Elastic wave device, filter, multiplexer, and method for manufacturing an elastic wave device

Abstract

To provide an elastic wave device capable of suppressing the peeling of piezoelectric films. [Solution] The elastic wave device 100 comprises a substrate 10, a lower electrode 20 provided on the substrate 10, a piezoelectric film 30 provided on the lower electrode 20, which mainly contains aluminum nitride and has a ratio of Lc to La (Lc / La) greater than 1.6008 and less than or equal to 1.6053 when the lattice constant of the a axis of aluminum nitride is La and the lattice constant of the c axis is Lc, and an upper electrode 40 provided on the piezoelectric film 30, which forms a resonance region 50 facing the lower electrode 20 with the piezoelectric film 30 in between.

Description

Technical Field

[0001] The present invention relates to an elastic wave device, a filter, a multiplexer, and a method for manufacturing an elastic wave device.

Background Art

[0002] Piezoelectric thin film resonators are used in filters and duplexers for high-frequency circuits of wireless terminals such as mobile terminals. The piezoelectric thin film resonator has a structure in which a lower electrode, a piezoelectric film, and an upper electrode are laminated on a substrate. The region where the lower electrode and the upper electrode face each other with the piezoelectric film sandwiched therebetween is the resonance region. In order to increase the electromechanical coupling coefficient, it is known to use an aluminum nitride film containing a divalent element and a tetravalent element, or a divalent element and a pentavalent element, as the piezoelectric film (for example, Patent Document 1). Further, in order to suppress abnormal growth, in a piezoelectric film made of an aluminum nitride film to which another element such as scandium is added, the ratio of the lattice constant of the c-axis to the lattice constant of the a-axis in the region on the lower electrode side is made larger than that in the region on the upper electrode side (for example, Patent Document 2).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0004] After a certain period has elapsed since the formation of the piezoelectric thin film resonator, peeling may occur between the piezoelectric film and the lower electrode.

[0005] The present invention has been made in view of the above problems, and an object thereof is to suppress peeling of the piezoelectric film.

Means for Solving the Problems

[0006] The present invention relates to an elastic wave device comprising: a substrate; a lower electrode provided on the substrate; a piezoelectric film provided on the lower electrode, the piezoelectric film having aluminum nitride as its main component, wherein the ratio of Lc to La (Lc / La) is greater than 1.6008 and less than or equal to 1.6053 when the lattice constant of the a-axis of the aluminum nitride is La and the lattice constant of the c-axis is Lc; and an upper electrode provided on the piezoelectric film, forming a resonant region that sandwiches the piezoelectric film and faces the lower electrode.

[0007] In the above configuration, the piezoelectric film can be configured such that the ratio of Lc to La (Lc / La) is 1.6027 or more and 1.6053 or less.

[0008] In the above configuration, the lower electrode can be provided with an air gap between it and the upper surface of the substrate in the resonance region.

[0009] In the above configuration, the piezoelectric film may have an opening that extends to the lower electrode outside the resonance region.

[0010] The present invention is a filter that includes the elastic wave device described above.

[0011] The present invention is a multiplexer comprising the filter described above.

[0012] The present invention is a method for manufacturing an elastic wave device, comprising the steps of: forming a lower electrode on a substrate; heating the substrate at a temperature of 157°C or lower after forming the lower electrode; forming a piezoelectric film mainly composed of aluminum nitride on the substrate after heating; and forming an upper electrode on the piezoelectric film such that a resonance region is formed between the piezoelectric film and the lower electrode.

[0013] In the above configuration, the heating step can be configured to heat the substrate at a temperature of 87°C or higher and 157°C or lower.

[0014] In the above configuration, a step of forming a sacrificial layer on the substrate before forming the lower electrode, and a step of removing the sacrificial layer after forming the upper electrode to form a gap between the lower electrode and the substrate in the resonance region can be provided.

Effects of the Invention

[0015] According to the present invention, peeling of the piezoelectric film can be suppressed.

