Piezoelectric film device, transducer, and method for manufacturing piezoelectric film device
Patent Information
- Application Number
- JP2023131327
- Authority / Receiving Office
- JP · JP
- Patent Type
- Applications
- Current Assignee / Owner
- Filing Date
- 2023-08-10
- Publication Date
- 2025-02-21
Smart Images
Figure 2025026010000001_ABST
Abstract
Description
[Technical field]
[0001] The present disclosure relates to a piezoelectric membrane device, a transducer, and a method for manufacturing a piezoelectric membrane device. [Background technology]
[0002] Patent document 1 describes a transducer that includes a support having a cavity, a vibrating membrane that is disposed opposite the cavity and is capable of vibrating in the opposing direction, and a piezoelectric element formed on the surface of the vibrating membrane opposite the cavity, forming a cantilever having a fixed end and a free end. [Prior art documents] [Patent documents]
[0003] [Patent Document 1] Patent Publication No. 2021-52305
[0004] [overview] In the cantilever-type transducer described above, the only part that is constrained is the base of the cantilever, so the vibration can be made larger. On the other hand, the cantilever-type transducer may be more susceptible to sound leakage at the free end.
[0005] The present disclosure provides a piezoelectric film device, a transducer, and a method for manufacturing a piezoelectric film device that can suppress sound leakage while vibrating sufficiently.
[0006] A piezoelectric film device according to one aspect of the present disclosure comprises a substrate having a first surface and a second surface facing in opposite directions, a support portion supporting the first surface and fixed to an installation surface, and a piezoelectric film provided on the second surface and displacing in a first direction together with the substrate in response to application of a bias voltage and a drive voltage, the substrate having, as regions of the first surface, a first region supported by the support portion and a second region not supported by the support portion, and when the bias voltage and the drive voltage are not applied, the substrate is warped so that the second region is located closer to the installation surface in the first direction than the first region. [Brief description of the drawings]
[0007] [Figure 1] FIG. 1 is a plan view of a sound wave generating device according to the present embodiment. [Diagram 2] FIG. 2 is a bottom view of the sound wave generating device according to the present embodiment. [Diagram 3] FIG. 3 is a vertical cross-sectional view of the sound wave generating device taken along line III-III in FIG. 1, and is a vertical cross-sectional view of the sound wave generating device to which no bias voltage is applied. [Figure 4] FIG. 4 is a longitudinal cross-sectional view of an acoustic wave generating device with a bias voltage applied thereto. [Diagram 5] FIG. 5 is a longitudinal sectional view of an acoustic wave generating device to which a bias voltage and a driving voltage are applied. [Figure 6] FIG. 6(a) is a diagram showing the amount of displacement when a drive voltage is applied with a bias voltage of 0, and FIG. 6(b) is a diagram showing the amount of displacement when a drive voltage is applied with a bias voltage of Va. [Figure 7] FIG. 7 is a longitudinal cross-sectional view showing steps in a manufacturing procedure for an acoustic wave generating device. [Figure 8] FIG. 8 is a vertical cross-sectional view showing a step carried out following the step shown in FIG. [Figure 9] FIG. 9 is a vertical cross-sectional view showing a step carried out following the step shown in FIG. [Figure 10] FIG. 10 is a vertical cross-sectional view showing a step carried out following the step shown in FIG. [Figure 11] FIG. 11 is a vertical cross-sectional view showing a step carried out following the step shown in FIG. [Figure 12] FIG. 12 is a vertical cross-sectional view showing a step carried out following the step shown in FIG. [Figure 13] FIG. 13 is a vertical cross-sectional view showing a step carried out following the step shown in FIG. [Figure 14] FIG. 14 is a vertical cross-sectional view showing a step carried out following the step shown in FIG. [Figure 15] FIG. 15(a) is a diagram illustrating a sound wave generating device according to a comparative example, and FIG. 15(b) is a diagram illustrating a sound wave generating device according to another comparative example. [Figure 16] FIG. 16 is a diagram showing an example of sound pressure levels according to states when a bias voltage is applied. [Figure 17] FIG. 17 is a diagram illustrating further operational effects of the sound wave generating device according to this embodiment. [Figure 18] FIG. 18 is a plan view of a sound wave generating device according to a modified example. [Figure 19] FIG. 19 is a longitudinal sectional view of the sound wave generating device taken along line XIX-XIX in FIG. [Figure 20] FIG. 20 is a plan view of a sound wave generating device 1B according to a modified example. [Figure 21] FIG. 21 is a perspective view of the sound wave generating device 1B shown in FIG. 20 as viewed from above. [Figure 22] FIG. 22 is a perspective view of the sound wave generating device 1B shown in FIG. 20 as viewed from below.
[0008] [Detailed Description] Hereinafter, an embodiment will be described with reference to the drawings. In the description, the same elements or elements having the same functions are denoted by the same reference numerals, and duplicated description will be omitted.
[0009] Fig. 1 is a plan view of a sound wave generating device (transducer) according to the present embodiment. Fig. 2 is a bottom view of the sound wave generating device according to the present embodiment. Fig. 3 is a vertical cross-sectional view of the sound wave generating device taken along line III-III in Fig. 1, showing the sound wave generating device to which no bias voltage is applied.
[0010] The sound wave generating device 1 is, for example, a device including one piezoelectric film device 90. The sound wave generating device 1 is a device that generates sound waves by vibrating the air through the vibration of the piezoelectric film of the piezoelectric film device 90. The sound wave generating device 1 may be, for example, a speaker, or an ultrasonic transducer that generates ultrasonic waves. In this case, the speaker and the ultrasonic transducer both vibrate the air and can operate with roughly the same structure, but the frequency ranges used are different between the audible range and the ultrasonic range. The piezoelectric film device 90 is, for example, a MEMS (Micro Electro Mechanical Systems) device that utilizes piezoelectric characteristics. In the following, the sound wave generating device 1 will be described as a speaker.
[0011] 1 to 3, the sound wave generating device 1 includes a substrate assembly 2 and a subframe 3. The substrate assembly 2 includes a support substrate 4, a vibration film forming layer 6, and a piezoelectric element .
