Radio frequency piezoelectric micromachined ultrasonic transducer and method of manufacturing the same

Abstract

This application discloses a radio frequency piezoelectric micromechanical ultrasonic transducer and its fabrication method. In the above method, high-resistivity silicon is used to replace the original SOI substrate as the substrate of the radio frequency piezoelectric micromechanical ultrasonic transducer, which can greatly simplify the manufacturing process of the radio frequency piezoelectric micromechanical ultrasonic transducer and reduce the manufacturing cost. Furthermore, the piezoelectric material wafer formed by epitaxy on the silicon substrate is transferred to the high-resistivity silicon substrate with a defect-rich oxide layer through metal-to-metal direct wafer bonding. Both the defect-rich oxide layer and the defect-rich layer play the role of capturing free parasitic charges in the entire device, improving the resistivity of the entire radio frequency MDOL substrate structure, and enabling the radio frequency piezoelectric micromechanical ultrasonic transducer to have excellent radio frequency performance. This solves the problems of low signal linearity, high insertion loss, poor signal integrity, and high signal distortion in PMUT.

Description

Technical Field

[0001] This application relates to the technical field of piezoelectric micromechanical ultrasonic transducers, and in particular to a radio frequency piezoelectric micromechanical ultrasonic transducer and its fabrication method. Background Technology

[0002] With the rapid development of ultrasound imaging technology, it has been widely used in the medical field. An important component of medical ultrasound equipment is the ultrasound transducer (UT), which is a transducer element that can be used to both transmit and receive ultrasound waves.

[0003] In recent years, piezoelectric micro-machined ultrasonic transducers (PMUTs) have gradually attracted attention as a novel type of MEMS device. PMUT technology utilizes the piezoelectric effect of piezoelectric materials to vibrate piezoelectric films, thereby spontaneously absorbing ultrasonic signals. It boasts advantages such as high transmission efficiency, high sensitivity, low internal resistance, and no need for high-voltage drive. Furthermore, PMUTs offer advantages such as simple device structure, highly compatible MEMS manufacturing processes, low manufacturing cost, high sensitivity, and high signal-to-noise ratio, making them suitable for large-scale applications. However, current PMUTs are typically fabricated on expensive silicon-on-insulator (SOI) substrates, resulting in not only high manufacturing costs but also problems such as low signal linearity, high insertion loss, poor signal integrity, and high signal distortion. Summary of the Invention

[0004] This application provides a radio frequency piezoelectric micromechanical ultrasonic transducer and its fabrication method. It uses high-resistivity silicon as a substrate and adds a defect-rich oxide layer and a defect-rich layer to improve the resistivity of the substrate structure and enable the PMUT to have excellent radio frequency performance. This solves the problems of high manufacturing cost, low signal linearity, high insertion loss, poor signal integrity and high signal distortion of PMUT devices.

[0005] In a first aspect, this application provides a method for fabricating a radio frequency piezoelectric micromechanical ultrasonic transducer, comprising:

[0006] A silicon substrate is provided, and a piezoelectric material layer and a bottom metal layer are sequentially formed on the silicon substrate to obtain a piezoelectric material wafer;

[0007] A high-resistivity silicon substrate is provided, and a defect-rich layer, a defect-rich oxide layer and a bottom metal layer are sequentially formed on the high-resistivity silicon substrate to obtain a defect-rich layer wafer.

[0008] A piezoelectric material wafer and a defect-rich layer wafer are wafer-bonded to form a radio frequency piezoelectric micromechanical ultrasonic transducer. The defect-rich layer and defect-rich oxide layer in the radio frequency piezoelectric micromechanical ultrasonic transducer are used to capture free parasitic charges in the radio frequency piezoelectric micromechanical ultrasonic transducer so that the radio frequency piezoelectric micromechanical ultrasonic transducer has radio frequency performance.

[0009] Optionally, the piezoelectric material wafer and the defect-rich layer wafer are subjected to wafer bonding to form a radio frequency piezoelectric micromechanical ultrasonic transducer, comprising:

[0010] The piezoelectric material wafer and the defect-rich layer wafer are subjected to metal-to-metal direct wafer bonding to obtain a metal MDOL substrate structure on a radio frequency oxide defect-rich layer.

