Method for fabricating a piezoelectric layer and method for manufacturing a bulk acoustic wave filter using the same.

Abstract

This piezoelectric layer fabrication method comprises: a substrate preparation step for preparing a substrate (11) that is formed from a single crystal having a crystal structure with three-fold rotational symmetry or six-fold rotational symmetry around an axis perpendicular to a prescribed plane, and has said prescribed plane on a surface thereof; a sacrificial layer fabrication step for fabricating a sacrificial layer (13) on the substrate (11) by means of an epitaxial method; a piezoelectric layer fabrication step for fabricating an epitaxial zinc oxide piezoelectric layer (14) that is a layer of zinc oxide on the sacrificial layer (13) by means of the epitaxial method; and a sacrificial layer dissolution step for selectively dissolving the sacrificial layer (13). By fabricating an acoustic Bragg reflector (16) on the epitaxial zinc oxide piezoelectric layer (14) between the piezoelectric layer fabrication step and the sacrificial layer dissolution step, it is possible to manufacture a bulk acoustic wave filter (10).

Description

Technical Field

[0001] The present invention relates to a method for fabricating a piezoelectric layer that can be used in the manufacture of a bulk acoustic wave (BAW) filter, which is a frequency filter used in communication devices such as smartphones, and particularly to a method for fabricating a BAW filter of the solidly mounted resonator (SMR) type, and a method for manufacturing a BAW filter using the method.

Background Art

[0002] A BAW filter is a frequency filter that allows an electrical signal in a specific frequency band to pass through by utilizing the bulk vibration of a piezoelectric layer, which is a layer made of a piezoelectric material. As a frequency filter using a piezoelectric layer, a surface acoustic wave (SAW) filter that utilizes surface acoustic waves generated on the surface of the piezoelectric layer is also used. However, the BAW filter has the characteristics of higher voltage resistance than the SAW filter and can be used in a higher frequency band.

[0003] There are SMR-type and FBAR (Film Bulk Acoustic Resonator)-type BAW filters. The SMR-type BAW filter has a piezoelectric layer on an acoustic Bragg reflector formed by alternately laminating layers with different acoustic impedances, while the FBAR-type BAW filter has a piezoelectric layer on the upper surface including the cavity of a substrate provided with a cavity in part. Due to such a difference in structure, the SMR-type BAW filter has the characteristic of higher mechanical strength than the FBAR-type one.

[0004] Currently, SMR-type BAW filters, which are practically used as frequency filters for smartphones, generally consist of an acoustic Bragg reflector made by alternately stacking a low acoustic impedance layer made of silicon dioxide (SiO2) and a high acoustic impedance layer made of metals such as tungsten (W) or molybdenum (Mo), which have heavier atoms than the constituent elements of the low acoustic impedance layer, and a piezoelectric layer made of a piezoelectric material composed of numerous microcrystals whose predetermined crystal axes are oriented perpendicular to the layers. In order to handle even higher frequency bands in the future, there is a need for frequency filters that have a piezoelectric layer made of a piezoelectric material composed of polycrystalline or monocrystalline materials with high orientation precision.

[0005] Patent Document 1 describes an SMR-type BAW filter that includes a piezoelectric layer on an acoustic Bragg reflector, which is made by cutting a bulk single crystal of a piezoelectric material such as lithium tantalate (LiTaO3) or lithium niobate (LiNiO3) into a plate shape on a predetermined plane. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2021-153263 [Overview of the project] [Problems that the invention aims to solve]

[0007] When mass-producing SMR-type BAW filters, a laminate of a large-area acoustic Bragg reflector and piezoelectric layer is fabricated, and then this laminate is cut into numerous individual pieces. Therefore, in order to increase the production efficiency of SMR-type BAW filters, it is necessary to increase the area of ​​the laminate before cutting into individual pieces. However, in the SMR-type BAW filter described in Patent Document 1, there is a limit to the volume that can be fabricated using bulk single crystals such as LiTaO3 and LiNiO3, and currently, it is only possible to obtain a piezoelectric layer with a maximum diameter of about 6 inches. In other words, the laminate before cutting cannot be made larger than 6 inches in diameter.

[0008] Up to this point, the explanation has focused on SMR-type BAW filters, but similar problems arise when manufacturing devices with piezoelectric layers other than SMR-type BAW filters, such as when you want to efficiently produce a large number of elements by cutting them into individual pieces, or when you want to make the piezoelectric layer of the element itself cover a large area.

[0009] The problem that this invention aims to solve is to provide a method for obtaining a piezoelectric layer made of a polycrystalline or single-crystal piezoelectric material with high orientation accuracy over a large area. [Means for solving the problem]

[0010] The piezoelectric layer fabrication method according to the present invention, which was developed to solve the above problems, a) A substrate preparation step of preparing a substrate consisting of a single crystal having a crystal structure having 3-fold or 6-fold rotational symmetry about an axis perpendicular to a predetermined plane, and having the predetermined plane as its surface, b) A sacrificial layer fabrication step in which a sacrificial layer is fabricated on the substrate by an epitaxial method, c) A piezoelectric layer fabrication step of fabricating an epitaxial zinc oxide piezoelectric layer, which is a layer of zinc oxide, on the sacrificial layer by an epitaxial method, d) A sacrificial layer dissolution step for selectively dissolving the sacrificial layer, It has.

[0011] Generally, the epitaxial method is known as a method for fabricating a single-crystal or polycrystalline layer by depositing a film on a substrate made of a single crystal having a crystal structure similar to that of the target substance, so that the predetermined crystal planes of the target substance are aligned with the crystal planes of the single crystal on the surface of the substrate. In the case of single crystals, as well as when polycrystalline layers are formed, the predetermined crystal axes (axes perpendicular to the predetermined crystal planes) are aligned in one direction, resulting in high orientation accuracy. Furthermore, the epitaxial method allows for the fabrication of large-area piezoelectric layers by using large-area substrates. For example, substrates made of sapphire (Al2O3) single crystals with a diameter of 8 inches are used, and an 8-inch diameter piezoelectric layer can be fabricated using this substrate.

