Piezoelectric film stack structure and manufacturing method thereof

Abstract

A piezoelectric film stack structure includes a substrate (11, 25, 27, 32, 34, 41, 54) and a ScAlN film formed on the substrate. The ScAlN film has an unpaired electron density of 1.7×10 18 electrons / cm 3 (inclusive) and 1.1×10 19 electrons / cm 3 (inclusive) within the range.

Description

Technical Field

[0001] The present disclosure relates to a piezoelectric film stack structure and a method for manufacturing the same, in which a piezoelectric film and a substrate are stacked. Background Art

[0002] JP 2019-145677 A discloses a piezoelectric film stack structure comprising a substrate and a scandium-containing aluminum nitride (ScAlN) film as a piezoelectric film. ScAlN films have higher piezoelectricity than AlN films. Therefore, piezoelectric film stack structures provided with ScAlN films are expected to be applied to various devices. Summary of the Invention

[0003] When piezoelectric film stacks with ScAlN films are used in various devices, it is desirable for the ScAlN films to have a low tanδ. Tanδ is an electrical property known as dielectric loss tangent. More specifically, tanδ is a numerical value that indicates the degree to which electrical energy is lost as heat in a dielectric when an AC electric field is applied to the dielectric.

[0004] In view of the above points, an object of the present disclosure is to provide a piezoelectric film stack structure having an ScAlN film with low tan δ and a method for manufacturing the same.

[0005] A piezoelectric film stack structure includes a substrate and a scandium-containing aluminum nitride (ScAlN) film formed on the substrate. The unpaired electron density of the ScAlN film is 1.7×10 18 electrons / cm 3 (inclusive) and 1.1×10 19 electrons / cm 3 (inclusive) between.

[0006] A method for manufacturing a piezoelectric film stack structure includes forming a ScAlN film on a substrate at a first temperature, and annealing the ScAlN film by heating the ScAlN film at a second temperature higher than the first temperature so that the unpaired electron density of the ScAlN film is within 1.7×10 18 electrons / cm 3 (inclusive) and 1.1×10 19 electrons / cm 3 (inclusive) between.

[0007] Here, the present inventors have found that the controlling factor of tan δ of the ScAlN film is the unpaired electron density, and that the unpaired electron density of the ScAlN film can be controlled by setting the unpaired electron density of the ScAlN film to 1.7×10 18 electrons / cm 3 (inclusive) and 1.1×10 19 electrons / cm 3 (inclusive) to reduce the tanδ of the ScAlN film.

[0008] Therefore, according to one aspect of the present disclosure, a piezoelectric film stack structure including a ScAlN film having low tan δ can be obtained. According to another aspect of the present disclosure, a method for manufacturing a piezoelectric film stack structure including a ScAlN film having low tan δ is provided. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Figure 1 is a cross-sectional view of the piezoelectric film stack structure according to the first embodiment.

[0010] Figure 2 is a flowchart illustrating a process of forming a piezoelectric film stack structure according to the first embodiment.

[0011] Figure 3 : is a graph showing the relationship between the unpaired electron density and tan δ of a piezoelectric film stack structure prepared by the inventors.

[0012] Figure 4 : is a graph showing the relationship between annealing temperature and tan δ of a piezoelectric film stack structure prepared by the present inventors.

[0013] Figure 5 is a cross-sectional view of a microphone according to a second embodiment.

[0014] Figure 6 is a perspective view of a BAW resonator according to a third embodiment.

[0015] Figure 7 is a perspective view of a SAW device according to a fourth embodiment.

[0016] Figure 8 is a cross-sectional view of a MEMS resonator according to a fifth embodiment. DETAILED DESCRIPTION

[0017] Hereinafter, embodiments of the present disclosure will be described with reference to the accompanying drawings. In the following embodiments, the same or equivalent parts are represented by the same reference numerals.

[0018] (First embodiment)

[0019] like Figure 1 As shown, the piezoelectric film stack structure 10 of this embodiment includes a substrate 11 and a scandium-containing aluminum nitride (ScAlN) film 12. The substrate 11 and the ScAlN film 12 are stacked.

[0020] The substrate 11 serves as a base for the ScAlN film 12. The substrate 11 is in contact with the ScAlN film 12 and supports the ScAlN film 12. As a material forming the substrate 11, an electrode material or a material other than an electrode material is used. Examples of the electrode material include semiconductor materials, metal materials, conductive ceramic materials, and the like.