[0011] A radio frequency piezoelectric micromechanical ultrasonic transducer is formed based on the radio frequency MDOL substrate structure.

[0012] Optionally, the piezoelectric material wafer and the defect-rich layer wafer are subjected to metal-to-metal direct wafer bonding to obtain an RF MDOL substrate structure, including:

[0013] Wafer bonding is performed between the bottom metal layer of a piezoelectric material wafer and the bottom metal layer of a defect-rich layer wafer to obtain the bottom electrode;

[0014] The silicon substrate is removed by mechanical thinning, dry etching, or wet etching to obtain the radio frequency MDOL substrate structure.

[0015] Optionally, a radio frequency piezoelectric micromechanical ultrasonic transducer is formed according to the radio frequency MDOL substrate structure, including:

[0016] The top metal layer and piezoelectric material layer in the RF MDOL substrate structure are etched to form a mesa structure.

[0017] A high-resistivity silicon substrate is etched to form a cavity high-resistivity silicon substrate in order to obtain a radio frequency piezoelectric micromechanical ultrasonic transducer.

[0018] Optionally, the process of obtaining the top metal layer in the RF MDOL substrate structure includes:

[0019] A top metal layer is formed before forming a piezoelectric material layer on a silicon substrate to obtain a top electrode.

[0020] Secondly, this application provides a radio frequency piezoelectric micromechanical ultrasonic transducer, which includes:

[0021] The high-resistivity silicon substrate, the defect-rich layer, the defect-rich oxide layer, the bottom metal layer, the piezoelectric material layer and the top metal layer are stacked sequentially from bottom to top. The high-resistivity silicon substrate is a high-resistivity silicon substrate with a cavity. The defect-rich layer and the defect-rich oxide layer are used to capture free parasitic charges in the radio frequency piezoelectric micromechanical ultrasonic transducer so that the radio frequency piezoelectric micromechanical ultrasonic transducer has radio frequency performance.

[0022] Optionally, the high-resistivity silicon substrate is etched to form cavities.

[0023] Optionally, the piezoelectric material layer and the top metal layer are etched to form a mesa structure.

[0024] Optionally, the etching process includes at least one of the following methods: wet etching, dry etching, or a combination of dry etching and wet etching.

[0025] Optionally, the piezoelectric material layer is a multilayer piezoelectric material structure with alternating stacks.

[0026] Therefore, this application has the following beneficial effects:

[0027] This application provides a method for fabricating a radio frequency piezoelectric micromechanical ultrasonic transducer. Since the high-resistivity silicon substrate has a defect-rich oxide layer, which can function as an insulating layer in an SOI wafer, the high-resistivity silicon can replace the original SOI substrate as the substrate for the radio frequency piezoelectric micromechanical ultrasonic transducer. This greatly simplifies the manufacturing process and reduces manufacturing costs. Furthermore, the piezoelectric material wafer epitaxially formed on the silicon substrate is transferred to the high-resistivity silicon substrate with the defect-rich oxide layer via metal-to-metal direct wafer bonding. Both the defect-rich oxide layer and the defect-rich layer capture free parasitic charges throughout the device, increasing the resistivity of the entire RF oxide defect-rich layer metal-on-defective oxide layer (MDOL) substrate structure. This results in excellent RF performance for the radio frequency piezoelectric micromechanical ultrasonic transducer, thus solving problems such as low signal linearity, high insertion loss, poor signal integrity, and high signal distortion inherent in PMUTs. Attached Figure Description

[0028] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments recorded in this application. For those skilled in the art, other drawings can be obtained based on these drawings.

[0029] Figure 1 This is a schematic flowchart illustrating a method for fabricating a radio frequency piezoelectric micromechanical ultrasonic transducer according to an embodiment of this application.

[0030] Figure 2 This is a schematic diagram of the structure of a radio frequency piezoelectric micromechanical ultrasonic transducer according to an embodiment of this application;

[0031] Figure 3 This is a schematic flowchart of an embodiment of a method for fabricating a radio frequency piezoelectric micromechanical ultrasonic transducer according to this application.

