JP2020092322A - Piezoelectric film and manufacturing method thereof, piezoelectric device, resonator, filter and multiplexer - Google Patents

Abstract

To improve piezoelectricity.SOLUTION: A piezoelectric film according to the present invention includes a plurality of crystal grains 44 containing a piezoelectric material, and the average crystal grain size in the central region in the thickness direction is larger than the average crystal grain size in regions on both sides of the central region in the thickness direction.SELECTED DRAWING: Figure 4

Description

The present invention relates to a piezoelectric film and a method for manufacturing the same, a piezoelectric device, a resonator, a filter, and a multiplexer.

Piezoelectric films are used for piezoelectric devices such as piezoelectric thin film resonators and acoustic wave devices. It is known that an aluminum nitride film is used as a piezoelectric film and scandium is added to the aluminum nitride film to improve the piezoelectricity of the piezoelectric film (for example, Patent Document 1). It is known that the piezoelectricity is improved by adding a Group 2 element or a Group 12 element and a Group 4 element or a Group 5 element to the aluminum nitride film (for example, Patent Document 2).

JP, 2011-15148, A JP, 2013-219743, A

When impurities are added to the aluminum nitride film as in Patent Documents 1 and 2, the piezoelectricity of the aluminum nitride film is improved but the electromechanical coupling coefficient is decreased. As described above, it is difficult to improve the piezoelectricity without lowering the electromechanical coupling coefficient of the piezoelectric film.

The present invention has been made in view of the above problems, and an object thereof is to improve piezoelectricity.

The present invention is a piezoelectric film including a plurality of crystal grains containing a piezoelectric material, wherein the average crystal grain size in the central region in the thickness direction is larger than the average crystal grain size in regions on both sides of the central region in the thickness direction. is there.

In the above structure, the average crystal grain size in the central region may be 1.5 times or more the average crystal grain size in the regions on both sides.

In the above structure, the piezoelectric film includes a first piezoelectric film and a second piezoelectric film laminated in a thickness direction, and the plurality of crystal grains are continuous at an interface between the first piezoelectric film and the second piezoelectric film. The central region includes the interface, and the average crystal grain size of the first piezoelectric film in the central region and the average crystal grain size of the second piezoelectric film in the central region are in the regions on both sides. It can be configured to be larger than the average crystal grain size.

In the above structure, the average crystal grain size in the first piezoelectric film and the second piezoelectric film increases from the surface opposite to the interface of the first piezoelectric film and the second piezoelectric film to the interface. can do.

In the above configuration, the plurality of crystal grains have a columnar structure, the average crystal grain size viewed from the thickness direction in the central region of the thickness direction is the thickness in the regions on both sides in the thickness direction. It can be configured to be larger than the average crystal grain size viewed from the direction.

In the above structure, the piezoelectric material may be aluminum nitride.

The present invention provides a substrate, a piezoelectric film provided on the substrate, a first electrode and a second electrode that face each other with at least a part of the piezoelectric film sandwiched in the thickness direction, and are in contact with the regions on both sides. And a piezoelectric device including.

The present invention provides a substrate, a piezoelectric film provided on the substrate, a first electrode and a second electrode that face each other with at least a part of the piezoelectric film sandwiched in the thickness direction, and are in contact with the regions on both sides. And a resonator including.

The present invention provides a substrate, a piezoelectric film provided on the substrate, a first electrode and a second electrode that face each other with at least a part of the piezoelectric film sandwiched in the thickness direction, and are in contact with the regions on both sides. And an insertion film provided between the first piezoelectric film and the second piezoelectric film.

The present invention is a filter including the above resonator.

The present invention is a multiplexer including the above filter.

The present invention comprises a step of forming a first piezoelectric film on a first substrate so that the crystal grain size becomes larger as it grows, and a crystal grain size becomes larger and grows as it grows on a second substrate. Forming a second piezoelectric film so that its surface is a polarization surface opposite to the growth surface of the first piezoelectric film; a surface of the first piezoelectric film opposite to the first substrate; and a second piezoelectric film. And a surface of the second substrate opposite to the surface of the second substrate are bonded together.

According to the present invention, piezoelectricity can be improved.

1A is a plan view of the piezoelectric thin film resonator according to the first embodiment, and FIGS. 1B and 1C are cross-sectional views taken along the line AA of FIG. 1A. 2A to 2E are cross-sectional views (No. 1) showing the method of manufacturing the series resonator according to the first embodiment. 3A to 3E are cross-sectional views (No. 2) showing the method for manufacturing the series resonator according to the first embodiment. FIG. 4 is a schematic cross-sectional view near the piezoelectric film in Example 1. 5A to 5C are a schematic cross-sectional view taken along the line AA, a schematic cross-sectional view taken along the line BB, and a schematic cross-sectional view taken along the line C-C in FIG. 4, respectively. FIG. 6A is a cross-sectional view of the piezoelectric thin film resonator in Simulation 1, and FIG. 6B is a diagram showing Q values at the antiresonance frequencies of Samples A to C. FIG. 7A to FIG. 7C are diagrams showing strain, stress, and elastic wave energy density in the simulation 1. 8A and 8B are schematic cross-sectional views of the piezoelectric film according to the first embodiment. FIG. 9 is a diagram showing the standard error with respect to the number of samples. 10A to 10C are cross-sectional views of the piezoelectric thin film resonators according to Modifications 1 to 3 of Example 1, respectively. FIG. 11A is a circuit diagram of the filter according to the second embodiment, and FIG. 11B is a circuit diagram of the duplexer according to the first modification of the second embodiment.

Hereinafter, embodiments will be described with reference to the drawings.