[0021] Examples of semiconductor materials forming the base material 11 include silicon (Si). That is, a silicon substrate can be used as the base material 11. When a silicon substrate is used, the specific resistance of the silicon substrate is arbitrarily set depending on the application.

[0022] As the metal material forming the substrate 11, a metal material whose crystallinity does not change during annealing in the production of the piezoelectric film stack structure 10, which will be described later, is preferably used. In other words, a metal material with a high melting point is preferably used. In addition, as the metal material forming the substrate 11, a metal material commonly used in semiconductor processes including micro-electromechanical systems (MEMS) is preferably used. Examples of metal materials that meet such requirements include Mo, Ti, Pt, Ru, etc.

[0023] Examples of the conductive ceramic material forming the substrate 11 include titanium compounds such as titanium nitride (TiN). In semiconductor processes, TiN is often used as an electrode material.

[0024] The substrate 11 is not limited to a single member, but may be a plurality of members. That is, the ScAlN film 12 may be formed on a plurality of members serving as the substrate 11 .

[0025] The ScAlN film 12 is a piezoelectric film made of ScAlN (ie, scandium-containing aluminum nitride) and is formed on the surface of the substrate 11 .

[0026] The scandium concentration (Sc concentration) of the ScAlN film 12 can be any concentration within the range between 0 atomic % (inclusive) and 45 atomic % (inclusive). The scandium concentration is the ratio of the number of scandium atoms to the total number of scandium atoms and aluminum atoms, which is 100 atomic %. Atom % refers to atomic percentage. The Sc concentration is measured by RBS. RBS is an abbreviation for Rutherford Backscattering Spectroscopy. The Sc concentrations shown in this specification are values ​​measured under the following measurement conditions using the following equipment.

[0027] Equipment Name: Pelletron 3SDH produced by National Electrostatics Corporation

[0028] Measurement conditions:

[0029] RBS measurement

[0030] Incident ion: 4He++

[0031] Incident energy: 2300keV

[0032] Angle of incidence: 0 degrees

[0033] Scattering angle: 160 degrees

[0034] Sampling current: 13nA

[0035] Beam diameter: 2mmφ

[0036] In-plane rotation: No

[0037] Radiation: 70μC

[0038] The unpaired electron density of ScAlN film 12 is around 1.7×10 18 electrons / cm 3 (inclusive) and 1.1×10 19 electrons / cm 3 The unpaired electron density is also referred to as the dangling bond density or the electron spin density. The unpaired electron density is measured using the electron spin resonance method. The unpaired electron density of the ScAlN film 12 shown in this specification is a value measured using the following ESR device under the following measurement conditions. ESR is the abbreviation for electron spin resonance.

[0039] Equipment name: Elexsys E580 manufactured by BRUKER

[0040] Accessory equipment: ER036TM Gaussmeter manufactured by BRUKER

[0041] Measurement conditions:

[0042] Measurement temperature: 20K

[0043] Central magnetic field: close to 3362G

[0044] Magnetic field scanning range: 1000G

[0045] Modulation: 100kHz, 5G

[0046] Microwave: 9.42GHz, 0.01mW

[0047] Scan time: 167.77s×4 times

[0048] Time constant: 327.68ms

[0049] Data points: 2048 points

[0050] Holes: Ultra-high Q

[0051] For example, the upper electrode may be present on the upper surface of the ScAlN film 12, which is the target of electron spin density measurement. Alternatively, the lower electrode may be present on the lower surface of the ScAlN film 12, which is the target of electron spin density measurement, and the substrate may be present on the lower surface of the lower electrode. In this case, when the upper electrode or the lower electrode is present, it is difficult to measure the electron spin density of the ScAlN film 12.

[0052] Therefore, when an upper electrode is present, it is removed by etching or other means. This makes it possible to measure electron spin density. When a lower electrode and substrate are present on the lower surface of the ScAlN film, the upper surface of the ScAlN film is fixed to another substrate, and the substrate located on the lower surface of the ScAlN film is removed by machining or etching. Furthermore, the lower electrode is removed by etching. This makes it possible to measure electron spin density.

