A piezoelectric ultrasonic transducer based on a MEMS chip

Abstract

This invention discloses a piezoelectric ultrasonic transducer based on a MEMS chip and its fabrication method. The transducer includes a substrate with opposing front and back sides. A piezoelectric layer is formed on the front side of the substrate, and a back cavity penetrating the piezoelectric layer is formed on the back side of the substrate. A structural layer is formed on the surface of the piezoelectric layer. The piezoelectric layer and structural layer within the longitudinal projection area of ​​the back cavity serve as 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. This invention's piezoelectric ultrasonic transducer and its fabrication method use silicon oxide / silicon nitride as the structural layer, with the neutral plane elevated to the structural layer, replacing 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, thus benefiting the design and fabrication of low-frequency devices.

Description

Technical Field

[0001] This 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 its fabrication method. Background Technology

[0002] An ultrasonic sensor is a sensor that converts ultrasonic signals into other energy signals (usually electrical signals). Ultrasonic waves are mechanical waves with vibration frequencies higher than 20 kHz. They are characterized by high frequency, short wavelength, minimal diffraction, and, most importantly, good directionality, enabling them to propagate directionally as rays. Ultrasonic waves have a strong penetrating ability in liquids and solids, especially in solids that are opaque to sunlight. When ultrasonic waves encounter impurities or interfaces, they produce significant reflections, forming reflected echoes. When they encounter moving objects, they produce the Doppler effect. Ultrasonic sensors are widely used in industry, defense, and biomedicine.

[0003] In existing technologies, low-frequency ultrasonic sensors are required for long-distance measurement, monitoring, and information transmission using ultrasound. Therefore, low-frequency MEMS chip devices have a large market. However, traditional piezoelectric MEMS chips consist of a stacked SOI substrate with a back cavity and a piezoelectric layer. The top silicon in the SOI substrate acts as a structural layer and serves as the 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. In order to reduce the stress on the device and improve the sensitivity, gaps are made on the chip. The gaps cause the chip to generate multiple resonant frequencies. Moreover, after the gaps divide the diaphragm into several parts, it cannot maintain good performance under vibration and shock environments, resulting in low mechanical stability.

[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 this application 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. A hexagonal opening is formed in the substrate layer through its upper and lower end faces. The chip sensitive layer includes a lower electrode layer, a first piezoelectric layer and an upper electrode layer stacked from bottom to top. The lower electrode layer is stacked on top of the device layer. The upper electrode layer has a hexagonal electrode structure and is located directly above the hexagonal opening. A plurality of first recesses are formed on the buried oxide layer, the device layer, the lower electrode layer and the first piezoelectric layer. The plurality of first recesses are distributed in a hexagonal outline on the outer periphery of the upper electrode layer. The hexagonal outline is close to or located at the edge of the hexagonal opening.

[0005] This proposed solution improves the effective vibration displacement of the chip's sensitive layer, thereby enhancing the chip's performance as both a driver and a sensor. However, the solution also has drawbacks: its structure is a traditional one, with the top silicon layer in the SOI substrate serving as the neutral plane, which presents challenges for the design and manufacturing of low-frequency devices.

[0006] Therefore, a piezoelectric ultrasonic transducer based on MEMS chips and its fabrication method are needed to solve the above-mentioned problems. Summary of the Invention

[0007] To address the aforementioned problems, the present invention aims to provide a piezoelectric ultrasonic transducer based on a MEMS chip and its fabrication method. A structural layer is produced on the surface of the piezoelectric layer to replace the traditional top silicon, effectively reducing the resonant frequency. At the same time, the various vibration modules are integrated into a whole using a bending beam to avoid the occurrence of multiple resonant frequencies, maintain the mechanical stability of the chip, and enable it to maintain good performance under vibration and shock environments.