1A is a plan view of the piezoelectric thin film resonator according to the first embodiment, and FIGS. 1B and 1C are cross-sectional views taken along the line AA of FIG. 1A. FIG. 1B shows a cross-sectional view of, for example, a series resonator of a ladder type filter, and FIG. 1C shows a parallel resonator of, for example, a ladder type filter.

The structure of the series resonator S will be described with reference to FIGS. 1(a) and 1(b). The lower electrode 12 is provided on the substrate 10. A void 30 having a dome-shaped bulge is formed between the flat main surface of the substrate 10 and the lower electrode 12. The dome-shaped bulge is, for example, a bulge in which the height of the void 30 is small around the void 30 and the height of the void 30 increases toward the inside of the void 30. The substrate 10 is, for example, a silicon (Si) substrate. The lower electrode 12 includes a lower layer 12a and an upper layer 12b. The lower layer 12a and the upper layer 12b are, for example, a chromium (Cr) film and a ruthenium (Ru) film, respectively.

A piezoelectric film 14 is provided on the lower electrode 12. The piezoelectric film 14 is polycrystalline and includes a plurality of crystal grains. The piezoelectric film 14 is, for example, an aluminum nitride film containing aluminum nitride whose main axis is the (0001) direction (that is, having C-axis orientation) as a main component. The piezoelectric film 14 includes a lower piezoelectric film 14 a that contacts the lower electrode 12 and an upper piezoelectric film 14 b that contacts the upper electrode 16.

An insertion film 28 is provided between the lower piezoelectric film 14a and the upper piezoelectric film 14b. The insertion film 28 is, for example, a silicon oxide film. The insertion film 28 is provided in the outer peripheral region 52 in the resonance region 50, and is not provided in the central region 54. The insertion film 28 is continuously provided from the outer peripheral region 52 to the outside of the resonance region 50.

The upper electrode 16 is provided on the piezoelectric film 14 so as to have a region (resonance region 50) facing the lower electrode 12 with the piezoelectric film 14 interposed therebetween. The resonance region 50 has an elliptical shape and is a region where elastic waves in the thickness extensional vibration mode resonate. The resonance region 50 overlaps the air gap 30 in a plan view, and the air gap 30 is equal to or larger than the resonance region 50. That is, the entire resonance region 50 overlaps the void 30 in a plan view. The upper electrode 16 includes a lower layer 16a and an upper layer 16b. The lower layer 16a and the upper layer 16b are, for example, a ruthenium film and a chromium film, respectively.

A silicon oxide film is formed as the frequency adjustment film 24 on the upper electrode 16. The laminated film 18 in the resonance region 50 includes the lower electrode 12, the piezoelectric film 14, the insertion film 28, the upper electrode 16, and the frequency adjustment film 24. The frequency adjustment film 24 may function as a passivation film.

As shown in FIG. 1A, the introduction path 33 for etching the sacrificial layer is formed in the lower electrode 12. The sacrificial layer is a layer for forming the void 30. The vicinity of the tip of the introduction path 33 is not covered with the piezoelectric film 14, and the lower electrode 12 has a hole 35 at the tip of the introduction path 33.

The structure of the parallel resonator P will be described with reference to FIGS. 1(a) and 1(c). The parallel resonator P is different from the series resonator S in that a mass load film 20 made of a titanium (Ti) layer is provided between the lower layer 16a and the upper layer 16b of the upper electrode 16. Therefore, the laminated film 18 includes, in addition to the laminated film of the series resonator S, the mass load film 20 formed on the entire surface in the resonance region 50. The other configuration is the same as that of the series resonator S shown in FIG.

The difference in resonance frequency between the series resonator S and the parallel resonator P is adjusted by using the thickness of the mass load film 20. The resonance frequencies of both the series resonator S and the parallel resonator P are adjusted by adjusting the film thickness of the frequency adjustment film 24.

In the case of a piezoelectric thin film resonator having a resonance frequency of 2 GHz, the lower layer 12a made of a chromium film of the lower electrode 12 has a film thickness of 100 nm, and the upper layer 12b made of a ruthenium film has a film thickness of 210 nm. The film thickness of the piezoelectric film 14 made of an aluminum nitride film is 1100 nm. The thickness of the insertion film 28 made of a silicon oxide film is 150 nm. The lower layer 16a made of a ruthenium film of the upper electrode 16 has a thickness of 230 nm, and the upper layer 16b made of a chromium film has a thickness of 50 nm. The thickness of the frequency adjustment film 24 made of a silicon oxide film is 50 nm. The thickness of the mass load film 20 made of a titanium film is 120 nm. The film thickness of each layer can be appropriately set to obtain desired resonance characteristics.

As the substrate 10, other than the silicon substrate, a sapphire substrate, an alumina substrate, a spinel substrate, a quartz substrate, a quartz substrate, a glass substrate, a ceramic substrate, a GaAs substrate, or the like can be used. As the lower electrode 12 and the upper electrode 16, besides ruthenium and chromium, aluminum (Al), titanium, copper (Cu), molybdenum (Mo), tungsten (W), tantalum (Ta), platinum (Pt), rhodium ( A single layer film such as Rh) or iridium (Ir) or a laminated film thereof can be used. For example, the lower layer 16a of the upper electrode 16 may be Ru and the upper layer 16b may be Mo.