[0053] Next, a method for manufacturing the piezoelectric film stack structure 10 of this embodiment will be described. Figure 2 As shown, the method for manufacturing the piezoelectric film stack structure 10 includes a substrate preparation step S1 , a film formation step S2 , and an annealing step S3 .

[0054] In the step S1 of preparing a substrate, a substrate 11 is prepared. In the step S1 of preparing a substrate, a substrate 11 made of any one of the above-mentioned materials is prepared.

[0055] In the film forming step S2, the ScAlN film 12 is formed on the surface of the substrate 11. In the film forming step S2, the substrate 11 is positioned on the film forming apparatus, and the ScAlN film 12 is formed at a predetermined film forming temperature by reactive sputtering. The film forming temperature corresponds to the first temperature in the present disclosure. After the ScAlN film 12 is formed and before annealing, the unpaired electron density of the ScAlN film 12 is greater than 1.1×10 19 electrons / cm 3 .

[0056] In the annealing step S3, the ScAlN film 12 is annealed. In the annealing step S3, the ScAlN film 12 is heated at an annealing temperature higher than the film formation temperature. The annealing temperature corresponds to the second temperature in the present disclosure. As a result, the unpaired electron density of the ScAlN film 12 is reduced compared to before annealing. Specifically, the unpaired electron density of the ScAlN film 12 is set to 1.7×10 18 electrons / cm 3 (inclusive) and 1.1×10 19 electrons / cm 3 The annealing temperature may be a value within the range of (inclusive). Examples of the annealing temperature for setting the unpaired electron density include a temperature 30°C or higher than the film formation temperature, or a temperature equal to or higher than 400°C, as described below. The annealing temperature may be a temperature lower than the melting point of the substrate 11 and the ScAlN film 12.

[0057] To illustrate an example of the process of annealing step S3, the piezoelectric film stack structure 10 having the ScAlN film 12 formed on the surface of the substrate 11 is moved from the film forming apparatus into the annealing apparatus. The annealing apparatus then heats the piezoelectric film stack structure 10 at a predetermined annealing temperature. The piezoelectric film stack structure 10 can be heated within the film forming apparatus without having to move the piezoelectric film stack structure 10 from the film forming apparatus into the annealing apparatus. In this case, after forming the ScAlN film 12, the heating temperature of the film forming apparatus is increased from the film forming temperature to the annealing temperature. Alternatively, the heating temperature of the film forming apparatus can be lowered to a value lower than the film forming temperature and then increased to the annealing temperature.

[0058] The annealing atmosphere during annealing of the ScAlN film 12 is preferably an atmosphere inert to ScAlN. Specifically, the annealing atmosphere is preferably an inert gas atmosphere or a vacuum state. Examples of inert gases include nitrogen, argon, hydrogen, or mixed gases thereof.

[0059] Here, Table 1 shows the measurement results of the electron spin density and tan δ of the piezoelectric film stacked structures 10 of Samples 1 to 10 prepared by the present inventors. Samples 1 to 10 correspond to Sample Nos. 1 to 10 in Table 1, respectively.

[0060] [Table 1]

[0061]

[0062] Samples 4 to 10 were prepared using the manufacturing method described above for this embodiment. Sample 1 is a comparative example, produced using a manufacturing method that omitted annealing step S3 from the manufacturing method described above for this embodiment. Samples 2 and 3 are comparative examples, produced using a manufacturing method in which the conditions for annealing step S3 differed from those of the manufacturing method described above for this embodiment. Electron spin density was measured using the measurement method described in this embodiment. Tan δ is the value at 1 kHz.

[0063] When manufacturing the piezoelectric film stack structures 10 of Samples 1 to 10, the present inventors used a reactive sputtering apparatus to generate plasma discharge in the film formation step S2, thereby forming the ScAlN film 12 on the surface of the silicon substrate serving as the base material 11. The film formation conditions for the ScAlN film 12 were as follows: The Si substrate temperature was the film formation temperature when the ScAlN film 12 was formed.

[0064] Target type: ScAl target

[0065] Target size: 100mm diameter

[0066] Distance between silicon substrate and target: 200mm

[0067] DC power supply: 800W

[0068] Pulse frequency: 20kHz

[0069] Pulse length: 4μs

[0070] Gas flow rate N2: 28 sccm, Ar: 28 sccm

[0071] Air pressure: 0.2Pa

[0072] Silicon substrate temperature: 370°C

[0073] Specific resistance of silicon substrate: ≥1×10 3 Ω·cm

[0074] The Sc concentration of the ScAl target used to form the ScAlN film 12 was 40 atomic %. The Sc concentration of the formed ScAlN film 12 was 30 atomic %.