[0012] A piezoelectric layer made of single crystal or polycrystalline zinc oxide (ZnO), which is a piezoelectric material, can be fabricated not only directly on a single crystal substrate such as sapphire by epitaxial growth, but also on the single crystal substrate after epitaxial growth of a layer made of another material (referred to here as an "intervening layer") and then fabrication on the intervening layer by epitaxial growth.

[0013] In the case where an epitaxial zinc oxide piezoelectric layer is fabricated via an intervening layer, if the intervening layer is made of a material such as gold, the intervening layer can be dissolved, for example, by using an etching solution used in selective etching to selectively dissolve the material as the dissolving solution.

[0014] Therefore, in this invention, a sacrificial layer (one of the intervening layers made of a predetermined material) is fabricated on a substrate made of a single crystal having a crystal structure which will be described in detail later. Then, an epitaxial zinc oxide piezoelectric layer, which is a layer of zinc oxide, is fabricated on the sacrificial layer by an epitaxial method, and the sacrificial layer is dissolved. This separates the epitaxial zinc oxide piezoelectric layer from the substrate.

[0015] According to the present invention, since an epitaxial zinc oxide piezoelectric layer is fabricated by an epitaxial method, a piezoelectric layer made of polycrystalline or single-crystal piezoelectric material with higher orientation accuracy can be obtained over a large area compared to the case where a piezoelectric layer is fabricated by cutting a bulk single crystal.

[0016] The substrate used is a single crystal having a crystal structure with 3-fold or 6-fold rotational symmetry about an axis perpendicular to a predetermined plane, and having the predetermined plane on its surface. Examples of such crystal structures and predetermined planes include the (0001) plane of a trigonal or hexagonal crystal structure, and the (111) plane of a face-centered cubic lattice structure of a cubic crystal. If a sacrificial layer is epitaxially grown on a substrate made of a single crystal having such a surface, the sacrificial layer will also have a crystal structure with 3-fold or 6-fold rotational symmetry about an axis perpendicular to the surface. The material of the sacrificial layer is limited to a material that can be epitaxially grown on a substrate made of such a single crystal. Zinc oxide has a hexagonal crystal structure with 6-fold rotational symmetry about the c-axis, and can be epitaxially grown on a sacrificial layer having a crystal structure with 6-fold rotational symmetry, as well as on a sacrificial layer having a crystal structure with 3-fold rotational symmetry. The resulting epitaxial zinc oxide piezoelectric layer is a single crystal or polycrystalline layer with its c-axis oriented perpendicular to the layer.

[0017] Specifically, the substrate can be made of sapphire, silicon carbide (SiC), titanium (Ti), gallium phosphide (GaP), etc., having a trigonal or hexagonal crystal structure, or a substrate made of strontium titanate (SrTiO3), magnesium oxide (MgO), aluminum (Al), silicon (Si), germanium (Ge), gallium arsenide (GaAs), etc., having a cubic crystal structure.

[0018] In the piezoelectric layer manufacturing method according to the present invention, The sacrificial layer consists of a material that is dissolved by a predetermined dissolving solution that does not dissolve zinc oxide. In the sacrificial layer dissolution step, the sacrificial layer is selectively dissolved by the dissolution treatment liquid. Such a configuration can be adopted. As a result, even if the epitaxial zinc oxide piezoelectric layer comes into contact with the etching solution during the sacrificial layer dissolution process, the sacrificial layer can be dissolved without affecting the epitaxial zinc oxide piezoelectric layer. As such an etching solution, for example, an iodine-based etching solution is known. Note that using such an etching solution is not essential in the present invention. For example, the sacrificial layer dissolution process may be performed while protecting the side surface of the epitaxial zinc oxide piezoelectric layer with a protective layer, or measures may be taken to minimize the time the etching solution is exposed by immediately removing the etching solution when the epitaxial zinc oxide piezoelectric layer is separated from the substrate.

[0019] In the piezoelectric layer manufacturing method according to the present invention, Between the substrate preparation process and the sacrificial layer manufacturing process, a buffer layer made of a material having a crystal structure with three-fold rotational symmetry or six-fold rotational symmetry is formed on the surface of the substrate by an epitaxial method. In the sacrificial layer manufacturing process, the sacrificial layer is formed on the surface of the buffer layer. Such a configuration may be adopted. In this method, the sacrificial layer is formed on the substrate via the buffer layer.

[0020] By appropriately selecting the material of such a buffer layer, the crystallinity of the sacrificial layer is improved compared to the case where the sacrificial layer is directly formed on the surface of the substrate, and thereby the crystallinity of the epitaxial zinc oxide piezoelectric layer formed on the surface of the sacrificial layer is also improved. As an index of crystallinity, when the ω scan (2θ) is performed after fixing the value of 2θ when the peak of the (0002) plane is obtained by the 2θ-ω scan of the X-ray diffraction measurement for the epitaxial zinc oxide piezoelectric layer (piezoelectric film), a profile (rocking curve) having a peak with a full width at half maximum within the range of 0.4° to 0.8° is obtained. The smaller the width of the peak in this rocking curve, the better the crystallinity.

[0021] For example, when the substrate is made of sapphire and the sacrificial layer is made of gold, it is preferable to use a buffer layer made of platinum. Both gold and platinum have a face-centered cubic lattice structure. The distance between the nearest neighboring atoms in the crystal of gold 2 -1 / 2 a (a is the lattice constant of the cubic crystal) is smaller than the distance between the nearest neighboring atoms in the crystal of platinum 2 -1 / 2 a, and the distance between the nearest neighboring oxygen atoms in sapphire 2 -1 / 3 a (here, a is the lattice constant in the a-axis direction of the hexagonal crystal) is closer. Therefore, by interposing a buffer layer made of platinum, the lattice mismatch between the sapphire of the substrate and the gold of the sacrificial layer is less likely to occur, and the crystallinity of the epitaxial zinc oxide piezoelectric layer formed on the surface of the sacrificial layer is improved. As will be described later, in the epitaxial zinc oxide piezoelectric layer formed using these sapphire substrates, platinum buffer layers, and gold sacrificial layers, a high crystallinity is achieved such that the full width at half maximum of the rocking curve obtained for the (0002) plane of zinc oxide by X-ray diffraction measurement is within the range of 0.4 to 0.8°.

