Aluminum nitride film, piezoelectric device, resonator, filter and multiplexer

Abstract

To provide an aluminum nitride film which has a small tanδ.SOLUTION: An aluminum nitride film has a plurality of crystal grains, and includes at least one kind of a group 2 element and a group 12 element, and also includes a grain boundary of crystal grains. The at least one kind of element in a grain boundary region of 4 nm in width has a concentration of 1.0 atomic% or less, and the concentration of metal elements other than the at least one kind of element and aluminum when measured across the plurality of crystal grains is lower than that of the at least one kind of element. Further, the aluminum nitride film has a plurality of crystal grains, and includes a group 1 element as a metal element. When the concentration of the group 1 element is 0.3 atomic% or less when measured across the plurality of crystal grains, and the concentration of the metal element other than the group 1 element and aluminum is less than that of the group 1 element.SELECTED DRAWING: Figure 10

Description

Technical Field

[0001] The present invention relates to aluminum nitride films, piezoelectric devices, resonators, filters, and multiplexers.

Background Art

[0002] Aluminum nitride films are used as piezoelectric films in piezoelectric devices such as piezoelectric thin film resonators and elastic wave devices. It is known that the piezoelectricity is improved by adding scandium to the aluminum nitride 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). It is known that a Group 2 element (alkaline earth metal) or the like is added to the aluminum nitride film (for example, Patent Documents 3 to 5).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Summary of the Invention

Problems to be Solved by the Invention

[0004] However, when an aluminum nitride film is used in a piezoelectric device or the like, it is required to reduce tanδ.

[0005] The present invention has been made in view of the above problems, and an object thereof is to provide an aluminum nitride film having a small tanδ. **Means for Solving the Problems**

[0006] The present invention has a plurality of crystal grains, contains at least one element of a group 2 element and a group 12 element, and the concentration of the at least one element in a grain boundary region having a width of 4 nm including the grain boundaries of the crystal grains The value calculated by multiplying the average concentration of the at least one element determined by EPMA analysis by the ratio of the concentration of the at least one element in the grain boundary region to the average concentration of the at least one element determined by EDS analysis is 1.0 atomic% or less, and Using EPMA analysis when measured across the plurality of crystal grains of the concentration of a metal element other than the at least one element and aluminum average is lower than the concentration of the at least one element average is an aluminum nitride film.

[0007] In the above configuration, the value can be set to 0.07 atomic% or more.

[0008] In the above configuration, Using EPMA analysis when measured across the plurality of crystal grains of the at least one element other than aluminum concentration of the metal element average can be set to 1 / 10 or less of the concentration of the at least one element. average

[0009] In the above configuration, the at least one element can be magnesium.

[0010] Using EPMA analysis The present invention has a plurality of crystal grains, contains a group 1 element which is a metal element, Using EPMA analysis when measured across the plurality of crystal grains of the average concentration of the group 1 element is 0.3 atomic% or less, and the concentration of a metal element other than the group 1 element and aluminum average is lower than the concentration of the group 1 element average is an aluminum nitride film.

[0011] In the above configuration, the concentration of the Group 1 element in the grain boundary region having a width of 4 nm including the grain boundaries of the crystal grains The value calculated by multiplying the average concentration of the Group 1 element determined by EPMA analysis by the ratio of the concentration of the Group 1 element in the grain boundary region to the average concentration of the Group 1 element determined by EDS analysis is 0.5 atomic% or less.

[0012] In the above configuration, Using EPMA analysis when measured across the plurality of crystal grains of the average concentration of the Group 1 element

[0013] can be set to 0.05 atomic% or more. Using EPMA analysis In the above configuration, of the Group 1 element other than aluminum concentration of the metal element average when measured across the plurality of crystal grains average can be set to 1 / 10 or less of the concentration of the Group 1 element.

[0014] The present invention is a piezoelectric device including the above aluminum nitride film.

[0015] The present invention is a resonator including the above aluminum nitride film and a pair of electrodes provided so as to sandwich at least a part of the aluminum nitride film in the film thickness direction and overlap in plan view.

[0016] The present invention is a filter including the above resonator.

[0017] The present invention is a multiplexer including the above filter.

Advantages of the Invention

[0018] According to the present invention, an aluminum nitride film having a small tanδ can be provided.

Brief Description of the Drawings

[0019]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Figure 12

Figure 13

Figure 14

Mode for Carrying Out the Invention

[0020] Hereinafter, embodiments of the present invention will be described with reference to the drawings. The names of the groups of elements are based on the IUPAC (International Union of Pure and Applied Chemistry) notation.

Example

[0021] Example 1 is an example of an aluminum nitride film. The following Experiments 1 to 3 were conducted. [Experiment 1: Measurement of the active carrier concentration near grain boundaries] Samples were prepared in which an aluminum nitride film was formed on a silicon substrate. There are the following three types of samples. AlN film A: An aluminum nitride film formed by sputtering without intentionally adding impurities AlN film B: An aluminum nitride film formed by sputtering with 0.6 atomic% of magnesium (Mg) added AlN film C: An aluminum nitride film formed by MOCVD (Metal Organic Chemical Vapor Deposition) AlN films A and B are polycrystals with a columnar structure, and are almost single crystals except for the presence of dislocations in AlN film C. AlN films A and C correspond to comparative examples, and AlN film B corresponds to Example 1.