[0075] The ScAlN film 12 of Sample 1 was not annealed. In Samples 2 and 3, the ScAlN film was annealed at 380° C. In Samples 4 to 10, the ScAlN film 12 was annealed at a temperature equal to or higher than 400° C., as shown in Table 1.

[0076] In the annealing step S3, the inventors used a quartz tube furnace as an annealing device. The sample was placed in an annealing device with a standby temperature of 200°C. Thereafter, the internal temperature of the annealing device was raised to a set temperature, which was determined so that the temperature of the silicon substrate and the temperature of the ScAlN film 12 were set to the annealing temperature, respectively. Then, the set temperature was maintained for 60 minutes. Then, the internal temperature of the annealing device was lowered to 200°C and the sample was removed. During the annealing process, N2 was used as the atmospheric gas in the annealing device. The pressure in the annealing device was set to 80 kPa.

[0077] Figure 3 : is a graph showing the relationship between tan δ and electron spin density (ie, unpaired electron density) of the ScAlN film 12 of samples 1 to 10 in Table 1. Figure 3 As shown, the greater the unpaired electron density, the greater the tanδ.

[0078] More specifically, with an unpaired density of 1.1×10 19 electrons / cm 3 The rate of change of tanδ is the ratio of the increase in tanδ to the increase in unpaired electron density. 18 electrons / cm 3 (inclusive) and 1.1×10 19 electrons / cm 3 When the range is between 1.1×10-1 and 1.2×10-1, as in samples 4 to 10, the rate of change of tanδ is less than that when the unpaired electron density is greater than 1.1×10-1.19 electrons / cm 3 The rate of change of tan δ when (such as in samples 1 to 3).

[0079] That is, when the unpaired electron density falls below 1.7×10 18 (inclusive) and 1.1×10 19 When the unpaired electron density is greater than 1.1×10 19 electrons / cm 3 When the unpaired electron density increases, tanδ increases significantly.

[0080] As shown in Table 1, the annealing temperatures of Samples 4 to 10 fall within the range between 400° C. (inclusive) and 800° C. (inclusive). Figure 4 : is a graph showing the relationship between tan δ and annealing temperature of the ScAlN film 12 in samples 1 to 10 of Table 1. Figure 4 As shown, when the annealing temperature falls within the range between 400° C. (inclusive) and 800° C. (inclusive) (as in Samples 4 to 10), tan δ is 0.001 or less.

[0081] As described above, the present inventors have discovered that the controlling factor of tan δ of the ScAlN film 12 is the unpaired electron density of the ScAlN film 12. It is believed that the cause of tan δ of the ScAlN film 12 is leakage current, which is caused by defects in the film, particularly dangling bonds. Therefore, the dangling bond density (i.e., the unpaired electron density) is considered to be the controlling factor of tan δ.

[0082] The present inventors have found that by setting the unpaired electron density of the ScAlN film 12 to 1.7×10 18 electrons / cm 3 (inclusive) and 1.1×10 19 electrons / cm 3 Therefore, according to the piezoelectric film stack structure 10 of this embodiment, the unpaired electron density of the ScAlN film 12 falls within 1.7×10 18 electrons / cm 3 and 1.1×10 19 electrons / cm 3 Therefore, the tan δ of the ScAlN film 12 can be maintained at a low value.

[0083] Incidentally, the following document discloses that tan δ of a ScAlN film having a Sc concentration of 30 atomic % or more is higher than 0.001.

[0084] Journal of Applied Physics, 122, 035301 (2017)

[0085] Proc. SPIE 9517, Smart Sensors, Actuators, and MEMS VII; and Cyber-Physical Systems, 95171C (May 21, 2015)

[0086] APL Materials 3, 116102 (2015)

[0087] Applied Physics Letters 97, 112902 2010

[0088] In contrast, according to the piezoelectric film stack structure 10 of the present embodiment, the tan δ of the ScAlN film 12 having the Sc concentration within the range between 30 atomic % and 45 atomic % inclusive can be equal to or less than 0.001.

