Piezoelectric ultrasonic transducer based on MEMS chip

Abstract

The transducer comprises a substrate, the substrate is provided with a front face and a back face which are opposite, a piezoelectric layer is formed on the front face of the substrate, a back cavity penetrating to the piezoelectric layer is formed in the back face of the substrate, a structural layer is formed on the surface of the piezoelectric layer, and the piezoelectric layer is arranged on the surface of the structural layer. The piezoelectric layer and the structural layer in the longitudinal projection area range of the back cavity are vibrating diaphragms, and the thickness of the structural layer is larger than that of a piezoelectric film in the piezoelectric layer, so that the neutral plane is located in the transverse projection area range of the structural layer. According to the piezoelectric ultrasonic transducer and the preparation method thereof, the silicon oxide / silicon nitride is adopted as the structural layer, the neutral surface is lifted to the structural layer to replace traditional top silicon, the Young modulus of the silicon oxide / silicon nitride is low, the process stress is low, the resonant frequency can be effectively reduced, and design and manufacturing of low-frequency devices are facilitated.

Description

Technical Field

[0001] The present invention relates to the field of semiconductor process technology based on MEMS chips, and in particular to a piezoelectric ultrasonic transducer based on MEMS chips and a preparation method thereof. Background Art

[0002] Ultrasonic sensors are sensors that convert ultrasonic signals into other energy signals (usually electrical signals). Ultrasonic waves are mechanical waves with a vibration frequency higher than 20kHz. They have the characteristics of high frequency, short wavelength, small diffraction phenomenon, especially good directionality, and the ability to propagate in a directional manner as rays. Ultrasonic waves have a great ability to penetrate liquids and solids, especially in solids that are opaque to sunlight. When ultrasonic waves encounter impurities or interfaces, they will produce significant reflections to form reflected echoes, and when they encounter moving objects, they can produce the Doppler effect. Ultrasonic sensors are widely used in industry, national defense, biomedicine, etc.

[0003] In the prior art, low-frequency ultrasonic sensors are needed in technical fields such as long-distance measurement, monitoring, and remote information transmission using ultrasound. Therefore, low-frequency MEMS chip devices also have a large market. However, traditional piezoelectric MEMS chips are composed of a stacked SOI substrate with a back cavity and a piezoelectric layer. The top silicon in the SOI substrate serves as a structural layer and assumes the function of a neutral plane. Silicon has a high Young's modulus and high process stress, which leads to a high resonant frequency, which is not conducive to the production design and manufacturing of low-frequency devices. On this basis, in order to reduce the stress of the device and improve the sensitivity, a gap is opened on the chip. The gap will cause the chip to produce multiple resonant frequencies, and after the gap divides the diaphragm into several parts, it cannot maintain good performance in a vibration and impact environment, and the mechanical stability is low.

[0004] Patent document with application number 202323565644.9 discloses a piezoelectric micromechanical ultrasonic transducer chip structure and an ultrasonic transducer array structure. The chip structure of the application scheme includes an SOI wafer substrate and a chip sensitive layer; the SOI wafer substrate includes a substrate layer, a buried oxide layer and a device layer stacked from bottom to top, and a hexagonal opening penetrating the upper and lower end faces of the substrate layer is provided in the substrate layer; the chip sensitive layer includes a lower electrode layer, a first piezoelectric layer and an upper electrode layer stacked from bottom to top, and the lower electrode layer is stacked on the upper part of the device layer; the upper electrode layer is a hexagonal electrode structure, and the upper electrode layer is located directly above the hexagonal opening; a plurality of first depressions aligned one by one are provided on the buried oxide layer, the device layer, the lower electrode layer and the first piezoelectric layer; the plurality of first depressions are distributed on the periphery of the upper electrode layer in a hexagonal outline, and the hexagonal outline is close to or located at the edge of the hexagonal opening.

[0005] The application scheme improves the effective vibration displacement of the chip's sensitive layer and improves the chip's performance as a driver and sensor. However, the scheme also has the following problems: its structural scheme is a traditional one, and the top silicon in its SOI substrate serves as a structural layer to bear the function of the neutral plane, which is not conducive to the production design and manufacturing of low-frequency devices.