Brief Description of the Drawings

[0016] [Figure 1] FIG. 1(a) is a plan view of the elastic wave device according to Example 1, FIG. 1(b) is a cross-sectional view taken along line A-A of FIG. 1(a), and FIG. 1(c) is a cross-sectional view taken along line B-B of FIG. 1(a). [Figure 2] FIGS. 2(a) to 2(h) are cross-sectional views showing a manufacturing method of the elastic wave device according to Example 1. [Figure 3] FIG. 3 is a cross-sectional view of the sample fabricated in the experiment. [Figure 4] FIGS. 4(a) and 4(b) are diagrams showing the experimental results of Experiment 1. [Figure 5] FIG. 5 is a cross-sectional view showing problems occurring in the piezoelectric thin film resonator. [Figure 6] FIGS. 6(a) and 6(b) are diagrams schematically showing the crystal structure of the piezoelectric film formed on the lower electrode. [Figure 7] FIG. 7 is a diagram showing the experimental results of Experiment 3. [Figure 8] FIGS. 8(a) and 8(b) are cross-sectional views of the elastic wave devices according to Modification 1 and Modification 2 of Example 1. [Figure 9] FIGS. 9(a) to 9(c) are diagrams showing the measurement results by X-ray diffraction of the piezoelectric film. [Figure 10] FIG. 10(a) is a circuit diagram of the filter according to Example ②, and FIG. 10(b) is a circuit diagram of the duplexer according to the modification of Example ②.

Modes for Carrying Out the Invention

[0017] Hereinafter, embodiments of the present invention will be described with reference to the drawings.

Embodiment

[0018] FIG. 1(a) is a plan view of the elastic wave device 100 according to Embodiment 1, FIG. 1(b) is a cross-sectional view taken along the line A-A of FIG. 1(a), and FIG. 1(c) is a cross-sectional view taken along the line B-B of FIG. 1(a). The plane directions of the substrate 10 are the X direction and the Y direction, and the thickness direction of the substrate 10 is the Z direction. As shown in FIGS. 1(a) to 1(c), the elastic wave device 100 is a piezoelectric thin film resonator having a lower electrode 20, a piezoelectric film 30, and an upper electrode 40.

[0019] The lower electrode 20 is provided on the substrate 10. A gap 12 is formed between the flat upper surface of the substrate 10 and the lower electrode 20. The gap 12 has a dome-shaped bulge. The dome-shaped bulge is a bulge having a shape such that the height of the gap 12 is small at the periphery of the gap 12 and the height of the gap 12 becomes larger toward the inside of the gap 12. It can also be said that the dome-shaped gap 12 has an arch shape in a cross-sectional view. The substrate 10 is, for example, a silicon (Si) substrate and has a thickness of, for example, 100 μm to 1000 μm. The lower electrode 20 is a laminated film of, for example, a lower layer that is a chromium (Cr) film and an upper layer that is a ruthenium (Ru) film, and has a thickness of, for example, 30 nm to 400 nm.

[0020] A piezoelectric film 30 is provided on the lower electrode 20. The piezoelectric film 30 is an aluminum nitride film mainly composed of aluminum nitride (AlN) with the (0001) direction as its principal axis (i.e., having C-axis orientation). "Main component" means that the total amount of aluminum atoms and nitrogen atoms is 50 atomic percent or more, or 80 atomic percent or more. The piezoelectric film 30 has a ratio (Lc / La) of the c-axis lattice constant to the a-axis lattice constant La, where La is the a-axis lattice constant and Lc is the c-axis lattice constant, that is greater than 1.6008 and less than or equal to 1.6053. The lattice constant of the piezoelectric film 30 can be measured, for example, using electron backscattered diffraction (EBSD). The thickness of the piezoelectric film 30 is, for example, 400 nm to 1500 nm.

[0021] An upper electrode 40 is provided on the piezoelectric film 30. The upper electrode 40 is provided on the piezoelectric film 30 with a region facing the lower electrode 20, sandwiching the piezoelectric film 30 above the air gap 12. The region where the lower electrode 20 and the upper electrode 40 face each other, sandwiching the piezoelectric film 30 above the air gap 12, is the resonance region 50. The resonance region 50 is the region where elastic waves of the thickness longitudinal vibration mode resonate. The resonance region 50 is, for example, elliptical in plan view. In plan view, the size of the air gap 12 is the same as or larger than the resonance region 50. Note that the resonance region 50 may have a polygonal shape such as a pentagon in plan view. The upper electrode 40 is, for example, a laminated film of a lower layer which is a Ru film and an upper layer which is a Cr film, with a thickness of 30 nm to 400 nm.

[0022] Below the lower electrode 20, an introduction passage 14 is provided, which is formed when etchant is introduced when forming the gap 12. The area near the tip of the introduction passage 14 is not covered with the piezoelectric film 30, and the tip of the introduction passage 14 is a hole 16. The hole 16 is an introduction opening for introducing etchant when forming the gap 12. The piezoelectric film 30 has an opening 18 on the lower electrode 20 outside the resonance region 50 to enable electrical connection with the lower electrode 20. The shortest distance L in the X direction between the edge of the piezoelectric film 30 at the opening 18 and the gap 12 is, for example, less than or equal to twice the thickness T of the lower electrode 20.