[0022] For AlN films A to C, AFM (Atomic Force Microscope) analysis and sMIM (scanning Microwave Impedance Microscope) analysis were performed on the cross-section in the planar direction of the aluminum nitride film. In the AFM analysis, the unevenness of the cross-section can be observed, and the crystal grains and grain boundaries can be observed due to the unevenness. The C intensity obtained by the sMIM analysis corresponds to the active carrier concentration contributing to conduction.

[0023] Figs. 1(a) and 1(b) are diagrams showing the AFM image and the C-intensity image of the sMIM of AlN film A in Experiment 1. In Fig. 1(a), the shading indicates the unevenness of the cross-section of the aluminum nitride film. Black indicates concave and white indicates protruding. The white region corresponds to the crystal grains 50, and the black region corresponds to the grain boundaries 52. In Fig. 1(b), it shows that the C-intensity of the sMIM increases from black to white. The C-intensity increases with a high active carrier density. Figs. 1(a) and 1(b) are the results of observing the same location with the same equipment. Comparing Fig. 1(a) and Fig. 1(b), the C-intensity is high and the active carrier concentration is high at the grain boundary 52.

[0024] Figs. 2(a) and 2(b) are diagrams showing the AFM image and the C-intensity image of the sMIM of AlN film B in Experiment 1. As shown in Figs. 2(a) and 2(b), also in AlN film B, the C-intensity of the sMIM is higher at the grain boundary 52 than at the crystal grains 50. However, in both Figs. 2(a) and 2(b), the shading is less distinct in AlN film B than in AlN film A.

[0025] Fig. 3 is a diagram showing the C-intensity image of the sMIM of AlN film C in Experiment 1. As shown in Fig. 3, in AlN film C, the shading of the C-intensity of the sMIM is more uniform than in AlN films A and B.

[0026] The C intensity of the sMIM along the dashed arrow 59 in FIGS. 1(b), 2(b) and 3 was plotted. FIG. 4 is a diagram showing the C intensity of the sMIM with respect to the positions in Experiment 1. In the AlN film A, the C intensity of the sMIM is large at the grain boundary indicated by the arrow 65, and it is considered that the active carrier concentration at the grain boundary is high. In the AlN film C, the C intensity of the sMIM is small. The C intensity of the sMIM is low at the dislocation indicated by the arrow 66. In the AlN film C, which is close to a single crystal, the overall C intensity of the sMIM is lower than that of the AlN film A. Accordingly, it is considered that the overall active carrier concentration of the AlN film C is low. In the AlN film B, the C intensity of the sMIM is about the same as that of the AlN film C. The C intensity of the sMIM is also about the same as that of the AlN film C at the grain boundary indicated by the arrow 65.

[0027] As described above, in the aluminum nitride film (AlN film A) formed by sputtering without intentionally adding impurities, the active carrier concentration near the grain boundary 52 becomes high. Accordingly, it is considered that the leakage current flowing through the aluminum nitride film increases. In the aluminum nitride film (AlN film C) formed by MOCVD, there are almost no grain boundaries and the active carrier concentration is low. In the AlN film B, by adding Mg with a valence smaller than that of Al, active carriers can be compensated. Accordingly, the active carrier concentration in the entire region and at the grain boundaries can be reduced.

[0028] [Experiment 2: Method for calculating grain boundary concentration and average concentration] The concentration of magnesium was investigated for regions sufficiently larger than the crystal grain boundaries 52 and the crystal grains 50. An aluminum nitride film doped with magnesium and hafnium (Hf) was fabricated using the sputtering method. As a target used for sputtering, an Al-Mg-Hf alloy target was used. Four targets were used to form the AlN films D to G. The Mg:Hf:Al compositions of the formed AlN films D to G and the targets used are as follows. AlN film D: Mg:Hf:Al = 4:12:84 AlN film E: Mg:Hf:Al = 7:12:81 AlN film F: Mg:Hf:Al = 12:12:76 AlN film G:Mg:Hf:Al = 14:12:74 AlN films D to G are AlN films for examining the calculation methods of the grain boundary Mg concentration and the average Mg concentration, and do not correspond to Example 1.

[0029] For the formed AlN films D to G, EDS (Energy Dispersive X-ray Spectrometry) analysis and EPMA (Electron Probe Micro Analyzer) analysis were performed. EDS analysis enables surface analysis of a local region. On the other hand, surface analysis is difficult with EPMA analysis. In EDS analysis, since characteristic X-rays of light elements are easily absorbed by the sample, the quantitative values of light elements such as N tend to be lower than the actual values. Characteristic X-rays of light elements are also easily absorbed in EPMA analysis. However, by using the ZAF method as a quantitative calculation method assuming absorption and performing correction using a standard sample, it is possible to suppress the decrease in quantitativeness due to the absorption of characteristic X-rays in light elements. Thus, EDS analysis has low quantitativeness, while EPMA analysis has high quantitativeness.