[0006] Therefore, a piezoelectric ultrasonic transducer based on a MEMS chip and a preparation method thereof are needed to solve the above-mentioned problems. Summary of the invention

[0007] In view of the above-mentioned problems, the purpose of the present invention is to provide a piezoelectric ultrasonic transducer based on MEMS chip and a preparation method, produce a structural layer on the surface of the piezoelectric layer to replace the traditional top silicon, effectively reduce the resonant frequency, and at the same time use bent beams to form each vibration module into a whole to avoid the occurrence of multiple resonant frequencies and maintain the mechanical stability of the chip, so that it can still maintain good performance in vibration and impact environments.

[0008] The object of the present invention can be achieved through the following technical solutions: A piezoelectric ultrasonic transducer based on a MEMS chip, comprising a substrate, the substrate having a front side and a back side opposite to each other, a piezoelectric layer formed on the front side of the substrate, a back cavity penetrating to the piezoelectric layer is opened on the back side of the substrate, a structural layer is formed on the surface of the piezoelectric layer, the piezoelectric layer and the structural layer within the longitudinal projection area of ​​the back cavity are a diaphragm, the thickness of the structural layer is greater than the thickness of the piezoelectric film in the piezoelectric layer, so that the neutral plane is located within the lateral projection area of ​​the structural layer;

[0009] The diagonal lines of the structural layer are provided with gaps, so that the area of ​​the structural layer located at the diaphragm is divided into four vibration modules, and adjacent vibration modules are connected by bending beams.

[0010] The neutral plane is located in the structural layer, replacing the traditional top silicon. The process stress is low, which can effectively reduce the resonant frequency and is conducive to the production design and manufacturing of low-frequency devices. The gap can reduce the stress effect, and the bending beam can be used to form each vibration module into a whole to avoid the occurrence of multiple resonant frequencies.

[0011] As a further solution of the present invention, the structural layer is a double-layer structure or a triple-layer structure in which silicon nitride and silicon oxide are alternately stacked.

[0012] Silicon oxide / silicon nitride is used as the main layer of the structural layer. Silicon oxide / silicon nitride has a low Young's modulus and low process stress, which can effectively reduce the resonant frequency and is beneficial to the production design and manufacturing of low-frequency devices.

[0013] As a further solution of the present invention, the pattern of the piezoelectric layer diaphragm region is consistent with the pattern of the structural layer diaphragm region.

[0014] As a further solution of the present invention, the bent beams are divided into four groups, which are symmetrically distributed, and each group of bent beams includes straight beams and U-shaped beams connected to each other.

[0015] The straight beam and U-shaped beam structure of the bent beam can enhance the mechanical stability of the chip, allowing it to maintain good performance under vibration and shock environments.

[0016] As a further solution of the present invention, a structure in which four vibration modules are connected by bending beams is:

[0017] The first vibration module connects the U-shaped beam in the first group of bent beams and the straight beam in the fourth group of bent beams;

[0018] The second vibration module is connected to the U-shaped beam in the straight beam in the first set of bent beams;

[0019] The third vibration module is connected to the U-shaped beam in the straight beam in the second group of bent beams;

[0020] The fourth vibration module is connected to the U-shaped beam in the straight beam in the third group of bent beams.

[0021] As a further solution of the present invention, another structure in which four vibration modules are connected by bending beams is:

[0022] The first vibration module connects the U-shaped beam in the first group of bent beams and the U-shaped beam in the fourth group of bent beams;

[0023] The second vibration module connects the straight beam in the first group of bent beams and the straight beam in the second group of bent beams;

[0024] The third vibration module connects the U-shaped beam in the second group of bent beams and the U-shaped beam in the third group of bent beams;

[0025] The fourth vibration module connects the straight beams in the third group of bent beams and the straight beams in the fourth group of bent beams.

[0026] Different types of connection structures can form the various vibration modules into a whole while avoiding the occurrence of multiple resonant frequencies.

[0027] As a further solution of the present invention, each group of bent beams includes a plurality of U-shaped beams arranged in a linear array and a straight beam, adjacent U-shaped beams are connected, and the U-shaped beam at the end is connected to the straight beam.

[0028] A preparation method for the piezoelectric ultrasonic transducer, the preparation method comprising the following steps:

[0029] S1, preparing a substrate and growing a piezoelectric layer on the front side of the substrate;

[0030] S2, etching the piezoelectric layer to pattern the piezoelectric layer;

[0031] S3, growing a structural layer on the surface of the piezoelectric layer;

[0032] S4, etching the structural layer to pattern the structural layer, and after the structural layer is etched, a gap penetrating the structural layer is formed along the diagonal line of the diaphragm region;

[0033] S5, opening a back cavity in the substrate that runs through the substrate to the piezoelectric layer;

[0034] Wherein, the region where the structural layer is located on the diaphragm forms four vibration modules due to the gaps, and adjacent vibration modules are connected by bending beams formed by etching.