[0023] As the substrate 10, in addition to a silicon substrate, insulating substrates or semiconductor substrates such as sapphire substrates, spinel substrates, alumina substrates, quartz substrates, glass substrates, crystal substrates, ceramic substrates, or gallium arsenide substrates can be used. As the lower electrode 20 and upper electrode 40, in addition to Ru and Cr, single-layer films or multilayer films of aluminum (Al), titanium (Ti), copper (Cu), molybdenum (Mo), tungsten (W), tantalum (Ta), platinum (Pt), rhodium (Rh), or iridium (Ir) can be used.

[0024] The piezoelectric film 30 is mainly composed of AlN and may contain other elements to improve resonance characteristics or piezoelectricity. The added elements may be, for example, a combination of a group 3 element, a group 2 element, or a group 12 element and a group 4 element, or a combination of a group 2 element or a group 12 element and a group 5 element. This improves the piezoelectricity of the piezoelectric film 30 and improves the effective electromechanical coupling coefficient. Examples of group 2 elements are magnesium (Mg), calcium (Ca), or strontium (Sr). Examples of group 12 elements are zinc (Zn). Examples of group 4 elements are titanium (Ti), zirconium (Zr), or hafnium (Hf). Examples of group 5 elements are vanadium (V), niobium (Nb), or tantalum (Ta). Examples of group 3 elements are scandium (Sc). Furthermore, the piezoelectric film 30 is mainly composed of AlN and may also contain fluorine (F) or boron (B).

[0025] [Manufacturing method] Figures 2(a) to 2(h) are cross-sectional views showing the manufacturing method of the elastic wave device 100 according to Example 1. Figures 2(a) to 2(d) are cross-sections of the area corresponding to the section A and A in Figure 1(a), and Figures 2(e) to 2(h) are cross-sections of the area corresponding to the section B and C in Figure 1(a).

[0026] As shown in Figures 2(a) and 2(e), a sacrificial layer 52 is formed on the flat upper surface of the substrate 10 to form the void 12 and the introduction channel 14. The thickness of the sacrificial layer 52 is, for example, 10 nm to 100 nm. The sacrificial layer 52 is selected from materials that readily dissolve in etching solutions or etching gases, such as magnesium oxide (MgO), zinc oxide (ZnO), germanium (Ge), or silicon oxide (SiO). The sacrificial layer 52 is formed by, for example, depositing a film using a sputtering method or vacuum deposition method, and then patterning it into a desired shape using a photolithography method and an etching method. The sacrificial layer 52 may also be formed by a lift-off method. The shape of the sacrificial layer 52 corresponds to the planar shape of the void 12 and the introduction channel 14.

[0027] Next, the substrate 10 is heated to a predetermined temperature of 450°C or lower, for example, 40°C or lower. After the substrate 10 reaches the predetermined temperature, the lower electrode 20 is deposited on the substrate 10 so as to cover the sacrificial layer 52. For example, sputtering is used to deposit the lower electrode 20. After that, the lower electrode 20 is patterned into a desired shape using photolithography and etching. Note that the lower electrode 20 is deposited by vacuum deposition or CVD (Chemical Vapor Deposition). A vapor deposition method may also be used. Heating of the substrate 10 is performed to improve the film quality of the lower electrode 20. Alternatively, the lower electrode 20 may be formed without heating the substrate 10.

[0028] As shown in Figures 2(b) and 2(f), after forming the lower electrode 20, the substrate 10 is heated to a predetermined temperature of 157°C or lower. After the substrate 10 reaches the predetermined temperature, a piezoelectric film 30 is deposited on the substrate 10 so as to cover the lower electrode 20. The piezoelectric film 30 is an aluminum nitride film mainly composed of aluminum nitride (AlN), and is deposited using, for example, a sputtering method. Note that the piezoelectric film 30 may also be deposited using a CVD method or a vacuum deposition method. Heating the substrate 10 is performed to improve the film quality of the piezoelectric film 30. Next, the upper electrode 40 is deposited on the piezoelectric film 30 using, for example, a sputtering method. Note that the upper electrode 40 may also be deposited using a vacuum deposition method or a CVD method.

[0029] As shown in Figures 2(c) and 2(g), the upper electrode 40 is patterned into a desired shape using photolithography and etching. The upper electrode 40 may also be formed by the lift-off method. Next, the piezoelectric film 30 is patterned into a desired shape using photolithography and etching. Then, the holes 16 are formed by patterning the lower electrode 20 and the sacrificial layer 52 using photolithography and etching.