[0089] Furthermore, the method for manufacturing the piezoelectric film stack structure 10 of the present embodiment includes a substrate preparation step S1, a film formation step S2, and an annealing step S3. In the annealing step S3, the ScAlN film 12 is heated at an annealing temperature higher than the film formation temperature, thereby setting the unpaired electron density of the ScAlN film 12 to 1.7×10 18 electrons / cm 3 (inclusive) and 1.1×10 19 electrons / cm 3 Therefore, since the unpaired electron density of the ScAlN film 12 falls within the range of 1.7×10 18 electrons / cm 3 (inclusive) and 1.1×10 19 electrons / cm 3 Within the range between (inclusive), the piezoelectric film stack structure 10 with low tan δ can be manufactured.

[0090] Here, when ScAlN is a single crystal without grain boundaries or crystal defects, there are no unpaired electrons in ScAlN. In contrast, when ScAlN is a polycrystalline film with grain boundaries or crystal defects, unpaired electrons exist in ScAlN. In other words, when ScAlN has crystal defects, unpaired electrons appear. Annealing the ScAlN film can repair ScAlN crystal defects and reduce unpaired electrons. Therefore, it is believed that the unpaired electron density of the ScAlN film 12 can be reduced by annealing.

[0091] In addition, as shown in Table 1, among samples 1 to 10, the unpaired electron density of samples 4 to 10 was 1.1×10 19 electrons / cm 3The annealing temperature of samples 4 to 10 was 400°C or higher. Therefore, in order to reduce the unpaired electron density of the ScAlN film 12 to 1.1×10 19 electrons / cm 3 Hereinafter, it is considered that an annealing temperature of 400° C. or higher in the annealing step S3 is effective.

[0092] The annealing temperature of samples 4 to 10 is 30°C higher than the film formation temperature. Therefore, from another perspective, in order to reduce the unpaired electron density of the ScAlN film 12 to 1.1×10 19 electrons / cm 3 Hereinafter, it is effective to set the annealing temperature in the annealing step S3 to a temperature higher than the film formation temperature by 30° C. or more.

[0093] (Second embodiment)

[0094] Figure 5 The microphone 20 of the present embodiment shown uses the piezoelectric film stack structure 10 of the first embodiment. The microphone 20 includes a pressure receiving portion 21 and a support body 22. The pressure receiving portion 21 is a film-shaped portion that receives sound pressure. The support body 22 supports the pressure receiving portion 21.

[0095] The support body 22 defines a space 23 into which the pressure receiving portion 21 deforms by receiving sound pressure. The support body 22 supports the pressure receiving portion 21 above the space 23 so that the pressure receiving portion 21 can deform when receiving sound pressure. The support body 22 is mainly made of silicon.

[0096] The pressure receiving portion 21 includes a piezoelectric film 24, a lower electrode 25, an upper electrode 26, and an insulating film 27. The piezoelectric film 24 is the same as the ScAlN film 12 of the first embodiment. The lower electrode 25 contacts the lower surface of the piezoelectric film 24. The upper electrode 26 contacts the upper surface of the piezoelectric film 24. The lower electrode 25 and the upper electrode 26 are electrodes for recovering the charge generated in the piezoelectric film 24 when the pressure receiving portion 21 is deformed. The lower electrode 25 and the upper electrode 26 are mainly made of molybdenum (Mo). The insulating film 27 covers the space 23 of the support body 22 and its periphery. The insulating film 27 is a silicon oxide film.

[0097] The lower electrode 25 is provided on a portion of the insulating film 27 located above the space 23. The piezoelectric film 24 is formed on the upper surface of the lower electrode 25 and on the surface of the portion of the insulating film 27 where the lower electrode 25 is not formed. Therefore, the lower electrode 25 and the insulating film 27 correspond to the base material 11 of the first embodiment.

[0098] In the microphone 20 configured in this manner, when the pressure receiving portion 21 receives sound pressure, the pressure receiving portion 21 deflects. When the pressure receiving portion 21 deforms into a downwardly convex shape, compressive stress is generated in the in-plane direction of the piezoelectric film 24. At this time, due to the piezoelectric effect, electric charge is generated on the surface of the piezoelectric film 24. In addition, when the pressure receiving portion 21 deforms into an upwardly convex shape, tensile stress is generated in the in-plane direction of the piezoelectric film 24. At this time, due to the piezoelectric effect, electric charge of opposite polarity to that when the compressive stress is generated is generated on the surface of the piezoelectric film 24. The generated electric charge is recovered by the lower electrode 25 and the upper electrode 26, and the sound pressure applied to the pressure receiving portion 21 can be detected.