[0035] Through the above processing method, the neutral plane is moved up and the process stress is low, which can effectively reduce the resonant frequency. At the same time, the sub-modules are connected by bending beams to reduce the generated resonant frequency. At the same time, the bending beam structure can enhance the mechanical stability of the chip, so that it can still maintain good performance in vibration and impact environments.

[0036] As a further solution of the present invention, when the gap is formed in S4, the pattern of the piezoelectric layer diaphragm region is consistent with the pattern of the structural layer diaphragm region, and the preparation method includes:

[0037] S4-1, after the piezoelectric layer is generated on the substrate, the piezoelectric layer is etched to pattern the piezoelectric layer; a gap is formed along the diagonal of the diaphragm region of the piezoelectric layer due to etching, and the gap divides the diaphragm region of the piezoelectric layer into four vibration modules, and adjacent vibration modules are connected by bending beams formed by etching;

[0038] S4-2. Generate a structural layer on the surface of the piezoelectric layer and pattern the structural layer; make the diaphragm area of ​​the structural layer form a pattern consistent with the diaphragm area of ​​the piezoelectric layer due to etching, obtain a gap running through the piezoelectric layer and the structural layer, divide the diaphragm area into four vibration modules, and connect adjacent vibration modules through bent beams formed by etching.

[0039] The gaps penetrate the chip, improving sensitivity and interconnecting vibration modules, enhancing the mechanical stability of the chip and reducing the resonant frequency.

[0040] As a further solution of the present invention, the back cavity opening process in S5 includes:

[0041] Firstly, a dry process is used to etch part of the substrate from the back side of the substrate, and then a wet process is used to etch through the back cavity of the substrate.

[0042] When etching the SOI substrate, the buried oxide layer acts as a cutoff line, and a back cavity can be formed by penetrating the buried oxide layer. Pure dry etching may damage the piezoelectric layer. In order to improve the yield rate, dry etching is used to etch part of the silicon wafer substrate, and then a wet process is used to etch the remaining part. The wet process will not affect the piezoelectric layer.

[0043] Beneficial effects of the present invention:

[0044] 1. The piezoelectric ultrasonic transducer and preparation method thereof of the present invention adopt silicon oxide / silicon nitride as the structural layer, and the neutral plane is raised to the structural layer to replace the traditional top silicon. Silicon oxide / silicon nitride has a low Young's modulus and low process stress, which can effectively reduce the resonant frequency and is useful for the design and manufacture of low-frequency devices.

[0045] 2. The present invention opens a gap in the diaphragm area and utilizes the gap to reduce stress. At the same time, each vibration module is formed into a whole by a bent beam to avoid the occurrence of multiple resonant frequencies. The structure of the bent beam can enhance the mechanical stability of the chip, so that it can still maintain good performance under vibration and impact environments.

[0046] 3. Compared with the traditional preparation process, the preparation method of the piezoelectric ultrasonic transducer based on MEMS chip proposed in the present invention first grows the piezoelectric layer and patterns it, then grows the structural layer on the surface of the piezoelectric layer, and then patterns the structural layer. The patterning steps of the piezoelectric layer and the structural layer are independent and do not interfere with each other. The piezoelectric layer does not need to be hollowed out unnecessary due to the patterning of the structural layer, thereby ensuring the integrity of the piezoelectric layer to the greatest extent.

[0047] 4. The present invention can improve the sensitivity of the piezoelectric ultrasonic transducer by opening a gap that penetrates the piezoelectric layer and the structural layer, interconnect the vibration modules, enhance the mechanical stability of the chip, and reduce the resonant frequency. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Figure 1 A schematic diagram of the structure of a piezoelectric ultrasonic transducer based on a MEMS chip proposed in an embodiment of the present invention;

[0049] Figure 2 for Figure 1 Exploded diagram of the structure;

[0050] Figure 3 for Figure 1 A front view of

[0051] Figure 4 for Figure 1 Bottom view of

[0052] Figure 5 for Figure 1 A top view of

[0053] Figure 6 for Figure 5 The enlarged view of point A in the middle;

[0054] Figure 7 for Figure 5 Structural diagram of a medium-bend beam;

[0055] Figure 8 A schematic flow chart of step S1 in the method for preparing a piezoelectric ultrasonic transducer based on a MEMS chip according to an embodiment of the present invention;

