A piezoelectric MEMS microphone, packaging method, piezoelectric ceramic preparation method

Abstract

A piezoelectric MEMS microphone, a packaging method and a piezoelectric ceramic preparation method, the microphone comprising: a substrate, an annular structure to surround a first sound cavity; a first piezoelectric diaphragm, a triangular cantilever beam structure comprising a piezoelectric film, the bottom edge is fixed to the substrate and the tip of the triangle is a free end; a second piezoelectric diaphragm, separated by a circular groove structure in the bottom edge area of the triangular cantilever beam structure to form an independent diaphragm area; a packaging shell, accommodating and fixing the substrate, the first piezoelectric diaphragm and the second piezoelectric diaphragm, and surrounding a second sound cavity with the substrate. The application effectively avoids the leakage current phenomenon by adding excess K and Na elements in the KNN sol to compensate for subsequent volatilization, combined with Mn doping. The application ensures that the sensitivity curve has a fluctuation range at the resonance frequency of the KNN piezoelectric film by two piezoelectric film areas, thereby improving the resonance frequency and further improving the sensitivity.

Description

Technical Field

[0001] This invention belongs to the field of acoustic-electric conversion technology, specifically relating to a piezoelectric MEMS microphone, a packaging method, and a method for preparing piezoelectric ceramics. Background Technology

[0002] MEMS microphones are widely used in consumer electronics products such as mobile computers, smartphones, and headphones. Traditional MEMS microphones are mainly condenser microphones. However, their double-diaphragm design cannot avoid the air damping problem between the back electrode and the diaphragm, which affects the microphone's signal-to-noise ratio. Piezoelectric microphones have a simple manufacturing process, and their single-diaphragm design avoids the effects of backplate and air damping, improving their signal-to-noise ratio. However, their sensitivity is not as good as that of condenser microphones.

[0003] Piezoelectric MEMS microphones are MEMS devices in the field of sound-to-electric conversion, capable of converting sound signals into electrical signals in various ways. Their basic principle is based on the piezoelectric effect, which converts sound signals into electrical signals. Commonly used piezoelectric films in piezoelectric microphones include PZT / ZnO / AlN. For a long time, lead zirconate titanate (PZT)-based piezoelectric materials have emerged due to their excellent piezoelectric properties and high Curie temperature (TC), widely used in various electronic devices and dominating the piezoelectric material market. However, they contain up to 60% lead. From the perspective of economic and social sustainable development, lead and related chemical substances are inevitably released during calcination, sintering, and post-processing, posing a serious threat to the environment and human health. Since the beginning of this century, a research boom in lead-free piezoelectric materials has emerged both domestically and internationally, resulting in an explosive growth in research achievements. Traditional lead-free piezoelectric materials include polyvinylidene fluoride (PVDF), aluminum nitride (AlN), and zinc oxide (ZnO). PVDF is a flexible piezoelectric material, first discovered by Japanese researchers in 1969. Its core characteristics are low density and flexibility, but its piezoelectric performance is relatively low and it exhibits humidity sensitivity, limiting its applications to wearable flexible devices and restricting its environmental adaptability. AlN and ZnO are two widely used lead-free piezoelectric materials with low dielectric constants and losses, and they are well-integrated with semiconductor processes. However, their piezoelectric coefficients are low, typically d33 between 5 and 15 pC / N, making them unsuitable for high-performance sensing requirements. Currently, three main types of lead-free piezoelectric materials show potential to replace PZT-based lead-based piezoelectric materials: barium titanate (BT), sodium bismuth titanate (BNT), and sodium potassium niobate (KNN). Among them, BT-based piezoelectric materials have high dielectric constant and electromechanical coupling coefficient, but their TC is only 120℃, resulting in a narrow operating temperature range; BNT-based piezoelectric materials have high field-induced strain, but their temperature stability needs further improvement; in contrast, by constructing polymorphic phase boundaries (PPB) or quasi-isomorphic phase boundaries (MPB), KNN-based piezoelectric ceramics can possess both excellent piezoelectric properties and high TC. Therefore, KNN-based piezoelectric materials have broad application prospects as a replacement for PZT lead-based piezoelectric materials.