For the piezoelectric film 14, zinc oxide (ZnO), lead zirconate titanate (PZT), lead titanate (PbTiO ₃ ) or the like can be used other than aluminum nitride. Further, for example, the piezoelectric film 14 may contain aluminum nitride as a main component, and may contain other elements for improving resonance characteristics or piezoelectric properties. For example, the piezoelectricity of the piezoelectric film 14 is improved by using scandium (Sc), two elements of group 2 elements and group 4 elements, or two elements of group 2 elements and group 5 elements as additional elements. To do. Therefore, the effective electromechanical coupling coefficient of the piezoelectric thin film resonator can be improved. The Group 2 element is, for example, calcium (Ca), magnesium (Mg), strontium (Sr) or zinc (Zn). The Group 4 element is, for example, titanium, zirconium (Zr) or hafnium (Hf). The Group 5 element is, for example, tantalum, niobium (Nb) or vanadium (V). Furthermore, the piezoelectric film 14 may contain aluminum nitride as a main component and may contain boron (B).

The insertion film 28 is a material whose Young's modulus and/or acoustic impedance is smaller than that of the piezoelectric film 14. As the insertion film 28, in addition to silicon oxide, a single layer film of aluminum (Al), gold (Au), copper, titanium, platinum, tantalum, chromium or the like or a laminated film thereof can be used.

As the frequency adjusting film 24, a silicon nitride film, an aluminum nitride film, or the like can be used instead of the silicon oxide film. As the mass load film 20, other than titanium, a metal film exemplified as the lower electrode 12 and the upper electrode 16 or an insulating film such as silicon nitride or silicon oxide can be used.

[Production method of Example 1]
2A to 3E are cross-sectional views showing a method of manufacturing the series resonator according to the first embodiment. As shown in FIG. 2A, a sacrificial layer 38 for forming voids is formed on the substrate 10 having a flat main surface. The sacrifice layer 38 has a film thickness of, for example, 10 to 100 nm, and can be easily dissolved in an etching solution or etching gas such as magnesium oxide (MgO), zinc oxide (ZnO), germanium (Ge), or silicon oxide (SiO ₂ ). Selected from materials. Then, the sacrificial layer 38 is patterned into a desired shape by using the photolithography method and the etching method. The shape of the sacrificial layer 38 is a shape corresponding to the planar shape of the void 30, and includes, for example, a region serving as the resonance region 50. Next, the lower layer 12a and the upper layer 12b are formed as the lower electrode 12 on the sacrificial layer 38 and the substrate 10. The sacrificial layer 38 and the lower electrode 12 are formed by, for example, a sputtering method, a vacuum evaporation method or a CVD (Chemical Vapor Deposition) method. After that, the lower electrode 12 is patterned into a desired shape by the photolithography method and the etching method. The lower electrode 12 may be formed by a lift-off method.

As shown in FIG. 2B, a lower piezoelectric film 14a is formed on the lower electrode 12 and the substrate 10 by using, for example, a sputtering method or a vacuum evaporation method. When the lower piezoelectric film 14a is formed by the Volmer-Weber growth method, the crystal grain size in the lower piezoelectric film 14a increases as the lower piezoelectric film 14a grows. Therefore, in the lower piezoelectric film 14a, the crystal grain size near the upper surface is larger than the crystal grain size near the lower electrode 12. Further, for example, when the lower piezoelectric film 14a is an aluminum nitride film, the upper surface of the lower piezoelectric film 14a is the N surface. After that, the upper surface of the lower piezoelectric film 14a is planarized by using, for example, a CMP (Chemical Mechanical Polishing) method.

An aluminum nitride film having a thickness of 1 μm was formed on a silicon substrate by a sputtering method. A cross section parallel to the upper surface of the silicon substrate was observed using a TEM (Transmission Electron Microscope) method. The area of a plurality of crystal grains was converted into a diameter corresponding to a circle. That is, when the area of the crystal grains is S, the crystal grain size R is 2×√(S/π). The average of a plurality of crystal grain sizes was calculated. The average crystal grain size at the position 0.08 μm from the upper surface of the silicon substrate (the number of crystal grains is 94) is 15.8 nm, and the position 0.08 nm from the surface of the aluminum nitride film (that is, from the upper surface of the silicon substrate). The average crystal grain size (the number of samples: 71) at a position of 0.92 μm) was 44.8 nm. The above is an example, and the average crystal grain size can be appropriately set depending on the material of the piezoelectric film 14, the material of the underlying layer (the lower electrode 12 and the substrate 10) and the method of forming the piezoelectric film 14.

As shown in FIG. 2C, a part of the upper surface of the lower piezoelectric film 14a is removed by photolithography and etching. As a result, the concave portion 36a and the convex portion 36b are formed on the upper surface of the lower piezoelectric film 14a. The recess 36a is a region where the insertion film 28 is formed.

As shown in FIG. 2D, an insertion film 28 is formed on the upper surface of the lower piezoelectric film 14a by using, for example, a sputtering method, a vacuum evaporation method or a CVD method. As shown in FIG. 2E, the upper surface of the insertion film 28 is flattened by, for example, the CMP method. As a result, the insertion film 28 on the protrusion 36b is removed and the upper surface of the protrusion 36b is exposed from the insertion film 28. The insertion film 28 is formed in the recess 36a.

As shown in FIG. 3A, a substrate 32 is prepared separately from the substrate 10. The substrate 32 is, for example, a silicon substrate. The substrate 32 may be a substrate made of the materials exemplified in the substrate 10. A seed layer 34 is formed on the flat upper surface of the substrate 32. The seed layer 34 is, for example, an aluminum film and is formed by using, for example, a vacuum vapor deposition method or a sputtering method. The upper piezoelectric film 14b is formed on the seed layer 34 by, for example, a sputtering method or a vacuum evaporation method. As the upper piezoelectric film 14b grows, the crystal grain size in the upper piezoelectric film 14b increases. Therefore, in the upper piezoelectric film 14b, the crystal grain size near the upper surface (the lower surface in FIG. 3B and subsequent figures) of the upper piezoelectric film 14b is larger than the crystal grain size near the seed layer 34. Further, for example, when the upper piezoelectric film 14b is an aluminum nitride film, when the upper piezoelectric film 14b is formed on the aluminum film, the upper surface (lower surface in FIG. 3B and subsequent figures) of the upper piezoelectric film 14b becomes an Al surface.