[0056] Fig. 9 A schematic flow chart of step S2 in the method for preparing a piezoelectric ultrasonic transducer based on a MEMS chip according to an embodiment of the present invention;

[0057] Fig.10 A schematic flow chart of step S3 in the method for preparing a piezoelectric ultrasonic transducer based on a MEMS chip according to an embodiment of the present invention;

[0058] Fig.11 for Fig.10 A partial enlarged view of point B in the middle;

[0059] Fig.12 A schematic flow chart of step S4 in the method for preparing a piezoelectric ultrasonic transducer based on a MEMS chip according to an embodiment of the present invention;

[0060] Fig.13 for Fig.12 A partial enlarged view of point C in the middle;

[0061] Fig.14 This is a flow chart of step S5 in the method for preparing a piezoelectric ultrasonic transducer based on a MEMS chip according to an embodiment of the present invention.

[0062] In the reference numerals:

[0063] 1. Substrate;

[0064] 2. Piezoelectric layer; 21. Bottom electrode; 22. Piezoelectric film; 23. Top electrode;

[0065] 3. structural layer; 31. first structural layer; 32. second structural layer; 33. third structural layer;

[0066] 3a, gap;

[0067] 3b, bent beam; 3b-1, U-shaped beam; 3b-2, straight beam. DETAILED DESCRIPTION

[0068] Embodiments of the present invention are described in detail below, examples of which are shown in the accompanying drawings, wherein the same or similar symbols throughout represent the same or similar elements or elements with the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present invention, and cannot be understood as limiting the present invention.

[0069] Embodiment 1:

[0070] like Figure 1 and Figure 2 As shown, the present invention discloses a piezoelectric ultrasonic transducer based on a MEMS chip, comprising a substrate 1, a piezoelectric layer 2 and a structural layer 3 which are stacked in sequence.

[0071] The substrate 1 has a front side and a back side relative to each other. A piezoelectric layer 2 is formed on the front side of the substrate 1. A back cavity penetrating through the piezoelectric layer 2 is provided on the back side of the substrate 1. The piezoelectric layer 2 and the structural layer 3 within the longitudinal projection area of ​​the back cavity are a diaphragm. The back cavity provides a vibration space for the diaphragm.

[0072] Optionally, the substrate 1 is a silicon wafer substrate, and the cost is reduced by replacing the SOI substrate with the silicon wafer substrate.

[0073] The piezoelectric layer 2 includes a bottom electrode 21 , a piezoelectric film 22 , and a top electrode 23 which are sequentially stacked on the surface of the substrate 1 , and a structural layer 3 is formed on the surface of the piezoelectric layer 2 .

[0074] A structure of the structural layer 3 is as follows: the structural layer 3 includes a first structural layer 31 and a second structural layer 32, the thickness of the second structural layer 32 is much greater than the thickness of the first structural layer 31, the material of the first structural layer 31 is silicon nitride (Si3N4), and the material of the second structural layer 32 is silicon oxide (SiO2), forming a double-layer structure of silicon nitride-silicon oxide.

[0075] A structure of the structural layer 3 is as follows: the structural layer 3 includes a first structural layer 31 and a second structural layer 32, the thickness of the second structural layer 32 is much greater than the thickness of the first structural layer 31, the material of the first structural layer 31 is silicon oxide (SiO2), and the material of the second structural layer 32 is silicon nitride (Si3N4), forming a double-layer structure of silicon oxide-silicon nitride.

[0076] In the above two structures, the second structural layer 32 serves as the main structural layer. The thickness of the entire structural layer 3 is greater than the thickness of the piezoelectric film 22 in the piezoelectric layer 2, so that the neutral plane is located within the lateral projection area of ​​the structural layer 3. The second structural layer 32 mainly assumes the function of adjusting the neutral plane of the chip, and the device performance is mainly determined by the material and thickness of the second structural layer 32.

[0077] Another structural layer 3 structure, such as Figure 3 As shown, the structure layer 3 includes a first structure layer 31, a second structure layer 32 and a third structure layer 33; wherein the thickness of the second structure layer 32 is much greater than the thickness of the first structure layer 31 and / or the third structure layer 33. The material of the first structure layer 31 is silicon nitride (Si3N4), the material of the second structure layer 32 is silicon oxide (SiO2), and the material of the third structure layer 33 is silicon nitride (Si3N4), forming a three-layer structure of silicon nitride-silicon oxide-silicon nitride.