[0022] The piezoelectric layer manufacturing method according to the present invention can be suitably used in the manufacturing method of a BAW filter. That is, in the manufacturing method of a BAW filter using the present invention, an acoustic Bragg reflector manufacturing step of manufacturing an acoustic Bragg reflector on the epitaxial zinc oxide piezoelectric layer is performed between the piezoelectric layer manufacturing step and the sacrificial layer dissolution step in the piezoelectric layer manufacturing method according to the present invention.

[0023] According to the manufacturing method of a BAW filter using the present invention, a laminate in which an epitaxial zinc oxide piezoelectric layer and an acoustic Bragg reflector are laminated can be manufactured over a large area. Then, by cutting such a large-area laminate into individual pieces, a large number of SMR type BAW filters can be efficiently manufactured.

[0024] Furthermore, acoustic Bragg reflectors do not need to be fabricated by the epitaxial method. In addition, to prevent the acoustic Bragg reflector from dissolving during the sacrificial layer dissolution process, it is advisable to use a material for the acoustic Bragg reflector that does not dissolve in the dissolution solution, to apply a protective film to the surface of the acoustic Bragg reflector that does not dissolve in the dissolution solution, or to supply the dissolution solution only to the sacrificial layer and its vicinity so as not to come into contact with the acoustic Bragg reflector.

[0025] When a single crystal or polycrystalline zinc oxide is grown on a sacrificial layer by an epitaxial method according to the method of the present invention, oxygen atoms are located on the surface facing the sacrificial layer, and zinc atoms are located on the surface opposite the sacrificial layer. In a BAW filter manufactured using the method of the present invention, an acoustic Bragg reflector is fabricated on the epitaxial zinc oxide piezoelectric layer after the epitaxial zinc oxide piezoelectric layer is fabricated on the sacrificial layer. As a result, zinc atoms are located on the surface facing the acoustic Bragg reflector, and oxygen atoms are located on the surface opposite the acoustic Bragg reflector.

[0026] In other words, a bulk acoustic wave filter (BAW filter) fabricated using the method of the present invention is Acoustic Bragg reflector, A layer provided on the acoustic Bragg reflector, comprising a single crystal or polycrystalline zinc oxide oriented perpendicular to the c-axis with respect to the layer, wherein zinc atoms are located on the surface facing the acoustic Bragg reflector and oxygen atoms are located on the surface opposite to the acoustic Bragg reflector, and an epitaxial zinc oxide piezoelectric layer. It is equipped with.

[0027] Furthermore, as described above, the method of the present invention yields an epitaxial zinc oxide piezoelectric layer having high crystallinity, such that the half-width at half maximum of the rocking curve obtained with respect to the (0002) plane of zinc oxide by X-ray diffraction measurement is in the range of 0.4 to 0.8°. Therefore, a BAW filter fabricated using the method of the present invention is Acoustic Bragg reflector, A layer provided on the acoustic Bragg reflector, comprising a single crystal or polycrystalline zinc oxide oriented perpendicular to the c-axis with respect to the layer, wherein the epitaxial zinc oxide piezoelectric layer has a full width at half maximum of the peak of the rocking curve obtained with respect to the (0002) plane of zinc oxide by X-ray diffraction measurement in the range of 0.4° to 0.8°. It is equipped with.

[0028] In the explanations so far, for convenience, expressions such as "on the substrate," "on the sacrificial layer," "on the epitaxial zinc oxide piezoelectric layer," and "on the acoustic Bragg reflector" have been used to indicate the relative positions of each component. However, these do not limit the orientation of the piezoelectric layer or BAW filter during fabrication or use. [Effects of the Invention]

[0029] According to the present invention, polycrystalline or single-crystal epitaxial zinc oxide piezoelectric layers with high orientation accuracy can be obtained over a large area. [Brief explanation of the drawing]