[0032] Figure 4 This is a schematic diagram of the process for forming a piezoelectric material wafer in an embodiment of this application;

[0033] Figure 5 This is a schematic diagram of the process for forming a defect-rich layer wafer in an embodiment of this application;

[0034] Figure 6 This is a schematic diagram of the process for forming the radio frequency MDOL substrate structure in an embodiment of this application;

[0035] Figure 7 This is a schematic diagram of the process for forming the platform structure in an embodiment of this application;

[0036] Figure 8 This is a schematic diagram illustrating the process of forming the bottom electrode and the top electrode in an embodiment of this application;

[0037] Figure 9 This is a schematic diagram of the substrate backside patterning process in an embodiment of this application. Detailed Implementation

[0038] The technical solutions of 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 some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0039] It should be noted that the user information (including but not limited to user device information, user personal information, etc.) and data (including but not limited to data used for analysis, data stored, data displayed, etc.) involved in this application are all information and data authorized by the user or fully authorized by all parties, and the collection, use and processing of the relevant data shall comply with the relevant laws, regulations and standards of the relevant countries and regions.

[0040] Currently, commercial PMUTs are typically fabricated on high-cost SOI substrates. However, PMUT devices manufactured using SOI technology also suffer from problems such as low signal linearity, high insertion loss, poor signal integrity, and high signal distortion. The main reason for this is the presence of numerous free parasitic charges in the buried oxide layer (BOX) and the top silicon layer of the SOI substrate, leading to a low resistivity of the SOI substrate. Therefore, eliminating free parasitic charges in the SOI substrate is one of the important research topics for realizing high-performance PMUT devices.

[0041] In this embodiment, adding a defect-rich oxide layer and a defect-rich layer during the fabrication of the radio frequency piezoelectric micromechanical ultrasonic transducer (PMUT) can capture free parasitic charges throughout the device and give the PMUT excellent radio frequency performance. This solves the problems of low signal linearity, high insertion loss, poor signal integrity, and high signal distortion inherent in PMUT devices. Specifically, the fabrication method may include: first, providing a silicon substrate, and sequentially forming a bottom metal layer and a piezoelectric material layer on the silicon substrate to obtain a piezoelectric material wafer; then, providing a high-resistivity silicon substrate, and sequentially forming a defect-rich layer, a defect-rich oxide layer, and a bottom metal layer on the high-resistivity silicon substrate to obtain a defect-rich layer wafer; finally, performing wafer bonding on the piezoelectric material wafer and the defect-rich layer wafer to form the radio frequency piezoelectric micromechanical ultrasonic transducer.

[0042] As can be seen, the defect-rich layer and defect-rich oxide layer in the radio frequency piezoelectric micromechanical ultrasonic transducer provided in this application can be used to capture free parasitic charges in the radio frequency piezoelectric micromechanical ultrasonic transducer, thereby increasing the resistivity of the entire radio frequency piezoelectric micromechanical ultrasonic transducer and enabling the radio frequency piezoelectric micromechanical ultrasonic transducer to have excellent radio frequency performance. This solves the problems of low signal linearity, high insertion loss, poor signal integrity, and high signal distortion in PMUT devices.

[0043] In addition, transferring the piezoelectric material layer onto a high-resistivity silicon substrate wafer with a defect-rich layer via wafer bonding greatly simplifies the manufacturing process of radio frequency piezoelectric micromechanical ultrasonic transducers and can also reduce manufacturing costs.

[0044] To facilitate understanding of the specific implementation of the fabrication method of the radio frequency piezoelectric micromechanical ultrasonic transducer provided in the embodiments of this application, the following description will be provided in conjunction with the accompanying drawings.

[0045] Figure 1 This is a schematic flowchart illustrating a method for fabricating a radio frequency piezoelectric micromechanical ultrasonic transducer, as provided in an embodiment of this application. Figure 1 As shown, the method includes the following steps S101 to S103:

[0046] S101: Provide a silicon substrate, and sequentially form a piezoelectric material layer and a bottom metal layer on the silicon substrate to obtain a piezoelectric material wafer.