[0030] As shown in Figures 2(d) and 2(h), the etching solution is introduced through the hole 16 to etch the sacrificial layer 52. The stress in the laminated film consisting of the lower electrode 20, the piezoelectric film 30, and the upper electrode 40 is set to compressive stress. As a result, when the sacrificial layer 52 is removed, a dome-shaped void 12 is formed between the substrate 10 and the lower electrode 20. An introduction path 14 is also formed connecting the hole 16 and the void 12. Thus, the elastic wave device 100 according to Example 1 is formed.

[0031] [Experiment 1] Experiment 1 was conducted to evaluate the relationship between the temperature of the heat treatment performed on the substrate before deposition of the piezoelectric film and the lattice constant of the piezoelectric film. Figure 3 is a cross-sectional view of the sample prepared in the experiment. As shown in Figure 3, a sacrificial layer 9 made of MgO film was deposited on a Si substrate 1 using the sputtering method. The thickness T1 of the sacrificial layer 9 was set to 60 nm. After heating the substrate 1 to 157°C, a lower electrode 2, a laminated film consisting of a lower layer 2a made of Cr film and an upper layer 2b made of Ru film, was deposited on the sacrificial layer 9 using the sputtering method. The thickness T2 of the lower layer 2a was set to 100 nm, and the thickness T3 of the upper layer 2b was set to 220 nm. After heating the substrate 1 to 87°C, 157°C, or 251°C, a piezoelectric film 3, an aluminum nitride film mainly composed of AlN, was deposited on the lower electrode 2 using the sputtering method. The thickness T4 of the piezoelectric film 3 was set to 900 nm.

[0032] Figures 4(a) and 4(b) show the experimental results of Experiment 1. In Figure 4(a), the horizontal axis represents the heating temperature of the substrate 1 before deposition of the piezoelectric film 3, and the vertical axis represents the (002) lattice plane spacing of AlN in the piezoelectric film 3. In Figure 4(b), the horizontal axis represents the heating temperature of the substrate 1 before deposition of the piezoelectric film 3, and the vertical axis represents the (110) lattice plane spacing of AlN in the piezoelectric film 3. The lattice plane spacing is measured by X-ray diffraction (XRD). The measurement was performed using the Diffraction method.

[0033] As shown in Figures 4(a) and 4(b), the higher the heating temperature of the substrate 1 before deposition of the piezoelectric film 3, the larger the (002) lattice plane spacing of AlN in the piezoelectric film 3, and the smaller the (110) lattice plane spacing. The (002) lattice plane spacing is equal to the c-axis lattice constant (Lc) of AlN divided by 2, and the (110) lattice plane spacing is equal to the a-axis lattice constant (La) divided by 2. Therefore, it can be seen that there is a correlation between the heating temperature of the substrate 1 before deposition of the piezoelectric film 3 and the a-axis lattice constant La and c-axis lattice constant Lc of AlN in the piezoelectric film 3. In other words, the higher the heating temperature of the substrate 1 before deposition of the piezoelectric film 3, the larger the ratio of the c-axis lattice constant Lc to the a-axis lattice constant La (Lc / La) of AlN in the piezoelectric film 3.

[0034] [Challenges arising from piezoelectric thin-film resonators] Figure 5 is a cross-sectional view illustrating the problems that arise in a piezoelectric thin-film resonator. In Figure 5, a lower electrode 2 is provided on a substrate 1. A piezoelectric film 3, which is an aluminum nitride film mainly composed of aluminum nitride (AlN), is provided on the lower electrode 2. An upper electrode 4 is provided on the piezoelectric film 3. A dome-shaped gap 5 is formed between the substrate 1 and the lower electrode 2. A resonant region 6 is formed in the gap 5 where the lower electrode 2 and the upper electrode 4 face each other with the piezoelectric film 3 in between. The piezoelectric film 3 has an opening 7 on the lower electrode 2 to enable electrical connection with the lower electrode 2.

[0035] After a certain period of time (e.g., several weeks to several months) has elapsed since the manufacture of the piezoelectric thin-film resonator, the edges of the piezoelectric film 3 may peel off from the lower electrode 2, creating a gap 8. For example, if an opening 7 extending to the lower electrode 2 is provided in the piezoelectric film 3 outside the resonant region 6, the piezoelectric film 3 may peel off from the lower electrode 2 at the opening 7. One possible reason for this is that the edge of the piezoelectric film 3 at the opening 7 is located close to the gap 5. When the piezoelectric film 3 peels off from the lower electrode 2 and a gap 8 is created, the reliability and characteristics of the piezoelectric thin-film resonator deteriorate.