[0030] [Figure 1A] A longitudinal cross-sectional view showing a substrate preparation step in one embodiment of a method for manufacturing an SMR-type BAW filter using the piezoelectric layer fabrication method according to the present invention. [Figure 1B] A longitudinal cross-sectional view showing the state after the buffer layer has been fabricated in the manufacturing method of the SMR type BAW filter of this embodiment. [Figure 1C] A longitudinal cross-sectional view showing the state after the sacrificial layer has been fabricated in the manufacturing method of the SMR type BAW filter of this embodiment. [Figure 1D] A longitudinal cross-sectional view showing the state after the piezoelectric layer has been fabricated in the manufacturing method of the SMR type BAW filter of this embodiment. [Figure 1E] This is a vertical cross-sectional view showing the state after the first electrode layer has been fabricated in the manufacturing method of the SMR type BAW filter of this embodiment. [Figure 1F] A vertical cross-sectional view showing the state in which an acoustic Bragg reflector has been fabricated in the manufacturing method of the SMR type BAW filter of this embodiment. [Figure 1G]A longitudinal cross-sectional view showing the state after the protective layer has been fabricated in the manufacturing method of the SMR type BAW filter of this embodiment. [Figure 1H] A longitudinal cross-sectional view showing the sacrificial layer dissolution step in the manufacturing method of the SMR type BAW filter of this embodiment. [Figure 1I] This is a vertical cross-sectional view showing a second intermediate laminate (an SMR-type BAW filter in which the second electrode layer has not yet been fabricated) separated from the substrate and buffer layer by the sacrificial layer dissolution step in the manufacturing method of the SMR-type BAW filter of this embodiment. [Figure 2] A longitudinal cross-sectional view showing an SMR-type BAW filter manufactured by the manufacturing method of this embodiment. [Figure 3] Scanning electron microscope image of a longitudinal section of the second intermediate laminate in an SMR-type BAW filter manufactured by the manufacturing method of this embodiment. [Figure 4A] This graph shows the results of X-ray diffraction measurements performed using a 2θ-ω scan on the epitaxial zinc oxide piezoelectric layer of the SMR-type BAW filter of Fabrication Example 1, which was manufactured using the manufacturing method of this embodiment. [Figure 4B] This graph shows the results of ω-scan rocking curve (with 2θ fixed) measurements performed on the epitaxial zinc oxide piezoelectric layer fabricated in Fabrication Example 1. [Figure 4C] This graph shows the results of measuring the intensity of X-ray diffraction by the (10-11) plane while rotating the epitaxial zinc oxide piezoelectric layer fabricated in Fabrication Example 1 around an axis perpendicular to the layer. [Figure 4D] (10-11) plane pole diagram obtained by X-ray diffraction for the epitaxial zinc oxide piezoelectric layer fabricated in Fabrication Example 1. [Figure 5A] A transmission electron microscope image of the second intermediate layer fabricated in Fabrication Example 1. [Figure 5B] A transmission electron microscope image taken at magnified magnification within the imaging area of ​​the epitaxial zinc oxide piezoelectric layer fabricated in Fabrication Example 1. [Figure 5C] Electron diffraction pattern obtained in the imaging area of ​​the transmission electron microscope image in Figure 5B. [Figure 6]This graph shows the measured frequency characteristics of the SMR-type BAW filter fabricated in Fabrication Example 1. [Figure 7A] A longitudinal cross-sectional view showing the state after the first electrode layer and acoustic Bragg reflector have been fabricated in Fabrication Example 2. [Figure 7B] A longitudinal cross-sectional view showing the state in which the epitaxial zinc oxide piezoelectric layer and the first electrode layer are bonded together in fabrication example 2. [Figure 7C] A longitudinal cross-sectional view showing the configuration of the SMR-type BAW filter obtained in example 2. [Figure 8A] This graph shows the results of X-ray diffraction measurements performed using a 2θ-ω scan on the epitaxial zinc oxide piezoelectric layer in the SMR-type BAW filter of Fabrication Example 2. [Figure 8B] This graph shows the results of ω-scan rocking curve measurements performed on the epitaxial zinc oxide piezoelectric layer fabricated in Fabrication Example 2. [Figure 8C] This graph shows the results of measuring the intensity of X-ray diffraction by the (10-11) plane while rotating the epitaxial zinc oxide piezoelectric layer fabricated in Fabrication Example 2 around an axis perpendicular to the layer. [Figure 9] This graph shows the measured frequency characteristics of the SMR-type BAW filter fabricated in Fabrication Example 2. [Figure 10A] A longitudinal cross-sectional view showing the sacrificial layer fabrication process in a modified SMR-type BAW filter manufacturing method. [Figure 10B] A longitudinal cross-sectional view showing the piezoelectric layer fabrication step in a modified SMR type BAW filter manufacturing method. [Figure 11] This graph shows the results of X-ray diffraction measurements performed using a 2θ-ω scan with the sacrificial layer as the surface for this embodiment and its modified form. [Modes for carrying out the invention]

[0031] Using Figures 1 to 11, an embodiment of the piezoelectric layer fabrication method according to the present invention will be explained, focusing on an embodiment of a method for manufacturing an SMR-type BAW filter using this method.

[0032] (1) One embodiment of a method for manufacturing an SMR type BAW filter First, an embodiment of the manufacturing method for an SMR type BAW filter will be described using Figure 1. Initially, a substrate 11 is prepared (Figure 1A: Substrate preparation process). The substrate 11 is made by cutting a single crystal of sapphire, a material having a hexagonal crystal structure, into a plate shape so that the (0001) plane of the single crystal becomes the surface.

[0033] Next, a buffer layer 12 made of platinum is fabricated on the surface of the substrate 11 by an epitaxial method (Figure 1B), and then a sacrificial layer 13 made of gold is fabricated on the surface of the buffer layer 12 by an epitaxial method (Figure 1C: Sacrificial layer fabrication process). Here, the buffer layer 12 is fabricated on the surface of the substrate 11 by an epitaxial method, inheriting the crystal structure of the surface of the substrate 11, and the platinum atoms are arranged to have 6-fold rotational symmetry around an axis perpendicular to the surface of the substrate 11 (also satisfying 3-fold rotational symmetry). However, the crystal structure of platinum is a face-centered cubic structure, and the surface of the buffer layer 12 is a (111) plane. Similarly, the sacrificial layer 13 is fabricated on the surface of the buffer layer 12 by an epitaxial method, inheriting the crystal structure of the buffer layer 12, and the gold atoms are arranged to have 6-fold rotational symmetry around an axis perpendicular to the surface of the buffer layer 12 (also satisfying 3-fold rotational symmetry). The crystal structure of gold is also a face-centered cubic structure, and the surface of the sacrificial layer 13 is a (111) plane.

[0034] Next, an epitaxial zinc oxide piezoelectric layer 14 made of zinc oxide (ZnO) is fabricated on the surface of the sacrificial layer 13 by an epitaxial method (Figure 1D: Piezoelectric layer fabrication process). The epitaxial zinc oxide piezoelectric layer 14 inherits the crystal structure of the sacrificial layer 13, which has six rotational symmetry around an axis perpendicular to the surface of the layer, and grows as a single crystal or polycrystal with a hexagonal wurtzite-type crystal structure in which its (0001) plane is parallel to the surface of the sacrificial layer 13 (the c-axis is perpendicular to the surface). As a result, the epitaxial zinc oxide piezoelectric layer 14 becomes a single crystal with its (0001) plane oriented parallel to the epitaxial zinc oxide piezoelectric layer 14 (the c-axis is perpendicular to the layer), or a polycrystal oriented with high precision in that direction. Here, oxygen atoms are located on the surface of the epitaxial zinc oxide piezoelectric layer 14 that is on the sacrificial layer 13 side, and zinc atoms are located on the surface opposite to the sacrificial layer 13. The thickness of the epitaxial zinc oxide piezoelectric layer 14 is determined so that resonance occurs due to vibration in the thickness direction at frequencies included in the frequency band to be filtered. If the thickness of the epitaxial zinc oxide piezoelectric layer 14 is less than 100 nm, the resonance frequency will be 30 GHz or higher, resulting in large acoustic attenuation, and a so-called sharp resonance cannot be obtained. On the other hand, if the thickness of the epitaxial zinc oxide piezoelectric layer 14 exceeds 5000 nm, there is a risk of damage due to stress. Therefore, the thickness of the epitaxial zinc oxide piezoelectric layer 14 is preferably within the range of 100 to 5000 nm.