[0008] The objective of this invention can be achieved through the following technical solution: a piezoelectric ultrasonic transducer based on a MEMS chip, comprising a substrate having a front side and a back side, a piezoelectric layer formed on the front side of the substrate, a back cavity extending through the piezoelectric layer on the back side of the substrate, a structural layer formed on the surface of the piezoelectric layer, the piezoelectric layer and the structural layer within the longitudinal projection area of ​​the back cavity serving as a diaphragm, the thickness of the structural layer being greater than the thickness of the piezoelectric film in the piezoelectric layer, such that the neutral plane is located within the lateral projection area of ​​the structural layer;

[0009] The structural layer has slits along its diagonal, dividing the area of ​​the structural layer located on the diaphragm into four vibration modules, with adjacent vibration modules connected by bent beams.

[0010] The neutral plane is located in the structural layer, replacing the traditional top silicon, resulting in lower process stress and effectively reducing the resonant frequency, which is beneficial for the production design and manufacturing of low-frequency devices. The slots reduce stress, and the bent beams allow the various vibration modules to be integrated into a single unit, preventing multiple resonant frequencies from occurring.

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

[0012] Using silicon oxide / silicon nitride as the main 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 embodiment 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 embodiment of the present invention, the bent beams are in four groups, which are symmetrically distributed. Each group of bent beams includes a straight beam and a U-shaped beam connected to each other.

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

[0016] As a further embodiment of the present invention, a structure in which the four vibration modules are connected by a bent beam is as follows:

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

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

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

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

[0021] As a further embodiment of the present invention, another structure in which the four vibration modules are connected by a bent beam is as follows:

[0022] The first vibration module connects the U-shaped beam in the first set of bending beams to the U-shaped beam in the fourth set of bending beams;

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

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

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

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

[0027] As a further embodiment of the present invention, each set of bent beams includes multiple linearly arranged U-shaped beams and a straight beam, with adjacent U-shaped beams connected and the U-shaped beam at the end connected to the straight beam.

[0028] A method for fabricating the above-mentioned piezoelectric ultrasonic transducer, the method comprising the following steps:

[0029] S1. Prepare the substrate and grow a piezoelectric layer on the front side of the substrate;

[0030] S2. Etch the piezoelectric layer to pattern it;

[0031] S3. Growing a structural layer on the surface of the piezoelectric layer;

[0032] S4. Etch the structural layer to pattern the structural layer. After etching, a through-slot is formed along the diagonal of the diaphragm region.

[0033] S5. A back cavity is formed in the substrate, extending through the substrate to the piezoelectric layer;

[0034] The area where the structural layer is located on the diaphragm is formed by gaps to create four vibration modules, and adjacent vibration modules are connected by bent beams formed by etching.

[0035] By using the above processing method, the neutral plane is moved up, the process stress is low, and the resonant frequency can be effectively reduced. At the same time, the resonant frequency generated is reduced by connecting each sub-module with a bending beam. In addition, the bending beam structure can enhance the mechanical stability of the chip, so that it can maintain good performance under vibration and shock environment.

[0036] As a further embodiment 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 it. The piezoelectric layer forms a gap through the diaphragm region along the diagonal due to the etching. The gap divides the diaphragm region of the piezoelectric layer into four vibration modules. Adjacent vibration modules are connected by a bent beam formed by the 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 that runs through the piezoelectric layer and the structural layer, divide the diaphragm area into four vibration modules, and connect adjacent vibration modules through the bending beams formed by etching.

[0039] The gaps allow for improved sensitivity, and the interconnected vibration modules enhance the chip's mechanical stability and reduce the resonant frequency.

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

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

[0042] During the etching of SOI substrates, the buried oxide layer serves as a cutoff line. 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, dry etching is used to etch part of the silicon wafer substrate, and then wet etching is used to etch through the remaining part. The wet etching process does not affect the piezoelectric layer.

[0043] The beneficial effects of this invention are:

[0044] 1. The piezoelectric ultrasonic transducer and its fabrication method of the present invention use 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 low Young's modulus and low process stress, which can effectively reduce the resonant frequency and is beneficial to the design and manufacturing of low frequency devices.