[0004] With the development of MEMS technology, MEMS microphones are becoming increasingly smaller, allowing for integration into tiny spaces. Current research on piezoelectric MEMS microphone design, both domestically and internationally, mainly focuses on structural design and performance optimization, with sensitivity and signal-to-noise ratio being the two most critical performance parameters. From a design perspective, the sensitivity of a MEMS microphone primarily depends on its mechanical and electrical sensitivity. Mechanical sensitivity is mainly related to the diaphragm structure; low residual stress and high compliance films are the main directions for optimizing diaphragm mechanical sensitivity.

[0005] In summary, existing MEMS microphone diaphragms suffer from residual stress and resonant frequency issues, which hinder further improvements in microphone sensitivity. Summary of the Invention

[0006] The purpose of this invention is to solve the problems of residual stress and resonant frequency of microphone diaphragms, so as to improve the sensitivity of piezoelectric MEMS microphones and realize their application in a wider frequency band.

[0007] The objective of this invention is achieved through the following technical solution:

[0008] A piezoelectric MEMS microphone, the microphone comprising:

[0009] The base has a ring-shaped structure to enclose the first acoustic cavity;

[0010] The first piezoelectric diaphragm is a triangular cantilever beam structure including the piezoelectric diaphragm, with the bottom edge fixed to the substrate and the apex of the triangle being a free end;

[0011] The second piezoelectric diaphragm is an independent diaphragm region separated by a circular groove structure in the bottom area of ​​the triangular cantilever beam structure;

[0012] The encapsulation housing accommodates and fixes the substrate, the first piezoelectric diaphragm, and the second piezoelectric diaphragm, and encloses a second sound cavity between the housing and the substrate;

[0013] The first piezoelectric diaphragm includes a KNN piezoelectric film, a top electrode, and a bottom electrode.

[0014] Preferably, the material of the piezoelectric diaphragm is KNN lead-free piezoelectric ceramic material with a nominal composition of (K0.5Na0.5)NbO3.

[0015] Preferably, the piezoelectric diaphragm above the circular groove is partially supported by the insulating layer.

[0016] Preferably, the substrate includes a quadrilateral cavity.

[0017] Preferably, there are four first piezoelectric diaphragms that cover the quadrilateral cavity; there are gaps between the first piezoelectric diaphragms.

[0018] Preferably, the substrate includes a bottom substrate and an intermediate layer substrate fixed to the top of the bottom substrate; an insulating layer is present between the bottom substrate and the intermediate layer substrate.

[0019] Preferably, the insulating layer forms a groove structure with the first piezoelectric diaphragm and the intermediate substrate.

[0020] Based on the same inventive concept, the present invention also provides a packaging method for a piezoelectric MEMS microphone, used to package a piezoelectric MEMS microphone as described above, the method comprising the following steps:

[0021] The substrate is obtained by chemically etching multiple substrate layers to form the first acoustic cavity;

[0022] The first piezoelectric diaphragm and the second piezoelectric diaphragm are processed on the top of the substrate;

[0023] The microphone's internal structure is obtained by connecting the top and bottom electrodes of the first piezoelectric diaphragm to the base electrodes respectively.

[0024] The piezoelectric MEMS microphone is obtained by sealing the internal structure of the microphone with a packaging shell to form the second sound cavity.

[0025] Based on the same inventive concept, this invention also provides a method for preparing a thin-film KNN lead-free piezoelectric ceramic material, wherein the thin-film KNN lead-free piezoelectric ceramic material is used in a piezoelectric MEMS microphone as described above, and the method includes the following steps:

[0026] An intermediate sol was obtained by adding excess alkali metal elements K and Na to (K0.5Na0.5)NbO3 sol;

[0027] The intermediate sol was doped with Mn to obtain a doped gel;

[0028] The doped gel is spin-coated on a substrate, dried, and subjected to RTP heat treatment to obtain the thin-film KNN lead-free piezoelectric ceramic material.

[0029] Preferably, the material used for Mn doping includes Mn(CH3COO)2.

[0030] Preferably, the substrate material comprises SrTiO3 (STO) single crystal.

[0031] Preferably, the heat treatment is divided into two stages: pyrolysis and annealing.

[0032] Preferably, the pyrolysis temperature range is 450-550℃.

[0033] Preferably, the annealing temperature is 800℃±10%.