[0035] The buffer layer 12, sacrificial layer 13, and epitaxial zinc oxide piezoelectric layer 14, as described above, can be fabricated using, for example, an RF (radio frequency) magnetron sputtering apparatus or a DC (direct current) magnetron sputtering apparatus. Examples of fabrication conditions when using an RF magnetron sputtering apparatus will be described later.

[0036] Next, a first electrode layer 15 made of platinum is fabricated on the surface of the epitaxial zinc oxide piezoelectric layer 14 (Figure 1E). The first electrode layer 15 does not need to be epitaxially grown and can be fabricated by, for example, a conventional vapor deposition method.

[0037] Next, an acoustic Bragg reflector 16 is fabricated by alternately stacking multiple layers of a first acoustic impedance layer 161 made of silicon dioxide (SiO2) and a second acoustic impedance layer 162 made of molybdenum (Mo) on the surface of the first electrode layer 15 (Figure 1F: Acoustic Bragg reflector fabrication process). In this embodiment, two layers each of the first acoustic impedance layer 161 and the second acoustic impedance layer 162 are fabricated, but one layer each, or three or more layers each may be fabricated. The thickness of the first acoustic impedance layer 161 and the second acoustic impedance layer 162 is set to be half the wavelength of a sound wave having the resonance frequency generated by the epitaxial zinc oxide piezoelectric layer 14. These first acoustic impedance layers 161 and the second acoustic impedance layer 162 do not need to be fabricated by the epitaxial method.

[0038] In this embodiment, silicon dioxide, used as the material for the first acoustic impedance layer 161, has been widely used as the material for one of the acoustic impedance layers in acoustic Bragg reflectors, but it is amorphous (glass) and cannot be fabricated by the epitaxial method. Therefore, it is not possible to fabricate an epitaxial zinc oxide piezoelectric layer on the surface of an acoustic Bragg reflector containing such amorphous silicon dioxide as a substrate. For this reason, in this embodiment, a separate substrate 11 is prepared, an epitaxial zinc oxide piezoelectric layer 14 is fabricated on it, and then the acoustic Bragg reflector 16 is fabricated on it.

[0039] After the acoustic Bragg reflector 16 is fabricated, a protective layer 17 made of silicon dioxide and thinner than the first acoustic impedance layer 161 is fabricated on its surface (in this embodiment, the surface of the second acoustic impedance layer 162) and sides (Figure 1G). The protective layer 17 plays a role in protecting the second acoustic impedance layer 162, which is made of molybdenum, in the sacrificial layer dissolution process described below.

[0040] Next, the intermediate laminate 100, which is laminated from the substrate 11 to the protective layer 17, is immersed in a dissolution solution 19 consisting of an iodine-based etching solution known to selectively dissolve gold, thereby dissolving the sacrificial layer 13 (Figure 1H: Sacrificial layer dissolution process). As a result, the second intermediate laminate 101, which consists of the epitaxial zinc oxide piezoelectric layer 14, the first electrode layer 15, the acoustic Bragg reflector 16, and the protective layer 17, is separated from the substrate 11 and the buffer layer 12 with little effect on the layers other than the sacrificial layer 13 (Figure 1I). The dissolution solution 19 also has the ability to dissolve Mo, which is the material of the second acoustic impedance layer 162, but as described above, the surface and sides of the second acoustic impedance layer 162 are covered with the protective layer 17, so the dissolution of the second acoustic impedance layer 162 can be prevented.

[0041] Finally, an SMR-type BAW filter 10 is obtained by fabricating a second electrode layer 18 made of gold on the surface of the epitaxial zinc oxide piezoelectric layer 14 (the surface opposite to the surface in contact with the first electrode layer 15) (Figure 2). The second electrode layer 18 does not need to be fabricated by the epitaxial method. In the obtained SMR-type BAW filter 10, zinc atoms are located on the surface of the epitaxial zinc oxide piezoelectric layer 14 on the acoustic Bragg reflector 16 side (first electrode layer 15 side), and oxygen atoms are located on the surface opposite the acoustic Bragg reflector 16 (second electrode layer 18 side).

[0042] The materials of the substrate 11, buffer layer 12, sacrificial layer 13, acoustic Bragg reflector 16, protective layer 17, first electrode layer 15, and second electrode layer 18 described above are not limited to the examples above. Depending on the combination of materials for the substrate 11 and the sacrificial layer 13, a sacrificial layer 13 with sufficiently good crystallinity may be obtained even if it is fabricated directly on the substrate 11, in which case the buffer layer 12 may be omitted. In addition, the sacrificial layer 13 may be dissolved using a dissolution solution 19 different from the iodine-based etching solution, and if a material other than gold is used for the sacrificial layer 13, a dissolution solution appropriate to that material should be selected. If the dissolution solution does not affect the acoustic Bragg reflector 16, the protective layer 17 may be omitted.

[0043] For example, instead of sapphire, the substrate 11 can be made of various materials having a crystal structure with 3-fold or 6-fold rotational symmetry around an axis perpendicular to a predetermined plane, such as other materials having a trigonal or hexagonal crystal structure, or materials having a cubic face-centered cubic crystal structure. Specifically, substrates made of silicon carbide (SiC), strontium titanate (SrTiO3), magnesium oxide (MgO), silicon (Si), etc., can be used.

[0044] The material for the sacrificial layer 13 can be metals other than gold that have a crystalline structure with 3-fold or 6-fold rotational symmetry, such as aluminum (Al), tungsten (W), tantalum (Ta), molybdenum (Mo), niobium (Nb), chromium (Cr), vanadium (V), zirconium (Zr), titanium (Ti), nickel (Ni), silver (Ag), palladium (Pd), or alloys thereof, or alloys of these metals with platinum (Pt). These metals and alloys can also be dissolved with an iodine-based etching solution.

[0045] The first acoustic impedance layer 161 and the second acoustic impedance layer 162 can be made by any combination of two materials with different densities and / or sound velocities. As mentioned above, silicon dioxide (SiO2) is widely used for one of the acoustic impedance layers, in which case tungsten is widely used for the other acoustic impedance layer in addition to molybdenum used in the above embodiment.

[0046] Any conductive material can be used for the first electrode layer 15 and the second electrode layer 18.