[0012] The support substrate 4 has a quadrangular shape (approximately rectangular) in plan view. In the following description, the longitudinal direction of the approximately rectangular support substrate 4 in plan view is defined as the X direction, the lateral direction is defined as the Y direction, and the direction intersecting the X direction and the Y direction (i.e., the thickness direction of the support substrate 4) is defined as the Z direction. Within the Z direction, the direction in which the piezoelectric element 10 is located relative to the support substrate 4 will be described as "up" and the direction in which the support substrate 4 is located relative to the piezoelectric element 10 will be described as "down".
[0013] The support substrate 4 is made of, for example, a part of an SOI substrate. Specifically, the SOI substrate includes a silicon (Si) substrate 32 as a support layer, an oxide film layer 33 as a BOX layer formed on the surface of the silicon substrate 32, and a silicon (Si) layer 34 as an active layer formed on the surface of the silicon substrate 32. In the configuration according to this embodiment, a silicon oxide (SOI2) film 35 is formed on the surface of the silicon layer 34. Among them, the support substrate 4 includes the silicon substrate 32 and the oxide film layer 33 formed on the surface of the silicon substrate 32. The thickness of the support substrate 4 is about 380 μm.
[0014] The support substrate 4 has a cavity 5 formed by a through hole penetrating in the Z direction. In a plan view, the cavity 5 has a rectangular parallelepiped shape with a rectangular opening 53 on the lower surface (bottom surface) side of the cavity 5 as the bottom surface.
[0015] The support substrate 4 includes a support portion 41, a first portion 42, a second portion 43, and a third portion 44. The support portion 41 supports a lower surface 81 of the substrate 80 and is fixed to an installation surface as described later. The support portion 41 and the second portion 43 extend along the Y direction so as to sandwich the cavity 5 between them in the X direction. The first portion 42 and the third portion 44 extend along the X direction so as to sandwich the cavity 5 between them in the Y direction. That is, the support portion 41, the first portion 42, the second portion 43, and the third portion 44 are provided so as to surround the cavity 5. In the following description, the direction in the X direction in which the second portion 43 is located with respect to the support portion 41 as a reference is referred to as the "front", and the direction in which the support portion 41 is located with respect to the second portion 43 as a reference is referred to as the "rear".
[0016] The vibration membrane forming layer 6 is formed on the support substrate 4. The vibration membrane forming layer 6 is made of a laminated film in which a silicon layer 34 and a silicon oxide film 35 are laminated in this order from the support substrate 4 side. The silicon layer 34 has a thickness of, for example, about 20 μm, and the silicon oxide film 35 has a thickness of, for example, about 0.5 μm.
[0017] The vibration membrane forming layer 6 includes a vibration membrane 7 facing the cavity 5 in a plan view, and a frame portion 8 formed to surround the cavity 5. The vibration membrane 7 has a rectangular shape that is almost similar to the cavity 5 in a plan view. The frame portion 8 includes a support frame body 181 (see FIG. 3), a first portion 182 (see FIG. 1), a second portion 183 (see FIG. 3), and a third portion 184 (see FIG. 1). The support frame body 181 constitutes a part of a substrate 80 described later, and is supported by a support portion 41. The first portion 182 is formed along the X direction on the first portion 42 of the support substrate 4 and is supported by the first portion 42. The second portion 183 is formed along the Y direction on the second portion 43 of the support substrate 4 and is supported by the second portion 43. The third portion 184 is formed along the X direction on the third portion 44 of the support substrate 4 and is supported by the third portion 44.
[0018] In the manufacturing process, the slits 9 are formed before the cavities 5 are formed in the support substrate 4 (details will be described later). In the step of forming the slits 9, the slits 9 are formed so as to penetrate the vibration membrane 7 from the surface of the interlayer insulating film 15 (described later) formed on the vibration membrane forming layer 6 and reach the support substrate 4.
[0019] 1, the slit 9 is composed of a first portion 9a and a third portion 9c formed along the X direction, and a second portion 9b formed along the Y direction. The first portion 9a is formed along a side extending in the X direction at one end side of the cavity 5 in the Y direction. The third portion 9c is formed along a side extending in the X direction at the other end side of the cavity 5 in the Y direction. The second portion 9b is formed so as to connect a front end portion of the first portion 9a and a front end portion of the third portion 9c.
[0020] The piezoelectric element 10 is formed after forming a hydrogen barrier film 14 on the vibration film forming layer 6. In detail, the piezoelectric element 10 is formed so that at least a part of it is disposed on the vibration film 7. In this embodiment, the entire piezoelectric element 10 is disposed on the vibration film 7. The piezoelectric element 10 includes a lower electrode 11 formed on the vibration film 7, a piezoelectric film 12 formed on the lower electrode 11, and an upper electrode 13 formed on the piezoelectric film 12. In addition, a hydrogen barrier film 140 is formed on the upper electrode 13 of the piezoelectric element 10.
[0021] The lower electrode 11 and the upper electrode 13 are made of a conductive metal thin film such as platinum, molybdenum, iridium, titanium, etc. The thickness of the lower electrode 11 is, for example, about 200 nm, and the thickness of the upper electrode 13 is, for example, about 80 nm.
[0022] The piezoelectric film 12 is made of, for example, lead zirconate titanate (PZT). The piezoelectric film 12 may be made of niobium-doped lead zirconate titanate (PNZT) film, lanthanum-doped lead zirconate titanate (PLZT) film, strontium barium titanate (BST) film, strontium bismuth tantalate (SBT) film, strontium barium niobate (SBN) film, lithium niobate (LiNbO3) film, barium titanate (TiBaO3) film, lanthanum strontium copper oxide (LSCO) film, potassium dihydrogen phosphate (KDP) film, potassium tantalum niobate (KTN) film, magnesium lead niobate titanate (PMN-PT) ceramic film, zinc lead niobate titanate (PZN-PT) ceramic film, aluminum nitride (AIN), zinc oxide (ZnO), lead titanate (PbTiO3), etc. The thickness of the piezoelectric film 12 is, for example, about 2 μm.
[0023] A hydrogen barrier film 14 is formed on the vibration film 7 and frame portion 8 that constitute the vibration film forming layer 6. The hydrogen barrier film 14 is formed on the vibration film 7 so as to cover the piezoelectric element 10. The hydrogen barrier film 14 is made of, for example, Al2O3 (alumina). The hydrogen barrier film 14 has a thickness of about 20 nm to 100 nm. The hydrogen barrier film 14 is provided to prevent deterioration of the characteristics of the piezoelectric film 12 due to hydrogen reduction.