[0034] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0035] This invention provides a novel piezoelectric MEMS microphone and related methods. The microphone includes: a substrate with a ring structure to enclose a first acoustic cavity; a first piezoelectric diaphragm, which is a triangular cantilever beam structure including a piezoelectric film, with its base fixed to the substrate and its apex a free end; a second piezoelectric diaphragm, an independent diaphragm region separated from the base region of the triangular cantilever beam structure by a circular groove structure; and a housing that houses and fixes the substrate, the first piezoelectric diaphragm, and the second piezoelectric diaphragm, and encloses the second acoustic cavity between the housing and the substrate; the first piezoelectric diaphragm includes a KNN piezoelectric film, a top electrode, and a bottom electrode. This invention constructs the first acoustic cavity through the substrate, while also providing structural support for the first piezoelectric diaphragm; reduces residual stress in the diaphragm through the triangular cantilever beam structure; improves the microphone's sensitivity through the second piezoelectric diaphragm; and by using two piezoelectric films of different areas, achieves a fluctuation range in the sensitivity curve of the piezoelectric microphone at the resonant frequency, thereby generating a flat broadband range, increasing the resonant frequency, and thus improving the sensitivity of the piezoelectric microphone. Attached Figure Description

[0036] Figure 1 This is a top view of the piezoelectric microphone chip in an example of the present invention.

[0037] Figure 2 This is a cross-sectional view of the piezoelectric microphone chip in an example of the present invention.

[0038] Figure 3 This is a packaging diagram of the piezoelectric microphone in an example of the present invention.

[0039] Figure 4 This is a piezoelectric amplitude response diagram of different orientation KNN epitaxial films tested on the PFM platform in this invention example.

[0040] Figure 5 This is a piezoelectric phase response diagram of different orientation KNN epitaxial films tested on the PFM platform in this invention example.

[0041] Figure 6 This is a flowchart illustrating the preparation process of KNN sol in an example of the present invention.

[0042] Figure 7 This is a flowchart of the KNN thin film preparation process in an example of the present invention.

[0043] Wherein: 1-substrate, 2-first piezoelectric diaphragm, 3-second piezoelectric diaphragm, 4-first semi-ring gap, 5-second semi-ring gap, 6-linear slit, 7-second groove, 8-first groove, 9-encapsulation shell, 11-bottom substrate, 12-first insulating layer, 13-intermediate layer substrate, 14-second insulating layer, 15-LGA substrate, 21-piezoelectric thin film, 22-top electrode, 23-bottom electrode, 24-wire, 25-intermediate electrode, 26-pad, 31-local piezoelectric thin film, 32-local top electrode, 33-local bottom electrode, 91-encapsulation base plate, 92-metal upper shell, 100-first sound cavity, 200-second sound cavity, 300-acoustic hole. Detailed Implementation

[0044] The technical solution will be further described below with reference to the accompanying drawings and specific embodiments to help understand the content of the present invention.

[0045] Example 1

[0046] like Figure 1-3 As shown, the present invention provides a piezoelectric MEMS microphone, the microphone comprising:

[0047] Base 1, with a ring structure to enclose the first acoustic cavity 100;

[0048] The first piezoelectric diaphragm 2 is a triangular cantilever beam structure including the piezoelectric diaphragm, with the bottom edge fixed to the base 1 and the apex of the triangle being a free end;

[0049] The second piezoelectric diaphragm 3 is an independent diaphragm region separated by a circular groove structure in the bottom area of ​​the triangular cantilever beam structure;

[0050] The encapsulation housing 9 accommodates and fixes the substrate 1, the first piezoelectric diaphragm 2, and the second piezoelectric diaphragm 3, and forms a second sound cavity 200 between the encapsulation housing 9 and the substrate 1; the encapsulation housing 9 includes an encapsulation base plate 91 for supporting the substrate 1 and a metal upper shell 92 that forms a sealing structure with the encapsulation base plate 91.

[0051] The first piezoelectric diaphragm 2 includes a KNN piezoelectric thin film 21, a top electrode 22, and a bottom electrode 23;

[0052] The second piezoelectric diaphragm 3 includes a local piezoelectric thin film 31, a local top electrode 32, and a local bottom electrode 33. The local top electrode 32 is electrically connected to the top electrode 22, and the local bottom electrode 33 is electrically connected to the bottom electrode 23.

[0053] Figure 1This is a top view of the piezoelectric microphone chip in an embodiment of the present invention. A first semi-annular gap 4 and a second semi-annular gap 5 exist between the first piezoelectric diaphragm 2 and the second piezoelectric diaphragm 3. The local piezoelectric film 31, the local top electrode 32, the local bottom electrode 33, and the first semi-annular gap 4 together form a circular groove structure, and the second semi-annular gap 5 and the second semi-annular gap 5 form a sound inlet. The local top electrode 32 is electrically connected to the top electrode 22, and the local bottom electrode 33 is electrically connected to the bottom electrode 23. A bottom electrode 23 is located at the fixed end of one edge portion to facilitate electrical signal output.