[0045] 2. This invention creates gaps in the diaphragm area to reduce stress, and uses bent beams to form the various vibration modules into a whole, avoiding the occurrence of multiple resonant frequencies. The structure of the bent beams can enhance the mechanical stability of the chip, enabling it to maintain good performance under vibration and shock environments.

[0046] 3. The fabrication method of the piezoelectric ultrasonic transducer based on MEMS chip proposed in this invention, compared with the traditional fabrication process, first grows a piezoelectric layer and patterns it, then grows a 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 generate unnecessary cutouts due to the patterning of the structural layer, thus ensuring the integrity of the piezoelectric layer to the greatest extent.

[0047] 4. By creating a gap that penetrates the piezoelectric layer and the structural layer, this invention can improve the sensitivity of the piezoelectric ultrasonic transducer, and the interconnected vibration modules can enhance the mechanical stability of the chip and reduce the resonant frequency. Attached Figure Description

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

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

[0050] Figure 3 for Figure 1 The front view;

[0051] Figure 4 for Figure 1 A bottom view;

[0052] Figure 5 for Figure 1 Top view;

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

[0054] Figure 7 for Figure 5 Schematic diagram of a medium-bending beam;

[0055] Figure 8 This is a flowchart illustrating step S1 in the fabrication method of the piezoelectric ultrasonic transducer based on a MEMS chip proposed in this embodiment of the invention.

[0056] Figure 9 This is a flowchart illustrating step S2 in the fabrication method of the piezoelectric ultrasonic transducer based on a MEMS chip proposed in this embodiment of the invention.

[0057] Figure 10 This is a flowchart illustrating step S3 in the fabrication method of the piezoelectric ultrasonic transducer based on a MEMS chip proposed in this embodiment of the invention.

[0058] Figure 11 for Figure 10 A magnified view of a section at point B in the middle;

[0059] Figure 12 This is a flowchart illustrating step S4 in the fabrication method of the piezoelectric ultrasonic transducer based on a MEMS chip proposed in this embodiment of the invention.

[0060] Figure 13 for Figure 12 A magnified view of a section at point C;

[0061] Figure 14 This is a flowchart illustrating step S5 in the fabrication method of the piezoelectric ultrasonic transducer based on a MEMS chip proposed in this embodiment of the invention.

[0062] In the attached diagram:

[0063] 1. Substrate;

[0064] 2. Piezoelectric layer; 21. Bottom electrode; 22. Piezoelectric thin 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, Bending beam; 3b-1, U-shaped beam; 3b-2, Straight beam. Detailed Implementation

[0068] Embodiments of the present invention are described in detail below. Examples of these embodiments are illustrated in the accompanying drawings, wherein the same or similar symbols denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present invention, and should not be construed as limiting the present invention.

[0069] Example 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 stacked sequentially.

[0071] The substrate 1 has a front side and a back side. A piezoelectric layer 2 is formed on the front side of the substrate 1. A back cavity extending through the piezoelectric layer 2 is formed 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 serve as a diaphragm, and the back cavity provides space for the diaphragm to vibrate.

[0072] Optionally, substrate 1 is a silicon wafer substrate, which reduces costs by replacing the SOI substrate with a silicon wafer substrate.

[0073] The piezoelectric layer 2 includes a bottom electrode 21, a piezoelectric thin film 22 and a top electrode 23 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] One structural layer 3 has the following structure: 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 silicon nitride-silicon oxide bilayer structure.

[0075] One structural layer 3 has the following structure: 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 silicon oxide-silicon nitride bilayer structure.

[0076] In the two structures described above, 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 thin 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 undertakes 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 type of three-layer structure, such as Figure 3 As shown, structural layer 3 includes a first structural layer 31, a second structural layer 32, and a third structural layer 33; wherein the thickness of the second structural layer 32 is much greater than the thickness of the first structural layer 31 and / or the third structural layer 33. The first structural layer 31 is made of silicon nitride (Si3N4), the second structural layer 32 is made of silicon oxide (SiO2), and the third structural layer 33 is made of silicon nitride (Si3N4), forming a three-layer structure of silicon nitride-silicon oxide-silicon nitride.