[0047] (2) Example of an SMR type BAW filter fabricated by the method of this embodiment (2-1) Example 1 Next, an example of an SMR-type BAW filter fabricated by the method of this embodiment will be described. In this fabrication example 1, the buffer layer 12, the epitaxial zinc oxide piezoelectric layer 14, the first acoustic impedance layer 161, and the second acoustic impedance layer 162 were fabricated using an RF magnetron sputtering apparatus. The sacrificial layer 13 was fabricated using a DC magnetron sputtering apparatus. The fabrication conditions for each layer are as follows.

[0048] (Buffer layer 12) A platinum target was placed on the cathode of an RF magnetron sputtering apparatus, and a substrate 11 was positioned opposite the target at a distance of 30 mm. Film deposition was carried out at a temperature of 600°C for 2 minutes in an argon gas atmosphere at a pressure of 0.5 Pa while applying 80 W of high-frequency power until a thickness of 100 nm was achieved.

[0049] (Sacrifice layer 13) A gold target was placed on the cathode of a DC magnetron sputtering apparatus, and a buffer layer 12 was positioned opposite the target at a distance of 15 mm. A DC power of 350 V and 150 mA was applied in an argon gas atmosphere at a pressure of less than 1 Pa, and the film was deposited at a temperature of 300°C for 5 minutes until a thickness of 200 nm was achieved.

[0050] (Epitaxial zinc oxide piezoelectric layer 14) A zinc oxide target was placed on the cathode of an RF magnetron sputtering apparatus, and a sacrificial layer 13 was positioned opposite the target at a distance of 30 mm. The film was deposited at a temperature of 500°C for 40 minutes while applying 150 W of high-frequency power in an atmosphere of mixed argon and oxygen gas (partial pressure ratio of argon 3 to oxygen 1) at a pressure of 1 Pa, until a thickness of 1.8 μm was achieved.

[0051] (First acoustic impedance layer 161) A silicon dioxide target was placed on the cathode of an RF magnetron sputtering apparatus, and a first electrode layer 15 or a second acoustic impedance layer 162 was positioned opposite the target at a distance of 30 mm. A film was deposited for 80 minutes without heating while applying 150 W of high-frequency power in an atmosphere of argon and oxygen mixed gas (partial pressure ratio the same as above) at a pressure of 0.2 Pa, until a thickness of 1.0 μm was achieved.

[0052] (Second acoustic impedance layer 162) A molybdenum target was placed on the cathode of an RF magnetron sputtering apparatus, and a first acoustic impedance layer 161 was positioned opposite the target at a distance of 30 mm. The film was deposited in an argon gas atmosphere at a pressure of 0.5 Pa while applying 100 W of high-frequency power without heating for 12 minutes until a thickness of 0.85 μm was achieved.

[0053] Figure 3 shows a scanning electron microscope (SEM) image of a longitudinal section of the fabricated second intermediate laminate 101 (i.e., the SMR-type BAW filter 10 in a state where the second electrode layer 18 has not yet been fabricated). The epitaxial zinc oxide piezoelectric layer 14 appears as a plate-like single crystal with no visible grain boundaries. No other layers are present on the upper side of the epitaxial zinc oxide piezoelectric layer 14 (opposite the first electrode layer 15 side), indicating that the sacrificial layer 13 has been selectively dissolved and the substrate 11 and buffer layer 12 have been removed. The second acoustic impedance layer 162 is polycrystalline with visible grain boundaries, while the first acoustic impedance layer 161 is amorphous, although this cannot be determined from Figure 3.

[0054] Next, X-ray diffraction measurements were performed on the epitaxial zinc oxide piezoelectric layer 14. The X-rays used for the measurement were CuKα rays (wavelength 1.54 Å). First, a 2θ-ω scan was performed, where ω was the incident angle of the incident X-rays on the surface of the epitaxial zinc oxide piezoelectric layer 14, and 2θ was the angle between the incident X-rays and the emitted X-rays detected by the detector. The scan was performed while maintaining ω=θ and varying ω and 2θ. As a result, a profile with a peak around 2θ=34.2° was obtained, as shown in Figure 4A. This peak corresponds to the X-ray diffraction peak at the (0002) plane of zinc oxide.

[0055] Therefore, we fixed 2θ at 34.2° and performed an ω scan, varying only ω. As a result, as shown in Figure 4B, a profile was obtained with a peak around ω = 17.1° with a full width at half maximum of 0.80°. A profile obtained by an ω scan in this way is called a "rocking curve". The presence of a peak in the ω scan indicates that the epitaxial zinc oxide piezoelectric layer 14 is either a single crystal or a polycrystalline material with aligned (0002) planes (note that if it is a non-oriented polycrystalline material with misaligned (0002) planes, X-rays will be detected in the ω scan regardless of the value of ω).

[0056] Next, with 2θ fixed at 36.276° (corresponding to the diffraction peak in the (10-11) plane of zinc oxide) and ω at 18.138°, X-ray diffraction measurements were performed while rotating the epitaxial zinc oxide piezoelectric layer 14, whose (0001) plane was oriented parallel to the substrate (c axis perpendicular to the substrate), around an axis perpendicular to the layer. As shown in Figure 4C, a profile with a peak was obtained every 60° of rotation angle φ. These peaks correspond to the diffraction peak in the (10-11) plane of zinc oxide, indicating that the epitaxial zinc oxide piezoelectric layer 14 is a single crystal with six-fold rotational symmetry, or a polycrystalline material with aligned orientations in the plane parallel to the layer. Furthermore, the pole figure measurement results shown in Figure 4D also show similar six-fold rotational symmetry.

[0057] A transmission electron microscope (TEM) image of the fabricated second intermediate laminate 101 was taken. In the TEM image of the second intermediate laminate 101 shown in Figure 5A, an electron beam was incident from the <1-100> direction of the zinc oxide crystal near measurement point 141 within the epitaxial zinc oxide piezoelectric layer 14, and a higher magnification TEM image was taken. As a result, as shown in Figure 5B, although some transition was observed, it was confirmed that the atoms were aligned in a straight line. Furthermore, when an electron diffraction pattern was obtained at measurement point 21, as shown in Figure 5C, only diffraction points corresponding to the hexagonal crystal structure were clearly visible. These measurement results indicate that a good epitaxial zinc oxide piezoelectric layer 14 has been obtained.

