A piezoelectric MEMS acoustic sensor

Abstract

The present application relates to the technical field of acoustic sensor, and more particularly to a piezoelectric MEMS acoustic sensor; the acoustic sensor comprises a substrate, a diaphragm and at least one mass block; a back cavity is formed at the bottom of the substrate; the diaphragm is arranged above the substrate and covers the back cavity; under the action of sound pressure, the diaphragm deforms and generates a piezoelectric response; the mass block is arranged on the upper surface and / or the lower surface of the diaphragm; the line connecting the center of mass of the mass block and the center of mass of the diaphragm is perpendicular to the plane where the substrate is located; the mass block is used to increase the equivalent mass of the diaphragm, so as to reduce the final resonant frequency of the diaphragm, effectively expand the low-frequency response bandwidth of the device, and greatly improve the detection sensitivity of low-frequency acoustic underwater acoustic signals.

Description

Technical Field

[0001] This invention relates to the field of acoustic sensor technology, and more particularly to a piezoelectric MEMS acoustic sensor. Background Technology

[0002] With the rapid development of marine resource exploration, marine ecological environment and seabed geological activity monitoring, the demand for high sensitivity and high stability detection of very low frequency and ultra-low frequency underwater acoustic signals is becoming increasingly urgent. However, traditional piezoelectric MEMS acoustic sensors with back cavity structure are limited by the inherent structural characteristics of the diaphragm. The substrate under the diaphragm is completely hollowed out to form a fully transparent back cavity structure. This results in common bottlenecks such as small equivalent mass of the movable diaphragm, high resonant frequency, and insufficient response sensitivity in the low frequency band, making it difficult to effectively pick up low frequency signals in the high hydrostatic pressure and complex noise environment of the deep sea.

[0003] Existing research schemes for low-frequency piezoelectric MEMS acoustic sensors mainly focus on increasing diaphragm size and improving diaphragm topology and support structure. However, these schemes suffer from drawbacks such as increased device size, increased fabrication process complexity, decreased structural mechanical reliability, and increased thermal noise. As a result, they cannot truly achieve excellent low-frequency response performance while maintaining device miniaturization.

[0004] Therefore, there is an urgent need to develop a piezoelectric MEMS acoustic sensor that can be easily fabricated without increasing the device size. Summary of the Invention

[0005] To address the shortcomings of the existing technologies, this invention provides a small-sized and easily fabricated piezoelectric MEMS acoustic sensor.

[0006] This invention provides a piezoelectric MEMS acoustic sensor, comprising: A substrate having a back cavity; A diaphragm is disposed above the substrate and covers the back cavity. The diaphragm deforms under the action of sound pressure and generates a piezoelectric response. At least one mass block is disposed on the upper and / or lower surface of the diaphragm; the line connecting the center of mass of the mass block and the center of mass of the diaphragm is perpendicular to the plane of the substrate, and the mass block is used to increase the equivalent mass of the diaphragm to reduce the final resonant frequency of the diaphragm.

[0007] In some embodiments of this application, the mass of the mass block and the final resonant frequency satisfy the following relationship:

[0008] Where f is the final resonant frequency of the diaphragm plus the mass block; f0 is the initial resonant frequency of the diaphragm before the addition of the mass block; m eq The equivalent mass of the diaphragm; M is the mass of the mass block.

[0009] In some embodiments of this application, the substrate is a bottom silicon substrate, on which a buried oxide layer and a top silicon device layer are sequentially disposed; The diaphragm includes a support layer and a piezoelectric composite layer. The support layer is disposed below the piezoelectric composite layer. The support layer is composed of the buried oxide layer and the top silicon device layer. The piezoelectric composite layer includes a lower electrode layer, a piezoelectric functional layer and an upper electrode layer from bottom to top. The lower electrode layer is disposed above the top silicon device layer. The mass block is positioned below the buried oxide layer and / or above the upper electrode layer.

[0010] In some embodiments of this application, the mass of the mass block satisfies the following relationship:

[0011] Where M is the mass of the mass block, h is the thickness of the diaphragm, ν is the Poisson's ratio of the diaphragm material, g is the acceleration due to gravity, and σ allow This represents the allowable stress of the piezoelectric functional layer.