[0054] In this embodiment, the second piezoelectric diaphragm 3 is circular; in another embodiment, the shape of the second piezoelectric diaphragm 3 can be elliptical, semi-circular, polygonal, or star-shaped. Using different shapes can adjust the sensitivity and frequency sensing characteristics of the second piezoelectric diaphragm 3.

[0055] During the processing, a bottom electrode layer is deposited on the SOI wafer. The bottom electrode layer has a thickness of 0.1 μm and is a Pt electrode. Then, a piezoelectric thin film layer is deposited and circular trenches are etched to form the first piezoelectric diaphragm 2 and the second piezoelectric diaphragm 3. At the same time, there are two bridge-shaped structures on the edge of the second piezoelectric diaphragm 3 that are connected to the first piezoelectric diaphragm 2 to achieve mechanical support and electrode output for the second piezoelectric diaphragm 3. Specifically, the bridge-shaped structure on one side retains the connection of the top electrode layer to achieve electrode output of the local top electrode 32, while the bottom electrode layer of the bridge-shaped structure on the other side remains connected to achieve electrode output of the local bottom electrode 33. At least one side of the substrate and insulating layer at the bridge-shaped structure on both sides is kept connected to ensure sufficient connection strength of the second piezoelectric diaphragm 3. The structure shown in the attached figure of this example is such that the substrate and insulating layer are kept connected at the bridge-shaped structure at the bottom electrode connection position, and the substrate and insulating layer are etched away at the bridge-shaped structure at the top electrode connection position. In another embodiment, the substrate and insulating layer are kept connected at the bridge-shaped structure at the top electrode connection position to further increase the connection strength between the second piezoelectric diaphragm 3 and the first piezoelectric diaphragm 2. The specific connection structure is selected according to the microphone application and sensitivity requirements. Better connection strength is required for high sound pressure application environments. On the other hand, using lower connection strength can reduce residual stress and improve the sensitivity of the piezoelectric microphone.

[0056] The piezoelectric thin film layer has a thickness of 3 μm. The top electrode layer is formed by depositing Cr / Au and patterning it to create the desired connection structure, thus realizing the local top electrode 32 connection circuit. A second stage of etching is performed on KNN / Pt / SiO2. The triangular cantilever beam structure and the circular trench are etched using deep reactive ion etching (DRIE) and BOE.

[0057] Figure 2This is a cross-sectional view of a piezoelectric microphone chip according to an embodiment of the present invention. It includes: a top electrode 22, a piezoelectric thin film 21, a bottom electrode 23, a second insulating layer 14, an intermediate substrate 13, a first insulating layer 12, and a bottom substrate 11 at the fixed end of the edge portion; the bottom substrate 11 and the intermediate substrate 13 are separated by the first insulating layer 12, the area of ​​which is smaller than the substrate area, forming a first groove 8. The top electrode 22, the piezoelectric thin film 21, and the bottom electrode 23 form a first piezoelectric diaphragm 2, and a first semi-annular gap 4 and a second semi-annular gap 5 exist between the first piezoelectric diaphragm 2 and the second piezoelectric diaphragm 3. The second piezoelectric diaphragm is composed of a local top electrode 32, a local piezoelectric thin film 31, and a local bottom electrode 33. From a top view, the second piezoelectric diaphragm 3 is primarily an island structure in the middle of a circular groove. From a three-dimensional perspective, the second piezoelectric diaphragm 3 is an island structure connected to the first piezoelectric diaphragm 2 by bridge-like structures on both sides, thus forming two piezoelectric diaphragm structures with different areas. The first semi-annular gap 4 and the second semi-annular gap 5 on both sides of the island structure are through-hole annular grooves, which can serve as sound inlets. A second groove 7 exists between the fixed end of the first piezoelectric diaphragm, the substrate 13, and the second insulating layer 14.