[0099] According to this embodiment, the ScAlN film 12 of the first embodiment is used as the piezoelectric film 24. As described in the first embodiment, the tan delta of the ScAlN film 12 is kept low. Comparing the contributions of various factors to microphone noise, it can be seen that tan delta contributes the most across the entire frequency range used by the microphone. Consequently, the noise of the microphone 20 can be reduced.

[0100] In this embodiment, the pressure receiving portion 21 includes the insulating film 27. However, the insulating film 27 may be a conductive film different from the lower electrode 25. In addition, in this embodiment, the insulating film 27 is arranged so that the deflection neutral axis of the pressure receiving portion 21 does not exist in the piezoelectric film 24. When the deflection neutral axis of the pressure receiving portion 21 does not exist in the piezoelectric film 24 by making the lower electrode 25 thicker than the upper electrode 26, etc., the pressure receiving portion 21 does not need to include the insulating film 27. In addition, in this embodiment, the piezoelectric film 24, the lower electrode 25, and the upper electrode 26 have Figure 5 However, its shape is not limited to the shape shown in FIG. Figure 5 Furthermore, each of the lower electrode 25, the upper electrode 26, the support body 22, and the insulating film 27 may be made of a material other than the above-described materials.

[0101] (Third embodiment)

[0102] Figure 6 The BAW resonator 30 of this embodiment is a BAW device using the piezoelectric film stack structure 10 of the first embodiment. BAW stands for bulk acoustic wave (i.e., volume elastic wave). BAW resonator 30 includes a piezoelectric film 31, a lower electrode 32, an upper electrode 33, and a support 34.

[0103] The piezoelectric film 31 is the same as the ScAlN film 12 of the first embodiment. The lower electrode 32 is in contact with the lower surface of the piezoelectric film 31. The upper electrode 33 is in contact with the upper surface of the piezoelectric film 31. The lower electrode 32 and the upper electrode 33 are electrodes that apply an AC electric field to the piezoelectric film 31 to vibrate the piezoelectric film 31 in the film thickness direction. The lower electrode 32 and the upper electrode 33 are mainly made of molybdenum (Mo).

[0104] Support 34 supports piezoelectric film 31, lower electrode 32, and upper electrode 33. Support 34 defines a space 35 in which piezoelectric film 31 vibrates when an AC electric field acts on piezoelectric film 31. Support 34 is primarily made of silicon (Si). Lower electrode 32 faces space 35 in support 34. In this embodiment, piezoelectric film 31 is formed on the surface of lower electrode 32 and on a portion of the surface of support 34. Therefore, lower electrode 32 and support 34 correspond to substrate 11 of the first embodiment.

[0105] In the BAW resonator 30 configured in this manner, when a voltage is applied between the upper electrode 33 and the lower electrode 32, the piezoelectric film 31 is in the state of Figure 6 The tensile vibrations vibrate in the film thickness direction indicated by the arrow in the middle. When a sinusoidal voltage waveform is applied, this tensile vibration also has a sinusoidal vibration waveform. When the frequency of the tensile vibration matches the resonant frequency of the mechanical vibration, the impedance between the upper electrode 33 and the lower electrode 32 changes significantly. As a result, the BAW resonator 30 of this embodiment becomes an electric resonator. By using multiple resonators configured as described above and connecting the resonators to a circuit, filtering operation can be achieved.

[0106] According to this embodiment, the ScAlN film 12 of the first embodiment is used as the piezoelectric film 31. As described in the first embodiment, the tan δ of the ScAlN film 12 is kept low. Therefore, the Q value of the resonator can be increased. Therefore, the filter characteristics of the BAW resonator 30 can be improved.

[0107] In the BAW resonator 30 of the present embodiment, the support body 34 defines the space 35. However, the support body 34 may not define the space 35. In this case, the BAW resonator 30 may include an acoustic multilayer film located between the lower electrode 32 and the support body 34. In addition, each of the lower electrode 32, the upper electrode 33, and the support body 34 may be made of a material different from the above-mentioned materials.