[0036] The reason why the piezoelectric film 3 peels off from the lower electrode 2 is not clear, but the following are possible explanations. Figures 6(a) and 6(b) schematically show the crystal structure of the piezoelectric film 3 formed on the lower electrode 2. The piezoelectric film 3 mainly contains AlN, and the crystal structure of AlN is a hexagonal wurtzite type. Generally, the lattice constant of the a-axis of AlN is known to be 3.111 Å, and the lattice constant of the c-axis is 4.980 Å. Figure 6(a) schematically shows the crystal structure of AlN when the lattice constant of the a-axis is 3.111 Å and the lattice constant of the c-axis is 4.980 Å.

[0037] The lattice constant of AlN varies depending on the temperature of the heat treatment performed on the substrate 1 before the deposition of the piezoelectric film 3, as shown in Figures 4(a) and 4(b). As the heating temperature of the substrate 1 increases, the lattice constant of the a-axis decreases, and the lattice constant of the c-axis increases. Figure 6(b) schematically shows the crystal structure of AlN when the lattice constant of the a-axis is less than 3.111 Å and the lattice constant of the c-axis is greater than 4.980 Å.

[0038] When the lattice constant of the a-axis decreases and the lattice constant of the c-axis increases, it is thought that compressive stress is generated in the piezoelectric film 3 in a direction parallel to the upper surface of the lower electrode 2. It is thought that the strain caused by the compressive stress generated in the piezoelectric film 3 becomes apparent over time, and delamination occurs at the interface between the piezoelectric film 3 and the lower electrode 2.

[0039] [Experiment 2] Based on the inferences in Figures 6(a) and 6(b), it is conceivable that there is a correlation between the ratio of the c-axis lattice constant Lc to the a-axis lattice constant La of AlN in the piezoelectric film 3 (Lc / La) and the delamination between the piezoelectric film 3 and the lower electrode 2. Therefore, the presence or absence of delamination of the piezoelectric film 3 was investigated for several piezoelectric thin film resonators manufactured by performing a heat treatment on the substrate 1 at 87°C, 157°C, or 251°C before the deposition of the piezoelectric film 3. The investigation of the presence or absence of delamination was performed on piezoelectric thin film resonators that had been manufactured for approximately two months or more. In addition, the ratio of the c-axis lattice constant Lc to the a-axis lattice constant La of AlN in the piezoelectric film 3 (Lc / La) was measured when the substrate 1 was subjected to a heat treatment at 87°C, 157°C, or 251°C before the deposition of the piezoelectric film 3 in the structure shown in Figure 3. The lattice constant was measured using X-ray diffraction. Then, the relationship between the presence or absence of delamination of the piezoelectric film 3 and the ratio of the c-axis lattice constant Lc to the a-axis lattice constant La of AlN (Lc / La) was evaluated.

[0040] Table 1 shows the experimental results for Example 2. [Table 1] In Table 1, the ratio of the lattice constant Lc on the c axis to the lattice constant La on the a axis (Lc / La) is shown as the average, maximum, and minimum values ​​measured at 25 points within the plane of substrate 1.

[0041] As shown in Table 1, when the substrate 1 was heated to 87°C or 157°C before the deposition of the piezoelectric film 3, no piezoelectric thin-film resonators exhibited delamination between the piezoelectric film 3 and the lower electrode 2. On the other hand, when the substrate 1 was heated to 251°C before the deposition of the piezoelectric film 3, some piezoelectric thin-film resonators exhibited delamination between the piezoelectric film 3 and the lower electrode 2.

[0042] When substrate 1 was heated at 87°C, the average Lc / La ratio was 1.6038, the maximum was 1.6050, and the minimum was 1.6027. When substrate 1 was heated at 157°C, the average Lc / La ratio was 1.6043, the maximum was 1.6053, and the minimum was 1.6033. When substrate 1 was heated at 251°C, the average Lc / La ratio was 1.6048, the maximum was 1.6056, and the minimum was 1.6038.

[0043] These results indicate that delamination between the piezoelectric film and the lower electrode can be suppressed by using a piezoelectric film mainly composed of AlN with an Lc / La ratio of 1.6053 or less. Furthermore, as mentioned above, the lattice constant of the a-axis of AlN is known to be 3.111 Å, and the lattice constant of the c-axis is 4.980 Å. These are considered to be the values ​​when AlN is in its most stable state. In other words, when Lc / La is 1.6008, AlN is in its most stable state, and the stress generated in the piezoelectric film is considered to be small. Therefore, taking this into consideration, it can be said that delamination between the piezoelectric film and the lower electrode can be suppressed by using a piezoelectric film mainly composed of AlN with an Lc / La ratio greater than 1.6008 and 1.6053 or less.