[0012] In some embodiments of this application, the back cavity extends through the underlying silicon substrate and terminates at the lower surface of the buried oxide layer; When the mass block is positioned below the buried oxygen layer, the mass block extends into the back cavity.

[0013] In some embodiments of this application, the piezoelectric functional layer is any one of aluminum nitride piezoelectric film, ZnO film, lead zirconate titanate film, and lead magnesium niobate titanate film.

[0014] In some embodiments of this application, an insulating protective layer is further provided above the upper electrode layer to isolate moisture and impurities, protect the internal piezoelectric functional layer, and improve the reliability and environmental adaptability of the acoustic sensor.

[0015] In some embodiments of this application, the acoustic sensor is metal-encapsulated to adapt to the extreme high-pressure environment of the deep sea.

[0016] In some embodiments of this application, when the mass block is disposed below the buried oxide layer, the mass block is made of silicon or ceramic material; When the mass block is positioned above the upper electrode, the mass block is made of metal or a metal alloy.

[0017] In some embodiments of this application, the back cavity is formed by the substrate through a back cavity etching process. A light-shielding area is provided in the light-transmitting area of ​​the back cavity photomask. The line connecting the center of the light-shielding area and the centroid of the diaphragm is perpendicular to the substrate. When the substrate is etched, the substrate corresponding to the light-shielding area is retained to form a mass block located below the diaphragm. The mass block extends into the back cavity, and the line connecting the centroid of the mass block and the centroid of the diaphragm is perpendicular to the plane where the substrate is located.

[0018] In some embodiments of this application, after the upper electrode layer of the diaphragm is prepared, the mass block is sputtered above the upper electrode layer, and the line connecting the center of mass of the mass block and the center of mass of the diaphragm is perpendicular to the plane of the substrate.

[0019] Based on the above technical solution, the mass block is set on the diaphragm, and the line connecting its center of mass and the center of mass of the diaphragm is perpendicular to the substrate. This allows the mass block to increase the equivalent mass of the diaphragm, significantly reduce its inherent resonant frequency, effectively expand the low-frequency response bandwidth of the device, and thus greatly improve the detection sensitivity of low-frequency acoustic underwater signals. When the mass block is located above the diaphragm, the same material as the upper electrode layer of the diaphragm can be selected. When the mass block is located below the diaphragm, the same material as the back cavity substrate can be selected. No additional process steps are required, which maintains the compatibility of MEMS process and the mechanical stability of the device. It is especially suitable for the development needs of low-frequency detection devices such as MEMS hydrophones in deep-sea high-pressure environments. Attached Figure Description

[0020] The accompanying drawings, which are included to provide a further understanding of the invention and form part of this application, illustrate exemplary embodiments of the invention and, together with their description, serve to explain the invention and do not constitute an undue limitation thereof. In the drawings: Figure 1 This is a schematic diagram of the acoustic sensor structure according to a specific embodiment 1 of the present invention; Figure 2 This is a schematic diagram of the acoustic sensor structure according to specific embodiment 2 of the present invention; Figure 3 This is a schematic diagram of the acoustic sensor structure in specific embodiment 3 of the present invention; Figure 4 This is a schematic diagram of the film structure of the acoustic sensor of the present invention; Figure 5 This is a schematic diagram of the back cavity photomask structure in specific embodiment 1 of the present invention; Figure 6 This is a schematic diagram of the back cavity of the acoustic sensor in specific embodiment 1 of the present invention; In the picture: 10. Substrate; 20. Diaphragm; 30. Mass block; 40. Back cavity photolithography plate; 11. Back cavity; 21. Support layer; 22. Piezoelectric composite layer; 211. Buried oxide layer; 212. Top silicon device layer; 221. Bottom electrode layer; 222. Piezoelectric functional layer; 223. Top electrode layer; 41. Light-transmitting area; 42. Light-blocking area. Detailed Implementation

[0021] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0022] In the description of this invention, it should be understood that the terms "center", "lateral", "longitudinal", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0023] The terms "first," "second," and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Therefore, a feature defined as "first," "second," or "third" may explicitly or implicitly include one or more of that feature.

