A novel wideband pMUT unit and its array

Abstract

This invention discloses a novel wideband pMUT unit and array. The novel wideband pMUT unit and array, from bottom to top, comprises a base layer, a driving layer, and a multi-gradient vibration layer. The base layer includes a support, a back cavity, a back load, and a silicon dioxide insulating layer. The driving layer includes a bottom electrode, a top electrode, and an adhesion layer. The gradient of the multi-gradient vibration layer is guided by its thickness dimension, and the multi-gradient vibration layer is a multi-gradient piezoelectric thin film. The pMUT unit with the multi-gradient vibration layer has multiple resonant modes, giving it multi-frequency performance. The back load is formed by etching to amplify the damped oscillation of the pMUT unit, while adjusting the equivalent vibrating mass, stiffness, and effective vibrating area distribution of the pMUT unit, and coupling the multi-frequency bands into a wideband. This invention is applicable to the acoustic field and, compared with conventional pMUT structures, can effectively broaden the operating frequency band of the pMUT unit.

Description

[0001] This application relates to the field of ultrasonic transducers, and mainly to the structural design of a novel broadband pMUT unit and its array. Background Technology

[0002] Ultrasonic transducers, as ultrasonic devices that can convert electrical energy into mechanical acoustic energy, play a crucial role in the generation and detection of ultrasonic waves. Thanks to the development of MEMS technology, ultrasonic transducers have been miniaturized, with pMUTs requiring only a few volts of excitation voltage. This gives pMUTs higher application safety and biocompatibility.

[0003] The main performance parameters of a piezoelectric ultrasonic transducer include resonant frequency, electromechanical coupling coefficient, bandwidth, operating efficiency, and transmit and receive sensitivity. Depending on the specific application of the transducer, the performance parameters that need to be emphasized will vary.

[0004] For a long time, research on parameters such as the sensitivity of pMUTs has been relatively mature. However, compared with this, research on how to effectively broaden the operating bandwidth of pMUT units and arrays is relatively scarce. A narrow operating bandwidth greatly limits the resolution of pMUT units and arrays, restricting their application in many fields. Currently, scholars at home and abroad have achieved a series of results in the research of pMUT bandwidth. For example, Fudan University (CN202110235018.X) has successfully increased the bandwidth of pMUT units and arrays by more than double by creating blind-hole cavities on the back of traditional pMUT units and filling the cavities with backing material. However, the cost of these pMUT units and arrays is high, which is not conducive to their transformation into engineering applications, greatly limiting their application scenarios. Wuhan University has developed an array-type broadband ultrasonic transducer (CN202011621958.4), but the uniformity of the directivity of the ultrasonic transducer array devices is poor, limiting its application areas.

[0005] Under the current technological background, the design of wideband pMUT arrays and cells has shortcomings, including:

[0006] (1) Multi-frequency excitation sources or multi-drive methods cannot achieve practical effective bandwidth, and the device design process is cumbersome.

[0007] (2) Low stability and low fault tolerance. There are one or more frequency points in the operating frequency band that are close to the lower limit of the -6dB frequency band, resulting in low axial resolution and low stability.

[0008] (3) The materials used are expensive and the development cycle of new processes is long. This invention uses an oxide substrate and a mature piezoelectric thin film preparation process. Summary of the Invention

[0009] Based on the above problems, the present invention proposes a novel wideband pMUT unit and array.

[0010] This invention can be achieved through the following technical solutions:

[0011] A novel wideband pMUT unit and array. The structure, from bottom to top, includes a substrate layer, a driving layer, and a multi-gradient vibration layer; the substrate layer includes a support, a back cavity, a back load, and a silicon dioxide insulating layer; the driving layer includes a bottom electrode, a top electrode, and an adhesion layer, the adhesion layer being used to fix the bottom electrode and the top electrode; the multi-gradient vibration layer includes a multi-gradient piezoelectric thin film.

[0012] The shape of the pMUT unit can be one of the following: prism, toroid, cube, or cuboid.

[0013] The back load is located inside the back cavity, and the distribution of the back load includes symmetrical and asymmetrical distribution; the shape of the back load (1-3) is one of the following: cylinder, cone, ring, cube, cuboid.