[0047] To improve the resistivity and enhance the radio frequency (RF) performance of a radio frequency (RF) piezoelectric micromechanical ultrasonic transducer, the fabrication process involves first providing a silicon substrate, on which a bottom metal layer and a piezoelectric material layer are sequentially formed to obtain a piezoelectric material wafer. Next, a high-resistivity silicon substrate is provided, on which a defect-rich layer, a defect-rich oxide layer, and a bottom metal layer are sequentially formed to obtain a defect-rich layer wafer. Finally, the piezoelectric material wafer and the defect-rich layer wafer are wafer-bonded to form the RF piezoelectric micromechanical ultrasonic transducer. This allows the defect-rich layer and defect-rich oxide layer to capture free parasitic charges within the RF piezoelectric micromechanical ultrasonic transducer, thereby improving its resistivity and enhancing its RF performance. Therefore, in this embodiment, obtaining the piezoelectric material wafer in step S101 prepares the way for subsequent wafer-bonding.

[0048] As an example, S101 may specifically include: first providing a silicon substrate; forming a piezoelectric material layer on the silicon substrate; and finally forming a bottom metal layer on the piezoelectric material layer to obtain a piezoelectric material wafer.

[0049] S102: Provides a high-resistivity silicon substrate, and sequentially forms a defect-rich layer, a defect-rich oxide layer, and a bottom metal layer on the high-resistivity silicon substrate to obtain a defect-rich layer wafer.

[0050] As an example, S102 may specifically include: first, providing a high-resistivity silicon substrate; forming a defect-rich layer on the high-resistivity silicon substrate, then forming a defect-rich oxide layer on the defect-rich layer, and finally forming a bottom metal layer on the defect-rich oxide layer, thereby obtaining a defect-rich layer wafer.

[0051] S103: A piezoelectric material wafer and a defect-rich layer wafer are subjected to wafer bonding to form a radio frequency piezoelectric micromechanical ultrasonic transducer. The defect-rich layer and defect-rich oxide layer in the radio frequency piezoelectric micromechanical ultrasonic transducer are used to capture free parasitic charges in the radio frequency piezoelectric micromechanical ultrasonic transducer so that the radio frequency piezoelectric micromechanical ultrasonic transducer has excellent radio frequency performance.

[0052] As an example, S103 may include: performing metal-to-metal direct wafer bonding on a piezoelectric material wafer and a defect-rich layer wafer to obtain a radio frequency MDOL substrate structure; and forming a radio frequency piezoelectric micromechanical ultrasonic transducer based on the radio frequency MDOL substrate structure.

[0053] The above-mentioned method for obtaining the RF MDOL substrate structure specifically includes: wafer bonding of the bottom metal layer of the piezoelectric material wafer and the bottom metal layer of the defect-rich layer wafer to obtain the bottom electrode; and then removing the silicon substrate using mechanical thinning, dry etching, or wet etching methods to obtain the RF MDOL substrate structure. Specifically, the silicon substrate removal step includes: firstly, removing most of the silicon substrate using mechanical thinning. Since mechanical thinning cannot completely remove the silicon substrate, it must be used quickly to remove most of it; then, removing the remaining portion of the silicon substrate using dry etching or wet etching methods to complete the silicon substrate removal.

[0054] The above-mentioned formation of a radio frequency piezoelectric micromechanical ultrasonic transducer based on a radio frequency MDOL substrate structure specifically includes: etching the top metal layer and piezoelectric material layer in the radio frequency MDOL substrate structure to form a mesa structure; etching the high-resistivity silicon substrate to form a cavity high-resistivity silicon substrate, thereby obtaining the radio frequency piezoelectric micromechanical ultrasonic transducer. The process of obtaining the top metal layer in the radio frequency MDOL substrate structure includes: forming the top metal layer before forming the piezoelectric material layer on the silicon substrate to obtain the top electrode. That is, a top metal layer, a piezoelectric material layer, and a bottom metal layer are sequentially formed on the silicon substrate.

[0055] In this process, the fabrication method in this application embodiment directly uses low-cost silicon substrate as raw material, and then transfers the piezoelectric material wafer formed by epitaxy on the silicon substrate to a high-resistivity silicon substrate with a defect-rich oxide layer through metal-to-metal direct wafer bonding. The defect-rich oxide layer acts as an insulating layer in the SOI wafer. Both the defect-rich oxide layer and the defect-rich layer play the role of capturing free parasitic charges in the entire device, so that the radio frequency piezoelectric micromechanical ultrasonic transducer has excellent radio frequency performance. Unlike SOI-based PMUT devices, the defect-rich oxide layer in the radio frequency piezoelectric micromechanical ultrasonic transducer proposed in this application embodiment is adjacent to the bottom metal layer of the device, which greatly simplifies the device manufacturing process and can achieve the same effect as SOI-based PMUT devices.