[0058] like Figure 3 As shown, the substrate 1 is constructed by stacking a bottom substrate 11, a first insulating layer 12, an intermediate substrate 13, and a second insulating layer 14, forming a ring structure to enclose a first acoustic cavity 100. This embodiment also includes an optional LGA substrate 15 to expand the rear cavity and enhance acoustic performance. The top electrode 22 is connected to the intermediate electrode 25 via a wire 24, and then connected to pads 26 via internal wiring of the package base plate 91 for SMT soldering, thereby enabling the top electrode to output electrical signals. Simultaneously, the bottom electrode 33 is also connected to another intermediate electrode via additional wires (fixed on the other two sides, not shown in the figure, but with the same structure as the top electrode connection), and similarly connected to pads via internal wiring of the package base plate 91 for SMT soldering. The substrate 1 and the encapsulation base plate 91 form a semi-enclosed first sound cavity 100, and external sound waves enter the first sound cavity 100 through the sound hole 300; the metal upper shell 92, the substrate 1, the first piezoelectric diaphragm 2, the second piezoelectric diaphragm 3 and the encapsulation base plate 91 form a second sound cavity 200, and the first sound cavity 100 is connected to the second sound cavity 200 through the first semi-annular gap 4, the second semi-annular gap 5 and the linear gap 6.

[0059] In this invention, the piezoelectric microphone chip uses a second piezoelectric diaphragm with a circular groove structure and a triangular cantilever beam structure. Due to the different areas of the piezoelectric film, these structures can form different resonant frequencies, ensuring that the sensitivity curve of the piezoelectric microphone has a fluctuation range within the resonant frequency of the piezoelectric film. This results in a flat, wide bandwidth, increasing the resonant frequency and thus improving the sensitivity of the piezoelectric microphone. The triangular cantilever beam structure allows for acoustic signal testing over a wider frequency range. The second insulating layer 14 is SiO2 with a thickness of 1 μm, and the intermediate substrate 13 of the base portion is also SiO2 with an etched thickness of 400 μm. The substrate areas of the first semi-annular gap 4, the second semi-annular gap 5, the linear gap 6, and the second groove 7 are larger than the area of ​​the second insulating layer 14. This reduces the gap width and also reduces the leakage of mid-frequency and low-frequency signals from the gaps, lowering residual stress and improving the sensitivity of the piezoelectric microphone.

[0060] An alternative implementation involves depositing a bottom electrode layer of a piezoelectric film on an SOI wafer. The bottom electrode layer has a thickness of 0.1 μm and is a Pt electrode. A KNN piezoelectric film with a thickness of 5 μm is then deposited and patterned to define the connection pathways for the bottom electrode. The top electrode is then patterned by depositing Cr / Au to define the top electrode connection circuit. A second stage of etching is performed on the KNN / Pt / SiO2. The triangular cantilever structure and the circular trench are etched using deep reactive ion etching (DRIE) and boron ionization (BOE).

[0061] Preferably, the material of the piezoelectric diaphragm is KNN lead-free piezoelectric ceramic material with a nominal composition of (K0.5Na0.5)NbO3.

[0062] Preferably, the piezoelectric diaphragm above the circular groove is partially supported by the insulating layer.

[0063] Preferably, the substrate includes a quadrilateral cavity.

[0064] Preferably, there are four first piezoelectric diaphragms that cover the quadrilateral cavity; there are gaps between the first piezoelectric diaphragms.

[0065] Preferably, the substrate includes a bottom substrate and an intermediate layer substrate fixed on top of the bottom substrate; an insulating layer is present between the bottom substrate and the intermediate layer substrate. The substrate is a silicon substrate, and the insulating layer is SiO2.

[0066] Preferably, the insulating layer forms a groove structure with the first piezoelectric diaphragm and the intermediate substrate.

[0067] Furthermore, the top electrode is etched with a wider linewidth, and the area of ​​the top electrode in the etched trench is smaller than that of the substrate.

[0068] Furthermore, the top electrode in the circular trench is etched with a wider linewidth, and the top electrode in the etched trench is smaller than the area of ​​the substrate.