As shown in FIG. 3B, the substrate 32 and the upper piezoelectric film 14b of FIG. 3A are turned upside down, and the upper surfaces of the lower piezoelectric film 14a and the insertion film 28 and the lower surface of the upper piezoelectric film 14b are bonded. .. For joining, a joining method at room temperature such as a surface activation method is used. In the surface activation method, the upper surfaces of the lower piezoelectric film 14a and the insertion film 28 and the lower surface of the upper piezoelectric film 14b are irradiated with ions as an ion beam, a neutralized beam, or plasma. The ions are ions of an inert element (for example, a rare gas element) such as argon (Ar) ions. When the upper surfaces of the lower piezoelectric film 14a and the insertion film 28 and the lower surface of the upper piezoelectric film 14b are adhered to each other while maintaining the vacuum, the upper surfaces of the activated lower piezoelectric film 14a and the insertion film 28 and the upper piezoelectric film 14b are bonded. The lower surface is joined.

Since such joining is performed at room temperature (eg, 100° C. or lower and −20° C. or higher, preferably 80° C. or lower and 0° C. or higher), thermal stress can be suppressed. Further, the upper surfaces of the lower piezoelectric film 14a and the insertion film 28 and the lower surface of the upper piezoelectric film 14b can be directly bonded to each other without interposing another material such as a bonding layer. When the surface activation method is used, an amorphous layer having a thickness of 10 nm or less may be formed on the bonding surface. However, since the amorphous layer is very thin, the upper surfaces of the lower piezoelectric film 14a and the insertion film 28 and the upper piezoelectric film 14b are formed. Is substantially directly joined to the lower surface of the. As a result, the piezoelectric film 14 having the lower piezoelectric film 14a and the upper piezoelectric film 14b is formed. The upper surfaces of the lower piezoelectric film 14a and the upper piezoelectric film 14b are both N surfaces. Thereby, the lower piezoelectric film 14a and the upper piezoelectric film 14b have the same direction of electrostatic polarization.

As shown in FIG. 3C, the substrate 32 is removed by using, for example, the smart cut method. The seed layer 34 is removed by using, for example, the CMP method or the etching method. As a result, the upper surface of the piezoelectric film 14 is exposed.

As shown in FIG. 3D, the lower layer 16a and the upper layer 16b of the upper electrode 16 are formed on the piezoelectric film 14 by using, for example, a sputtering method, a vacuum deposition method or a CVD method. The upper electrode 16 is patterned into a desired shape by using the photolithography method and the etching method. The upper electrode 16 may be formed by a lift-off method.

In the parallel resonator shown in FIG. 1C, after forming the lower layer 16a, the mass load film 20 is formed by using, for example, a sputtering method, a vacuum evaporation method or a CVD method. The mass load film 20 is patterned into a desired shape by photolithography and etching. Then, the upper layer 16b is formed.

The frequency adjustment film 24 is formed by using, for example, a sputtering method or a CVD method. The frequency adjustment film 24 is patterned into a desired shape by using a photolithography method and an etching method.

As shown in FIG. 3E, the frequency adjustment film 24 and the upper piezoelectric film 14b are patterned into a desired shape by photolithography and etching. The insertion film 28 and the lower piezoelectric film 14a are patterned into a desired shape by photolithography and etching. As a result, the end surface of the piezoelectric film 14 has a structure having a step.

After that, the etching solution for the sacrificial layer 38 is introduced into the sacrificial layer 38 below the lower electrode 12 through the hole 35 and the introduction path 33 (see FIG. 1A). As a result, the sacrificial layer 38 is removed. The medium for etching the sacrificial layer 38 is preferably a medium that does not etch the material constituting the resonator other than the sacrificial layer 38. In particular, the etching medium is preferably a medium in which the lower electrode 12 with which the etching medium is in contact is not etched. The stress of the laminated film 18 (see FIGS. 1B and 1C) is set so as to be a compressive stress. As a result, when the sacrificial layer 38 is removed, the laminated film 18 swells on the opposite side of the substrate 10 away from the substrate 10. A space 30 having a dome-shaped bulge is formed between the lower electrode 12 and the substrate 10. As described above, the series resonator S shown in FIGS. 1A and 1B and the parallel resonator P shown in FIGS. 1A and 1C are manufactured.

[Cross-sectional schematic view of Example 1]
FIG. 4 is a schematic cross-sectional view near the piezoelectric film in Example 1. As shown in FIG. 4, the lower piezoelectric film 14a and the upper piezoelectric film 14b are directly bonded to each other on the bonding surface 15. The lower piezoelectric film 14a and the upper piezoelectric film 14b have a columnar structure in which crystal grains 44 extend in the stacking direction. The boundaries of the crystal grains 44 are grain boundaries 45. At the bonding surface 15, the crystal grains 44 and the grain boundaries 45 of the lower piezoelectric film 14a and the upper piezoelectric film 14b are discontinuous. Part of the crystal grain 44 and the grain boundary 45 penetrates the lower piezoelectric film 14a in the thickness direction. In the lower piezoelectric film 14a, many crystal grains 44 are formed near the lower electrode 12. Some crystal grains 44 converge as they go in the stacking direction. The crystal grains 44 that do not converge increase in size in the stacking direction. The upper piezoelectric film 14b has a structure in which the lower piezoelectric film 14a is turned upside down.