[0030] Using the EPMA analysis method, the Mg concentration and Hf concentration in a region with a diameter of about 100 μm are measured as the average Mg concentration and the average concentration in a region sufficiently wide for the crystal grains 50 (for example, a region containing 10 or more crystal grains 50).

[0031] Line EDS analysis was performed on the same AlN film as the AlN film on which EPMA analysis was performed. Fig. 5 is a diagram showing an image of a cross-section of aluminum nitride using the HAADF-STEM method in Experiment 2, and is the analysis result of AlN film F. As shown in Fig. 5, crystal grains 50 and grain boundaries 52 can be observed using the HAADF-STEM (High-Angle Annular Dark-Field Scanning Transmission Electron Microscopy) method.

[0032] FIG. 6 is a diagram showing the element concentrations along the straight line in FIG. 5 using the EDS analysis method in Experiment 2. In FIG. 6, the Mg concentration (solid line) and the Hf concentration (dashed line) with respect to the distance along the straight line 57 in FIG. 5 are shown. The spot diameter of the EDS analysis is 0.2 nm. As shown in FIG. 6, the Hf concentration is almost constant regardless of the location. Five peaks of Mg appear. Let the positions of the peaks of the Mg concentration be 54a to 54e. The positions of the peaks 54a to 54e almost coincide with the positions of the grain boundaries 52 in FIG. 5. Thus, Hf is uniformly distributed regardless of the crystal grains 50 and the grain boundaries 52. The Mg concentration is higher than that of the crystal grains 50 in the vicinity of the grain boundary 52.

[0033] In FIG. 6, the range of ±2 nm at the positions 54a to 54e is defined as the grain boundary region 56. The average value of the Mg concentration and the average value of the Hf concentration in the grain boundary region 56 are defined as the Mg concentration and the Hf concentration at the grain boundary, respectively. The average value of the overall Mg concentration and the average value of the Hf concentration in FIG. 6 are defined as the average Mg concentration and the average Hf concentration, respectively.

[0034] Table 1 is a table showing the Mg concentration and the Hf concentration calculated by EPMA analysis and EDS analysis.

Table 1

[0035] In Table 1, the average concentration of EPMA is the average Mg concentration and the average Hf concentration calculated by EPMA analysis. The average concentration of EDS is the average Mg concentration and the average Hf concentration calculated by EDS analysis. The grain boundary of EDS is the Mg concentration and the Hf concentration of the grain boundary calculated by EDS analysis. The grain boundary / average of EDS is the Mg concentration of the grain boundary with respect to the average Mg concentration and the Hf concentration of the grain boundary with respect to the average Hf concentration.

[0036] As shown in Table 1, the average concentration of EPMA is different from that of EDS. Since the EPMA analysis has higher quantification than the EDS analysis method, it is more accurate to use the average concentration of the EPMA analysis as the average concentration. On the other hand, grain boundary / average is the ratio of the values of the EDS analysis and is considered to have high accuracy. Therefore, the average Mg concentration calculated using the EPMA analysis is taken as the average Mg concentration.

[0037] Figure 7 is a diagram showing the grain boundary / average by EDS analysis with respect to the average concentration by EPMA analysis in Experiment 2. As shown in Figure 7, for Hf, the grain boundary / average is approximately 1 regardless of the average concentration. For Mg, the grain boundary / average is approximately 1.56 regardless of the average concentration. Thus, the Mg concentration at the grain boundary is about 1.56 times the Mg concentration in the entire region. Therefore, the Mg concentration obtained by multiplying the average Mg concentration calculated using the EPMA analysis by 1.56 is taken as the Mg concentration at the grain boundary.

[0038] [Experiment 3: Evaluation of Mg Concentration Dependence of tanδ] A MIM (Metal Insulator Metal) capacitor was fabricated using an aluminum nitride film doped with magnesium as a dielectric film.

[0039] Figure 8(a) is a plan view of the capacitor in Experiment 3, and Figure 8(b) is a cross-sectional view taken along line A-A of Figure 8(a). As shown in Figures 8(a) and 8(b), a lower electrode 12 is provided on a silicon (Si) substrate 10. The lower electrode 12 includes a lower film 12a and an upper film 12b. A piezoelectric film 14 is provided as a dielectric film on the lower electrode 12. An upper electrode 16 is provided on the piezoelectric film 14. The upper electrode 16 includes a lower film 16a and an upper film 16b. The region where the lower electrode 12 and the upper electrode 16 overlap with the piezoelectric film 14 interposed therebetween is the capacitor region 61. Pads 22 are provided on the lower electrode 12 and the upper electrode 16 outside the capacitor region 61.

