (each molar concentration of Sc) of the ScAlN thin film with a thickness of 100 nm, and graph L223 is obtained by linearly connecting points P223. As shown in Fig. 13(a), the Sc solid solubility limit of the wurtzite phase when the film thickness is 100 nm is 63.5 mol%.

[0071] Point P124 indicates the formation enthalpy of the wurtzite phase for each composition (each molar concentration of Sc) of the ScAlN thin film with a thickness of 50 nm, and graph L124 is obtained by linearly connecting points P124. Point P224 indicates the formation enthalpy of the rock-salt phase for each composition (each molar concentration of Sc) of the ScAlN thin film with a thickness of 50 nm, and graph L224 is obtained by linearly connecting points P224. As shown in Fig. 13(b), the Sc solid solubility limit of the wurtzite phase when the film thickness is 50 nm is 67.5 mol%.

[0072] Point P125 indicates the formation enthalpy of the wurtzite phase for each composition (each molar concentration of Sc) of the ScAlN thin film with a thickness of 10 nm, and graph L125 is obtained by linearly connecting points P125. Point P225 indicates the formation enthalpy of the rock-salt phase for each composition (each molar concentration of Sc) of the ScAlN thin film with a thickness of 10 nm, and graph L225 is obtained by linearly connecting points P225. As shown in Fig. 13(c), the Sc solid solubility limit of the wurtzite phase when the film thickness is 10 nm is 86 mol%.

[0073] As described above, it can be seen that even when the nearest-neighbor atomic distance a0 of the underlayer 20 is 3.50 Å or 3.60 Å, the wurtzite-phase Sc solid solubility limit increases as the film thickness (h) of the ScAlN thin film 30 decreases. When the film thickness of the ScAlN thin film is 10 nm or less, the wurtzite-phase Sc solid solubility limit exceeds 70 mol%. Therefore, the film thickness of the ScAlN thin film is preferably 10 nm or less.

[0074] <4. Method for manufacturing ScAlN laminate> The manufacturing method is not particularly limited, and any manufacturing method may be used as long as it can obtain the ScAlN laminate 1 having the above-described characteristics. As an example of the manufacturing method, a manufacturing method including a step of forming an underlayer 20 on a substrate 10 and a step of forming a ScAlN thin film 30 on the underlayer 20 can be mentioned. The underlayer 20 and the ScAlN thin film 30 are formed by, for example, a sputtering method. Here, the molar concentration of Sc and the material of the underlayer 20 are selected so that the nearest-neighbor atomic distance of the underlayer 20 is shorter than the a-axis length of the ScAlN thin film 30.

[0075] <5. Application examples of ScAlN laminate> The ScAlN laminate 1 is applied to various fields. For example, the ScAlN laminate 1 may be used for any one or more selected from the group consisting of a transistor, an inverter, a ferroelectric memory, and a MEMS device (including a piezoelectric device). Further, the ScAlN laminate 1 may be used for an FBAR high-frequency filter for mobile communication, a piezoelectric sensor, an energy harvester, a MEMS microphone, and a fingerprint authentication sensor.

[0076] As described above, the preferred embodiments of the present invention have been described in detail with reference to the accompanying drawings, but the present invention is not limited to such examples. It is obvious that those having ordinary knowledge in the technical field to which the present invention pertains can conceive of various modification examples or correction examples within the scope of the technical idea described in the claims, and it is naturally understood that these also belong to the technical scope of the present invention.

Explanation of reference numerals

[0077] 1 ScAlN laminate 10 Substrate 20 Underlayer 30 ScAlN thin film

Claims

1. A substrate, an underlayer formed on the substrate, and a ScAlN thin film formed on the underlayer, wherein the nearest-neighbor atomic distance in the lattice plane parallel to the surface of the underlayer, which is the nearest-neighbor atomic distance, is shorter than the a-axis length of the ScAlN thin film, the ScAlN thin film contains more than 43 mol% of Sc with respect to the total number of atoms of Al and Sc, and the thickness of the ScAlN thin film is 100 nm or less. A ScAlN laminate characterized by this.

2. The ScAlN thin film contains 59 mol% or more of Sc with respect to the total number of atoms of Al and Sc. The ScAlN laminate according to Claim 1, characterized by this.

3. The ScAlN thin film contains 81.5 mol% or less of Sc with respect to the total number of atoms of Al and Sc. The ScAlN laminate according to Claim 1 or 2, characterized by this.

4. The ScAlN thin film contains 67 mol% or less of Sc with respect to the total number of atoms of Al and Sc. The ScAlN laminate according to Claim 1 or 2, characterized by this.

5. The nearest-neighbor atomic distance of the underlayer is 3.34 Å or more and 3.71 Å or less. The ScAlN laminate according to Claim 1 or 2, characterized by this.

6. The underlayer contains a <111>-oriented film of YN. The ScAlN laminate according to Claim 5, characterized by this.

7. Used in any one or more selected from the group consisting of a transistor, an inverter, a ferroelectric memory, and a MEMS device. The ScAlN laminate according to Claim 1 or 2, characterized by this.

8. A method for manufacturing a ScAlN laminate, including a step of forming an underlayer on a substrate and a step of forming a ScAlN thin film on the underlayer, wherein the nearest-neighbor atomic distance in the lattice plane parallel to the surface of the underlayer, which is the nearest-neighbor atomic distance, is shorter than the a-axis length of the ScAlN thin film, the ScAlN thin film contains more than 43 mol% of Sc with respect to the total number of atoms of Al and Sc, and the thickness of the ScAlN thin film is 100 nm or less. ​

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