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

[0079] In the two structures described above, the second structural layer 32 serves as the main structural layer. The second structural layer 32 mainly functions to adjust the neutral plane of the chip. The device performance is mainly determined by the material and thickness of the second structural layer 32. The first structural layer 31 mainly improves the adhesion of the intermediate layer and enhances the film quality. The third structural layer 33 mainly enhances the tensile strength of the intermediate layer and improves the stability of the structural layer 3. By adopting the above three-layer structure, the structural layers are more stable, more reliable, and more conducive to improving the yield rate.

[0080] With 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. 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 for the design and manufacturing of low-frequency devices.

[0081] Example 2:

[0082] Based on Example 1, such as Figure 4 As shown, a slit 3a is provided diagonally in the structural layer 3, dividing the area of ​​the structural layer 3 located on the diaphragm into four vibration modules, as follows: Figure 5 As shown, adjacent vibration modules are connected by a bent beam 3b. The diagonal gap 3a can effectively release the stress in the structural layer 3, reduce stress concentration and fatigue damage, thereby extending the service life of the piezoelectric ultrasonic transducer. The gap 3a also improves the chip's sensitivity.

[0083] Adjacent vibration modules are connected by bending beams 3b. When four vibration modules are used, there are four sets of bending beams 3b, which are symmetrically distributed. Each set of bending beams 3b includes straight beams 3b-2 and U-shaped beams 3b-1 that are connected to each other.

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

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

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

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

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

[0089] Four bent beams 3b are arranged 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 shock environments.

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

[0091] The first vibration module is connected to the U-beam 3b-1 in the first set of bent beams 3b and the U-beam 3b-1 in the fourth set of bent beams 3b;

[0092] The second vibration module is connected to the straight beam 3b-2 in the first set of bent beams 3b and the straight beam 3b-2 in the second set of bent beams 3b.

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

[0094] The fourth vibration module is connected to the straight beam 3b-2 in the third set of bent beams 3b and the straight beam 3b-2 in the fourth set of bent beams 3b.

[0095] The four bent beams 3b are arranged symmetrically to enhance the mechanical stability of the chip, enabling it to maintain good performance under vibration and shock conditions.

[0096] Furthermore, such as Figure 7 As shown in 7a and 7b, each set of bent beams 3b includes multiple linearly arranged U-shaped beams 3b-1 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] The piezoelectric ultrasonic transducer based on MEMS chip proposed in this embodiment uses a bent beam 3b to integrate the various vibration modules into a whole, avoiding the occurrence of multiple resonant frequencies. At the same time, the bent beam 3b structure can enhance the mechanical stability of the chip, enabling it to maintain good performance under vibration and shock environments.

[0098] Example 3:

[0099] Based on Example 2, the pattern of the diaphragm region of piezoelectric layer 2 is made consistent with the pattern of the diaphragm region of structural layer 3. Through etching, a through-slot 3a is formed between the piezoelectric layer 2 and the structural layer 3 along the diagonal of the diaphragm region. This through-slot 3a can improve the chip's sensitivity.

[0100] Example 4:

[0101] Regarding the MEMS chip-based piezoelectric ultrasonic transducers of Examples 1-3, this embodiment discloses a fabrication method for the above-mentioned piezoelectric ultrasonic transducers, the steps of which include:

[0102] S1. Prepare substrate 1, which has a front side and a back side. Grow a piezoelectric layer 2 on the front side of substrate 1; Figure 8 As shown, it specifically includes:

[0103] S1-1, a bottom electrode 21 is sputtered and grown on the front side of substrate 1;

[0104] S1-2, a piezoelectric thin film 22 is sputtered and grown on the surface of the bottom electrode 21;

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

[0106] S2. Etch piezoelectric layer 2 to pattern piezoelectric layer 2, such as... Figure 9 As shown, it specifically includes:

[0107] S2-1, The top electrode 23 is patterned by etching the top electrode 23 using the IBE dry etching method;

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

[0109] S2-3, IBE dry etching is used to pattern the bottom electrode 21.