[0040] The manufacturing conditions of the capacitor are as follows. Lower film 12a of the lower electrode 12: A chromium (Cr) film with a film thickness of 100 nm Upper film 12b of the lower electrode 12: A ruthenium (Ru) film with a film thickness of 200 nm Piezoelectric film 14: Aluminum nitride film with a film thickness of 1000 nm Lower film 16a of the upper electrode 16: Ruthenium film with a film thickness of 200 nm Upper film 16b of the upper electrode 16: Chromium film with a film thickness of 100 nm Pad 22: Titanium (Ti) film with a film thickness of 100 nm and gold (Au) film with a film thickness of 1000 nm from the substrate side Planar shape of the capacitor region 61: Square with a side length of 133 μm Capacitance: 1.6 pF The average Mg concentration measured by EPMA analysis in the fabricated capacitor is 0 atomic % to 0.81 atomic %.

[0041] Figure 9 is a diagram showing the equivalent circuit of the capacitor. As shown in Figure 9, in the equivalent circuit of the capacitor, a resistor R1 and a capacitor C0 are connected in parallel between nodes N01 and N02. A resistor Rs is connected between terminal T01 and node N01. Terminal T02 and node N02 are directly connected. Fitting is performed using the equivalent circuit of Figure 9 to obtain each lumped constant, and tanδ can be calculated from the formula of Equation 1.

[0042]

Equation

[0043] The tanδ of the fabricated capacitor was measured using an LCR meter. The measurement voltage was 10 V, and the four-terminal method was used. Figure 10(a) is a diagram showing tanδ with respect to the measurement frequency in Experiment 3, and Figure 10(b) is a diagram showing tanδ with respect to the average Mg concentration. Figure 10(a) shows the measurement results of the sample with an average Mg concentration of 0 atomic %. Figure 10(b) shows the measurement results at a measurement frequency of 1 MHz. The dots are the measurement points, and the solid line is the straight line connecting the dots. The dashed line is the line predicted from Experiment 3. As shown in Figure 10(a), tanδ increases as the measurement frequency increases. This is consistent with Equation 1 and does not conflict with the fact that tanδ increases due to the leakage current of the capacitor.

[0044] As shown in Fig. 10(b), when the average Mg concentration increases from 0 atomic %, tan δ decreases. This is because when the average Mg concentration increases, the Mg concentration near the grain boundary 52 increases. Since Mg substitutes for the Al site, it becomes an acceptor and is considered to compensate for the active carriers near the grain boundary. As a result, it is considered that the leakage current decreases and tan δ decreases. Tan δ is the smallest when the average Mg concentration is 0.2 atomic % to 0.3 atomic %, and tan δ increases when the average Mg concentration is greater than 0.2 atomic % to 0.3 atomic %. This is considered to be because the average Mg concentration increases and the active carrier concentration due to holes increases. At an average Mg concentration of 0.6 atomic %, tan δ becomes almost the same as that at an average Mg concentration of 0 atomic %.

[0045] As in Experiment 3, by setting the average Mg concentration of the aluminum nitride film to be greater than 0 atomic % and 0.6 atomic % or less, tan δ can be decreased. From Fig. 7, the Mg concentration at the grain boundary corresponds to 1.56 times the above Mg concentration. That is, by setting the Mg concentration at the grain boundary to be greater than 0 atomic % and 1.0 atomic % or less, the active carriers at the grain boundary 52 can be compensated and tan δ can be decreased. The average Mg concentration is preferably 0.05 atomic % or more, more preferably 0.1 atomic % or more. The average Mg concentration is preferably 0.5 atomic % or less, more preferably 0.4 atomic % or less. The Mg concentration at the grain boundary is preferably 0.07 atomic % or more, more preferably 0.15 atomic % or more. The Mg concentration at the grain boundary is preferably 0.8 atomic % or less, more preferably 0.7 atomic % or less.

[0046] Experiments 1 to 3 were conducted on aluminum nitride films added with magnesium. If the impurity added to the aluminum nitride film is at least one element of Group 2 elements and Group 12 elements, the active carriers at the grain boundary 52 are compensated. Magnesium, Group 2 elements, and Group 12 elements have the same valence. Therefore, the preferable ranges of the concentration at the grain boundary 52 and the overall concentration for Group 2 elements and Group 12 elements can be the ranges of Experiment 3 for magnesium.

[0047] According to Example 1, in an aluminum nitride film having a plurality of crystal grains 50 and containing at least one element M2 of a Group 2 element and a Group 12 element, the concentration of the element M2 in a grain boundary region 56 including a grain boundary 52 of the crystal grains 50 and having a width of 4 nm is set to 1.0 atomic % or less. Thereby, the active carrier concentration near the grain boundary 52 can be reduced and tan δ can be reduced. The concentration of the element M2 in the grain boundary region 56 is preferably 0.07 atomic % or more, and more preferably 0.15 atomic % or more. The concentration of the element M2 in the grain boundary region 56 is preferably 0.8 atomic % or less, and more preferably 0.7 atomic % or less. The average concentration of the element M2 is preferably 0.05 atomic % or more, and more preferably 0.1 atomic % or more. The average concentration of the element M2 is preferably 0.6 atomic % or less, more preferably 0.5 atomic % or less, and even more preferably 0.4 atomic % or less. The Group 2 element is, in addition to magnesium, for example, calcium (Ca), strontium (Sr), and barium (Ba), and the Group 12 element is, for example, zinc (Zn).