[0058] Next, Figure 6 shows the results of measuring the frequency characteristics of the fabricated SMR-type BAW filter 10. In this measurement, a network analyzer was used to measure the real part of the acoustic impedance of the epitaxial zinc oxide piezoelectric layer 14 at each frequency. From these measurement results, it was confirmed that the epitaxial zinc oxide piezoelectric layer 14 resonates at a resonant frequency of approximately 1.3 GHz.

[0059] (2-2) Example 2 In Fabrication Example 2, a buffer layer 12, a sacrificial layer 13, and an epitaxial zinc oxide piezoelectric layer 14 were first fabricated on the substrate 11 using the epitaxial method under the same conditions as in Fabrication Example 1. However, the thickness of the epitaxial zinc oxide piezoelectric layer 14 was set to 1.0 μm.

[0060] Separately, a laminate of the acoustic Bragg reflector 26 and the first electrode layer 15 was fabricated by sputtering (Figure 7A). The acoustic Bragg reflector 26 consists of two alternating layers of a first acoustic impedance layer 261 made of silicon dioxide and a second acoustic impedance layer 262 made of tungsten. The thickness of each layer is 100 nm for the first electrode layer 15, 725 nm per layer for the first acoustic impedance layer 261, and 655 nm per layer for the second acoustic impedance layer 262.

[0061] Next, the epitaxial zinc oxide piezoelectric layer 14 and the first electrode layer 15 were bonded together with a 10 nm thick adhesive layer 21 made of polycrystalline titanium (Figure 7B). Then, the sacrificial layer 13 was dissolved. Subsequently, a second electrode layer 18 made of gold was fabricated on the surface of the epitaxial zinc oxide piezoelectric layer 14 (on the side opposite to the first electrode layer 15) to obtain an SMR type BAW filter 20 (Figure 7C).

[0062] X-ray diffraction measurements were performed on the epitaxial zinc oxide piezoelectric layer 14 before the fabrication of the second electrode layer 18. In the 2θ-ω scan, a profile was obtained with a peak around 2θ = 34.2°, corresponding to the X-ray diffraction peak at the (0002) plane of zinc oxide, as shown in Figure 8A. The peak labeled "(111)Pt" in Figure 8A is the peak due to platinum in the first electrode layer 15. Next, when the ω scan was performed with 2θ fixed at 34.2°, a rocking curve was obtained with a peak around ω = 17.1° with a full width at half maximum of 0.43° (Figure 8B). Furthermore, when the epitaxial zinc oxide piezoelectric layer 14 was rotated around an axis perpendicular to the layer while 2θ was fixed at 36.276° and ω at 18.138°, an X-ray diffraction measurement was performed, and a profile with a peak was obtained every 60° of rotation angle φ, as shown in Figure 8C.

[0063] When the frequency characteristics of the fabricated SMR-type BAW filter 20 were measured using a network analyzer, it was confirmed that the epitaxial zinc oxide piezoelectric layer 14 resonated at a resonant frequency of approximately 2.1 GHz (Figure 9). In the second fabrication example, the thickness of the epitaxial zinc oxide piezoelectric layer 14 was reduced compared to the first fabrication example, resulting in a higher resonant frequency.

[0064] (3) Variations The present invention is not limited to the embodiments described above, and various modifications are possible.

[0065] For example, in the above embodiment, a buffer layer 12 was fabricated on the surface of the substrate 11 by an epitaxial method (Figure 1B), a sacrificial layer 13 was fabricated on the surface of the buffer layer 12 by an epitaxial method (Figure 1C), and then an epitaxial zinc oxide piezoelectric layer 14 was fabricated on the surface of the sacrificial layer 13 (Figure 1D). Alternatively, the sacrificial layer 13 may be fabricated directly on the surface of the substrate 11 by an epitaxial method (Figure 10A), and then the epitaxial zinc oxide piezoelectric layer 14 may be fabricated on the surface of the sacrificial layer 13 (Figure 10B). Even when the epitaxial zinc oxide piezoelectric layer 14 is fabricated by this method, the sacrificial layer 13 and the substrate 11 are removed by dissolving the sacrificial layer 13 in the sacrificial layer removal process, so the final epitaxial zinc oxide piezoelectric layer 14 and SMR type BAW filter 10 will have the same configuration as in the above embodiment.

[0066] However, when gold is used as the material for the sacrificial layer 13, even if the sacrificial layer 13 made of gold is fabricated by the epitaxial method as in this modified example, the gold atoms will be arranged to have 6 rotational symmetry. However, it is preferable to interpose a buffer layer 12 made of platinum as in the above embodiment, because this improves the crystallinity of the gold in the sacrificial layer 13 (the arrangement of atoms becomes more precise). This is because the distance between the nearest neighbor atoms in the gold crystal is 2 -1 / 2 The distance between nearest neighbor atoms in a platinum crystal is greater than a (where a is the lattice constant of the cubic crystal). -1 / 2 In case a, the distance between the nearest oxygen atoms in the sapphire is 2 -1 / 3 Since it is close to a (here, the lattice constant in the a-axis direction of the hexagonal crystal), it is thought that interposing the buffer layer 12 makes it less likely for lattice mismatch to occur.

[0067] In these embodiments and modified examples, when X-ray diffraction measurements were performed using a 2θ-ω scan with the sacrificial layer 13 stacked (Figure 1C in this embodiment, and Figure 10A in the modified example), as shown in Figure 11, the intensity of the diffraction peak from the sacrificial layer 13 (the (111) plane of gold), normalized by the intensity of the diffraction peak from the substrate 11 (the (0006) plane of Al2O3 = sapphire), was more than 10 times greater in this embodiment than in the modified example. This result means that the crystallinity of the gold sacrificial layer 13 was improved by interposing the platinum buffer layer 12.

[0068] Here, we have described the case where the substrate 11 is made of sapphire and the sacrificial layer 13 is made of gold. However, even when the materials of the substrate 11 and the sacrificial layer 13 are made of other combinations, if there is a material that has 3-fold or 6-fold rotational symmetry on a predetermined crystal plane and whose lattice constant is closer to that of the substrate 11 than that of the sacrificial layer 13, it is advisable to fabricate a buffer layer 12 using such a material.