[0044] [Experiment 3] In Figure 3, Experiment 3 was conducted to evaluate the ratio of the c-axis lattice constant Lc to the a-axis lattice constant La of AlN in the piezoelectric film 3 (Lc / La) by preparing multiple samples with different heating temperatures for the substrate 1 before deposition of the lower electrode 2 and the substrate 1 before deposition of the piezoelectric film 3. Figure 7 shows the experimental results of Experiment 3. In Figure 7, the horizontal axis represents the heating temperature of the substrate 1 before deposition of the piezoelectric film 3, and the vertical axis represents the ratio of the c-axis lattice constant Lc to the a-axis lattice constant La of AlN in the piezoelectric film 3 (Lc / La). White circles indicate the case where the substrate 1 was not heated before deposition of the lower electrode 2, double circles indicate the case where it was heated at 40°C, and black circles indicate the case where it was heated at 450°C.

[0045] As shown in Figure 7, when the heating temperature of the substrate 1 before deposition of the piezoelectric film 3 was 157°C or lower, the Lc / La ratio of AlN in the piezoelectric film 3 was 1.6053 or lower. Furthermore, in all cases—when the substrate 1 was not heated before deposition of the lower electrode 2, when heated at 40°C, and when heated at 450°C—when the heating temperature of the substrate 1 before deposition of the piezoelectric film 3 was 157°C or lower, the Lc / La ratio of AlN in the piezoelectric film 3 was 1.6053 or lower.

[0046] According to Example 1, as explained in Figures 1(a) to 1(c), the piezoelectric film 30 mainly contains AlN, and the ratio of the c-axis lattice constant Lc to the a-axis lattice constant La of AlN (Lc / La) is greater than 1.6008 and less than or equal to 1.6053. In this case, the compressive stress generated in the piezoelectric film 30 is reduced, and as shown in Table 1, delamination between the piezoelectric film 30 and the lower electrode 20 can be suppressed. Furthermore, when Lc / La is greater than 1.6008, the heating temperature of the substrate 10 before deposition of the piezoelectric film 30 can be increased, thereby improving the film quality of the piezoelectric film 30.

[0047] As shown in Table 1, when the ratio of the c-axis lattice constant Lc to the a-axis lattice constant La of AlN (Lc / La) is between 1.6027 and 1.6053, delamination between the piezoelectric film 30 and the lower electrode 20 can be suppressed. From the viewpoint of improving the film quality of the piezoelectric film 30, it is preferable to heat the substrate 10 at a high temperature before forming the piezoelectric film 30. However, as shown in Figures 4(a) and 4(b), Lc / La tends to increase as the heating temperature of the substrate 10 increases. Even in this case, by controlling the heating temperature of the substrate 10 to keep Lc / La between 1.6027 and 1.6053, delamination of the piezoelectric film 30 can be suppressed and the film quality of the piezoelectric film 30 can be improved. From the viewpoint of suppressing peeling of the piezoelectric film 30 and improving the film quality of the piezoelectric film 30, Lc / La is preferably 1.6035 or more and 1.6053 or less, more preferably 1.6040 or more and 1.6053 or less, and even more preferably 1.6045 or more and 1.6053 or less.

[0048] According to the manufacturing method of Example 1, as shown in Figures 2(b) and 2(f), after forming the lower electrode 20, the substrate 10 is heated to a temperature of 157°C or lower. Then, a piezoelectric film 30 mainly composed of AlN is formed on the substrate 10. As a result, as shown in Figure 7, a piezoelectric film 30 mainly composed of AlN can be formed in which the ratio of the lattice constant Lc of the c axis to the lattice constant La of the a axis (Lc / La) is greater than 1.6008 and less than or equal to 1.6053. Therefore, delamination between the piezoelectric film 30 and the lower electrode 20 can be suppressed.

[0049] As shown in Figure 7, when the heating temperature of the substrate 10 is set to 87°C or higher and 157°C or lower, a piezoelectric film 30 mainly composed of AlN can be formed, where the ratio of the c-axis lattice constant Lc to the a-axis lattice constant La (Lc / La) is 1.6027 or higher and 1.6053 or lower. Therefore, peeling of the piezoelectric film 30 can be suppressed and the film quality of the piezoelectric film 30 can be improved. From the viewpoint of suppressing peeling of the piezoelectric film 30 and improving the film quality of the piezoelectric film 30, the heating temperature of the substrate 10 is preferably 100°C or higher and 157°C or lower, more preferably 110°C or higher and 157°C or lower, and even more preferably 120°C or higher and 157°C or lower.