[0048] Also, it is considered that the mechanical bonding strength of the crystal grain boundaries increases due to the precipitation of dopants such as Mg at the crystal grain boundaries of the aluminum nitride film. Thereby, an improvement in the mechanical strength of the piezoelectric film 14 can be expected. When this piezoelectric film 14 is applied to a piezoelectric thin film resonator, it can be expected that the frequency of occurrence of cracks in the piezoelectric film 14 can be reduced.

[0049] The average concentration is the concentration obtained by measuring across a plurality of crystal grains 50. For example, it is obtained by performing EPMA analysis on a range including two or more or three or more adjacent crystal grains 50. The concentration in the grain boundary region 56 is obtained, for example, in a range including three or more adjacent crystal grains 50, by determining the concentration of the grain boundary region 56 and the EDS average concentration by line analysis of EDS, and calculating it as the grain boundary concentration of EDS / EDS average concentration × EPMA average concentration. When performing line analysis on the concentration of element M2 in the grain boundary region 56, the grain boundary region 56 forms a line and the grain boundary 52 is included within the grain boundary region 56. When performing surface analysis on the concentration of element M2 in the grain boundary region 56, the grain boundary region 56 preferably includes 80% or more of the grain boundary 52 located between two adjacent crystal grains 50, and more preferably includes 100% of the grain boundary 52. When there are two or more types of element M2, the concentration obtained by summing the respective concentrations of the two or more types of element M2 may be used as the concentration of element M2.

[0050] When the aluminum nitride film contains a metal element other than element M2, the suitable concentration of element M2 will change due to electrons or holes from the metal element. Therefore, the average concentration of aluminum and the metal element other than element M2 is preferably lower than the average concentration of element M2, more preferably 1 / 5 or less of the average concentration of element M2, and even more preferably 1 / 10 or less.

[0051] To compensate for active carriers at grain boundaries, a Group 1 element M1 may be added to the aluminum nitride film instead of the element M2. The Group 1 element M1 is a metallic element and does not contain hydrogen H. When the Group 1 element M1 substitutes for the Al site, twice as many holes are generated compared to the element M2. Therefore, the concentration of the element M1 in the grain boundary region 56 is greater than 0 atomic %, preferably 0.03 atomic % or more, and more preferably 0.07 atomic % or more. The concentration of the element M2 in the grain boundary region 56 is preferably 0.5 atomic % or less, more preferably 0.4 atomic % or less, and even more preferably 0.3 atomic % or less. The average concentration of the element M1 is greater than 0, preferably 0.02 atomic % or more, and more preferably 0.05 atomic % or more. The average concentration of the element M1 is preferably 0.3 atomic % or less, more preferably 0.25 atomic % or less, and even more preferably 0.2 atomic % or less. The Group 1 element M1 is, for example, lithium (Li), sodium (Na), and potassium (K).

[0052] The aluminum nitride film is a film mainly composed of aluminum nitride. Being mainly composed of aluminum nitride allows for the inclusion of impurities, either intentionally or unintentionally. For the aluminum nitride film, for example, the total concentration of aluminum and nitrogen is 50 atomic % or more, 80 atomic % or more, and each of the concentrations of aluminum and nitrogen is, for example, 10 atomic % or more, 20 atomic % or more.

Example

[0053] Example 2 is an example in which the aluminum nitride film of Example 1 is used in a piezoelectric thin film resonator. Fig. 11(a) is a plan view of the piezoelectric thin film resonator according to Example 2, and Figs. 11(b) and 11(c) are cross-sectional views taken along the line A-A of Fig. 11(a). Fig. 11(b) shows a cross-sectional view of a series resonator of, for example, a ladder type filter, and Fig. 11(c) shows a cross-sectional view of a parallel resonator of, for example, a ladder type filter.

[0054] Referring to FIGS. 11(a) and 11(b), the structure of the series resonator S will be described. A lower electrode 12 is provided on a substrate 10. A gap 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 having a shape such that the height of the gap 30 is small around the periphery of the gap 30 and the height of the gap 30 becomes larger toward the inside of the gap 30. The substrate 10 is, for example, a silicon substrate. The lower electrode 12 includes a lower film 12a and an upper film 12b. The lower film 12a and the upper film 12b are, for example, a chromium film and a ruthenium film, respectively.

[0055] A piezoelectric film 14 is provided on the lower electrode 12. The piezoelectric film 14 is an aluminum nitride film mainly composed of aluminum nitride having the (0001) direction as the main axis, and is the aluminum nitride film of Example 1. The piezoelectric film 14 includes a lower piezoelectric film 14a provided on the lower electrode 12 and an upper piezoelectric film 14b provided on the lower piezoelectric film 14a.