5A to 5C are a schematic cross-sectional view taken along the line AA, a schematic cross-sectional view taken along the line BB, and a schematic cross-sectional view taken along the line C-C in FIG. 4, respectively. As shown in FIGS. 5A to 5C, in the cross section of the plane orthogonal to the stacking direction of the lower piezoelectric film 14a and the upper piezoelectric film 14b, the crystal grains 44 have little direction dependency. The crystal grains 44 in FIG. 5B are larger than the crystal grains 44 in FIGS. 5A and 5C.

[Simulation 1]
When vibration such as thickness longitudinal vibration is applied to the piezoelectric film 14, slip occurs at the grain boundary 45 of the crystal grain 44. Therefore, the elastic wave energy is lost at the grain boundary 45. Therefore, the material Q is large in the region where the average crystal grain size is large, and the material Q is small in the region where the average crystal grain boundary is small. Therefore, when the material Q is different in the piezoelectric film 14, the Q value Qa at the antiresonance frequency when the material is longitudinally vibrated is simulated using the finite element method.

FIG. 6A is a cross-sectional view of the piezoelectric thin film resonator in Simulation 1. As shown in FIG. 6A, the piezoelectric film 14 was equally divided into six regions 13a to 13f. The regions 13a to 13c correspond to the lower piezoelectric film 14a, and the regions 13d to 13f correspond to the upper piezoelectric film 14b. The simulation conditions are as follows.
Lower electrode 12: Ruthenium film with a thickness of 0.25 μm Piezoelectric film 14: C-axis oriented aluminum nitride with a film thickness of 1.3 μm Upper electrode 16: Ruthenium film with a thickness of 0.25 μm Lower electrode 12 and upper electrode 16 And an amplitude of 1 V and a frequency of 1900 MHz to 2200 MHz were applied between the and, and the frequency dependence of the admittance was calculated. The calculation result of the admittance characteristic was fitted with an mBVD (Modified Butterworth-Van-Dyke) model to calculate the Q value Qa of the anti-resonance frequency.

Table 1 is a diagram showing the materials Q of the regions 13a to 13f in the samples A to C.

As shown in Table 1, in the sample A, the material Q increases in order in the regions 13a to 13c, and the material Q increases in order in the regions 13d to 13f. This means that in the lower piezoelectric film 14a, the crystal grain size increases as it goes from the lower electrode 12 to the bonding surface 15, and in the upper piezoelectric film 14b, the crystal grain size increases as it goes from the bonding surface 15 to the upper electrode 16. Correspond. For example, when the lower piezoelectric film 14a is formed on the lower electrode 12 and the upper piezoelectric film 14b is formed on the lower piezoelectric film 14a by a sputtering method or the like, a sample A is obtained.

In the sample B, the material Q increases in order in the regions 13a to 13c, and the material Q decreases in order in the regions 13d to 13f. This is because in the lower piezoelectric film 14a, the crystal grain size becomes larger as it goes from the lower electrode 12 to the bonding surface 15, and in the upper piezoelectric film 14b as it goes from the upper electrode 16 to the bonding surface 15 as in Example 1. Corresponding to larger particle size.

In the sample C, the material Q becomes smaller in order in the regions 13a to 13c, and the material Q becomes larger in order in the regions 13d to 13f. This is the reverse of Example 1, and the crystal grain size increases from the bonding surface 15 to the lower electrode 12 in the lower piezoelectric film 14a, and from the bonding surface 15 to the upper electrode 16 in the upper piezoelectric film 14b. Therefore, it corresponds to the increase of the crystal grain size.

FIG. 6B is a diagram showing the Q value at the anti-resonance frequency of samples A to C. As shown in FIG. 6B, the Q values Qa at the anti-resonance frequencies of Samples A, B and C are 2499, 2795 and 2349, respectively. Sample B has the largest Qa, and sample C has the smallest Qa.

FIG. 7A to FIG. 7C are diagrams showing strain, stress, and elastic wave energy density in the simulation 1. The strain corresponds to the amplitude of the thickness longitudinal vibration. The stress corresponds to the von Mises stress. The elastic wave energy density is the energy density due to thickness longitudinal vibration. The vertical axis represents the coordinates in the stacking direction with the lower surface of the lower electrode 12 being zero. The range of coordinates 0 to 0.25 μm corresponds to the lower electrode 12, the range of coordinates 0.25 μm to 1.55 μm corresponds to the piezoelectric film 14, and the range of coordinates 1.55 μm to 1.8 μm is the upper electrode. Corresponds to 16.

As shown in FIGS. 7A to 7C, the strain, stress, and elastic wave energy density of the piezoelectric film 14 are larger than those of the lower electrode 12 and the upper electrode 16. In the piezoelectric film 14, the strain, the stress, and the elastic wave energy density are the largest when the coordinate corresponding to the center of the piezoelectric film 14 in the thickness direction is 0.9 μm. The strain, stress, and elastic wave energy density decrease as the coordinates increase or decrease from 0.9 μm. In order to suppress the loss and improve the Q value, it is effective to increase the material Q in the region where the elastic wave energy density is large. In the sample, the material Q of the regions 13c and 13d near the center of the piezoelectric film 14 is large. Therefore, it is considered that the sample B had a higher Qa.

As in Patent Documents 1 and 2, when an impurity element substituting an Al site is added to the aluminum nitride film which is the piezoelectric film 14, the piezoelectric property is improved and the Q value is improved. However, the electromechanical coupling coefficient decreases. As described above, the piezoelectricity of the piezoelectric film 14 and the electromechanical coupling coefficient have a trade-off relationship. In order to improve both the piezoelectricity and the electromechanical coupling coefficient of the piezoelectric film 14, it is necessary to improve the film quality of the piezoelectric film 14 and reduce the loss of elastic waves.