[0024] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "joining" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal communication between two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0025] MEMS (Micro Electro Mechanical Systems) technology is an emerging technology originating from semiconductor technology. It involves the cross-integration of multiple physical fields such as force, sound, electricity, optics, magnetism, and heat in micro- and nano-fabrication techniques. MEMS chips are typically smaller than 1 cm in size. 2They possess outstanding characteristics such as small size, light weight, low inertia, low power consumption, high integration, and suitability for mass production, enabling one-time, large-scale, homogeneous production, thereby reducing the production cost per chip. Currently, various types of MEMS sensors are used in almost all military and civilian fields.

[0026] Acoustic sensors, as a type of passive piezoelectric sensor, utilize the piezoelectric effect of piezoelectric materials to directly convert underwater sound pressure signals into electrical signals. They possess core advantages such as simple structure, no need for bias voltage, extremely low power consumption, and ease of miniaturization and arraying. These characteristics make them ideal for marine resource exploration, marine ecological environment monitoring, and seabed geological activity monitoring. Currently, the operating frequency band or characteristic frequency of mainstream piezoelectric acoustic sensors is mainly concentrated above the kHz level, which cannot meet the detection requirements of low-frequency and ultra-low-frequency signals.

[0027] Therefore, this application provides a piezoelectric MEMS acoustic sensor, including a substrate 10 and a diaphragm 20 disposed on the substrate 10. A back cavity 11 is formed at the bottom of the substrate 10. The back cavity 11 is an open structure and its bottom does not need to be closed, making it suitable for deep-sea high static pressure environments. The diaphragm 20 is disposed above the substrate 10 and covers the back cavity 11. Under the action of sound pressure, the diaphragm 20 deforms and generates a piezoelectric response, converting the mechanical deformation of the diaphragm 20 into an electrical signal.

[0028] In some embodiments, such as Figure 1 As shown, a mass block 30 is provided on the lower surface of the diaphragm 20. The line connecting the center of mass of the mass block 30 and the center of mass of the diaphragm 20 is perpendicular to the plane where the base 10 is located. The mass block 30 extends into the back cavity 11. The mass block 30 increases the equivalent mass of the diaphragm, which can reduce the final resonant frequency of the diaphragm 20.

[0029] In some embodiments, such as Figure 2 As shown, a mass block 30 is provided on the upper surface of the diaphragm 20. The line connecting the center of mass of the mass block 30 and the center of mass of the diaphragm 20 is perpendicular to the plane of the base 10. The mass block 30 increases the equivalent mass of the diaphragm, which can reduce the final resonant frequency of the diaphragm 20.

[0030] In some embodiments, such as Figure 3 As shown, a mass block 30 is respectively provided on the upper and lower surfaces of the diaphragm 20. The centers of mass of the two mass blocks 30 are collinear with the center of mass of the diaphragm 20, and the line is perpendicular to the plane of the base 10. The two mass blocks 30 increase the equivalent mass of the diaphragm and reduce the final resonant frequency of the diaphragm 20.

[0031] In some embodiments, the mass of the mass block 30 and the final resonant frequency satisfy the following relationship:

[0032] Where f is the final resonant frequency after the diaphragm is fitted with a mass block; f0 is the initial resonant frequency of the diaphragm 20 without the addition of the mass block 30; m eq The equivalent mass of diaphragm 20; M is the mass of mass block 30.

[0033] In some embodiments, a silicon-on-insulator (SiI) wafer consists of three layers: a top silicon device layer, a buried oxide layer, and a bottom silicon substrate; such as Figure 4 As shown, the bottom silicon substrate of the silicon-on-insulator wafer serves as the substrate 10 in this embodiment, and the buried oxide layer 211 and the top silicon device layer 212 are sequentially arranged above the bottom silicon substrate. The diaphragm 20 includes a support layer 21 and a piezoelectric composite layer 22. The support layer 21 is disposed below the piezoelectric composite layer 22. The support layer 21 is composed of a buried oxide layer 211 and a top silicon device layer 212. The piezoelectric composite layer 22 includes a lower electrode layer 221, a piezoelectric functional layer 222 and an upper electrode layer 223 from bottom to top. The lower electrode layer 221 is disposed above the top silicon device layer 212. like Figure 1 The acoustic sensor shown has a mass block 30 positioned below the buried oxide layer 211; as shown Figure 2 The acoustic sensor shown has a mass block 30 positioned above the upper electrode layer 223; as shown Figure 3 The acoustic sensor and mass block 30 shown are respectively positioned below the buried oxygen layer 211 and above the upper electrode layer 223.