[0024] An interlayer insulating film 15 is laminated on the hydrogen barrier film 14. The interlayer insulating film 15 is made of, for example, a film (TEOS film) containing tetraethoxysilane (TEOS). The interlayer insulating film 15 may be made of silicon nitride (SiN) or the like. The thickness of the interlayer insulating film 15 is, for example, about 0.2 μm to 1.5 μm.
[0025] An upper wiring 18 and a lower wiring 19 (see FIG. 1) are formed on the interlayer insulating film 15. The upper wiring 18 and the lower wiring 19 are provided at the X-direction end (rear end) of the interlayer insulating film 15 on the side where the support portion 41 is provided. The upper wiring 18 and the lower wiring 19 extend in the X-direction with a gap between them in the Y-direction. The upper wiring 18 and the lower wiring 19 may be made of a metal material containing Al (aluminum). The thickness of the upper wiring 18 and the lower wiring 19 is, for example, about 1 μm.
[0026] The front end of the upper wiring 18 is disposed above the rear end of the upper electrode 13. A contact hole 16 (see FIGS. 1 and 3) is formed between the upper wiring 18 and the upper electrode 13, continuously penetrating the hydrogen barrier film 14 and the interlayer insulating film 15. The front end of the upper wiring 18 enters the contact hole 16 and is connected to the rear end of the upper electrode 13 within the contact hole 16. A wide upper pad portion 18a is formed at the rear end of the upper wiring 18. The upper pad portion 18a is disposed on a support frame 181 of the frame portion 8.
[0027] The front end of the lower wiring 19 is disposed above the rear end of the lower electrode 11. A contact hole 17 (see FIG. 1) is formed between the lower wiring 19 and the lower electrode 11, continuously penetrating the hydrogen barrier film 14 and the interlayer insulating film 15. The front end of the lower wiring 19 enters the contact hole 17 and is connected to the rear end of the lower electrode 11 within the contact hole 17. A wide lower pad portion 19a is formed at the rear end of the lower wiring 19. The lower pad portion 19a is disposed on a support frame 181 of the frame portion 8.
[0028] A passivation film 20 is formed on the interlayer insulating film 15 so as to cover the upper wiring 18 and the lower wiring 19. The passivation film 20 is made of, for example, a film (TEOS film) containing tetraethoxysilane (TEOS). The passivation film 20 may be made of silicon nitride (SiN) or the like. The thickness of the passivation film 20 is, for example, about 0.1 μm to 1.0 μm. The passivation film 20 has an upper pad opening 21 exposing a part of the upper pad portion 18a and a lower pad opening 22 (see FIG. 1) exposing a part of the lower pad portion 19a. Hereinafter, the hydrogen barrier film 14, the interlayer insulating film 15, and the passivation film 20 may be collectively referred to as an insulating film 30.
[0029] In the substrate assembly 2, the vibration membrane 7 and the support frame 181 of the frame portion 8 form a substrate 80 having a rectangular shape in a plan view. The substrate 80 has a lower surface 81 (first surface) and an upper surface 82 (second surface) facing in opposite directions. The lower surface 81 is the lower surface of the silicon layer 34, and the upper surface 82 is the upper surface of the silicon oxide film 35. The substrate 80 has, as regions of the lower surface 81, a first region 81x supported by the support portion 41 and a second region 81y not supported by the support portion 41.
[0030] The first region 81x is supported by the upper surface of the oxide film layer 33 of the support portion 41 and fixed to the upper surface. Being supported by (the upper surface of the oxide film layer 33 of) the support portion 41 means, for example, being in contact with the support portion 41 directly or via some member and being supported by the support portion 41. Being fixed to the support portion 41 means, for example, being fixed to the support portion 41 directly or via some member and not being separated from the support portion 41. Being fixed to the support portion 41 may be, for example, integrated with the support portion 41 or may be bonded to the support portion 41. Note that in the substrate 80, the entire region supported by the support portion 41 does not necessarily have to be fixed to the support portion 41, and only a part of the supported region may be fixed to the support portion 41.
[0031] The piezoelectric film 12 is provided on an upper surface 82 of the substrate 80 (i.e., an upper surface of the silicon oxide film 35) via a lower electrode 11, and displaces in the vertical direction (first direction) together with the substrate 80 in response to application of a bias voltage and a drive voltage, which will be described later. Such a substrate 80, a support portion 41, and a piezoelectric element 10 may be collectively referred to as a piezoelectric film device 90.
[0032] The piezoelectric film device 90 has a cantilever structure in which the rear end of the substrate 80 supported by the support portion 41 is a fixed end and the front end of the substrate 80 is a free end. In the piezoelectric film device 90, the piezoelectric film 12 is displaced in the vertical direction in response to application of a bias voltage and a drive voltage, and the free end side (front end side) of the substrate 80 is displaced in the vertical direction together with the piezoelectric film 12.
[0033] As shown in FIG. 3, when no bias voltage or driving voltage is applied to the piezoelectric film 12, the substrate 80 is warped so that the second region 81y is located lower than the first region 81x (on the installation surface side in the vertical direction). That is, in the piezoelectric film device 90 having a cantilever structure, in a position (neutral position) in a state in which the piezoelectric film 12 is not deformed by an input electric signal, the free end side of the substrate 80 is held in a warped position so as to hang down toward the cavity 5 side relative to the fixed end side. When no bias voltage or driving voltage is applied to the piezoelectric film 12, the difference in the position between the first region 81x and the front end of the second region 81y in the vertical direction may be 100 μm or more and 500 μm or less when the length in the X direction of the piezoelectric film device 90 having a cantilever structure is about 3 mm. The difference in the position between the first region 81x and the second region 81y may be equal to or more than the thickness of the silicon layer 34 constituting the substrate 80.
[0034] 4, in a state where only a bias voltage is applied to the piezoelectric film 12 (a state where a bias voltage is applied and a drive voltage is not applied), the difference in the vertical position between the front end of the first region 81x and the front end of the second region 81y may be 50 μm or less (i.e., approximately horizontal). In a state where only a bias voltage is applied to the piezoelectric film 12 (a state where a bias voltage is applied and a drive voltage is not applied), the difference in the vertical position between the front end of the first region 81x and the front end of the second region 81y may be less than the thickness of the substrate 80 in the vertical direction (i.e., approximately horizontal).