[0078] Another structural layer 3 structure, such as Figure 3 As shown, the structure layer 3 includes a first structure layer 31, a second structure layer 32 and a third structure layer 33; wherein the thickness of the second structure layer 32 is much greater than the thickness of the first structure layer 31 and / or the third structure layer 33. The material of the first structure layer 31 is silicon oxide (SiO2), the material of the second structure layer 32 is silicon nitride (Si3N4), and the material of the third structure layer 33 is silicon oxide (SiO2), forming a three-layer structure of silicon oxide-silicon nitride-silicon oxide.

[0079] In the above two structures, the second structural layer 32 serves as the main structural layer. The second structural layer 32 mainly undertakes the function of adjusting the neutral plane of the chip. The device performance is mainly determined by the material and thickness of the second structural layer 32. The main function of the first structural layer 31 is to improve the adhesion of the middle layer and improve the film forming quality. The main function of the third structural layer 33 is to enhance the tensile properties of the middle layer and improve the stability of the structural layer 3. With the above three-layer structure, the structural layer is more stable, more reliable, and more conducive to improving the yield rate.

[0080] Through the above four structures, the thickness of the entire structural layer 3 is greater than the thickness of the piezoelectric film 22 in the piezoelectric layer 2, so that the neutral plane is located within the lateral projection area of ​​the structural layer 3. By setting the structural layer 3, the position of the neutral plane is moved up to the structural layer 3. By utilizing the low Young's modulus and low process stress of silicon oxide / silicon nitride, the resonant frequency can be effectively reduced, which is beneficial to the design and manufacture of low-frequency devices.

[0081] Embodiment 2:

[0082] On the basis of Example 1, Figure 4 As shown, the diagonal line of the structural layer 3 is provided with a gap 3a, so that the area of ​​the structural layer 3 located at the diaphragm is divided into four vibration modules, such as Figure 5 As shown, adjacent vibration modules are connected by bending beams 3b, and the diagonal gaps 3a can effectively release the stress of the structural layer 3, reduce stress concentration and fatigue damage, and thus extend the service life of the piezoelectric ultrasonic transducer. The gaps 3a can also improve the sensitivity of the chip.

[0083] Adjacent vibration modules are connected by bent beams 3b. When four vibration modules are used, there are four groups of bent beams 3b. The four groups of bent beams 3b are symmetrically distributed. Each group of bent beams 3b includes a straight beam 3b-2 and a U-shaped beam 3b-1 connected to each other.

[0084] The specific structure of the four vibration modules connected by the bending beam 3b can be as follows Figure 6 As shown in a:

[0085] The first vibration module connects the U-shaped beam 3b-1 in the first group of bent beams 3b and the straight beam 3b-2 in the fourth group of bent beams 3b;

[0086] The second vibration module is connected to the U-shaped beam 3b-1 in the straight beam 3b-2 in the first group of bent beams 3b;

[0087] The third vibration module is connected to the U-shaped beam 3b-1 in the straight beam 3b-2 in the second group of bent beams 3b;

[0088] The fourth vibration module is connected to the U-shaped beam 3b-1 in the straight beam 3b-2 in the third group of bent beams 3b.

[0089] The four bent beams 3b are arranged centrally and symmetrically around the center of the diaphragm, which can enhance the mechanical stability of the chip and enable it to maintain good performance under vibration and impact environments.

[0090] Another structure in which four vibration modules are connected by a bending beam 3b can be as follows Figure 6 As shown in b:

[0091] The first vibration module connects the U-shaped beam 3b-1 in the first group of bent beams 3b and the U-shaped beam 3b-1 in the fourth group of bent beams 3b;

[0092] The second vibration module connects the straight beam 3b-2 in the first group of bent beams 3b and the straight beam 3b-2 in the second group of bent beams 3b;

[0093] The third vibration module connects the U-shaped beam 3b-1 in the second group of bent beams 3b and the U-shaped beam 3b-1 in the third group of bent beams 3b;

[0094] The fourth vibration module connects the straight beam 3b-2 in the third group of bent beams 3b and the straight beam 3b-2 in the fourth group of bent beams 3b.

[0095] The four bent beams 3b are arranged in an axisymmetric manner, which can enhance the mechanical stability of the chip and enable it to maintain good performance under vibration and impact environments.

[0096] Furthermore, if Figure 7 As shown in Figs. 7a and 7b, each group of bent beams 3b includes a plurality of U-shaped beams 3b-1 arranged in a linear array and a straight beam 3b-2, adjacent U-shaped beams 3b-1 are connected, and the U-shaped beam 3b-1 at the end is connected to the straight beam 3b-2.