[0056] As can be seen, in this embodiment, the high-resistivity silicon substrate has a defect-rich oxide layer, which can act as an insulating layer in the SOI wafer. Therefore, the high-resistivity silicon can replace the original SOI substrate as the substrate of the radio frequency piezoelectric micromechanical ultrasonic transducer, which can greatly simplify the manufacturing process of the radio frequency piezoelectric micromechanical ultrasonic transducer and reduce the manufacturing cost. Furthermore, both the defect-rich oxide layer and the defect-rich layer play the role of capturing free parasitic charges in the entire device, improving the resistivity of the entire radio frequency MDOL substrate structure, and giving the radio frequency piezoelectric micromechanical ultrasonic transducer excellent radio frequency performance. This solves the problems of low signal linearity, high insertion loss, poor signal integrity, and high signal distortion in PMUT.

[0057] The radio frequency piezoelectric micromechanical ultrasonic transducer in this embodiment can be found in [reference needed]. Figure 2 The radio frequency piezoelectric micromechanical ultrasonic transducer shown may include, for example:

[0058] The high-resistivity silicon substrate (1), the defect-rich layer (2), the defect-rich oxide layer (3), the bottom metal layer (4), the piezoelectric material layer (5) and the top metal layer (6) are stacked sequentially from bottom to top. The high-resistivity silicon substrate (1) is a high-resistivity silicon substrate (1) with a cavity (11). The defect-rich layer (2) and the defect-rich oxide layer (3) are used to capture free parasitic charges in the radio frequency piezoelectric micromechanical ultrasonic transducer so that the radio frequency piezoelectric micromechanical ultrasonic transducer has excellent radio frequency performance.

[0059] As an example, a cavity (11) is formed in a high-resistivity silicon substrate (1) by etching, so that the high-resistivity silicon substrate forms a suspended structure.

[0060] As an example, the piezoelectric material layer (5) and the top metal layer (6) are etched to form a mesa structure so that the bottom electrode and the top electrode can be formed subsequently.

[0061] As an example, the above etching process includes at least one of the following methods: wet etching method, dry etching method, or a combination of dry etching method and wet etching method.

[0062] As an example, the piezoelectric material layer (5) can be an alternating stacked multilayer piezoelectric material structure. The alternating stacked multilayer piezoelectric material structure has a certain quantum efficiency, which can improve the sensitivity.

[0063] As an example, both the top and bottom electrodes can be transparent electrodes.

[0064] like Figure 2 As shown, the radio frequency piezoelectric micromechanical ultrasonic transducer in this embodiment uses a low-cost high-resistivity silicon substrate (1) as raw material. The defect-rich oxide layer (3) acts as an insulating layer in the SOI wafer. Both the defect-rich oxide layer (3) and the defect-rich layer (2) capture the free parasitic charges in the entire device, giving the radio frequency piezoelectric micromechanical ultrasonic transducer excellent radio frequency performance. Moreover, unlike SOI-based PMUT devices, the defect-rich oxide layer in the radio frequency piezoelectric micromechanical ultrasonic transducer proposed in this embodiment is adjacent to the bottom metal layer of the device, which greatly simplifies the device manufacturing process and can achieve the same effect as SOI-based PMUT devices.

[0065] To make the preparation method provided in the embodiments of this application clearer and easier to understand, the following is combined with... Figure 3 , Figure 4 , Figure 5 , Figure 6 , Figure 7 , Figure 8 and Figure 9 A specific example of this method is then provided.

[0066] like Figure 3 As shown, this implementation may include, for example:

[0067] S301: Provide a silicon substrate, and sequentially form a top metal layer, a piezoelectric material layer and a bottom metal layer on the silicon substrate to obtain a piezoelectric material wafer.

[0068] like Figure 4 As shown, a silicon substrate (7) is first provided, a top metal layer (6) is formed on the silicon substrate, a piezoelectric material layer (5) is formed on the top metal layer (6), and a bottom metal layer (42) is formed on the piezoelectric material layer (5) to obtain a piezoelectric material wafer.