[0108] (Fourth embodiment)

[0109] Figure 7 The SAW device 40 of the present embodiment shown uses the piezoelectric film stack structure 10 of the first embodiment. SAW is an abbreviation for surface acoustic wave (ie, surface elastic wave).

[0110] The SAW device 40 includes a substrate 41, a piezoelectric film 42, and comb-tooth electrodes 43. The substrate 41 is primarily made of silicon. The substrate 41 corresponds to the base material 11 of the first embodiment. The piezoelectric film 42 is the same as the ScAlN film 12 of the first embodiment. The piezoelectric film 42 is provided on the surface of the substrate 41. The comb-tooth electrodes 43 are provided on the surface of the piezoelectric film 42. The comb-tooth electrodes 43 excite SAW on the piezoelectric film 42 or receive SAW propagated through the piezoelectric film 42. The comb-tooth electrodes 43 are primarily made of molybdenum (Mo). Examples of the SAW device 40 include SAW resonators, SAW filters, and the like.

[0111] Although not shown, a 1-port SAW resonator is an example of a SAW resonator. In this SAW resonator, reflectors are arranged on both sides of the comb-tooth electrode 43 on the surface of the piezoelectric film 42. In this SAW resonator, the SAW excited at the comb-tooth electrode 43 is reflected at the two reflectors, thereby generating a standing wave. As a result, resonance is achieved. According to this embodiment, the ScAlN film 12 of the first embodiment is used as the piezoelectric film 42. As described in the first embodiment, the tanδ of the ScAlN film 12 is kept low. Therefore, the Q value of the SAW resonator can be increased. Therefore, the filtering characteristics of the SAW resonator can be improved.

[0112] Although not shown, another example of a SAW device is a transverse SAW filter. In this SAW filter, the comb-tooth electrode 43 includes an input electrode and an output electrode. The SAW excited by the input electrode propagates along the surface of the piezoelectric film 42 and is detected by the output electrode. This makes it possible to extract electrical signals in a specific frequency band. According to this embodiment, the ScAlN film 12 of the first embodiment is used as the piezoelectric film 42. As described in the first embodiment, the tan δ of the ScAlN film 12 is kept low. Therefore, the filtering characteristics of the SAW filter can be improved.

[0113] Each of the substrate 41 and the comb-teeth electrodes 43 may be made of a material other than the above-mentioned materials.

[0114] (Fifth embodiment)

[0115] Figure 8 The MEMS resonator 50 of the present embodiment shown uses the piezoelectric film stacked structure 10 of the first embodiment. MEMS is an abbreviation for micro-electromechanical system.

[0116] The MEMS resonator 50 includes a three-layer structure 51 and a support body 52. ​​The three-layer structure 51 includes a piezoelectric film 53, a lower electrode 54, and an upper electrode 55.

[0117] The piezoelectric film 53 is identical to the ScAlN film 12 of the first embodiment. A lower electrode 54 contacts the lower surface of the piezoelectric film 53. An upper electrode 55 contacts the upper surface of the piezoelectric film 53. The lower and upper electrodes 54 and 55 apply an alternating current electric field to the piezoelectric film 53, causing it to expand and contract in the plane of the piezoelectric film 53. The lower and upper electrodes 54 and 55 are primarily made of molybdenum. In this embodiment, the piezoelectric film 53 is formed on the surface of the lower electrode 54. Therefore, the lower electrode 54 corresponds to the substrate 11 of the first embodiment.

[0118] The support body 52 defines a space 56. The support body 52 supports the three-layer structure 51 so that the three-layer structure 51 can vibrate on the upper side of the space 56. In this embodiment, one end of the three-layer structure 51 in one direction is fixed to the support body 52, while the other end of the three-layer structure 51 in one direction is free. That is, the three-layer structure 51 has a so-called cantilever beam structure. The support body 52 includes a substrate 57 and an insulating film 58. The substrate 57 is mainly made of silicon (Si). The insulating film 58 is formed on the surface of the substrate 57. The insulating film 58 is a silicon oxide film. The lower electrode 54 is formed on the surface of the insulating film 58.