When the piezoelectric film 14 is formed by a sputtering method, a vacuum evaporation method, or the like, it becomes a polycrystalline film. In the polycrystalline film, slip of a grain boundary may cause elastic wave loss. Therefore, the piezoelectric film 14 can be formed into a film close to a single crystal by epitaxially growing the piezoelectric film 14 by using a MOCVD (Metal Organic CVD) method or the like. As a result, grain boundaries in the piezoelectric film 14 are almost eliminated, and the loss of elastic waves is reduced. However, MOCVD equipment is expensive. Further, when the MOCVD method is used to form the piezoelectric film 14, the film forming temperature becomes high. As a result, the existing process will be significantly revised.

8A and 8B are schematic cross-sectional views of the piezoelectric film according to the first embodiment. As shown in FIG. 8A, according to the first embodiment, the piezoelectric film 14 includes a plurality of crystal grains containing a piezoelectric material, and the average crystal grain size in the region 15b (central region) in the thickness direction is: It is larger than the average crystal grain size in the regions 15a and 15c on both sides of the region 15b in the thickness direction. As a result, the material Q in the region 15b having the largest strain, stress, and elastic wave energy density can be increased. Therefore, the loss can be reduced and the piezoelectricity (Q value) can be improved. Further, since the amount of impurities added to the piezoelectric film 14 can be reduced, it is possible to suppress the decrease in the electromechanical coupling coefficient.

An example of how to obtain the average crystal grain size in the regions 15a to 15c will be described. The thicknesses of the regions 15a to 15c are Ta to Tc, respectively. Each of Ta to Tc is set to 1/3 of the thickness T of the piezoelectric film 14. That is, the piezoelectric film 14 is equally divided into regions 15a to 15c. The AA cross section, the BB cross section, and the CC cross section at the center positions of the respective regions 15a to 15c in the thickness direction are observed. The thickness of the regions 15a to 15c and the center position of each of the regions 15a to 15c are allowed to have an error of about 5% of the thickness T of the piezoelectric film 14.

For example, a TEM image of the AA cross section, the BB cross section, and the CC cross section is observed. When the diffraction contrast of the TEM image is used, the crystal grains can be easily observed and the crystal grains can be easily defined on the image. If the crystal grains cannot be defined, the sample is not sampled, but the crystal grains are defined. For example, crystal grains are sampled from a TEM image of 500 nm×500 nm. The number of crystal grains sampled is preferably 60 or more. When the number of crystal grains sampled is less than 60, the number of TEM images is increased.

The area of the sampled crystal grain is measured. The measured crystal grain area is converted into a diameter corresponding to a circle. When the area of the crystal grain is S, the crystal grain size R is 2×√(S/π). The average crystal grain size is obtained by averaging the crystal grain sizes R.

Using the example of the average crystal grain size described in FIG. 2B, the standard error of the average crystal grain size with respect to the sample number of crystal grains was calculated. FIG. 9 is a diagram showing the standard error with respect to the number of samples. The white circles are the calculation results of the standard error σ/√N at 0.08 μm from the surface of the piezoelectric film 14, and the black circles are the calculation results of the standard error σ/√N at 0.08 μm from the upper surface of the silicon substrate. As shown in FIG. 9, as the number of samples N increases, the standard error decreases and the accuracy of the average crystal grain size increases. When the number of samples is 60 or more, the standard error is 2 nm or less, and it is possible to sufficiently compare the average crystal grain sizes.

As shown in FIG. 8B, when the piezoelectric film 14 includes the lower piezoelectric film 14a and the upper piezoelectric film 14b, the average crystal grain boundaries in the region 15b can be obtained in the BB cross section and the B'-B' cross section. Good. The BB cross section is the center position in the thickness direction of the lower piezoelectric film 14a in the region 15b, and the B'-B' cross section is the center position in the thickness direction of the upper piezoelectric film 14b in the region 15b. An error of about 5% of the thickness T of the piezoelectric film 14 can be allowed in the center position.

The average crystal grain size in the region 15b is preferably 1.5 times or more, more preferably 2 times or more, and further preferably 2.5 times or more the average crystal grain size in the regions 15a and 15c on both sides.

The piezoelectric film 14 includes a lower piezoelectric film 14a (first piezoelectric film) and an upper piezoelectric film 14b (second piezoelectric film) stacked in the thickness direction, and an interface (bonding) between the lower piezoelectric film 14a and the upper piezoelectric film 14b. A plurality of crystal grains are not continuous on the surface 15), and the region 15b includes an interface. For example, when the insertion film 28 is formed in the piezoelectric film 14, the piezoelectric film 14 is formed twice. In this case, since the piezoelectric film 14 cannot be thickly formed at one time, the crystal grain becomes small. When the upper piezoelectric film 14b is formed on the lower piezoelectric film 14a, the Q value is lowered like the sample A in Table 1. Therefore, the average crystal grain size of the lower piezoelectric film 14a in the region 15b and the average crystal grain size of the upper piezoelectric film 14b in the region 15b are larger than the average crystal grain size in the regions 15a and 15c. Thereby, the piezoelectricity of the piezoelectric film 14 can be increased.