[0056] 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 62 within the resonance region 60 and is not provided in the central region 64. That is, the insertion film 28 is provided so as to surround the central region 64. The insertion film 28 is continuously provided from the outer peripheral region 62 to the outside of the resonance region 60.

[0057] An upper electrode 16 is provided on the piezoelectric film 14. The resonance region 60 is a region where the lower electrode 12 and the upper electrode face each other with the piezoelectric film 14 interposed therebetween in plan view, and is a region where elastic waves in the thickness longitudinal vibration mode resonate. In plan view, the gap 30 includes the resonance region 60. In plan view, the size of the gap 30 is the same as or larger than the resonance region 60. The planar shape of the resonance region 60 is an elliptical shape. The upper electrode 16 includes a lower film 16a and an upper film 16b. The lower film 16a and the upper film 16b are, for example, a ruthenium film and a chromium film, respectively.

[0058] On the upper electrode 16, a silicon oxide film is formed as the frequency adjustment film 24. The laminated film 18 in the resonance region 60 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.

[0059] In the region where the lower electrode 12 is drawn out from the resonance region 60, the outer periphery of the upper piezoelectric film 14b is located outside the outer periphery of the resonance region 60, and the outer periphery of the lower piezoelectric film 14a is located outside the outer periphery of the upper piezoelectric film 14b. The outer periphery of the insertion film 28 substantially coincides with the outer periphery of the lower piezoelectric film 14a. Thereby, a step is formed in the piezoelectric film 14.

[0060] As shown in Fig. 11(a), an 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 by the piezoelectric film 14, and the lower electrode 12 has a hole 35 at the tip of the introduction path 33.

[0061] Referring to Figs. 11(a) and 11(c), the structure of the parallel resonator P will be described. Compared with the series resonator S, a mass loading film 20 made of a titanium layer is provided between the lower film 16a and the upper film 16b of the upper electrode 16. Therefore, the laminated film 18 includes the mass loading film 20 formed on the entire surface in the resonance region 60 in addition to the laminated film of the series resonator S. Other configurations are the same as those in Fig. 1(b) of the series resonator S and the description thereof is omitted.

[0062] The difference in the resonance frequencies between the series resonator S and the parallel resonator P is adjusted using the film thickness of the mass loading 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.

[0063] In the case of a piezoelectric thin-film resonator having a resonance frequency of 2 GHz, the film thickness of the lower film 12a made of a Cr film of the lower electrode 12 is 100 nm, and the film thickness of the upper film 12b made of a ruthenium film is 250 nm. The film thickness of the piezoelectric film 14 made of an aluminum nitride film is 1100 nm. The film thickness of the insertion film 28 made of a silicon oxide film is 150 nm. The film thickness of the lower film 16a made of a ruthenium film of the upper electrode 16 is 250 nm, and the film thickness of the upper film 16b made of a chromium film is 50 nm. The film thickness of the frequency adjustment film 24 made of a silicon oxide film is 50 nm. The film thickness of the mass loading 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.

[0064] As the substrate 10, in addition to a silicon substrate, a sapphire substrate, an alumina substrate, a spinel 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, in addition to ruthenium and chromium, a single-layer film of aluminum (Al), titanium, copper (Cu), molybdenum (Mo), tungsten (W), tantalum (Ta), platinum (Pt), rhodium (Rh), iridium (Ir), or the like, or a laminated film thereof can be used. For example, the lower film 16a of the upper electrode 16 may be ruthenium and the upper film 16b may be molybdenum.

[0065] The insertion film 28 is a material having a Young's modulus and / or acoustic impedance smaller than those of the piezoelectric film 14. As the insertion film 28, in addition to silicon oxide, a single-layer film of aluminum, gold, copper, titanium, platinum, tantalum, chromium, or the like, or a laminated film thereof can be used.

[0066] As the frequency adjustment film, in addition to the silicon oxide film, a silicon nitride film, aluminum nitride, or the like can also be used. As the mass loading film 20, in addition to titanium, a single-layer film of the metals exemplified as the lower electrode 12 and the upper electrode 16 can also be used. As the mass loading film 20, for example, an insulating film made of a metal nitride or metal oxide such as silicon nitride or silicon oxide can also be used. The mass loading film 20 can be formed not only between the layers of the upper electrode 16 (between the lower film 16a and the upper film 16b), but also under the lower electrode 12, between the layers of the lower electrode 12, above the upper electrode 16, between the lower electrode 12 and the piezoelectric film 14, or between the piezoelectric film 14 and the upper electrode 16. The mass loading film 20 may be larger than the resonance region 60 as long as it is formed to include the resonance region 60.