[0014] The multi-gradient vibration layer includes a multi-gradient piezoelectric thin film, and the materials of the piezoelectric thin film include piezoelectric ceramics, piezoelectric crystals, composite piezoelectric materials, PVDF, AlN, and ZnO.

[0015] The gradient of a multigradient piezoelectric film is guided by a change in thickness, and the thickness of the multigradient piezoelectric film is 0.1-10 μm.

[0016] The shapes of multi-gradient piezoelectric films include non-uniform thickness vibrating layers in the form of multi-level steps, and uniform or non-uniform thickness vibrating layers in the form of concentric rings.

[0017] The multi-gradient vibration layer (3) and the back load (1-3) are coordinated to achieve frequency band coupling, and further realize the artificially controllable resonant frequency and frequency band range.

[0018] Furthermore, the silicon dioxide insulating layer is formed through a thermal oxidation process;

[0019] Furthermore, within a single pMUT cell, the top electrode area covers more than 50% of the pMUT cell.

[0020] Furthermore, within a single pMUT element, the area of ​​the multi-gradient vibration layer covers more than 50% of the pMUT element.

[0021] Furthermore, the multi-gradient vibration layer is selected as a multi-level stepped non-uniform thickness vibration layer, that is, a gradient guided by the change in thickness is selected.

[0022] Furthermore, a secondary stepped non-uniform thickness vibration layer is selected, and the preparation method includes the following steps:

[0023] S1. A piezoelectric thin film is grown on the bottom electrode.

[0024] S2. Spin-coat photoresist onto the piezoelectric thin film described in S1, perform the first photolithography using a mask, and then perform dry etching on the piezoelectric thin film after the first photolithography to remove the photoresist, thus completing the preparation of the first-order vibration layer.

[0025] S3. Spin-coat photoresist onto the piezoelectric thin film (first-stage vibration layer) described in S2, perform a second photolithography using a mask, and then perform dry etching on the piezoelectric thin film after the second photolithography. After removing the photoresist, the second-stage stepped vibration layer is prepared.

[0026] The above steps are repeated to prepare more stepped vibration layers.

[0027] Compared with the prior art, the present invention has at least the following beneficial technical effects:

[0028] (1) The vibration layer of the present invention adopts a multi-gradient vibration layer. Under the same excitation conditions, the vibration layer with gradients achieves multi-frequency response due to different mass and stiffness distributions. On the same fabrication area, it indirectly improves the integration of the pMUT array and effectively overcomes the limitation of the narrow working frequency band of the traditional pMUT unit due to problems such as the single resonant frequency and the nonlinear relationship between the size of the vibration layer and the resonant frequency. It is a novel structural design and concept of multi-frequency and wideband pMUT unit, providing a new research direction for the design of subsequent wideband pMUT units.

[0029] (2) The present invention adopts a combination structure of multi-gradient vibration layer and back load. The back load structure increases the oscillation damping of the pMUT unit, making the acoustic signal output by the pMUT unit smoother. At the same time, the multi-frequency bands responded by the multi-gradient vibration layer are coupled into a complete broadband band under the joint action of the back load.

[0030] (3) The present invention optimizes the vibration layer of the traditional pMUT unit into a multi-gradient vibration layer. By controlling the gradient, shape and material of the multi-gradient vibration layer, the resonant frequency and frequency band range can be artificially controlled. Attached Figure Description

[0031] The specific embodiments of the present invention will be described in further detail below with reference to the accompanying drawings:

[0032] Figure 1 This is a schematic diagram of a broadband pMUT unit and its array structure with a two-stage stepped vibration layer and a back load, as illustrated in an embodiment of the present invention.

[0033] Figure 2This is a schematic diagram illustrating the fabrication process of a broadband pMUT unit and array with a two-stage stepped vibration layer and a back load, as described in an embodiment of the present invention.