[0069] The embodiments of the SMR-type BAW filter and its manufacturing method according to the present invention have been described above, including modifications. However, the present invention is not limited to these embodiments and modifications, and further modifications are possible within the scope of the spirit of the present invention.

[0070] [Pattern] It will be obvious to those skilled in the art that the exemplary embodiments described above are specific examples of the following embodiments.

[0071] (Section 1) One aspect of the present invention is the piezoelectric layer manufacturing method according to Section 1, a) A substrate preparation step of preparing a substrate consisting of a single crystal having a crystal structure having 3-fold or 6-fold rotational symmetry about an axis perpendicular to a predetermined plane, and having the predetermined plane as its surface, b) A sacrificial layer fabrication step in which a sacrificial layer is fabricated on the substrate by an epitaxial method, c) A piezoelectric layer fabrication step of fabricating an epitaxial zinc oxide piezoelectric layer, which is a layer of zinc oxide, on the sacrificial layer by an epitaxial method, d) A sacrificial layer dissolution step for selectively dissolving the sacrificial layer, It has.

[0072] (Paragraph 2) The piezoelectric layer manufacturing method relating to Paragraph 2 is the piezoelectric layer manufacturing method relating to Paragraph 1, The sacrificial layer consists of a material that is dissolved by a predetermined dissolving solution that does not dissolve zinc oxide. In the sacrificial layer dissolution step, the sacrificial layer is selectively dissolved by the dissolution treatment liquid.

[0073] (Paragraph 3) The piezoelectric layer manufacturing method relating to Paragraph 3 is, in the piezoelectric layer manufacturing method relating to Paragraph 1 or Paragraph 2, Between the substrate preparation step and the sacrificial layer fabrication step, a buffer layer made of a material having a crystalline structure with 3-fold or 6-fold rotational symmetry is fabricated on the surface of the substrate by an epitaxial method. In the sacrificial layer fabrication step, the sacrificial layer is fabricated on the surface of the buffer layer.

[0074] (Paragraph 4) The piezoelectric layer manufacturing method relating to Paragraph 4 is, in the piezoelectric layer manufacturing method relating to Paragraph 3, The substrate is made of sapphire, the sacrificial layer is made of gold, and the buffer layer is made of platinum.

[0075] (Clause 5) Another aspect of the present invention, a bulk acoustic wave filter manufacturing method according to Clause 5, is an acoustic Bragg reflector manufacturing step of manufacturing an acoustic Bragg reflector on the epitaxial zinc oxide piezoelectric layer between the piezoelectric layer manufacturing step and the sacrificial layer dissolution step in the piezoelectric layer manufacturing method according to any one of Clauses 1 to 4.

[0076] (Clause 6) Another aspect of the present invention, the bulk acoustic wave filter according to Clause 6, Acoustic Bragg reflector, A layer provided on the acoustic Bragg reflector, comprising a single crystal or polycrystalline zinc oxide oriented perpendicular to the c-axis with respect to the layer, wherein zinc atoms are located on the surface facing the acoustic Bragg reflector and oxygen atoms are located on the surface opposite to the acoustic Bragg reflector, and an epitaxial zinc oxide piezoelectric layer. It is equipped with.

[0077] (Section 7) Another aspect of the present invention, the bulk acoustic wave filter according to Section 7, Acoustic Bragg reflector, A layer provided on the acoustic Bragg reflector, comprising a single crystal or polycrystalline zinc oxide oriented perpendicular to the c-axis with respect to the layer, wherein the epitaxial zinc oxide piezoelectric layer has a full width at half maximum of the peak of the rocking curve obtained with respect to the (0002) plane of zinc oxide by X-ray diffraction measurement in the range of 0.4° to 0.8°. It is equipped with. [Explanation of Symbols]

[0078] 10, 20... SMR type BAW filter 100…Intermediate laminate 101...Second intermediate layer 11… Circuit board 12... Buffer layer 13… Sacrificial layer 14…Epitaxial zinc oxide piezoelectric layer 15...First electrode layer 16, 26... Acoustic Bragg reflector 161, 261…First acoustic impedance layer 162, 262... Second acoustic impedance layer 17...Protective layer 18...Second electrode layer 19… Dissolving treatment solution 21...adhesive layer

Claims

1. a) A substrate preparation step of preparing a substrate consisting of a single crystal having a crystal structure having 3-fold or 6-fold rotational symmetry about an axis perpendicular to a predetermined plane, and having the predetermined plane as its surface, b) A sacrificial layer fabrication step in which a sacrificial layer is fabricated on the substrate by an epitaxial method, c) A piezoelectric layer fabrication step of fabricating an epitaxial zinc oxide piezoelectric layer, which is a layer of zinc oxide, on the sacrificial layer by an epitaxial method, d) A sacrificial layer dissolution step for selectively dissolving the sacrificial layer, It has, Between the substrate preparation step and the sacrificial layer fabrication step, a buffer layer made of a material having a crystalline structure with 3-fold or 6-fold rotational symmetry is fabricated on the surface of the substrate by an epitaxial method. In the sacrificial layer fabrication step, the sacrificial layer is fabricated on the surface of the buffer layer. Method for fabricating a piezoelectric layer.

2. The sacrificial layer consists of a material that is dissolved by a predetermined dissolving solution that does not dissolve zinc oxide. In the sacrificial layer dissolution step, the sacrificial layer is selectively dissolved by the dissolution treatment liquid. The method for producing a piezoelectric layer according to claim 1.

3. The piezoelectric layer manufacturing method according to claim 1, wherein the substrate is made of sapphire, the sacrificial layer is made of gold, and the buffer layer is made of platinum.

4. A method for manufacturing a bulk acoustic wave filter, wherein, in the piezoelectric layer manufacturing step according to any one of claims 1 to 3, an acoustic Bragg reflector manufacturing step is performed between the piezoelectric layer manufacturing step and the sacrificial layer dissolution step to manufacture an acoustic Bragg reflector on the epitaxial zinc oxide piezoelectric layer.

Images