[0067] [Experiment 4: Evaluation of the Correlation between Q Value and tanδ] Using the aluminum nitride film of Experiment 2, piezoelectric thin film resonators D to H were fabricated, and the correlation between the Q value and tanδ of the resonators was investigated. The targets used in resonators D to H are as follows. Resonator D: Mg:Hf:Al = 4:12:84 Resonator E: Mg:Hf:Al = 7:12:81 Resonator F: Mg:Hf:Al = 12:12:76 Resonator G: Mg:Hf:Al = 14:12:74 Resonator H: Mg:Hf:Al = 0:12:88 Resonators D to H are resonators for evaluating the correlation between the Q value and tanδ, and do not correspond to Example 2.

[0068] The fabrication conditions of the resonators are as follows. Lower film 12a of the lower electrode 12: Chromium film with a thickness of 100 nm Upper film 12b of the lower electrode 12: Ruthenium film with a thickness of 210 nm Piezoelectric film 14: Aluminum nitride film with a thickness of 1150 nm Insertion film 28: Silicon oxide film with a thickness of 150 nm Lower film 16a of the upper electrode 16: Ruthenium film with a thickness of 230 nm Upper film 16b of the upper electrode 16: Chromium film with a thickness of 20 nm Frequency adjustment film 24: Silicon oxide film with a film thickness of 50 nm Mass loading film 20: None

[0069] The Q value Qa of the anti-resonant frequency of resonators D to H was obtained by using a network analyzer to acquire the S11 characteristics from 10 MHz to 5000 MHz at 1 MHz intervals and fitting them to the equivalent circuit of the piezoelectric thin film resonator. Using the S11 from 50 MHz to 400 MHz and from 3000 MHz to 4500 MHz, the tanδ of resonators D to H at a frequency of 2000 MHz was obtained.

[0070] Fig. 12 is a diagram showing the Q value Qa of the anti-resonant frequency with respect to tanδ in Experiment 4. The dots indicate the measurement points, and the straight line indicates the approximate straight line. As shown in Fig. 12, there is a negative correlation between tanδ at 2000 MHz and the Q value Qa of the anti-resonant frequency. In order to improve the Q value, it is preferable to reduce tanδ. Therefore, by using the aluminum nitride film of Example 1 for the piezoelectric film 14 of Example 1, tanδ can be reduced and the Q value can be improved.

[0071] [Modification Example 1 of Example 2] Fig. 13(a) is a cross-sectional view of the piezoelectric thin film resonator according to Modification Example 1 of Example 2. As shown in Fig. 13(a), in Modification Example 1 of Example 2, the insertion film 28 is not provided. Other configurations are the same as those of Example 2 and the description thereof is omitted. The insertion film 28 may not be provided as in Modification Example 1 of Example 1.

[0072] [Modification Example 2 of Example 2] Modifications 2 and 3 of Example 2 are examples in which the configuration of the voids is changed. FIGS. 13(b) and 13(c) are cross-sectional views of the piezoelectric thin film resonator according to Modifications 2 and 3 of Example 1, respectively. As shown in FIG. 13(b), in Modification 2 of Example 2, a depression 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 60. Other configurations are the same as those of Example 2 and the description thereof is omitted. The void 30 may be formed so as to penetrate the substrate 10.

[0073] [Modification 3 of Example 2] As shown in FIG. 13(c), in Modification 3 of Example 2, an acoustic reflection film 31 is formed under the lower electrode 12 in the resonance region 60. The acoustic reflection film 31 is provided with a film 30a having a low acoustic impedance and a film 30b having a high acoustic impedance alternately. The film thicknesses of the films 30a and 30b are, for example, approximately λ / 4 (λ is the wavelength of the elastic wave) respectively. The number of stacked layers of the film 30a and the film 30b can be arbitrarily set. The acoustic reflection film 31 only needs to have at least two types of layers with different acoustic characteristics stacked with a gap therebetween. Further, 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 a film having a different acoustic impedance is provided in the substrate 10. Other configurations are the same as those of Example 2 and the description thereof is omitted.

[0074] In Example 2 and Modification 1 thereof, a void 30 similar to that of Modification 2 of Example 2 may be formed, or an acoustic reflection film 31 may be formed instead of the void 30 as in Modification 3 of Example 2.

[0075] As in Example 2 and its 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 60. Also, as in Modification 3 of Example 2, the piezoelectric thin film resonator may be an SMR (Solidly Mounted Resonator) including an acoustic reflection film 31 that reflects an elastic wave propagating through the piezoelectric film 14 under the lower electrode 12 in the resonance region 60. The acoustic reflection layer including the resonance region 60 may include the void 30 or the acoustic reflection film 31.

[0076] In Example 2 and its Modifications 2 and 3, the insertion film 28 is provided in the outer peripheral region 62 of the resonance region 60, but the insertion film 28 may be provided in at least a part of the outer peripheral region 62 of the resonance region 60. The insertion film 28 does not have to be provided outside the resonance region 60. As in Modification 1 of Example 2, the insertion film 28 does not have to be provided. Although the planar shape of the resonance region 60 has been described by taking an elliptical shape as an example, it may be a polygonal shape such as a square shape or a pentagonal shape.