[0050] Furthermore, in Example 1, as shown in Figures 1(b) and 1(c), the lower electrode 20 is formed with a gap 12 between it and the upper surface of the substrate 10 in the resonance region 50. The gap 12 is formed between the upper surface of the substrate 10 and the lower electrode 20 by forming a sacrificial layer 52 on the substrate 10 before forming the lower electrode 20, and then removing the sacrificial layer 52 after forming the upper electrode 40, as shown in Figures 2(a) to 2(h). When a gap 12 is formed between the upper surface of the substrate 10 and the lower electrode 20, stress is easily applied to the edge of the piezoelectric film 30 due to this gap 12, making delamination between the piezoelectric film 30 and the lower electrode 20 more likely. Therefore, in this case, it is desirable to use a piezoelectric film 30 that mainly contains AlN with an Lc / La ratio greater than 1.6008 and less than or equal to 1.6053. In particular, if the gap 12 has an arch shape in cross-section, stress is easily applied to the edge of the piezoelectric film 30. Therefore, in such cases, it is desirable to use a piezoelectric film 30 whose main component is AlN, with an Lc / La ratio greater than 1.6008 and less than or equal to 1.6053.

[0051] Furthermore, in Example 1, as shown in Figure 1(b), an opening 18 is provided in the piezoelectric film 30 outside the resonance region 50, reaching the lower electrode 20. In this case, the end of the piezoelectric film 30 at the opening 18 is formed near the void 12. For example, the shortest distance L between the end of the piezoelectric film 30 at the opening 18 and the void 12 is less than or equal to twice the thickness T of the lower electrode 20. In this case, the piezoelectric film 30 is more likely to peel off from the lower electrode 20, so it is desirable to use a piezoelectric film 30 mainly composed of aluminum nitride with an Lc / La ratio greater than 1.6008 and less than or equal to 1.6053. When the shortest distance L is less than or equal to 1.5 times the thickness T or less than or equal to 1.0 times the thickness T, the piezoelectric film 30 is even more likely to peel off from the lower electrode 20, so it is desirable to use a piezoelectric film 30 mainly composed of aluminum nitride with an Lc / La ratio greater than 1.6008 and less than or equal to 1.6053.

[0052] [Differentiation] Figure 8(a) is a cross-sectional view of an elastic wave device 110 according to a modified example 1 of Embodiment 1. As shown in Figure 8(a), the lower electrode 20 may be formed flat on a substrate 10 having a recess formed on its upper surface. The void 12 may be formed by a recess formed in the substrate 10. The void 12 may also be formed to penetrate the substrate 10.

[0053] Figure 8(b) is a cross-sectional view of an elastic wave device 120 according to a modified example 2 of Embodiment 1. As shown in Figure 8(b), an acoustic reflective film 70 may be provided below the lower electrode 20 instead of the gap 12. The acoustic reflective film 70 is made up of alternating layers of films 71 with low acoustic impedance and films 72 with high acoustic impedance. The thickness of the films 71 with low acoustic impedance and 72 with high acoustic impedance is, for example, λ / 4 each. The number of layers of films 71 with low acoustic impedance and 72 with high acoustic impedance can be set arbitrarily. The acoustic reflective film 70 only needs to consist of at least two layers with different acoustic properties stacked at intervals. Alternatively, the substrate 10 may be one of the at least two layers with different acoustic properties of the acoustic reflective film 70. For example, the acoustic reflective film 70 may consist of one layer of films with different acoustic impedances provided in the substrate 10.

[0054] In Figure 3, the film quality of piezoelectric film 3 was investigated by X-ray diffraction when the piezoelectric film 3 was deposited at 50 nm, 300 nm, and 900 nm. Figures 9(a) to 9(c) show the measurement results of piezoelectric film 3 by X-ray diffraction. Figure 9(a) shows the measurement results when piezoelectric film 3 was deposited at 50 nm. Figure 9(b) shows the measurement results when piezoelectric film 3 was deposited at 300 nm. Figure 9(c) shows the measurement results when piezoelectric film 3 was deposited at 900 nm.

[0055] As shown in Figure 9(a), when the piezoelectric film 3 was deposited at a thickness of 50 nm, the piezoelectric film 3 was amorphous when the heating temperature of the substrate 1 before deposition was 87°C or 157°C. On the other hand, when the heating temperature of the substrate 1 was 251°C, the piezoelectric film 3 was crystalline. As shown in Figures 9(b) and 9(c), when the piezoelectric film 3 was deposited at a thickness of 300 nm or more, the piezoelectric film 3 was crystalline regardless of whether the heating temperature of the substrate 1 before deposition was 87°C, 157°C, or 251°C.