[0110] S3. Grow structural layer 3 on the surface of piezoelectric layer 2; such as Figure 10 As shown, specifically, it includes: depositing structural layer 3 on the surface of piezoelectric layer 2 using PECVD process, wherein the thickness of structural layer 3 is greater than the thickness of piezoelectric film 22, so that the neutral surface is located within the lateral projection area of ​​structural layer 3.

[0111] S4. Etch structural layer 3 to pattern structural layer 3. After etching, a through-slit 3a is formed along the diagonal of the diaphragm region. Figure 12 and Figure 13As shown, the process specifically includes: etching the structural layer 3 to form a pattern of the structural layer 3; due to the etching, the diaphragm region of the structural layer 3 forms four vibration modules in the region of the structural layer 3 located on the diaphragm due to the gap 3a; and adjacent vibration modules are connected by a bent beam 3b formed by the etching.

[0112] S5. A back cavity is formed in substrate 1, extending through substrate 1 to piezoelectric layer 2; such as Figure 14 As shown, specifically, it includes: etching through the back of the substrate 1 to form a back cavity, wherein the piezoelectric layer 2 and the structural layer 3 within the longitudinal projection area of ​​the back cavity serve as a diaphragm, and the back cavity provides space for the diaphragm to vibrate.

[0113] Furthermore, when creating the back cavity process, a dry etching process can be used to etch a portion of substrate 1 from the back side of substrate 1, followed by a wet etching process to etch through the back cavity of substrate 1. Specifically, this includes:

[0114] S5-1, The back side of substrate 1 is etched by DRIE dry etching. Here, the substrate 1 will not be penetrated. Because DRIE dry etching will damage the piezoelectric layer 2, the substrate 1 will have a thickness of 5-15% remaining after DRIE etching.

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

[0116] During the etching of the SOI substrate, the buried oxide layer serves as a cutoff line. 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, a portion of the silicon wafer substrate 1 is etched using dry etching, and then the remaining portion is etched using a wet process. The wet process will not affect the piezoelectric layer 2.

[0117] Preferably, when forming the gap 3a, 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. The preparation method is as follows:

[0118] S4-1. After forming the piezoelectric layer 2 on the substrate 1, the piezoelectric layer 2 is etched to pattern the piezoelectric layer 2, such as... Figure 9 As shown, the piezoelectric layer 2 is etched to form a through-slit 3a along the diagonal of the diaphragm region. The through-slit 3a divides the diaphragm region of the piezoelectric layer 2 into four vibration modules. Adjacent vibration modules are connected by a bent beam 3b formed by the etching.

[0119] S4-2, Form a structural layer 3 on the surface of piezoelectric layer 2, such as Figure 10 and Figure 11 As shown, when graphically representing structure layer 3, as... Figure 12 and Figure 13As shown, the structural layer 3 is etched so that the diaphragm region of the structural layer 3 forms a pattern consistent with the diaphragm region of the piezoelectric layer 2, thereby obtaining a gap 3a that penetrates the piezoelectric layer 2 and the structural layer 3, dividing the diaphragm region into four vibration modules, and adjacent vibration modules are connected by a bent beam 3b formed by the etching.

[0120] By creating a gap 3a that penetrates the piezoelectric layer 2 and the structural layer 3, the sensitivity of the piezoelectric ultrasonic transducer can be improved, the interconnected vibration modules can enhance the mechanical stability of the chip, and the resonant frequency can be reduced.

[0121] The fabrication method of the MEMS chip-based piezoelectric ultrasonic transducer (PMUT) proposed in this embodiment, compared with the traditional fabrication process, first grows a piezoelectric layer 2 and patterns it, then grows a 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 generate unnecessary cutouts due to the patterning of the structural layer 3, thus ensuring the integrity of the piezoelectric layer 2 to the greatest extent.

[0122] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

[0123] It should be understood that the terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," "counterclockwise," "axial," "radial," and "circumferential" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing the present 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 the present invention.