[0035] Then, when a drive voltage is applied from the state in which the substrate 80 shown in Fig. 4 is substantially horizontal, the free end side (front end side) of the substrate 80 is displaced in the vertical direction (see Fig. 5). When the drive voltage is applied, the bias voltage is kept unchanged (fixed value).
[0036] Due to the characteristics of the piezoelectric film 12, in order to increase the vibration (sound) determined by the amount of displacement, it is preferable that the operating voltage range does not cross 0V and operates only on the positive side or only on the negative side. In the configuration according to this embodiment, a bias voltage is applied so that the operating voltage range of the piezoelectric film 12 is only on the positive side. Specifically, the bias voltage is set to more than half of the maximum driving voltage, and the operating voltage range of the piezoelectric film 12 is only on the positive side.
[0037] FIG. 6(a) shows the amount of displacement when a drive voltage is applied with the bias voltage set to 0, and FIG. 6(b) shows the amount of displacement when the bias voltage is V α 6(a) is a diagram showing the amount of displacement when a driving voltage (applied electric field) is applied as 2 V. As shown in FIG. α When the operating voltage range is set to 0V, the -V α ~+V α Therefore, the displacement of the piezoelectric film 12 is small, and here, d α On the other hand, as shown in Fig. 6(b), when the bias voltage is V α The driving voltage (applied electric field) is 2V. α If the operating voltage range is V α Centered on 0 to 2V α In this case, the displacement of the piezoelectric film 12 is d α greater than d β In this way, V pp Even if the voltage is the same, the amount of displacement of the piezoelectric film 12 can be increased by appropriately setting the bias voltage, and the vibration (sound) of the sound wave generating device 1 can be increased.
[0038] In the sound wave generating device 1, the amount of warping of the substrate 80 in a state where the bias voltage and the driving voltage are not applied (specifically, the amount of downward hanging at the free end side of the substrate 80) is set so that the substrate 80 is approximately horizontal in a state where the bias voltage is applied but the driving voltage is not applied. This allows the substrate 80 to be appropriately horizontal at the timing when the application of the driving voltage is started (a state where only the bias voltage is applied). As is clear from a comparison of the states of Figs. 3 to 5, the state where the substrate 80 is horizontal is a state where the gap between the front end (the end on the free end side) of the substrate 80 and the second part 183 of the frame part 8, etc. is minimized. Based on this state, the substrate 80 moves up and down in response to the application of the driving voltage, so that the size of the gap during the vertical movement of the substrate 80 can be minimized, and the effect of sound leakage can be suppressed.
[0039] As shown in FIG. 1 and FIG. 3, the subframe 3 is composed of, for example, a silicon substrate with a silicon oxide film (not shown) formed on the lower surface and the upper surface. The subframe 3 is disposed on the substrate assembly 2 so as to cover the piezoelectric film device 90. The subframe 3 is bonded to the frame portion 8 via an adhesive (not shown). The subframe 3 is composed of a first portion 131, a second portion 132, a third portion 133, and a connecting portion 134, all of which are erected in the Z direction. As shown in FIG. 1, the first portion 131 is formed along the X direction above the first portion 182 of the frame portion 8. The second portion 132 is formed along the Y direction above the second portion 183 of the frame portion 8. The third portion 133 is formed along the X direction above the third portion 184 of the frame portion 8. The connecting portion 134 is formed along the Y direction above the support frame body 181. The area surrounded by the first portion 131, the second portion 132, the third portion 133, and the connecting portion 134 is the area directly above the cavity 5, and is an accommodating recess 3a for accommodating the piezoelectric film device 90. A through hole 3b that functions as an outlet for the sound generated by the sound wave generating device 1 is formed above the accommodating recess 3a.
[0040] The provision of the above-mentioned subframe 3 prevents air leakage and improves the sound characteristics of the sound wave generating device 1. The provision of the subframe 3 also protects the cantilever-structured piezoelectric film device 90. Furthermore, the provision of the subframe 3 improves the rigidity of the chip.
[0041] In such a sound wave generating device 1, when a voltage is applied between the lower electrode 11 and the upper electrode 13, the piezoelectric film 12 is deformed by the inverse piezoelectric effect. As a result, the piezoelectric film 12 and the substrate 80 constituting the piezoelectric film device 90 are deformed with the first region 81x, which is the fixed end, as a fulcrum. When a voltage corresponding to an audio signal is continuously applied between the lower electrode 11 and the upper electrode 13, the piezoelectric film device 90 vibrates so that the free end side of the substrate 80 moves back and forth in the up and down direction. This vibration of the piezoelectric film device 90 vibrates the air around the piezoelectric film device 90, generating sound waves. The sound waves propagate to the external space through the through hole 3b of the subframe 3. Note that sound waves are also generated in the opposite direction.
[0042] Next, an example of a manufacturing procedure for the sound wave generating device 1 according to this embodiment will be described with reference to Fig. 7 to Fig. 14. Fig. 7 to Fig. 14 are vertical cross-sectional views (or plan views) showing a manufacturing procedure for the sound wave generating device 1 according to this embodiment shown in Fig. 1.
[0043] First, as shown in FIG. 7, a thermal oxidation process is performed on an SOI substrate. The SOI substrate includes a silicon substrate 32, an oxide film layer 33 formed on the surface thereof, and a silicon layer 34 formed on the surface thereof. By the thermal oxidation process, a silicon oxide film 35 is formed on the surface of the silicon layer 34 opposite to the oxide film layer 33, and a silicon oxide film 31 is formed on the surface of the silicon substrate 32 opposite to the oxide film layer 33. As a result, a workpiece 101 having one end surface 101a (upper surface of the silicon oxide film 35) and the other end surface 101b (lower surface of the silicon oxide film 31) facing in opposite directions is prepared (preparing process). Thus, the preparing process includes a process of obtaining the workpiece 101 by performing thermal oxidation on the SOI substrate. In the process of obtaining the workpiece, the film thickness of the workpiece 101 by thermal oxidation (specifically, the film thickness of the silicon oxide film 31 and the silicon oxide film 35) may be controlled so that the free end side of the substrate 80 is formed in a downwardly warped (drooping) state when each process is completed. The silicon substrate 32 and the oxide film layer 33 are parts that constitute the support substrate 4, and the silicon layer 34 and the silicon oxide film 35 are parts that constitute the vibration membrane forming layer 6.