[0056] From this, it can be said that in Example 1, since the heating temperature of the substrate 10 before the deposition of the piezoelectric film 30 was set to 157°C or lower, the piezoelectric film 30 in the range of 50 nm from the lower electrode 20 is amorphous. [Examples]

[0057] Figure 10(a) is a circuit diagram of the filter 200 according to Embodiment 2. As shown in Figure 10(a), one or more series resonators S1 to S4 are connected in series between the input terminal Tin and the output terminal Tout. One or more parallel resonators P1 to P3 are connected in parallel between the input terminal Tin and the output terminal Tout. The parallel resonators P1 to P3 are connected between the path between the input terminal Tin and the output terminal Tout and the ground terminal. At least one of the series resonators S1 to S4 and the parallel resonators P1 to P3 can be an elastic wave device according to Embodiment 1 and its modified form. The number of resonators in the ladder-type filter can be set as appropriate.

[0058] Figure 10(b) is a circuit diagram of a duplexer 210 according to a modified example of Embodiment 2. As shown in Figure 10(b), a transmit filter 75 is connected between the common terminal Ant and the transmit terminal Tx. A receive filter 76 is connected between the common terminal Ant and the receive terminal Rx. The transmit filter 75 allows signals in the transmit band from the signal input from the transmit terminal Tx to pass to the common terminal Ant as the transmit signal, and suppresses signals of other frequencies. The receive filter 76 allows signals in the receive band from the signal input from the common terminal Ant to pass to the receive terminal Rx as the received signal, and suppresses signals of other frequencies. At least one of the transmit filter 75 and the receive filter 76 can be the filter according to Embodiment 2. A duplexer has been described as an example of a multiplexer, but a triplexer or quadplexer may also be used.

[0059] Note that the elastic wave device is not limited to the above case; it may also be a MEMS or a sensor.

[0060] Although 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 changes are possible within the scope of the gist of the present invention as described in the claims. [Explanation of Symbols]

[0061] 1…Substrate, 2…Lower electrode, 2a…Lower layer, 2b…Upper layer, 3…Piezoelectric film, 4…Upper electrode, 5…Void, 6…Resonance region, 7…Aperture, 8…Gap, 9…Sacrificial layer, 10…Substrate, 12…Void, 14…Induction path, 16…Holes, 18…Aperture, 20…Lower electrode, 30…Piezoelectric film, 40…Upper electrode, 50…Resonance region, 52…Sacrificial layer, 70…Acoustic reflective film, 71…Low acoustic impedance film, 72…High acoustic impedance film, 75…Transmitting filter, 76…Receiving filter, 100, 110, 120…Elastic wave device, 200…Filter, 210…Duplexer

Claims

1. circuit board and The lower electrode provided on the substrate, A piezoelectric film provided on the lower electrode, containing aluminum nitride as its main component, wherein, when the lattice constant of the a-axis of the aluminum nitride is La and the lattice constant of the c-axis is Lc, the ratio of Lc to La (Lc / La) is greater than 1.6008 and less than or equal to 1.6053, An elastic wave device comprising: an upper electrode provided on the piezoelectric film, which forms a resonant region facing the lower electrode, sandwiching the piezoelectric film.

2. The elastic wave device according to claim 1, wherein the piezoelectric film has a ratio of Lc to La (Lc / La) of 1.6027 or more and 1.6053 or less.

3. The elastic wave device according to claim 1 or 2, wherein the lower electrode is provided with an air gap between it and the upper surface of the substrate in the resonance region.

4. The elastic wave device according to claim 3, wherein the piezoelectric film has an opening that reaches the lower electrode outside the resonant region.

5. A filter comprising the elastic wave device according to claim 1 or 2.

6. A multiplexer comprising the filter described in claim 5.

7. A process of forming a lower electrode on a substrate, After forming the lower electrode, the substrate is heated at a temperature of 157°C or lower. After heating, a piezoelectric film mainly composed of aluminum nitride is formed on the substrate; A method for manufacturing an elastic wave device, comprising the step of forming an upper electrode on a piezoelectric film such that a resonance region is formed on the piezoelectric film and facing the lower electrode.

8. The method for manufacturing an elastic wave device according to claim 7, wherein the heating step involves heating the substrate to a temperature of 87°C or higher and 157°C or lower.

9. A step of forming a sacrificial layer on the substrate before forming the lower electrode, A method for manufacturing an elastic wave device according to claim 7 or 8, comprising the step of removing the sacrificial layer after forming the upper electrode to form a gap between the lower electrode and the substrate in the resonance region.

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