[0097] In the piezoelectric ultrasonic transducer based on the MEMS chip proposed in this embodiment, the bent beam 3b can form each vibration module into a whole to avoid the occurrence of multiple resonant frequencies. At the same time, the bent beam 3b structure can enhance the mechanical stability of the chip, so that it can still maintain good performance under vibration and impact environments.

[0098] Embodiment 3:

[0099] On the basis of Example 2, the pattern of the diaphragm region of the piezoelectric layer 2 is made consistent with the pattern of the diaphragm region of the structural layer 3. By etching, the piezoelectric layer 2 and the structural layer 3 can form a gap 3a that penetrates the structural layer 3 and the piezoelectric layer 2 along the diagonal of the diaphragm region, and the overall through gap 3a can improve the sensitivity of the chip.

[0100] Embodiment 4:

[0101] With respect to the piezoelectric ultrasonic transducer based on the MEMS chip of Examples 1-3, this embodiment discloses a preparation method applied to the above-mentioned piezoelectric ultrasonic transducer, the steps comprising:

[0102] S1, preparing a substrate 1, the substrate 1 having a front side and a back side opposite to each other, and growing a piezoelectric layer 2 on the front side of the substrate 1; Figure 8 As shown, specifically including:

[0103] S1-1, sputtering and growing a bottom electrode 21 on the front surface of the substrate 1;

[0104] S1-2, sputtering and growing a piezoelectric film 22 on the surface of the bottom electrode 21;

[0105] S1 - 3 , a top electrode 23 is sputtered and grown on the surface of the piezoelectric film 22 .

[0106] S2, etching the piezoelectric layer 2 to pattern the piezoelectric layer 2, such as Fig. 9 As shown, specifically including:

[0107] S2-1, using IBE dry etching to pattern the top electrode 23;

[0108] S2-2, wet etching the piezoelectric film 22 to pattern the piezoelectric film 22;

[0109] S2-3, using IBE dry etching to etch the bottom electrode 21, so as to pattern the bottom electrode 21.

[0110] S3, growing a structural layer 3 on the surface of the piezoelectric layer 2; Fig.10 As shown, it specifically includes: using PECVD process to deposit the structural layer 3 on the surface of the piezoelectric layer 2, the thickness of the structural layer 3 is greater than the thickness of the piezoelectric film 22, so that the neutral plane is located within the lateral projection area of ​​the structural layer 3.

[0111] S4, etching the structural layer 3 to pattern the structural layer 3. After the structural layer 3 is etched, a gap 3a penetrating the structural layer 3 is formed along the diagonal line of the diaphragm region; Fig.12 and Fig.13As shown, it specifically includes: etching the structural layer 3 to make the structural layer 3 patterned, etching the diaphragm area of ​​the structural layer 3 so that the area of ​​the structural layer 3 located at the diaphragm forms four vibration modules due to gaps 3a, and adjacent vibration modules are connected by bending beams 3b formed by etching.

[0112] S5, opening a back cavity in the substrate 1 that penetrates the substrate 1 to the piezoelectric layer 2; Fig.14 As shown, it specifically includes: etching the substrate 1 through the back side of the substrate 1 to form a back cavity, the piezoelectric layer 2 and the structural layer 3 within the longitudinal projection area of ​​the back cavity are the diaphragm, and the back cavity provides a vibration space for the diaphragm.

[0113] Furthermore, when the back cavity process is opened, a dry process may be used to first etch a portion of the substrate 1 from the back of the substrate 1, and then a wet process may be used to etch through the back cavity of the substrate 1, specifically including:

[0114] S5-1, etching the back side of the substrate 1 by DRIE dry etching, where the substrate 1 will not be penetrated, because DRIE dry etching may damage the piezoelectric layer 2, so 5-15% of the thickness of the substrate 1 will remain after DRIE etching;

[0115] S5-2, etching the back side of the substrate 1 by a wet process, completely penetrating the remaining 5-15% thickness of the substrate 1, forming a U-shaped support structure, and forming a back cavity.

[0116] When the SOI substrate is etched, the buried oxide layer serves as a cutoff line, and a back cavity can be formed by penetrating the buried oxide layer. Pure dry etching may damage the piezoelectric layer 2. In order to improve the yield rate, a part of the silicon wafer substrate 1 is dry-etched, and then the remaining part is etched through by a wet process. The wet process will not affect the piezoelectric layer 2.