[0044] 8, a hydrogen barrier film 14 is formed on one end surface 101a (upper surface of the silicon oxide film 35) of the workpiece 101, and then a lower electrode film 111 which is a material film of the lower electrode 11, a piezoelectric material film 112 which is a material film of the piezoelectric film 12, and an upper electrode film 113 which is a material film of the upper electrode 13 are formed in that order. Then, the upper electrode film 113, the piezoelectric material film 112, and the lower electrode film 111 are patterned, for example, in that order by photolithography and etching to form the upper electrode 13, the piezoelectric film 12, and the lower electrode 11. As a result, the piezoelectric film 12 and electrodes (the lower electrode 11 and the upper electrode 13) are formed on the one end surface 101a of the workpiece 101, and the piezoelectric element 10 is formed (film forming process).
[0045] 9, a hydrogen barrier film 140 is formed to cover the exposed surface of the piezoelectric element 10. The hydrogen barrier film 140 is made of, for example, an alumina film. An interlayer insulating film 15 is then formed on the entire surface of the hydrogen barrier film 140. Furthermore, the hydrogen barrier film 140 and the interlayer insulating film 15 are continuously etched to form contact holes 16 and 17 (see FIG. 1). At this time, the interlayer insulating film 15 and the hydrogen barrier film 14 in the slit portion are also etched.
[0046] Then, a wiring film, which is a material film of the upper wiring 18 and the lower wiring 19, is formed on the interlayer insulating film 15 including the inside of the contact holes 16 and 17, and the wiring film is patterned by photolithography and etching to form the upper wiring 18 and the lower wiring 19 (see FIG. 1). Then, a passivation film 20 is formed on the interlayer insulating film 15 so as to cover the upper wiring 18 and the lower wiring 19. The interlayer insulating film 15 and the passivation film 20 are made of, for example, a TEOS film containing tetraethoxysilane (TEOS). The interlayer insulating film 15 and the passivation film 20 may be made of, for example, silicon nitride (SiN) or the like.
[0047] 10, an upper pad opening 21 exposing a part of the upper pad portion 18a is formed in the passivation film 20 by photolithography and etching, and a lower pad opening 22 (see FIG. 1) exposing a part of the lower pad portion 19a is formed. An opening 23 is formed in the interlayer insulating film 15 and the passivation film 20 by photolithography and etching. The opening 23 may not be formed or may be partially opened. Then, a slit 9 is formed by photolithography and etching, which penetrates the vibration film forming layer 6 (silicon oxide film 35 and silicon layer 34) and the oxide film layer 33 continuously and reaches the silicon substrate 32. By forming the slit 9, a frame portion 8 which is a peripheral portion of the vibration film forming layer 6 and a vibration film 7 which is a central portion of the vibration film forming layer 6 are obtained, and a substrate assembly work-in-progress 2A before the cavity 5 is formed is obtained.
[0048] 11, an adhesive (not shown) is applied to the surface of the subframe 3 facing the board assembly work-in-progress 2A, and the subframe 3 is fixed to the board assembly work-in-progress 2A.
[0049] Next, as shown in FIG. 12, a support substrate 400 is temporarily attached to the upper surface of the subframe 3, and the silicon oxide film 31 and silicon substrate 32, which are the other end surface of the substrate assembly work-in-progress 2A (the component to be processed after film formation), are polished to thin the silicon substrate 32.
[0050] 13, a resist mask (not shown) corresponding to a region where the cavity 5 is to be formed (specifically, a region where the opening 53 is to be formed) is formed on the lower surface of the silicon substrate 32, and the silicon substrate 32 is etched from the lower surface. This forms the cavity 5, and forms the sound wave generating device 1 including the piezoelectric film device 90 having the substrate 80, the support portion 41, and the piezoelectric film 12 (forming process).
[0051] In a state where the steps up to this point are completed, the substrate 80 is warped so that the second region 81y is located lower than the first region 81x (closer to the installation surface in the up-down direction). In order to hold the substrate 80 in a warped position so that the free end side of the substrate 80 hangs down from the fixed end side in this manner, for example, the thickness of the silicon oxide film 35 in the thermal oxidation process may be controlled as described above, or the thickness of the insulating film 30 on the vibrating film 7 may be controlled. Alternatively, the residual film stress of the vibrating film forming layer 6, the piezoelectric element 10, and the insulating film 30 may be controlled by changing the film forming conditions or partially adding openings.
[0052] Finally, as shown in FIG. 14, the support substrate 400 is peeled off, so that the piezoelectric film device 90 becomes operable, and the manufacture of the sound wave generating device 1 is completed.
[0053] Next, the effects of the piezoelectric film device 90, the sound wave generating device 1, and the manufacturing method of the piezoelectric film device 90 according to this embodiment will be described.
[0054] First, with reference to Figs. 15(a) and 15(b), a piezoelectric film device 590 (Fig. 15(a)) included in a sound wave generating device 500 according to a comparative example and a piezoelectric film device 690 (Fig. 15(b)) included in a sound wave generating device 600 according to another comparative example will be described. Fig. 15(a) is a diagram for explaining the sound wave generating device 500 according to a comparative example, and Fig. 15(b) is a diagram for explaining the sound wave generating device 600 according to another comparative example. Note that, for convenience of explanation, Figs. 15(a) and 15(b) show the configurations of the sound wave generating devices 500 and 600 in an extremely simplified manner, and illustration of configurations other than the piezoelectric film devices 590 and 690 shown in schematic form is omitted.
[0055] In the sound wave generating device 500 shown in Fig. 15(a), both ends in the X direction of the piezoelectric film device 590 are constrained. Since such a piezoelectric film device 590 does not have a free end, no gap occurs during vibration, and the effect of sound leakage can be suppressed. However, since both ends in the X direction of the piezoelectric film device 590 are constrained, the vibration of the piezoelectric film device 590 becomes small.
[0056] On the other hand, in the sound wave generating device 600 shown in FIG. 15(b), the piezoelectric film device 590 has a cantilever structure, and only the front end in the X direction is a free end. Such a piezoelectric film device can increase vibration. However, in the piezoelectric film device 590 with a cantilever structure, a gap occurs on the free end side, which may increase the effect of sound leakage. In the piezoelectric film device 590 with a cantilever structure, the vibrating substrate is in a nearly horizontal state when no bias or driving voltage is applied. From this state, when a bias voltage is applied and then a driving voltage is applied, the piezoelectric film device 590 is significantly warped upward, so the above-mentioned gap increases, and the effect of sound leakage increases. For this reason, a piezoelectric film device that can suppress sound leakage while vibrating sufficiently has been desired.