[0034] In some embodiments, the substrate 10 can also be replaced by a sapphire wafer or a quartz wafer, both of which have better insulation properties, can reduce leakage interference between electrodes, and are suitable for low-noise, low-frequency sensing requirements.

[0035] In some embodiments, the diaphragm 20 can be equivalent to a single-degree-of-freedom spring-mass system, whose fundamental resonant frequency satisfies:

[0036] Where f is the final resonant frequency of the diaphragm after adding the mass block, k is the equivalent stiffness of the diaphragm, and m eq M is the equivalent mass of diaphragm 20, and M is the mass of mass block 30; As can be seen from the above formula, theoretically, the resonant frequency f decreases when the mass M of the mass block increases, and f → 0 when M → ∞. However, in actual structures, the additional mass block cannot be increased indefinitely, and there is a maximum allowable mass M. The mass block 30 forms a concentrated load at the center of the diaphragm 20, causing stress at the edge of the diaphragm 20. This stress must not exceed the allowable stress σ of the buried oxide layer 211, the top silicon device layer 212, and the piezoelectric functional layer 222. allowOtherwise, the diaphragm 20 will break or crack and fail. Therefore, the maximum stress that the edge of the diaphragm 20 can withstand and the allowable stress must satisfy the following relationship:

[0037] Where, σ max Let M be the maximum stress that the edge of the diaphragm 20 can withstand, h be the mass of the mass block, h be the thickness of the diaphragm, ν be the Poisson's ratio of the diaphragm material, g be the acceleration due to gravity, and σ be the maximum stress that the diaphragm 20 can withstand. allow This represents the allowable stress of the piezoelectric functional layer. Therefore, the mass of mass block 30 satisfies the following relationship: .

[0038] In some embodiments, the back cavity penetrates through the bottom silicon substrate and ends at the lower surface of the buried oxide layer, which can precisely control the thickness of the diaphragm 20 and the size of the mass block 30, improve the consistency of the device structure and mechanical stability, and adapt to high-precision low-frequency detection scenarios. When the mass block 30 is positioned below the buried oxygen layer 211, the mass block 30 extends into the back cavity 11.

[0039] In some embodiments, the piezoelectric functional layer 222 is any one of aluminum nitride piezoelectric film, ZnO film, lead zirconate titanate film, and lead magnesium niobate titanate film.

[0040] In some embodiments, an insulating protective layer (not shown in the figure) is also provided above the upper electrode layer 223 to isolate moisture and impurities, protect the internal piezoelectric functional layer 222, and improve the reliability and environmental adaptability of the acoustic sensor.

[0041] In some embodiments, the acoustic sensor is encapsulated in a metal package to adapt to the extreme high-pressure environment of the deep sea.

[0042] In some embodiments, when the mass block 30 is disposed below the buried oxide layer 211, the material of the mass block 30 is silicon, which is the same as the material of the underlying silicon substrate. While etching the back cavity 11, a portion of the material at the centroid position of the diaphragm 20 is retained as the mass block 30, without the need for additional process steps.

[0043] In other embodiments, when the mass block 30 is disposed below the buried oxide layer 211, it can be made of ceramic materials such as SiC and Al2O3.

[0044] In some embodiments, when the mass block 30 is disposed above the upper electrode layer 223, the material of the mass block 30 can be Au or higher density metals such as Pt, W or metal alloys such as Au-Cu, taking into account mass density, process compatibility and cost requirements.

[0045] In other embodiments, when the mass block 30 is disposed above the upper electrode layer 223, the mass block can also be a metal alloy, a high-density metal or metal alloy mass load, which can achieve a more significant reduction in resonant frequency in a smaller size and a more prominent improvement in low-frequency sensitivity.

[0046] In some embodiments, the mass block 30 can be sheet-like or ring-shaped, with the line connecting the center of the ring and the centroid of the diaphragm perpendicular to the substrate, ensuring that the mass block 30 acts on the centroid of the diaphragm 20 in a concentrated manner, thereby achieving a reduction in the resonant frequency.