[0117] Preferably, when the gap 3a is formed, the pattern of the diaphragm region of the piezoelectric layer 2 is consistent with the pattern of the diaphragm region of the structural layer 3, and the preparation method is:

[0118] S4-1, after the piezoelectric layer 2 is formed on the substrate 1, the piezoelectric layer 2 is etched to pattern the piezoelectric layer 2, such as Fig. 9 As shown, the piezoelectric layer 2 is etched so that a gap 3a is formed along the diagonal line of the diaphragm region of the piezoelectric layer 2 due to etching. The gap 3a divides the diaphragm region of the piezoelectric layer 2 into four vibration modules, and adjacent vibration modules are connected by a bent beam 3b formed by etching;

[0119] S4-2, generating a structural layer 3 on the surface of the piezoelectric layer 2, such as Fig.10 and Fig.11 As shown, when the structure layer 3 is patterned, as shown in Fig.12 and Fig.13As shown, the structural layer 3 is etched so that the diaphragm area of ​​the structural layer 3 forms a pattern consistent with the diaphragm area of ​​the piezoelectric layer 2 due to the etching, thereby obtaining a gap 3a running through the piezoelectric layer 2 and the structural layer 3, and dividing the diaphragm area into four vibration modules. Adjacent vibration modules are connected by bent beams 3b formed by etching.

[0120] By opening the gap 3a penetrating the piezoelectric layer 2 and the structural layer 3, the sensitivity of the piezoelectric ultrasonic transducer can be improved, the vibration modules are connected to each other, the mechanical stability of the chip is enhanced, and the resonance frequency is reduced.

[0121] Compared with the traditional preparation process, the preparation method of the piezoelectric ultrasonic transducer (PMUT) based on the MEMS chip proposed in this embodiment first grows the piezoelectric layer 2 and patterns it, then grows the structural layer 3 on the surface of the piezoelectric layer 2, and then patterns the structural layer 3. The patterning steps of the piezoelectric layer 2 and the structural layer 3 are independent and do not interfere with each other. The piezoelectric layer 2 does not need to be unnecessary hollowed out due to the patterning of the structural layer 3, thereby ensuring the integrity of the piezoelectric layer 2 to the greatest extent.

[0122] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can make equivalent replacements or changes according to the technical scheme and inventive concept of the present invention within the technical scope disclosed by the present invention, which should be covered by the protection scope of the present invention.

[0123] It should be understood that the terms "center", "longitudinal", "lateral", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", "clockwise", "counterclockwise", "axial", "radial", "circumferential" and the like indicate orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the referred device or element must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as limiting the present invention.

[0124] In the present invention, unless otherwise clearly specified and limited, a first feature being "above" or "below" a second feature may mean that the first and second features are in direct contact, or the first and second features are in indirect contact through an intermediate medium. Moreover, a first feature being "above", "above" or "above" a second feature may mean that the first feature is directly above or obliquely above the second feature, or simply means that the first feature is higher in level than the second feature. A first feature being "below", "below" or "below" a second feature may mean that the first feature is directly below or obliquely below the second feature, or simply means that the first feature is lower in level than the second feature.

Claims

1. A piezoelectric ultrasonic transducer based on a MEMS chip, comprising a substrate (1), wherein the substrate (1) has a front side and a back side opposite to each other, and a piezoelectric layer (2) is formed on the front side of the substrate (1), characterized in that: The back side of the substrate (1) is provided with a back cavity penetrating to the piezoelectric layer (2); a structural layer (3) is formed on the surface of the piezoelectric layer (2); the piezoelectric layer (2) and the structural layer (3) within the longitudinal projection area of ​​the back cavity are diaphragms; the thickness of the structural layer (3) is greater than the thickness of the piezoelectric film (22) in the piezoelectric layer (2), so that the neutral plane is located within the transverse projection area of ​​the structural layer (3); The diagonal lines of the structural layer (3) are provided with gaps (3a), so that the area of ​​the structural layer (3) located on the diaphragm is divided into four vibration modules, and adjacent vibration modules are connected by bending beams (3b).

2. The piezoelectric ultrasonic transducer based on a MEMS chip according to claim 1, characterized in that: The structural layer (3) is a double-layer structure or a triple-layer structure in which silicon nitride and silicon oxide are alternately stacked.

3. The piezoelectric ultrasonic transducer based on a MEMS chip according to claim 1, characterized in that: The pattern of the diaphragm region of the piezoelectric layer (2) is consistent with the pattern of the diaphragm region of the structural layer (3).