[0069] like Figure 4 and Figure 5 As shown, this invention performs performance tests on STO substrates with a

[110] tangential orientation. During the tests, the piezoelectric response of three orientations of KNN epitaxial films was characterized using the ferroelectric reversal module of a PFM device, aiming to corroborate the results obtained on a Seiko device. Reference Figure 4 The (010) orientation was found to still exhibit the highest piezoelectric response, with its amplitude significantly greater than the other two orientations under the same voltage conditions. At the maximum DC voltage, the (110) oriented film showed slightly better amplitude data than the (111) oriented film, but as the DC voltage decreased, the amplitude of the (110) oriented film decreased faster. This should correspond to a greater amount of domain movement in the (110) oriented film than in the (111) oriented film. That is, when the external field polarization changes, the domain flipping motion and recovery state also change significantly, thus causing a significant difference in PFM amplitude. Figure 5 As shown, the larger coercivity field value clearly corresponds to domain flipping in the (010) oriented film. Due to the constraint of the substrate, domain movement in the film generally results in greater power consumption. The coercivity field of the (110) oriented film is the second largest, while the (111) oriented film exhibits a lower coercivity field due to the scarcity of domain movement. Performance tests revealed that the epitaxial film in the

[110] direction performs well when a

[110] tangential STO substrate is selected. Therefore, it is desirable to deposit an epitaxial film with a preferred (110) out-of-plane orientation, so that the self-polarization direction of the KNN film is consistent with the out-of-plane orientation, thereby obtaining higher electrical performance and improving the sensitivity of the piezoelectric microphone.

[0070] In this invention, the first piezoelectric diaphragm adopts a triangular cantilever beam structure, which naturally results in a lower frequency, enabling acoustic and vibration signal testing over a wider frequency range. The area of ​​the piezoelectric thin film KNN designed within the etched triangular cantilever beam structure is larger than the area of ​​the top electrode, thus increasing the area utilized for acoustic pressure measurement.

[0071] In this invention, the second piezoelectric diaphragm adopts a circular groove structure, and a wider linewidth etching is used between its top electrode and the substrate. The top electrode is smaller than the area of ​​the substrate, which reduces the gap width and also reduces the leakage of mid-frequency and low-frequency signals from the gap, thereby improving the sensitivity of the piezoelectric microphone. Grooves exist between the insulating layer, the piezoelectric diaphragm, and the substrate, further preventing signal leakage and improving the sensitivity of the piezoelectric microphone.

[0072] This invention employs both a first piezoelectric diaphragm with a triangular cantilever beam structure and a second piezoelectric diaphragm with a circular groove structure. The two piezoelectric diaphragms have different areas. By using the two piezoelectric film areas, the sensitivity curve of the piezoelectric microphone has a fluctuation range at the resonant frequency of the KNN piezoelectric film, thereby generating a flat broadband, increasing the resonant frequency, and thus improving the sensitivity of the piezoelectric microphone.

[0073] This invention encapsulates a piezoelectric microphone using the aforementioned packaging housing. The first acoustic cavity (front cavity) is positioned in the center of the base, ensuring the accuracy of signal reception by the piezoelectric microphone. The second acoustic cavity (rear cavity) employs a sealed structure with sufficient internal air space. This ensures that even after sealing, the air damping experienced by the piezoelectric film during vibration does not significantly affect the microphone's sensitivity. This solves the problems of residual stress and resonant frequency in the microphone diaphragm, thereby improving the sensitivity of the piezoelectric MEMS microphone and enabling its application in a wider frequency range.

[0074] This invention addresses issues such as low residual stress and resonant frequency by selecting an environmentally friendly sodium potassium niobate piezoelectric thin film and optimizing the structure of the piezoelectric microphone, thereby improving the sensitivity of the piezoelectric MEMS microphone and enabling its application in a wider frequency band.

[0075] Example 2

[0076] Based on the same inventive concept, the present invention also provides a packaging method for a piezoelectric MEMS microphone, used to package a piezoelectric MEMS microphone as described above, the method comprising the following steps:

[0077] The substrate is obtained by chemically etching multiple substrate layers to form the first acoustic cavity;

[0078] The first piezoelectric diaphragm and the second piezoelectric diaphragm are processed on the top of the substrate;

[0079] The microphone's internal structure is obtained by connecting the top and bottom electrodes of the first piezoelectric diaphragm to the base electrodes respectively.

[0080] The piezoelectric MEMS microphone is obtained by sealing the internal structure of the microphone with a packaging shell to form the second sound cavity.

[0081] An annular front cavity is formed on the substrate using a chemical method (thermal oxidation or wet etching). A fixed end is connected to the bottom surface of the substrate. The front cavity is positioned in the center of the base and has a cylindrical volume. The rear cavity employs a sealed structure with some space remaining. The top electrode of the edge support portion is bonded to the base electrode, thereby forming a vibration cavity.