As shown in FIG. 2B, the lower piezoelectric film 14a (first piezoelectric film) is formed on the substrate 10 (first substrate) so that the crystal grain size becomes larger as it grows. As shown in FIG. 3A, on the substrate 32 (second substrate), the crystal grain size becomes larger as it grows and the growth surface becomes the polarization surface opposite to the growth surface of the lower piezoelectric film 14a. Piezoelectric film 14b (second piezoelectric film 9 is formed. As shown in FIG. 3B, the surface of the lower piezoelectric film 14a opposite to the substrate 10 is joined to the surface of the upper piezoelectric film 14b opposite to the substrate 32. As a result, the average crystal grain size in the regions 15a and 15b increases from both surfaces in the thickness direction of the piezoelectric film 14 toward the interface (bonding surface 15), and thus the piezoelectricity of the piezoelectric film 14 becomes higher. Moreover, since the lower piezoelectric film 14a and the upper piezoelectric film 14b have the same dielectric polarization polarity, the piezoelectric thin film resonator resonates in an odd mode (eg, first-order mode) of thickness longitudinal vibration.

When the plurality of crystal grains have a columnar structure, the average crystal grain size in the thickness direction in the region 15b is larger than the average crystal grain size in the thickness direction in the regions 15a and 15c. In the columnar structure, the bond between the crystal grains at the grain boundary is weak, so that the elastic wave loss at the grain boundary tends to increase. Therefore, the average crystal grain size of the region 15b is made larger than the average crystal grain size of the regions 15a and 15c. Thereby, the piezoelectricity of the piezoelectric film 14 can be further increased. When the crystal grains do not have a columnar structure, the average crystal grain size in the regions 15a to 15c may be compared with the average crystal grain size viewed from the plane direction of the piezoelectric film 14.

When the piezoelectric material is aluminum nitride, the piezoelectric film 14 contains scandium or at least one element of calcium, magnesium, strontium and zinc and at least one element of titanium, zirconium, hafnium, niobium, vanadium and tantalum. These elements mainly replace aluminum. Thereby, the piezoelectricity of the piezoelectric film 14 can be further enhanced.

In the piezoelectric thin film resonator, the piezoelectric film 14 is provided on the substrate 10. The lower electrode 12 (first electrode) and the upper electrode 16 (second electrode) face each other with at least a portion of the piezoelectric film 14 sandwiched in the thickness direction, and are in contact with the regions 15a and 15c, respectively. In such a resonator, as shown in FIGS. 7A to 7C, the strain, stress and elastic wave energy density in the region 15b of the piezoelectric film 14 are large. Therefore, the average crystal grain size of the lower piezoelectric film 14a in the region 15b and the average crystal grain size of the upper piezoelectric film 14b in the region 15b are increased. Thereby, the Q value of the piezoelectric thin film resonator can be improved.

[Modification 1 of Embodiment 1]
FIG. 10A is a cross-sectional view of the piezoelectric thin film resonator according to the first modification of the first embodiment. As shown in FIG. 10A, no insertion film is provided in the piezoelectric film 14. The other configuration is the same as that of the first embodiment, and the description is omitted. Unlike the first modification of the first embodiment, the insertion film 28 may not be provided.

[Modification 2 of Embodiment 1]
FIG. 10B is a cross-sectional view of the piezoelectric thin film resonator according to the second modification of the first embodiment. As shown in FIG. 10B, a recess is formed on the upper surface of the substrate 10. The lower electrode 12 is formed flat on the substrate 10. Thereby, the void 30 is formed in the depression of the substrate 10. The void 30 is formed to include the resonance region 50. Other configurations are the same as those in the first embodiment, and the description thereof will be omitted. The void 30 may be formed so as to penetrate the substrate 10.

[Modification 3 of Embodiment 1]
FIG. 10C is a cross-sectional view of the piezoelectric thin film resonator according to Modification 3 of Example 1. As shown in FIG. 10C, the acoustic reflection film 31 is formed below the lower electrode 12 in the resonance region 50. The acoustic reflection film 31 is formed by alternately providing a film 31a having a low acoustic impedance and a film 31b having a high acoustic impedance. The film thickness of each of the films 31a and 31b is, for example, approximately λ/4 (λ is the wavelength of the elastic wave). The number of stacked films 31a and 31b can be set arbitrarily. The acoustic reflection film 31 may be formed by stacking at least two types of layers having different acoustic characteristics with a gap. The substrate 10 may be one of at least two types of layers having different acoustic characteristics of the acoustic reflection film 31. For example, the acoustic reflection film 31 may have a configuration in which one film having different acoustic impedances is provided in the substrate 10. Other configurations are the same as those in the first embodiment, and the description thereof will be omitted.

As in Example 1 and Modifications 1 and 2, the piezoelectric thin film resonator may be an FBAR (Film Bulk Acoustic Resonator) in which a void 30 is formed between the substrate 10 and the lower electrode 12 in the resonance region 50. .. In addition, as in the second modification of the first embodiment, the piezoelectric thin film resonator includes an SMR (Solidly Mounted Resonator) that includes an acoustic reflection film 31 that reflects acoustic waves propagating through the piezoelectric film 14 below the lower electrode 12 in the resonance region 50. ) Is OK. The acoustic reflection layer may include the void 30 or the acoustic reflection film 31.

In the first embodiment and the modifications 2 and 3, the insertion film 28 is provided in the outer peripheral region 52 of the resonance region 50. However, the insertion film 28 surrounds the central region 54 of the resonance region 50 and at least the outer peripheral region 52. It may be provided in part. The insertion film 28 may not be provided outside the resonance region 50. The insertion film 28 may be provided on at least a part of the resonance region 50. Although the elliptical shape is described as an example of the planar shape of the resonance region 50, it may be a polygonal shape such as a quadrangular shape or a pentagonal shape.