[0047] In some embodiments, the back cavity 11 of the acoustic sensor is formed by a back cavity etching process on the substrate 10, wherein, as... Figure 4 As shown, a light-shielding area 42 is provided within the light-transmitting area 41 of the back cavity photomask 40 used in the back cavity etching process. The line connecting the center of the light-shielding area 42 and the centroid of the diaphragm 20 is perpendicular to the substrate 10, so that the substrate 10 at the position corresponding to the light-shielding area 42 is preserved when etching the substrate 10. Figure 5 As shown, a mass block 30 is formed below the diaphragm 20, and the mass block 30 extends into the back cavity 11. The line connecting the center of mass of the mass block 30 and the center of mass of the diaphragm 20 is perpendicular to the plane where the base 10 is located.

[0048] In some embodiments, after the upper electrode layer 223 of the diaphragm 20 is prepared, a mass block 30 is sputtered above the upper electrode layer 223, and the line connecting the center of mass of the mass block 30 and the center of mass of the diaphragm 20 is perpendicular to the plane of the substrate 10.

[0049] The piezoelectric MEMS acoustic sensor provided by the present invention will be described below with reference to specific embodiments. Specific Implementation Example 1 This embodiment takes an AlN-based piezoelectric MEMS acoustic sensor as an example. Through a back cavity etching process, a silicon pillar-shaped mass block 30 is formed in situ at the center of the diaphragm 20 to achieve a lower resonant frequency and improved low-frequency sensitivity. S1. Substrate treatment: Using silicon-on-insulation (SOI) wafers as the substrate, RCA standard cleaning is performed: ultrasonic cleaning with acetone and isopropanol in sequence, immersion cleaning in SC1 (NH4OH:H2O2:H2O=1:1:5) and SC2 (HCl:H2O2:H2O=1:1:6) solutions at 70-80℃, rinsing with deionized water and drying with nitrogen gas to meet the cleanliness requirements of the substrate; S2. Fabrication of the lower electrode layer: A Ti (20–50 nm) / Mo (100–300 nm) composite lower electrode was deposited by magnetron sputtering at a flow rate of 20–30 sccm and a chamber pressure of 0.3–0.6 Pa at room temperature. After spin-coating photoresist, the electrode pattern was etched by ICP using an SF6 / O2 mixed gas. Finally, the photoresist was removed by oxygen plasma to obtain the lower electrode layer 221. S3, Preparation of piezoelectric functional layer 222: 0.5-2 μm AlN piezoelectric thin film was deposited by reactive magnetron sputtering in an Ar / N2 mixed atmosphere at a substrate temperature of 250-350℃ and a power of 150-300 W. After photolithography patterning, piezoelectric functional layer 222 was formed by ICP etching with a Cl2 / BCl3 mixed gas, followed by resist removal. S4. Fabrication of upper electrode layer 223: Mo electrode is sputtered at 100-300 nm with process parameters compatible with lower electrode layer 221; upper electrode patterning is completed by photolithography and ICP etching, and photoresist is removed by oxygen plasma to obtain upper electrode layer 223. S5. Insulating protective layer and pad opening: PECVD deposition of SiO2 or SiN x As an insulating protective layer, the thickness is 200–800 nm, and the deposition temperature is 250–350 °C; the pad windows are opened by ICP etching with photolithography and SF6 / O2 mixed gas to complete the device protection; S6, Back cavity 11 etching and mass block 30 forming: The bottom silicon substrate at the bottom of the insulating silicon wafer is precision ground and thinned to 150-500 μm, and the surface damage layer is removed simultaneously; a thick layer of photoresist is spin-coated on the back of the bottom silicon substrate, using a method such as... Figure 4 The back cavity photomask 40 shown is exposed and developed to form a mask, with its light-shielding area 42 precisely aligned with the centroid of the diaphragm 20. The back cavity is etched using the DRIEBosch process, alternating between SF6 etching and C4F8 passivation, with an etching power of 400–800 W and a cavity pressure of 1.0–2.0 Pa, until the buried oxide layer is reached. Due to the light-shielding area 42, the silicon material in this region is retained, and a silicon pillar collinear with the centroid of the diaphragm is formed in situ as a mass block 30, thus fabricating the back cavity as shown. Figure 1 The acoustic sensor structure shown is finally cleaned after removing the photoresist on the back. S7. Packaging and Testing: After the casing and electrode bonding are completed, testing is carried out. The results show that compared with traditional fully hollowed-out back cavity devices, the resonant frequency of the acoustic sensor in this embodiment can be reduced to 230~280kHz, a reduction of 22%~36%; the thickness of the mass block is consistent with the back cavity depth of 150~500μm; the low-frequency detection sensitivity is significantly improved, which is suitable for the needs of deep-sea low-frequency underwater acoustic detection. Specific Implementation Example 2 The difference from specific embodiment 1 is that a mass block 30 is integrated on the upper electrode layer 223; Steps S1-S3 are the same as in the specific embodiment, except that: Preparation of the upper electrode layer of S4: Mo base electrode layer of 100-200 nm was deposited by magnetron sputtering at room temperature in Ar atmosphere, and the upper electrode layer 223 was formed by photolithography and etching, and the photoresist was removed. S5. Fabrication of a mass block at the center of the upper electrode layer by magnetron sputtering: Photoresist is spin-coated onto the upper electrode layer 223, and an exposure and development process is performed using a photoresist plate with windows corresponding to the centroid of the diaphragm, exposing only the central region of the diaphragm; an Au or Mo thickening layer of 300–1000 nm is magnetron sputtered, with an Ar gas flow rate of 20–30 sccm and a cavity pressure of 0.4–0.6 Pa; excess metal and photoresist are removed by a lift-off process, forming a thickened metal mass block 30 at the centroid of the diaphragm 20, such as… Figure 2 As shown, the line connecting the center of mass of mass block 30 and the center of mass of diaphragm 20 is perpendicular to the substrate, and plasma de-adhesion is performed. S6. Back cavity etching: A traditional fully transparent back cavity photomask is used to form a fully hollowed-out back cavity through standard Bosch etching. The process parameters are the same as in Example 1. Then, the resist is removed and cleaned. S7. Packaging and Testing: After completing the metal packaging and bonding, the test results show that compared with traditional equal-thickness top electrode devices, this embodiment uses a high-density metal mass block, which can achieve a more significant reduction in resonant frequency in a smaller size, which can be reduced to 290~330kHz, with a reduction of 8%~19%; compared with the same thickness, the frequency tuning efficiency of high-density metal is more than 8 times that of silicon material, which can achieve significant low frequency in an extremely thin axial dimension, in line with the advantage of "frequency tuning in a smaller size", and the low-frequency sensitivity is more significantly improved.