[0069] As an example, the thickness of the top metal layer can range from 100 to 500 nm, the thickness of the piezoelectric material layer can range from 500 to 1000 nm, and the thickness of the bottom metal layer can range from 100 to 500 nm. The above is only for illustrative purposes, and the specific thickness range of each layer in the component is not specifically limited.

[0070] It should be noted that the piezoelectric material in the radio frequency piezoelectric micromechanical ultrasonic transducer can be a multilayer piezoelectric material structure with alternating stacks. The alternating stack structure has a certain quantum efficiency, which can improve the sensitivity.

[0071] S302: Provides a high-resistivity silicon substrate, and sequentially forms a defect-rich layer, a defect-rich oxide layer, and a bottom metal layer on the high-resistivity silicon substrate to obtain a defect-rich layer wafer.

[0072] like Figure 5 As shown, a high-resistivity silicon substrate (1) is first provided, a defect-rich layer (2) is formed on the high-resistivity silicon substrate (1), a defect-rich oxide layer (3) is formed on the defect-rich oxide layer (2), and finally a bottom metal layer (41) is formed on the defect-rich oxide layer (3), thus obtaining a defect-rich layer wafer.

[0073] As an example, the thickness of the defect-rich layer can range from 100 to 1000 nm, the thickness of the defect-rich oxide layer can range from 50 to 500 nm, and the thickness of the bottom metal layer can range from 100 to 500 nm. The above is only for illustrative purposes, and the specific thickness range of each layer in the component is not specifically limited.

[0074] S303: The bottom metal layer of the piezoelectric material wafer and the bottom metal layer of the defect-rich layer wafer are directly bonded to each other by metal-to-metal wafer bonding, and the silicon substrate is removed by mechanical thinning, dry etching or wet etching to obtain the radio frequency MDOL substrate structure.

[0075] like Figure 6 As shown, in order to transfer the piezoelectric material wafer formed by epitaxy on the silicon substrate to the high-resistivity substrate with a defect-rich oxide layer, the bottom metal layer (42) of the piezoelectric material wafer and the bottom metal layer (41) of the defect-rich layer wafer are first bonded together by metal-to-metal direct wafer bonding, so that the two wafers are bonded together, and most of the silicon substrate (7) is removed by mechanical thinning method, and then the remaining silicon substrate (7) is removed by dry etching method or wet etching method, so that only the high-resistivity substrate (1) remains, forming the radio frequency MDOL substrate structure.

[0076] In the aforementioned RF MDOL substrate structure, the defect-rich oxide layer acts as an insulating layer in the SOI wafer, and both the defect-rich oxide layer and the defect-rich layer capture free parasitic charges in the entire device, thereby forming an RF MDOL substrate structure with excellent RF performance.

[0077] S304: The top metal layer and piezoelectric material layer in the RF MDOL substrate structure are etched to form a mesa structure.

[0078] like Figure 7 As shown, the top metal layer (6) and piezoelectric material layer (5) in the RF MDOL substrate structure are etched using a wet etching method, a dry etching method, or a combination of dry etching and wet etching methods to form a mesa structure.

[0079] S305: Place metal 1 on the bottom metal layer to form the bottom electrode, and place metal 2 on the top metal layer to form the top electrode.

[0080] like Figure 8 As shown, metal 1 (8) is placed on the bottom metal layer (4) to form the bottom electrode; and metal 2 (9) is placed on the top metal layer (6) to form the top electrode.

[0081] It should be noted that both the top and bottom electrodes in the radio frequency piezoelectric micromechanical ultrasonic transducer can be transparent electrodes.

[0082] S306: The back side of a high-resistivity silicon substrate is patterned to form a cavity high-resistivity silicon substrate in order to obtain a radio frequency piezoelectric micromechanical ultrasonic transducer.

[0083] like Figure 9As shown, the high-resistivity silicon substrate (1) can also be etched using wet etching, dry etching, or a combination of dry and wet etching methods to form a suspended cavity (11) structure. The final structure is a radio frequency piezoelectric micromechanical ultrasonic transducer.