[0057] As shown in FIG. 3, the piezoelectric film device 90 of this embodiment comprises a substrate 80 having a lower surface 81 and an upper surface 82 facing in opposite directions, a support portion 41 supporting the lower surface 81 and fixed to the installation surface, and a piezoelectric film 12 provided on the upper surface 82 and displacing vertically together with the substrate 80 in response to application of a bias voltage and a drive voltage. The substrate 80 has, as areas of the lower surface 81, a first region 81x supported by the support portion 41 and a second region 81y not supported by the support portion 41, and when no bias voltage or drive voltage is applied, the substrate 80 is warped so that the second region 81y is located closer to the installation surface (lower) than the first region 81x.
[0058] As described above, if the substrate is horizontal when no bias voltage is applied, the substrate will warp upward and the gap will become larger when the bias voltage is applied thereafter. If the driving voltage is applied based on this state, the gap will remain large, and the effect of sound leakage will become greater. In this regard, as in the piezoelectric film device 90, when the bias voltage and driving voltage are not applied, the free end side of the substrate 80 is configured to hang down, so that when the bias voltage is applied thereafter, the substrate 80 will be displaced upward, and depending on the setting of the bias voltage, for example, the state in which the substrate 80 is almost horizontal can be used as the reference (the state in which the bias voltage is applied). The state in which the substrate 80 is horizontal is the state in which the gap is smallest (see FIG. 4), and by applying the driving voltage based on this state, the effect of sound leakage can be suppressed in the piezoelectric film device 90 with a cantilever structure. That is, the piezoelectric film device 90 according to this embodiment can suppress the effects of sound leakage, compared to a typical cantilever-structured piezoelectric film device (piezoelectric film device 690 in which the substrate is in a substantially horizontal position when no bias voltage is applied). The cantilever-structured piezoelectric film device 90 can generate larger vibrations, compared to the piezoelectric film device 590 in which both ends are constrained. As described above, the piezoelectric film device 90 according to this embodiment can suppress sound leakage while vibrating sufficiently.
[0059] The fact that sound leakage is most suppressed and the sound pressure level is highest when the substrate 80 is in a horizontal state during bias voltage application will be described with reference to FIG. 16. FIG. 16 is a diagram showing an example of sound pressure levels according to the state during bias voltage application. Since sound leakage is most affected by low tones, the sound pressure level at 20 Hz is used as an index here. In FIG. 16, the horizontal axis indicates the bias voltage, the vertical axis (left) indicates the amount of warping from the horizontal position on the free end side of the cantilever-structured piezoelectric film device 90, and the vertical axis (right) indicates the sound pressure level at a low tone of 20 Hz. In the example shown in FIG. 16, when the bias voltage is set to 13 V and the amount of warping on the free end side of the piezoelectric film device 90 is 0 (i.e. horizontal), the sound pressure level is highest (sound leakage is lowest). Thus, it can be said that sound leakage is most suppressed and the sound pressure level is highest when the substrate 80 is in a horizontal state.
[0060] When no bias voltage or drive voltage is applied, the difference in position between the first region 81x and the second region 81y in the vertical direction may be equal to or greater than the thickness of the silicon layer 34 constituting the substrate 80, and may be 100 μm or more and 500 μm or less. By setting the sagging amount to this extent, the substrate 80 can be appropriately kept horizontal when a bias voltage is applied.
[0061] When a bias voltage is applied but a drive voltage is not applied, the difference in position between the first region 81x and the second region 81y in the vertical direction may be equal to or less than the vertical thickness of the substrate 80. With this configuration, the substrate 80 is in a substantially horizontal state when a bias voltage is applied, so sound leakage can be appropriately suppressed.
[0062] When the bias voltage is applied but the drive voltage is not applied, the difference in position between the first region 81x and the second region 81y in the vertical direction may be 50 μm or less. With this configuration, the substrate 80 is in a substantially horizontal state when the bias voltage is applied, so that sound leakage can be appropriately suppressed.
[0063] When the driving voltage is applied, the bias voltage may be set to a fixed value, so that the piezoelectric film device 90 can be stably operated under preset conditions.
[0064] 1, the sound wave generating device 1 according to this embodiment is configured to include one of the above-mentioned piezoelectric film devices 90. Since such a sound wave generating device 1 includes the above-mentioned piezoelectric film device 90, it is possible to suppress sound leakage while sufficiently vibrating the piezoelectric film device 90.
[0065] The manufacturing method of the piezoelectric film device 90 according to this embodiment includes the steps of preparing a workpiece 101 having one end surface 101a and the other end surface 101b facing in opposite directions, depositing a piezoelectric film 12 and electrodes (upper electrode 13, lower electrode 11) on the one end surface 101a of the workpiece 101, and processing the silicon oxide film 31 and silicon substrate 32, which are the other end surface of the workpiece 101 after deposition, to form a substrate 80 and a support portion 41. When each step is completed, the substrate 80 is warped so that the second region 81y is located closer to the installation surface (lower) than the first region 81x. According to this manufacturing method, the piezoelectric film device 90 having the above-mentioned function can be appropriately manufactured.
[0066] The above-mentioned preparation step includes a step of obtaining a workpiece 101 by performing thermal oxidation on an SOI substrate, and in the step of obtaining a workpiece 101, the thickness of the workpiece 101 by thermal oxidation may be controlled so that the substrate 80 is formed with a downward warp when each step is completed. According to this manufacturing method, the piezoelectric film device 90 having the substrate 80 formed with a downward warp can be easily manufactured by controlling the thickness in thermal oxidation.