[0052] Specific embodiment 3 adopts steps S1-S3 of specific embodiment 1, steps S4-S5 of specific embodiment 2, and steps S6-S7 of specific embodiment 2 to prepare the following... Figure 3 The acoustic sensor shown has mass blocks 30 located on the upper and lower sides of the diaphragm with their centroids collinear with the centroid of the diaphragm 20, and the line connecting the centroids of the two mass blocks 30 and the centroid of the diaphragm 20 is perpendicular to the substrate. The acoustic sensor in this embodiment achieves a more significant reduction in resonant frequency, which can be reduced to 190~240kHz, a reduction of 33%~47%. The mass blocks added on both sides of the diaphragm are superimposed to maximize the equivalent total mass, resulting in better low-frequency performance than the single-sided solution and a more prominent improvement in low-frequency sensitivity.

[0053] In other embodiments, different manufacturing processes can be adapted to the mass blocks 30 with different materials and thicknesses, for example: For a mass block 30 with a thickness of 0.5-3μm and made of alumina (Al2O3) ceramic, alumina is deposited in the back cavity using reactive magnetron sputtering, followed by photolithography patterning and dry etching to prepare the alumina mass block. It has good stability and pressure resistance and is suitable for deep-sea high-pressure environments.

[0054] For a mass block with a thickness of 0.5–2 μm and made of molybdenum (Mo) metal, a Mo metal layer is deposited in the back cavity using magnetron sputtering, followed by photolithography patterning and ICP etching to prepare the Mo mass block.

[0055] For a mass block 30 with a thickness of 0.5–5 μm and made of gold (Au), a thick Au layer with a high density (19.3 g / cm³) can be quickly prepared on the upper electrode layer 223 using an electroplating process. A small thickness can significantly reduce the resonant frequency, making it suitable for ultra-low frequency detectors that require a large load mass block.