[0077] In Example 2 and its modifications, as a piezoelectric device using an aluminum nitride film, a piezoelectric thin film resonator having an upper electrode 16 and a lower electrode 12 (a pair of electrodes) provided so as to sandwich at least a part of the aluminum nitride film in the film thickness direction and overlap in a plan view has been described as an example. The piezoelectric device may be an elastic wave device having an electrode that excites an elastic wave propagating through the aluminum nitride film. For example, a resonator using a Lamb wave in which a comb-shaped electrode is provided on the aluminum nitride film may be used.

Example

[0078] Example 3 is an example of a filter and a duplexer using the piezoelectric thin film resonators of Example 2 and its modified examples. FIG. 14(a) is a circuit diagram of the filter according to Example 3. As shown in FIG. 14(a), between the input terminal Tin and the output terminal Tout, 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 P4 are connected in parallel. The piezoelectric thin film resonators of Example 2 and its modified examples can be used for at least one resonator among one or more series resonators S1 to S4 and one or more parallel resonators P1 to P4. The number of resonators of the ladder type filter and the like can be set as appropriate.

[0079] FIG. 14(b) is a circuit diagram of the duplexer according to Modified Example 1 of Example 3. As shown in FIG. 14(b), a transmission filter 40 is connected between the common terminal Ant and the transmission terminal Tx. A reception filter 42 is connected between the common terminal Ant and the reception terminal Rx. The transmission filter 40 passes the signal in the transmission band among the signals input from the transmission terminal Tx as a transmission signal to the common terminal Ant, 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 as a reception signal to the reception terminal Rx, and suppresses signals of other frequencies. At least one of the transmission filter 40 and the reception filter 42 can be the filter of Example 3.

[0080] Although the duplexer is described as an example of the multiplexer, a triplexer or a quadplexer may also be used.

[0081] As piezoelectric devices, elastic wave devices, filters, and multiplexers have been described, but the piezoelectric device may also be an actuator such as a micropump using an inkjet, an RF (Radio Frequency)-MEMS (Micro Electro Mechanical System) switch, or an optical mirror. The piezoelectric device may also be a sensor such as an acceleration sensor, a gyro sensor, or an energy harvester, or a MEMS element.

[0082] As described above in detail with respect to the embodiments of the present invention, the present invention is not limited to such specific embodiments, and various modifications and changes are possible within the scope of the gist of the present invention described in the claims.

Explanation of Reference Numerals

[0083] 10 Substrate 12 Lower electrode 14 Piezoelectric film 16 Upper electrode 50 Crystal grains 52 Grain boundaries

Claims

1. An aluminum nitride film having a plurality of crystal grains, containing at least one element selected from Group 2 elements and Group 12 elements, and having a ratio of the concentration of the at least one element in a grain boundary region including the grain boundaries of the crystal grains and having a width of 4 nm to the average concentration of the at least one element determined by EDS analysis with respect to the average concentration of the at least one element determined by EPMA analysis is 1.0 atomic % or less, and the average concentration of the at least one element and a metal element other than aluminum measured across the plurality of crystal grains using EPMA analysis is lower than the average concentration of the at least one element.

2. The aluminum nitride film according to Claim 1, wherein the value is 0.07 atomic % or more.

3. The aluminum nitride film according to Claim 1 or 2, wherein the average concentration of the at least one element and a metal element other than aluminum measured across the plurality of crystal grains using EPMA analysis is 1 / 10 or less of the average concentration of the at least one element.

4. The aluminum nitride film according to any one of Claims 1 to 3, wherein the at least one element is magnesium.

5. An aluminum nitride film having a plurality of crystal grains, containing a Group 1 element which is a metal element, and having an average concentration of the Group 1 element measured across the plurality of crystal grains using EPMA analysis of 0.3 atomic % or less, and the average concentration of a metal element other than the Group 1 element and aluminum is lower than the average concentration of the Group 1 element.

6. The aluminum nitride film according to Claim 5, wherein a value calculated by multiplying the ratio of the concentration of the Group 1 element in a grain boundary region including the grain boundaries of the crystal grains and having a width of 4 nm to the average concentration of the Group 1 element determined by EDS analysis with respect to the average concentration of the Group 1 element determined by EPMA analysis is 0.5 atomic % or less.

7. The aluminum nitride film according to Claim 5 or 6, wherein the average concentration of the Group 1 element measured across the plurality of crystal grains using EPMA analysis is 0.05 atomic % or more.

8. The average concentration of the Group 1 element and the metal elements other than aluminum, when measured across the plurality of crystal grains using EPMA analysis, is 1 / 10 or less of the average concentration of the Group 1 element. The aluminum nitride film according to any one of claims 5 to 7.

9. A piezoelectric device comprising the aluminum nitride film according to any one of claims 1 to 8.

10. The aluminum nitride film according to any one of claims 1 to 8, and a pair of electrodes provided so as to sandwich at least a part of the aluminum nitride film in the film thickness direction and overlap in a plan view. A resonator comprising:

11. A filter comprising the resonator according to claim 10.

12. A multiplexer comprising the filter according to claim 11.

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