[0067] The effects of the sound wave generating device 1 according to this embodiment are not limited to the above. For example, the sound wave generating device 1 according to this embodiment can improve the resistance to destruction compared to the sound wave generating device including the piezoelectric film device with the cantilever structure according to the comparative example. FIG. 17 is a diagram for explaining a further effect (improved resistance to destruction) of the sound wave generating device 1 according to this embodiment. FIG. 17(a) shows the operation of the piezoelectric film device 90 with the cantilever structure according to this embodiment. FIG. 17(b) shows the operation of the piezoelectric film device with the cantilever structure according to the comparative example. As described above, there are three states of the piezoelectric film device: no application (bias voltage and driving voltage are not applied), bias voltage application (bias voltage is applied and driving voltage is not applied), and bias application driving (bias voltage and driving voltage are applied). As shown in Fig. 17(b), if the substrate of the piezoelectric film device is horizontal when no bias voltage is applied, the substrate will warp upward when a bias voltage is applied, and the substrate will warp further upward when a bias voltage is applied and driven. This causes stress to concentrate at the fixed end of the piezoelectric film device, and depending on the drive voltage, the piezoelectric film device may be destroyed.
[0068] 17(a), in the piezoelectric film device 90 according to this embodiment, the free end of the piezoelectric film device 90 hangs downward in the non-applied state as described above, so that the amount of warping of the substrate 80 is smaller than that of the comparative example even if the device is subsequently placed in a bias voltage applied state or a bias applied driving state. Therefore, the stress concentration at the fixed end is alleviated, and the piezoelectric film device can be appropriately prevented from being destroyed by the driving voltage.
[0069] Although the embodiment according to the present disclosure has been described above, the present disclosure is not limited to the above embodiment. For example, in the above embodiment, the sound wave generating device 1 is described as including one piezoelectric film device 90, but the configuration of the sound wave generating device is not limited to this. The sound wave generating device may include multiple (for example, a pair) piezoelectric film devices.
[0070] Fig. 18 is a plan view of a sound wave generating device 1A according to a modified example. Fig. 19 is a longitudinal sectional view of the sound wave generating device 1A taken along line XIX-XIX in Fig. 18. As shown in Figs. 18 and 19, the sound wave generating device 1A includes two (a pair) piezoelectric film devices 90, 90. The configuration of the piezoelectric film device 90 may be as described above. The pair of piezoelectric film devices 90, 90 are arranged, for example, such that the free ends face each other.
[0071] Furthermore, application examples of the piezoelectric film device are not limited to sound wave generating devices (speakers or ultrasonic transducers) and may be, for example, various other transducers, MEMS mirrors, inkjet heads, or the like.
[0072] In addition, in the above embodiment, the sound wave generating device 1 is described as including a substrate assembly 2 and a subframe 3, but this is not limited thereto, and the sound wave generating device may not include a subframe.
[0073] Fig. 20 is a plan view of a sound wave generating device 1B according to a modified example. Fig. 21 is a perspective view of the sound wave generating device 1B shown in Fig. 20 as seen from above. Fig. 22 is a perspective view of the sound wave generating device 1B shown in Fig. 20 as seen from below. The sound wave generating device 1B does not include a subframe, and includes only a substrate assembly 2B. The substrate assembly 2B includes a support substrate 4B, a vibration membrane forming layer 6B, and a piezoelectric element 10B.
[0074] The support substrate 4B has a quadrangular shape (approximately rectangular) in plan view. The support substrate 4B is made of, for example, a part of an SOI substrate. The support substrate 4B may be configured to include a silicon substrate and an oxide film layer. The support substrate 4B has a cavity (hollow) 5B (see FIG. 22) having a regular hexagonal shape in plan view at its center. The cavity 5B has, for example, a hexagonal column shape. The support substrate 4B has a first portion 701, a second portion 702, a third portion 703, and a fourth portion 704 provided to surround the cavity 5B. The first portion 701 and the third portion 703 extend along the Y direction so as to sandwich the cavity 5B between them in the X direction. The second portion 702 and the fourth portion 704 extend along the X direction so as to sandwich the cavity 5B between them in the Y direction.
[0075] The vibration film forming layer 6B is formed on the support substrate 4B. The vibration film forming layer 6B may be composed of a laminated film in which a silicon layer and a silicon oxide film are laminated. The vibration film forming device 6B includes a vibration film 7B facing the cavity 5B in a plan view, and a frame portion 8B formed to surround the cavity 5B. The vibration film 7B has a regular hexagonal shape that is almost similar to the cavity 5B in a plan view. The frame portion 8B is configured to include a first frame body 801, a second frame body 802, a third frame body 803, and a fourth frame body 804. The first frame body 801 is supported by the first portion 701. The second frame body 802 is supported by the second portion 702. The third frame body 803 is supported by the third portion 703. The fourth frame body 804 is supported by the fourth portion 704.
[0076] The piezoelectric element 10B is formed so as to be disposed on the vibration film forming layer 6B, specifically, on the vibration film 7B. The piezoelectric element 10B may include a lower electrode formed on the vibration film 7B, a piezoelectric film formed on the lower electrode, and an upper electrode formed on the piezoelectric film.
[0077] Here, in the sound wave generating device 1B, slits 308, 309, and 310 are formed penetrating the piezoelectric element 10B and the vibration membrane 7B. The slit 308 extends along the Y direction. The slits 309 and 310 each extend so as to divide the area of the piezoelectric element 10B into six equal parts, taking into account the extension direction of the slit 308. That is, the slits 308, 309, and 310 each extend so as to divide the area of the piezoelectric element 10B into six equal parts. By forming such slits 308 and 309, the piezoelectric element 10B is separated into six piezoelectric elements 10Ba to 10Bf, and the vibration membrane 7B is separated into six vibration membranes 7Ba to 7Bf. As a result, the sound wave generating device 1B has six piezoelectric membrane devices 90Ba to 90Bf.
[0078] In each of such piezoelectric film devices 90Ba to 90Bf, when no bias voltage or drive voltage is applied, the "substrate" is "warped so that the second region is located closer to the installation surface than the first region," thereby achieving the same effect as the configuration of the embodiment.
[0079] Various embodiments of the present disclosure may be defined as follows:
[0080] (Supplementary Note 1, embodiment, FIG. 3) a substrate having a first surface and a second surface facing in opposite directions; A support portion that supports the first surface and is fixed to an installation surface; a piezoelectric film provided on the second surface and displaced in a first direction together with the substrate in response to application of a bias voltage and a drive voltage; the substrate has, as a region of the first surface, a first region supported by the support portion and a second region not supported by the support portion; A piezoelectric film device, wherein when the bias voltage and the drive voltage are not applied, the substrate is warped so that the second region is located closer to the installation surface in the first direction than the first region.