[0084] This embodiment provides a method for fabricating a radio frequency piezoelectric micromechanical ultrasonic transducer. Instead of a conventional SOI substrate, a low-cost silicon substrate is used as the raw material. A piezoelectric material wafer epitaxially formed on the silicon substrate is transferred to a high-resistivity silicon substrate with a defect-rich oxide layer via metal-to-metal direct wafer bonding. The defect-rich oxide layer acts as an insulating layer in the SOI wafer. Both the defect-rich oxide layer and the defect-rich layer trap free parasitic charges throughout the device, resulting in excellent radio frequency performance for the radio frequency piezoelectric micromechanical ultrasonic transducer. Unlike SOI-based PMUT devices, in this application, the defect-rich oxide layer is located adjacent to the bottom metal layer of the device, greatly simplifying the device manufacturing process while achieving comparable performance to SOI-based PMUT devices.

[0085] Furthermore, compared to SOI-based PMUT devices, this invention provides a PMUT device structure formed on an RF MDOL substrate and its fabrication scheme. Its realization will help promote the rapid development of PMUT devices in ultrasonic testing and has great research significance and economic benefits.

[0086] As can be seen from the above description of the embodiments, those skilled in the art can clearly understand that all or part of the steps in the methods of the above embodiments can be implemented by means of software plus a general-purpose hardware platform. Based on this understanding, the technical solution of this application can be embodied in the form of a software product. This computer software product can be stored in a storage medium, such as a read-only memory (ROM) / RAM, magnetic disk, optical disk, etc., including several instructions to cause a computer device (which may be a personal computer, a server, or a network communication device such as a router) to execute the methods described in various embodiments or some parts of the embodiments of this application.

[0087] The various embodiments in this specification are described in a progressive manner. Similar or identical parts between embodiments can be referred to mutually. Each embodiment focuses on its differences from other embodiments. In particular, the device embodiments are basically similar to the method embodiments, so the description is relatively simple; relevant parts can be referred to the descriptions in the method embodiments. The device embodiments described above are merely illustrative. Modules described as separate components may or may not be physically separate. Components shown as modules may or may not be physical modules; that is, they may be located in one place or distributed across multiple network units. Some or all of the modules can be selected to achieve the objectives of this embodiment according to actual needs. Those skilled in the art can understand and implement this without creative effort.

[0088] The above description is merely an exemplary implementation of this application and is not intended to limit the scope of protection of this application.

Claims

1. A method for fabricating a radio frequency piezoelectric micromechanical ultrasonic transducer, characterized in that, include: A silicon substrate is provided, and a piezoelectric material layer and a bottom metal layer are sequentially formed on the silicon substrate to obtain a piezoelectric material wafer; A high-resistivity silicon substrate is provided, and a defect-rich layer, a defect-rich oxide layer and a bottom metal layer are sequentially formed on the high-resistivity silicon substrate to obtain a defect-rich layer wafer. The thickness of the defect-rich layer ranges from 100 to 1000 nm, and the thickness of the defect-rich oxide layer ranges from 50 to 500 nm. The piezoelectric material wafer and the defect-rich layer wafer are wafer bonded to form a radio frequency piezoelectric micromechanical ultrasonic transducer. The defect-rich layer and defect-rich oxide layer in the radio frequency piezoelectric micromechanical ultrasonic transducer are used to capture free parasitic charges in the radio frequency piezoelectric micromechanical ultrasonic transducer so that the radio frequency piezoelectric micromechanical ultrasonic transducer has radio frequency performance. The step of performing wafer bonding processing on the piezoelectric material wafer and the defect-rich layer wafer to form a radio frequency piezoelectric micromechanical ultrasonic transducer includes: performing metal-to-metal direct wafer bonding on the bottom metal layer of the piezoelectric material wafer and the bottom metal layer of the defect-rich layer wafer to obtain a bottom electrode; removing the silicon substrate by mechanical thinning, dry etching, or wet etching to obtain a radio frequency MDOL substrate structure; etching the top metal layer and piezoelectric material layer in the radio frequency MDOL substrate structure to form a mesa structure; and etching the high-resistivity silicon substrate to form a cavity high-resistivity silicon substrate to obtain the radio frequency piezoelectric micromechanical ultrasonic transducer.

2. The method according to claim 1, characterized in that, The process of obtaining the top metal layer in the radio frequency MDOL substrate structure includes: The top metal layer is formed before the piezoelectric material layer is formed on the silicon substrate to obtain the top electrode.

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