4. The piezoelectric ultrasonic transducer based on a MEMS chip according to claim 3, characterized in that: The bent beams (3b) are divided into four groups, which are symmetrically distributed. Each group of bent beams (3b) comprises a straight beam (3b-2) and a U-shaped beam (3b-1) which are connected to each other.

5. The piezoelectric ultrasonic transducer based on a MEMS chip according to claim 4, characterized in that: A structure in which four vibration modules are connected by a bending beam (3b) is: The first vibration module connects the U-shaped beam (3b-1) in the first group of bent beams (3b) and the straight beam (3b-2) in the fourth group of bent beams (3b); The second vibration module is connected to the U-shaped beam (3b-1) in the straight beam (3b-2) in the first group of bent beams (3b); The third vibration module is connected to the U-shaped beam (3b-1) in the straight beam (3b-2) in the second group of bent beams (3b); The fourth vibration module is connected to the U-shaped beam (3b-1) in the straight beam (3b-2) in the third group of bent beams (3b).

6. The piezoelectric ultrasonic transducer based on a MEMS chip according to claim 4, characterized in that: Another structure in which four vibration modules are connected by a bending beam (3b) is: The first vibration module connects the U-shaped beam (3b-1) in the first group of bent beams (3b) and the U-shaped beam (3b-1) in the fourth group of bent beams (3b); The second vibration module connects the straight beam (3b-2) in the first group of bent beams (3b) and the straight beam (3b-2) in the second group of bent beams (3b); The third vibration module connects the U-shaped beam (3b-1) in the second group of bent beams (3b) and the U-shaped beam (3b-1) in the third group of bent beams (3b); The fourth vibration module connects the straight beam (3b-2) in the third group of bent beams (3b) and the straight beam (3b-2) in the fourth group of bent beams (3b).

7. The piezoelectric ultrasonic transducer based on a MEMS chip according to claim 4, characterized in that: Each group of bent beams (3b) comprises a plurality of U-shaped beams (3b-1) arranged in a linear array and a straight beam (3b-2); adjacent U-shaped beams (3b-1) are connected, and the U-shaped beam (3b-1) at the end is connected to the straight beam (3b-2).

8. A method for preparing the piezoelectric ultrasonic transducer according to any one of claims 1 to 7, characterized in that: The preparation method comprises the following steps: S1, preparing a substrate (1), and growing a piezoelectric layer (2) on the front surface of the substrate (1); S2, etching the piezoelectric layer (2) to pattern the piezoelectric layer (2); S3, growing a structural layer (3) on the surface of the piezoelectric layer (2); S4, etching the structural layer (3) to pattern the structural layer (3), and after the structural layer (3) is etched, a gap (3a) penetrating the structural layer (3) is formed along the diagonal line of the diaphragm region; S5, opening a back cavity in the substrate (1) that penetrates the substrate (1) to the piezoelectric layer (2); The region of the structural layer (3) located on the diaphragm forms four vibration modules due to the gaps (3a), and adjacent vibration modules are connected via bent beams (3b) formed by etching.

9. The method for preparing a piezoelectric ultrasonic transducer according to claim 8, characterized in that: When the gap (3a) is formed in S4, the pattern of the diaphragm region of the piezoelectric layer (2) is consistent with the pattern of the diaphragm region of the structural layer (3), and the preparation method comprises: S4-1, after the piezoelectric layer (2) is generated on the substrate (1), the piezoelectric layer (2) is etched to pattern the piezoelectric layer (2); a gap (3a) is formed along the diagonal line of the diaphragm region of the piezoelectric layer (2) due to etching, and the gap (3a) divides the diaphragm region of the piezoelectric layer (2) into four vibration modules, and adjacent vibration modules are connected by a bent beam (3b) formed by etching; S4-2, generating a structural layer (3) on the surface of the piezoelectric layer (2) and patterning the structural layer (3); forming a pattern in the diaphragm region of the structural layer (3) by etching that is consistent with the diaphragm region of the piezoelectric layer (2), obtaining a gap (3a) that runs through the piezoelectric layer (2) and the structural layer (3), dividing the diaphragm region into four vibration modules, and connecting adjacent vibration modules via bent beams (3b) formed by etching.

10. The method for preparing a piezoelectric ultrasonic transducer according to claim 8, characterized in that: The back cavity opening process in S5 includes: Firstly, a dry process is used to etch a portion of the substrate (1) from the back side of the substrate (1), and then a wet process is used to etch through the back cavity of the substrate (1).

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