[0124] In this invention, unless otherwise explicitly specified and limited, "above" or "below" the second feature can mean that the first feature is in direct contact with the second feature, or that the first feature is in indirect contact with the second feature through an intermediate medium. Furthermore, "above," "over," and "on top" of the second feature can mean that the first feature is directly above or diagonally above the second feature, or simply that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature can mean that the first feature is directly below or diagonally below the second feature, or simply that the first feature is at a lower horizontal level than the second feature.

Claims

1. A piezoelectric ultrasonic transducer based on a MEMS chip, comprising a substrate (1) having opposing front and back sides, wherein a piezoelectric layer (2) is formed on the front side of the substrate (1), characterized in that: The back of the substrate (1) has a cavity extending through the piezoelectric layer (2), and 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 thin 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 structural layer (3) has a slit (3a) on its diagonal, which divides the area of ​​the structural layer (3) located on the diaphragm into four vibration modules, and adjacent vibration modules are connected by a bent beam (3b). 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); The bent beams (3b) are in four groups, and 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) that are connected to each other.

2. The MEMS chip-based piezoelectric ultrasonic transducer of claim 1, wherein, The structural layer (3) is a two-layer or three-layer structure in which silicon nitride and silicon oxide are alternately stacked.

3. The MEMS chip-based piezoelectric ultrasonic transducer of claim 1, wherein, A structure in which four vibration modules are connected by a bent beam (3b) is as follows: The first vibration module connects the U-shaped beam (3b-1) in the first set of bent beams (3b) and the straight beam (3b-2) in the fourth set of bent beams (3b). The second vibration module connects the straight beam (3b-2) in the first set of bent beams (3b) and the U-shaped beam (3b-1) in the second set of bent beams (3b); The third vibration module connects the straight beam (3b-2) in the second set of bent beams (3b) and the U-shaped beam (3b-1) in the third set of bent beams (3b). The fourth vibration module connects the straight beam (3b-2) in the third set of bent beams (3b) and the U-shaped beam (3b-1) in the fourth set of bent beams (3b).

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

5. The piezoelectric ultrasonic transducer based on a MEMS chip according to claim 1, characterized in that, Each set of bent beams (3b) includes multiple linearly arranged U-shaped beams (3b-1) 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).

6. A method for preparing a piezoelectric ultrasonic transducer according to any one of claims 1 to 5, characterized in that, The preparation method includes the following steps: S1. Prepare substrate (1) and grow piezoelectric layer (2) on the front side of substrate (1). S2. Etch the piezoelectric layer (2) to pattern the piezoelectric layer (2); S3. A structural layer (3) is grown on the surface of the piezoelectric layer (2); S4. Etch the structural layer (3) to pattern the structural layer (3). After etching the structural layer (3), a gap (3a) is formed along the diagonal of the diaphragm region that penetrates the structural layer (3). S5. A back cavity is formed in the substrate (1) that extends through the substrate (1) to the piezoelectric layer (2); Among them, the area where the structural layer (3) is located on the diaphragm forms four vibration modules due to the gap (3a), and the adjacent vibration modules are connected by the bent beam (3b) formed by etching.

7. The method for preparing the piezoelectric ultrasonic transducer according to claim 6, 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). The preparation method includes: 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); the piezoelectric layer (2) forms a through-slit (3a) along the diagonal of the diaphragm region due to etching. The slit (3a) divides the diaphragm region of the piezoelectric layer (2) into four vibration modules. The adjacent vibration modules are connected by a bent beam (3b) formed by etching. S4-2. A structural layer (3) is generated on the surface of the piezoelectric layer (2) and the structural layer (3) is patterned. The diaphragm region of the structural layer (3) is etched to form a pattern consistent with the diaphragm region of the piezoelectric layer (2). A gap (3a) is obtained that runs through the piezoelectric layer (2) and the structural layer (3). The diaphragm region is divided into four vibration modules. Adjacent vibration modules are connected by a bent beam (3b) formed by etching.

8. The method for preparing the piezoelectric ultrasonic transducer according to claim 6, characterized in that, The back cavity opening process in S5 includes: First, a dry process is used to etch part 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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