[0081] (Supplementary Note 2, Embodiment, Figure 3) A piezoelectric film device as described in Appendix 1, wherein when the bias voltage and the driving voltage are not applied, the difference in position between the first region and the second region in the first direction is greater than or equal to the thickness in the first direction of an active layer constituting a substrate.
[0082] (Supplementary Note 3, embodiment, Figure 3) A piezoelectric film device described in Appendix 1 or Appendix 2, wherein when the bias voltage and the driving voltage are not applied, the positional difference between the first region and the second region in the first direction is 100 μm or more and 500 μm or less.
[0083] (Appendix 4, embodiment, Figure 4) A piezoelectric film device described in any one of Appendix 1 to Appendix 3, wherein when the bias voltage is applied and the driving voltage is not applied, the difference in position between the first region and the second region in the first direction is less than or equal to the thickness of the substrate in the first direction.
[0084] (Supplementary Note 5, embodiment, Figure 4) A piezoelectric film device described in any one of Appendix 1 to Appendix 4, wherein when the bias voltage is applied and the driving voltage is not applied, the positional difference between the first region and the second region in the first direction is 50 μm or less.
[0085] (Appendix 6, embodiment) 6. The piezoelectric film device according to claim 1, wherein the bias voltage is set to a fixed value when the driving voltage is applied.
[0086] (Appendix 7, embodiment, Figure 3) A transducer including the piezoelectric film device according to any one of claims 1 to 6.
[0087] (Appendix 8, embodiment, Figs. 7 to 14) A method for manufacturing the piezoelectric film device according to any one of claims 1 to 6, A step of preparing a workpiece having one end surface and another end surface facing in opposite directions; forming the piezoelectric film and an electrode on one end surface of the workpiece; and forming the substrate and the support by processing the other end surface of the workpiece after the film is formed. A manufacturing method in which, when each process is completed, the substrate is warped so that the second region is located closer to the installation surface in the first direction than the first region.
[0088] (Appendix 9, embodiment, FIG. 7) The preparing step includes a step of obtaining the workpiece by performing thermal oxidation on a substrate containing silicon, The manufacturing method according to claim 8, wherein in the step of obtaining the processed member, a film thickness of the processed member by thermal oxidation is controlled so that the substrate is formed warped when each step is completed. [Explanation of symbols]
[0089] Reference Signs List 1, 1A, 1B...sound wave generating device, 2, 2B...substrate assembly, 2A...substrate assembly work in progress, 3...subframe, 3a...accommodating recess, 3b...through hole, 4, 4B...support substrate, 5, 5B...cavity, 6, 6B...vibration membrane forming layer, 7, 7B (7Ba to 7Bf)...vibration membrane, 8, 8B...frame, 9...slit, 9a...first portion, 9b...second portion, 9c...third portion, 10, 10B (10Ba to 10Bf)...piezoelectric element, 11...lower electrode Electrode, 12...piezoelectric film, 13...upper electrode, 14...hydrogen barrier film, 15...interlayer insulating film, 16, 17...contact hole, 18...upper wiring, 18a...upper pad portion, 19...lower wiring, 19a...lower pad portion, 20...passivation film, 21...upper pad opening, 22...lower pad opening, 23...opening, 30...insulating film, 31...silicon oxide film, 32...silicon substrate, 33...oxide film layer, 34...silicon layer, 35...silicon oxide film, 41...support Holding portion, 42...first portion, 43...second portion, 44...third portion, 53...opening, 80...substrate, 81...lower surface, 81x...first region, 81y...second region, 82...upper surface, 90, 90Ba to 90Bf...piezoelectric film device, 101...workpiece, 101a...one end surface, 101b...other end surface, 111...lower electrode film, 112...piezoelectric material film, 113...upper electrode film, 131...first portion, 132...second portion, 133...third portion, 134...connecting portion, 14 0...hydrogen barrier film, 181...support frame, 182...first part, 183...second part, 184...third part, 308...slit, 309...slit, 310...slit, 400...support substrate, 500, 600...sound wave generating device, 590, 690...piezoelectric film device, 701...first part, 702...second part, 703...third part, 704...fourth part, 801...first frame, 802...second frame, 803...third frame, 804...fourth frame.
Claims
1. a substrate having a first surface and a second surface facing in opposite directions; A support portion that supports the first surface and is fixed to an installation surface; a piezoelectric film provided on the second surface and displaced in a first direction together with the substrate in response to application of a bias voltage and a drive voltage; the substrate has, as a region of the first surface, a first region supported by the support portion and a second region not supported by the support portion; A piezoelectric film device, wherein when the bias voltage and the drive voltage are not applied, the substrate is warped so that the second region is located closer to the installation surface in the first direction than the first region.
2. The piezoelectric film device of claim 1, wherein when the bias voltage and the driving voltage are not applied, the difference in position between the first region and the second region in the first direction is greater than or equal to the thickness in the first direction of an active layer constituting the substrate.
3. The piezoelectric film device according to claim 1 , wherein when the bias voltage and the driving voltage are not applied, the difference in position between the first region and the second region in the first direction is 100 μm or more and 500 μm or less.
4. The piezoelectric film device of claim 1, wherein when the bias voltage is applied and the drive voltage is not applied, the difference in position between the first region and the second region in the first direction is less than or equal to the thickness of the substrate in the first direction.
5. The piezoelectric film device according to claim 4 , wherein when the bias voltage is applied and the drive voltage is not applied, the difference in position between the first region and the second region in the first direction is 50 μm or less.
6. 2. The piezoelectric film device according to claim 1, wherein the bias voltage is set to a fixed value when the driving voltage is applied.
7. A transducer comprising the piezoelectric film device according to any one of claims 1 to 6.
8. A method for manufacturing the piezoelectric film device according to any one of claims 1 to 6, comprising the steps of: A step of preparing a workpiece having one end surface and another end surface facing in opposite directions; forming the piezoelectric film and an electrode on one end surface of the workpiece; and forming the substrate and the support by processing the other end surface of the workpiece after the film is formed. A manufacturing method in which, when each process is completed, the substrate is warped such that the second region is located closer to the installation surface in the first direction than the first region.
9. The preparing step includes a step of obtaining the workpiece by performing thermal oxidation on a substrate containing silicon, 9. The method according to claim 8, wherein in the step of obtaining the workpiece, a film thickness of the workpiece by thermal oxidation is controlled so that the substrate is formed with a warp when each step is completed.
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JP2021052305A