[0119] The thickness of the lower electrode 54 is equal to or greater than the total thickness of the upper electrode 55 and the piezoelectric film 53. Therefore, the deflection neutral axis of the three-layer structure 51 exists in the lower electrode 54. When a voltage is applied between the upper electrode 55 and the lower electrode 54, the piezoelectric film 53 expands and contracts in the plane direction of the film due to the inverse piezoelectric effect. Then, the entire three-layer structure 51 deflects. When a sinusoidal voltage waveform is applied, this deflection also becomes a sinusoidal vibration. When the frequency of the sinusoidal vibration matches the resonant frequency of the deflection vibration, the impedance between the upper electrode 55 and the lower electrode 54 changes significantly. Therefore, it becomes an electric resonator. This resonator can be used to generate the reference frequency required for the operation of an operational circuit, etc.

[0120] According to this embodiment, the ScAlN film 12 of the first embodiment is used as the piezoelectric film 53. As described in the first embodiment, the tan δ of the ScAlN film 12 is kept low. Therefore, the Q value of the resonator can be increased. Therefore, the accuracy of the generated reference frequency can be improved.

[0121] Each of the lower electrode 54, the upper electrode 55, the substrate 57, and the insulating film 58 may be made of a material other than the above-described materials. In addition, if the substrate 57 is an insulator, the insulating film 58 may not be provided.

[0122] (Other embodiments)

[0123] (1) In the first embodiment, in the method for manufacturing the piezoelectric film stack structure 10, the unpaired electron density of the ScAlN film 12 is set to 1.7×1018 electrons / cm 3 (inclusive) and 1.1×10 19 electrons / cm 3 However, if the annealing step S3 is not performed, the unpaired electron density of the ScAlN film 12 after film formation can be set to 1.7×10 18 electrons / cm 3 (inclusive) and 1.1×10 19 electrons / cm 3 For values ​​within the range of 5 and 10, inclusive, the annealing step S3 is not necessary.

[0124] (2) The present disclosure is not limited to the description of the above-mentioned embodiments, and modifications can be made within the scope of the present disclosure, and all such modifications are included in the scope of the present disclosure. The above-mentioned embodiments are not independent of each other, and can be appropriately combined with each other unless the combination is obviously impossible. In addition, in each of the above-mentioned embodiments, it is needless to say that the components of the embodiment are not necessarily essential, except for the case where the components are specifically clearly specified as essential components, the case where the components are clearly regarded as essential components in principle, etc. If specified in the above-mentioned exemplary embodiments, the quantity, value, amount, range, etc. are not necessarily limited to specific values, amounts, ranges, etc. unless it is clearly stated that the value, amount, range, etc. must be a specific value, amount, range, etc., or unless the value, amount, range, etc. must be a specific value, amount, range, etc. in principle. In addition, in each of the above-mentioned embodiments, when the materials, shapes, positional relationships, etc. of the components, etc. are mentioned, the components are not limited to the materials, shapes, positional relationships, etc. in each embodiment unless the components are specifically specified and the components are basically limited to specific materials, shapes, positional relationships, etc.

Claims

1. A piezoelectric film stack structure, comprising: substrate (11, 25, 27, 32, 34, 41, 54); and The ScAlN film (12, 24, 31, 42, 53) formed on the substrate, wherein The ScAlN film has an unpaired electron density of 8.0×10 18 electrons / cm 3 and 1.1×10 19 electrons / cm 3 Between and including 8.0×10 18 electrons / cm 3 and 1.1×10 19 electrons / cm 3 within the range.

2. The piezoelectric film stack structure according to claim 1, wherein Scandium concentration is defined as the atomic percentage of scandium, that is, the ratio of scandium atoms to the total number of scandium atoms and aluminum atoms, and The scandium concentration of the ScAlN film falls within a range between and including 30 atomic % and 45 atomic %.

3. The piezoelectric film stack structure according to claim 1, wherein The Tanδ of the ScAlN film falls within the range of 8.1×10 -4 to 1.0×10 -3 within the range.

4. A method for manufacturing a piezoelectric film stack structure, the method comprising: forming a ScAlN film (12) on a substrate (11) at a first temperature; and The ScAlN film is annealed by heating the ScAlN film at a second temperature higher than the first temperature so that the ScAlN film has an unpaired electron density of 8.0×10 18 electrons / cm 3 and 1.1×10 19 electrons / cm 3 Between and including 8.0×10 18 electrons / cm 3 and 1.1×10 19 electrons / cm 3 , wherein the second temperature is in the temperature range of 400°C to 450°C.

5. The method according to claim 4, wherein The second temperature is higher than the first temperature by more than 30°C.

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