[0082] Figure 3 This is a packaging diagram of the piezoelectric microphone in an embodiment of the present invention. The base is fixed to the base, and the top electrode at the fixed edge end is connected to the base. The triangular cantilever beam structure at the free end is placed in the center without deviation, ensuring the accuracy of acoustic testing. An annular front cavity is formed on two substrate layers by chemical methods (thermal oxidation or wet etching). A certain space is left between the front cavity and the rear cavity to ensure that even after sealing, the air damping experienced by the piezoelectric film during vibration will not significantly affect the sensitivity of the micro-microphone.

[0083] Example 3

[0084] Based on the same inventive concept, this invention also provides a method for preparing a thin-film KNN lead-free piezoelectric ceramic material, wherein the thin-film KNN lead-free piezoelectric ceramic material is used in a piezoelectric MEMS microphone as described above, and the method includes the following steps:

[0085] An intermediate sol was obtained by adding excess alkali metal elements K and Na to (K0.5Na0.5)NbO3 sol;

[0086] The intermediate sol was doped with Mn to obtain a doped gel;

[0087] The doped gel is spin-coated on a substrate, dried, and subjected to RTP heat treatment to obtain the thin-film KNN lead-free piezoelectric ceramic material.

[0088] The material used for Mn doping includes Mn(CH3COO)2.

[0089] The substrate material includes SrTiO3 (STO) single crystal.

[0090] The heat treatment is divided into two stages: pyrolysis and annealing.

[0091] The pyrolysis temperature range is 450-550℃.

[0092] The annealing temperature is 800℃±10%.

[0093] like Figure 6 As shown in the steps, the present invention first prepares potassium sodium niobate ((K) by the sol-gel method. 0.5 Na 0.5 NbO3, KNN sol was used to prepare KNN sol thin films, which were then subjected to performance testing to explore the properties of KNN piezoelectric films with different orientations and to determine the appropriate substrate orientation. The main experimental raw materials included those from Sinopharm Chemical Reagents Beijing Co., Ltd. 0.5 Na 0.5Other main raw materials required for NbO3 sol are high-purity niobium ethanol [(CH3COO)5Nb, >99.999%, Alpha Aisa (Tianjin) Chemical Co., Ltd.], acetylacetone [CH3COCH2COCH3, >99.0%, Sinopharm Chemical Reagent Beijing Co., Ltd.], ethylene glycol methyl ether [HOCH2CH2OCH3, >99.0%, Sinopharm Chemical Reagent Beijing Co., Ltd.], glacial acetic acid [CH3COOH, >99.5%, Sinopharm Chemical Reagent Beijing Co., Ltd.], ethylene glycol [(HOCH2)2, >99.0%, Sinopharm Chemical Reagent Beijing Co., Ltd.], etc. The substrate material is SrTiO3 (STO) single crystal, 2 mol% Mn doped [Mn(CH3COO)2, >99.0%; KNN]. In the KNN sol preparation process, the various raw materials are mixed in an orderly manner, heated under reflux, and then aged to obtain the stable sol that meets the requirements. Figure 7 As shown, the thin film preparation process mainly includes basic steps such as adhesive preparation, spin coating, drying, and RTP heat treatment, among which heat treatment is divided into two stages: pyrolysis and annealing.

[0094] This invention compensates for the potential volatilization of alkali metal elements such as K and Na during subsequent heat treatment by adding 10% excess. Simultaneously, 2 mol% Mn doping [Mn(CH3COO)2, >99.0%, Sinopharm Chemical Reagent Beijing Co., Ltd.] is applied to the sol to reduce film leakage current. The heat treatment of this invention has two stages: pyrolysis and annealing. The former aims to promote the volatilization of the solvent and the decomposition of organic groups in the spin-coated sample to obtain an amorphous film; the latter aims to ultimately crystallize the film. Based on experimental tests, the selected pyrolysis temperature range is 450-550℃, the annealing temperature is 800℃, and the holding time is 3 minutes for both.

[0095] This invention first compensates for the potential volatilization of K and Na alkali metal elements during subsequent heat treatment by adding 10% excess K and Na alkali metal elements when preparing the thin-film KNN-based lead-free piezoelectric ceramic sol. K and Na alkali metal elements are easily volatilized during heat treatment, which can easily lead to leakage current. At the same time, the sol is doped with 2 mol% Mn [Mn(CH3COO)2, >99.0%, Sinopharm Chemical Reagent Beijing Co., Ltd.] to reduce the leakage current of the film layer. Since the self-polarization direction of the KNN ceramic is along the paraelectric cubic phase

[110] direction, the experiment mainly selects the

[110] tangential to the STO substrate, hoping to deposit an epitaxial film with a (110) preferred orientation out of plane, so that the self-polarization direction of the KNN film is consistent with the out-of-plane orientation, which helps to obtain higher electrical performance.