[0056] For a mass block 30 with a thickness of 0.2–2 μm and made of tungsten (W), an electron beam evaporation process is used to prepare a W layer with a density of (19.25 g / cm³) on the upper electrode layer 223. The film is dense and the electron beam evaporation film has good uniformity, which is suitable for the preparation of high-precision, thin mass blocks.

[0057] For a mass block 30 with a thickness of 1–4 μm and made of copper (Cu), a Cu layer can be quickly prepared on the upper electrode layer 223 using an electroplating process, which is suitable for mass production of low- and medium-frequency MEMS acoustic sensors.

[0058] For a platinum (Pt) mass block 30 with a thickness of 0.1–1 μm, an electron beam evaporation process can be used to rapidly prepare a Pt mass block 30 with extremely high chemical stability on the upper electrode layer 223. This makes it suitable for harsh environments such as seawater corrosion and high temperature, and an acoustic sensor that can achieve effective mass loading with an ultra-thin thickness.

[0059] Comparative Example 1 The difference from Specific Embodiment 1 is that in step S6, the center of the light-shielding area 42 within the light-transmitting area 41 of the cavity photomask 40 is misaligned with the center of mass of the diaphragm 20. This causes the line connecting the center of mass of mass block 30 and the center of mass of diaphragm 20 to be tilted relative to the plane of the substrate. The resonant frequency of the acoustic sensor shows only a slight shift (usually <5%) in the main resonant peak, but is accompanied by multiple higher-order stray resonant peaks, resulting in a distorted resonant curve and no single stable first-order operating resonant frequency. The reason is that the misalignment between the center of mass block and the center of mass of diaphragm disrupts the axisymmetric vibration characteristics of the diaphragm, excites higher-order non-ideal modes such as torsion and bending, disperses the energy of the main operating mode, leading to resonant peak splitting, decreased piezoelectric conversion efficiency, and deterioration of low-frequency sensitivity.

[0060] Comparative Example 2 The difference from Specific Embodiment 2 is that in step S5, the window position of the photolithography plate is offset from the centroid of the diaphragm, causing the line connecting the centroid of the mass block 30 and the centroid of the diaphragm 20 to be tilted relative to the plane of the substrate. The resonant frequency of the acoustic sensor follows the same pattern as in Comparative Example 1, with a slight shift in the main resonant peak and the appearance of obvious higher-order clutter peaks, resulting in distorted resonant characteristics. This is because the front metal mass block deviates from the centroid of the diaphragm, disrupting the symmetry of the diaphragm's force and vibration, generating additional torsional torque, making the main working mode impure, causing the resonant peak to split, and reducing the device's sensitivity and signal-to-noise ratio.

[0061] Comparative Example 3 In step S5, the window position of the photomask is offset from the centroid of the diaphragm. In step S6, the center of the light-shielding area 42 within the light-transmitting area 41 of the back cavity photomask 40 is misaligned with the centroid of the diaphragm 20. This results in the centroids of the two mass blocks 30 located above and below the diaphragm 20 being non-collinear with the centroid of the diaphragm 20. The lines connecting the centroids of the two mass blocks 30 and the centroid of the diaphragm 20 are inclined to the plane of the substrate. As a result, the resonant frequency of the acoustic sensor exhibits severe distortion and splitting of the main resonant peak, with more and higher amplitude higher-order clutter peaks. Even the main working mode is suppressed, with no clearly discernible first-order main resonant peak. The reason is that both the upper and lower mass blocks are offset from the centroid of the diaphragm and are non-collinear, further exacerbating the asymmetric force and vibration imbalance of the diaphragm. Torsional and bending modes are significantly excited, the proportion of the main working mode is significantly reduced, the device's resonant characteristics are severely deteriorated, and it essentially loses its stable acoustic detection capability.

[0062] Based on the above technical solution, the mass block is set on the diaphragm, and the line connecting its center of mass and the center of mass of the diaphragm is perpendicular to the substrate. This allows the mass block to increase the equivalent mass of the diaphragm, significantly reduce its inherent resonant frequency, effectively expand the low-frequency response bandwidth of the device, and thus greatly improve the detection sensitivity of low-frequency acoustic underwater signals. When the mass block is located above the diaphragm, the same material as the upper electrode layer of the diaphragm can be selected. When the mass block is located below the diaphragm, the same material as the back cavity substrate can be selected. No additional process steps are required, which maintains the compatibility of MEMS process and the mechanical stability of the device. It is especially suitable for the development needs of low-frequency detection devices such as MEMS hydrophones in deep-sea high-pressure environments.