Although the piezoelectric thin film resonator is described as an example of the elastic wave device using the piezoelectric film in the first embodiment and its modification, an elastic wave device having an electrode for exciting an elastic wave propagating through the piezoelectric film may be used. For example, a resonator using a Lamb wave in which a comb-shaped electrode is provided on the piezoelectric film may be used. Such a resonator includes a substrate 10, a piezoelectric film 14 provided on the substrate 10, and a lower electrode 12 and an upper electrode 16 which are opposed to each other with at least a part of the piezoelectric film 14 sandwiched in the thickness direction.

Moreover, the piezoelectric film 14 may be used for a piezoelectric device. Piezoelectric devices that can use the piezoelectric film 14 are, for example, actuators and sensors in addition to acoustic wave devices. The actuator is, for example, a micro pump using an inkjet, an RF (Radio Frequency)-MEMS (Micro Electro Mechanical Systems), and an optical mirror. The sensor is, for example, an acceleration sensor, a gyro sensor, and an energy harvest sensor.

Example 2 is an example of a filter and a duplexer using the piezoelectric thin film resonators of Example 1 and its modifications. FIG. 11A is a circuit diagram of the filter according to the second embodiment. As shown in FIG. 11A, one or a plurality of series resonators S1 to S4 are connected in series between the input terminal T1 and the output terminal T2. One or more parallel resonators P1 to P4 are connected in parallel between the input terminal T1 and the output terminal T2. The piezoelectric thin film resonator of the first embodiment and its modification can be used for at least one resonator of the one or more series resonators S1 to S4 and the one or more parallel resonators P1 to P4. The number of resonators of the ladder type filter can be set appropriately.

FIG. 11B is a circuit diagram of the duplexer according to the first modification of the second embodiment. As shown in FIG. 11B, the transmission filter 40 is connected between the common terminal Ant and the transmission terminal Tx. The reception filter 42 is connected between the common terminal Ant and the reception terminal Rx. The transmission filter 40 allows a signal in the transmission band of the signals input from the transmission terminal Tx to pass through the common terminal Ant as a transmission signal and suppresses signals of other frequencies. The reception filter 42 passes the signal in the reception band among the signals input from the common terminal Ant to the reception terminal Rx as a reception signal, and suppresses signals of other frequencies. At least one of the transmission filter 40 and the reception filter 42 can be the filter of the second embodiment.

A duplexer has been described as an example of the multiplexer, but a triplexer or a quadplexer may be used.

Although the embodiments of the present invention have been described in detail above, the present invention is not limited to these specific embodiments, and various modifications and alterations are possible within the scope of the gist of the present invention described in the claims. It can be changed.

10 Substrate 12 Lower Electrodes 13a-13f, 15a-15c Region 14 Piezoelectric Film 14a Lower Piezoelectric Film 14b Upper Piezoelectric Film 15 Bonding Surface 16 Upper Electrode 44 Crystal Grain 45 Grain Boundary 40 Transmit Filter 42 Receive Filter

Claims (12)

Including a plurality of crystal grains including a piezoelectric material,
A piezoelectric film having an average crystal grain size in a central region in the thickness direction larger than an average crystal grain size in regions on both sides of the central region in the thickness direction. The piezoelectric film according to claim 1, wherein the average crystal grain size in the central region is 1.5 times or more the average crystal grain size in the regions on both sides. A first piezoelectric film and a second piezoelectric film stacked in the thickness direction of the piezoelectric film,
The plurality of crystal grains are not continuous at the interface between the first piezoelectric film and the second piezoelectric film,
The central region includes the interface,
The average crystal grain size of the first piezoelectric film in the central region and the average crystal grain size of the second piezoelectric film in the central region are larger than the average crystal grain sizes in the regions on both sides. The piezoelectric film according to 1. The average crystal grain size in the first piezoelectric film and the second piezoelectric film increases as going from the surface opposite to the interface of the first piezoelectric film and the second piezoelectric film to the interface. Piezoelectric film. The plurality of crystal grains have a columnar structure,
The average crystal grain size viewed from the thickness direction in the central region of the thickness direction is larger than the average crystal grain size viewed from the thickness direction in the regions on both sides in the thickness direction. The piezoelectric film according to any one of items. The piezoelectric film according to claim 1, wherein the piezoelectric material is aluminum nitride. Board,
The piezoelectric film according to any one of claims 1 to 6 provided on the substrate,
A first electrode and a second electrode that are opposed to each other with at least a part of the piezoelectric film sandwiched in the thickness direction, and are in contact with the regions on both sides, respectively.
A piezoelectric device including. Board,
The piezoelectric film according to any one of claims 1 to 6 provided on the substrate,
A first electrode and a second electrode that are opposed to each other with at least a part of the piezoelectric film sandwiched in the thickness direction, and are in contact with the regions on both sides, respectively.
A resonator comprising. Board,
The piezoelectric film according to claim 3, which is provided on the substrate,
A first electrode and a second electrode that are opposed to each other with at least a part of the piezoelectric film sandwiched in the thickness direction, and are in contact with the regions on both sides, respectively.
An insertion film provided between the first piezoelectric film and the second piezoelectric film,
A resonator comprising. A filter including the resonator according to claim 8. A multiplexer comprising the filter according to claim 10. A step of forming a first piezoelectric film on the first substrate so that the crystal grain size becomes larger as it grows,
Forming a second piezoelectric film on the second substrate such that the crystal grain size increases as it grows and the growth surface is a polarization surface opposite to the growth surface of the first piezoelectric film;
Bonding a surface of the first piezoelectric film opposite to the first substrate and a surface of the second piezoelectric film opposite to the second substrate;
A method for manufacturing a piezoelectric film including:

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