[0096] The above are merely embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention are included within the scope of the claims of the present invention.

Claims

1. A piezoelectric MEMS microphone, characterized in that, The microphone includes: The base has a ring-shaped structure to enclose the first acoustic cavity; The first piezoelectric diaphragm is a triangular cantilever beam structure including the piezoelectric diaphragm, wherein one of the base edges is fixed to the substrate, and the apex of the triangle opposite to the base edge is a free end; The second piezoelectric diaphragm is an independent diaphragm region separated by the groove structure in the fixed bottom edge region of the triangular cantilever beam structure; The encapsulation housing accommodates and fixes the substrate, the first piezoelectric diaphragm, and the second piezoelectric diaphragm, and encloses a second sound cavity between the housing and the substrate; The first piezoelectric diaphragm includes a KNN piezoelectric film, a top electrode, and a bottom electrode.

2. The piezoelectric MEMS microphone as described in claim 1, characterized in that, The second piezoelectric diaphragm is circular in shape.

3. A piezoelectric MEMS microphone as described in claim 1, characterized in that, The piezoelectric diaphragm of the groove structure is supported by local connections through an insulating layer.

4. A piezoelectric MEMS microphone as described in claim 1, characterized in that, The substrate includes a quadrilateral cavity.

5. A piezoelectric MEMS microphone as described in claim 4, characterized in that, The number of the first piezoelectric diaphragms is four, and their tips are joined together to form a quadrilateral. The sum of the areas of the first piezoelectric diaphragms is less than the top area of ​​the quadrilateral cavity.

6. A piezoelectric MEMS microphone as described in claim 1, characterized in that, The substrate includes a bottom substrate and an intermediate substrate fixed on top of the bottom substrate; an insulating layer is present between the bottom substrate and the intermediate substrate.

7. A piezoelectric MEMS microphone as described in claim 6, characterized in that, The insulating layer forms a groove structure with the first piezoelectric diaphragm and the intermediate substrate.

8. A packaging method for a piezoelectric MEMS microphone, characterized in that, For encapsulating a piezoelectric MEMS microphone as described in any one of claims 1-7, the method comprises the following steps: The substrate is obtained by chemically etching multiple substrate layers to form the first acoustic cavity; The first piezoelectric diaphragm and the second piezoelectric diaphragm are processed on the top of the substrate; The microphone's internal structure is obtained by connecting the top and bottom electrodes of the first piezoelectric diaphragm to the base electrodes respectively. The piezoelectric MEMS microphone is obtained by sealing the internal structure of the microphone with a packaging shell to form the second sound cavity.

9. A method for preparing a thin-film KNN lead-free piezoelectric ceramic, wherein the thin-film KNN lead-free piezoelectric ceramic material is used in a piezoelectric MEMS microphone as described in any one of claims 1-7, characterized in that, The method includes the following steps: To (K) 0.5 Na 0.5 Adding excess alkali metal elements such as K and Na to NbO3 sol yields an intermediate sol. The intermediate sol was doped with Mn to obtain a doped gel; The doped gel is spin-coated on a substrate, dried, and subjected to RTP heat treatment to obtain the thin-film KNN lead-free piezoelectric ceramic material.

10. The method for preparing a thin-film KNN lead-free piezoelectric ceramic as described in claim 9, characterized in that, The material used for Mn doping includes Mn(CH3COO)2.

11. The method for preparing a thin-film KNN lead-free piezoelectric ceramic as described in claim 9, characterized in that, The substrate material includes SrTiO3 (STO) single crystal.

12. The method for preparing a thin-film KNN lead-free piezoelectric ceramic as described in claim 9, characterized in that, The heat treatment is divided into two stages: pyrolysis and annealing.

13. The method for preparing a thin-film KNN lead-free piezoelectric ceramic as described in claim 12, characterized in that, The pyrolysis temperature range is 450-550 ℃.

14. The method for preparing a thin-film KNN lead-free piezoelectric ceramic as described in claim 12, characterized in that, The annealing temperature is 800 ℃ ± 10%.

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