[0063] Finally, it should be noted that the various embodiments in this specification are described in a progressive manner, with each embodiment focusing on the differences from other embodiments. The same or similar parts between the various embodiments can be referred to each other.

[0064] The above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them; although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications can still be made to the specific implementation of the present invention or equivalent substitutions can be made to some technical features without departing from the spirit of the technical solutions of the present invention, and all such modifications and substitutions should be covered within the scope of the technical solutions claimed in the present invention.

Claims

1. A piezoelectric MEMS acoustic sensor, characterized in that, include: A substrate having a back cavity; A diaphragm is disposed above the substrate and covers the back cavity. The diaphragm deforms under the action of sound pressure and generates a piezoelectric response. At least one mass block is disposed on the upper and / or lower surface of the diaphragm; the line connecting the center of mass of the mass block and the center of mass of the diaphragm is perpendicular to the plane of the substrate, and the mass block is used to increase the equivalent mass of the diaphragm to reduce the final resonant frequency of the diaphragm.

2. The piezoelectric MEMS acoustic sensor according to claim 1, characterized in that, The mass of the mass block and the final resonant frequency satisfy the following relationship: Where f is the final resonant frequency of the diaphragm plus the mass block; f0 is the initial resonant frequency of the diaphragm before the addition of the mass block; m eq The equivalent mass of the diaphragm; M is the mass of the mass block.

3. The piezoelectric MEMS acoustic sensor according to claim 1, characterized in that, The substrate is a bottom silicon substrate, on which a buried oxide layer and a top silicon device layer are sequentially disposed; The diaphragm includes a support layer and a piezoelectric composite layer. The support layer is disposed below the piezoelectric composite layer. The support layer is composed of the buried oxide layer and the top silicon device layer. The piezoelectric composite layer includes a lower electrode layer, a piezoelectric functional layer and an upper electrode layer from bottom to top. The lower electrode layer is disposed above the top silicon device layer. The mass block is positioned below the buried oxide layer and / or above the upper electrode layer.

4. The piezoelectric MEMS acoustic sensor according to claim 3, characterized in that, The mass of the mass block satisfies the following relationship: Where M is the mass of the mass block, h is the thickness of the diaphragm, ν is the Poisson's ratio of the diaphragm material, g is the acceleration due to gravity, and σ allow This represents the allowable stress of the piezoelectric functional layer.

5. The piezoelectric MEMS acoustic sensor according to claim 3, characterized in that, The back cavity penetrates the bottom silicon substrate and terminates at the lower surface of the buried oxide layer; When the mass block is positioned below the buried oxygen layer, the mass block extends into the back cavity.

6. The piezoelectric MEMS acoustic sensor according to claim 3, characterized in that, The piezoelectric functional layer is any one of aluminum nitride piezoelectric film, ZnO film, lead zirconate titanate film, and lead magnesium niobate titanate film.

7. The piezoelectric MEMS acoustic sensor according to claim 3, characterized in that, An insulating protective layer is also provided above the upper electrode layer.

8. The piezoelectric MEMS acoustic sensor according to claim 3, characterized in that, When the mass block is positioned below the buried oxide layer, the mass block is made of silicon or ceramic. When the mass block is positioned above the upper electrode, the mass block is made of metal or a metal alloy.

9. The piezoelectric MEMS acoustic sensor according to claim 3, characterized in that, The back cavity is formed by the substrate through a back cavity etching process. A light-shielding area is provided in the light-transmitting area of ​​the back cavity photomask. The line connecting the center of the light-shielding area and the centroid of the diaphragm is perpendicular to the substrate. When etching the substrate, the portion of the substrate corresponding to the light-shielding area is retained to form a mass block located below the diaphragm. The mass block extends into the back cavity, and the line connecting the centroid of the mass block and the centroid of the diaphragm is perpendicular to the plane of the substrate.

10. The piezoelectric MEMS acoustic sensor according to claim 9, characterized in that, After the upper electrode layer of the diaphragm is prepared, the mass block is sputtered above the upper electrode layer, and the line connecting the center of mass of the mass block and the center of mass of the diaphragm is perpendicular to the plane of the substrate.

Images