[0034] Figure 2 (a) Preparation of the substrate silicon wafer and silicon dioxide insulating layer;

[0035] Figure 2 (b)-(c) show the fabrication of the bottom electrode and the adhesion layer;

[0036] Figure 2 (d)-(g) represents the preparation of a second-order stepped vibration layer;

[0037] Figure 2 (h) describes the fabrication of the bottom electrode;

[0038] Figure 2 (i)-(k) represent the fabrication of the back cavity and the back load;

[0039] Figure 3 This is a -6dB frequency band diagram of a broadband pMUT unit with a two-stage stepped vibration layer and a back load, as illustrated in an embodiment of the present invention.

[0040] In the figure, 1. Base layer; 2. Driving layer; 3. Multi-gradient vibration layer; 1-1. Support; 1-2. Back cavity; 1-3. Back load; 1-4. Silicon dioxide insulating layer; 2-1. Bottom electrode; 2-2. Top electrode; 2-3. Adhesion layer; 3-1. Piezoelectric film; 3-2. Second photomask; 3-3. First piezoelectric film; 3-4. Third photomask; 3-5. Second-level stepped vibration layer; 1-1-1. Silicon wafer; 1-1-2. Fifth photomask; 1-1-3. Metal AI protective layer; 1-1-4. Sixth photomask; 2-1-1. First photomask; 2-2-1. Fourth photomask; 2-2-2. Multi-level top electrode and adhesion layer. Detailed Implementation

[0041] To illustrate the present invention in more detail, the technical solution of the present invention will be further described below with reference to preferred embodiments and accompanying drawings. The specific embodiments described herein are only for explaining the present invention and are not intended to limit the present invention.

[0042] In this invention, the terms "upper," "lower," "top," "bottom," "inner," and "center" are used to describe the structural relationships and orientations of the components in the accompanying drawings. They are solely for the convenience of describing specific embodiments of the invention and do not specify or imply that the invention must have a particular combination, specific structure, or relative positional relationship. Therefore, they should not be construed as limitations on the invention. The terms "first" and "second" are used to clearly describe the implementation process of the invention and do not specify or imply features that the invention must follow. Therefore, they should not be construed as limitations on the invention. The term "multi-level" includes two or more levels. The term "multiple times" includes two or more times. The term "connection" includes electrical connection and mechanical connection; it can also be a direct connection or an indirect connection through an intermediate structure; it can be an overall connection or a partial connection. Therefore, it should not be construed as a limitation on the invention. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0043] Example:

[0044] Reference Figure 1 This embodiment provides a broadband pMUT unit and its array having a two-stage non-uniform thickness stepped vibration layer and a back load. The pMUT unit structure includes, from bottom to top, a base layer (1), a driving layer (2), and a multi-gradient vibration layer (3). The base layer (1) includes a support (1-1), a back cavity (1-2), a back load (1-3), and a silicon dioxide insulating layer (1-4). The driving layer (2) includes a bottom electrode (2-1), a top electrode (2-2), and an adhesion layer (2-3). The adhesion layer (2-3) is used to fix the bottom electrode (2-1) and the top electrode (2-2). The multi-gradient vibration layer (3) includes a multi-gradient piezoelectric film.

[0045] The multi-gradient vibration layer (3) is made of piezoelectric material and is prepared by magnetron sputtering. Under the excitation of the driving layer (2), the multi-gradient vibration layer (3) generates ultrasonic waves through the piezoelectric effect and radiates and propagates in the liquid domain, or outputs corresponding electrical signals after receiving sound waves emitted by an external sound source. The thickness of the multi-gradient vibration layer (3) is controlled between 0.1um and 10um to prevent strain fracture and low sensitivity problems caused by excessively low or high thickness.

[0046] The purpose of the multi-gradient vibration layer (3) is to improve the single frequency response of the traditional pMUT unit. Based on electromechanical equivalence theory, the equivalent masses of the multi-gradient vibration layer are M1-M2. N The equivalent stiffnesses are D1-D N The effective vibration areas are A1-A NThe elastic deformation process of the pMUT element is equivalent to an equivalent spring with a spring constant K. A novel broadband pMUT element and array are described. The resonant frequencies include:

[0047]

[0048] …

[0049]

[0050] Since the multi-gradient vibration layer (3) has multiple levels of resonant frequencies, the traditional pMUT unit is improved into a multi-frequency response device.

[0051] Preferably, this embodiment takes into account both the manufacturing process and the display effect, and selects a broadband pMUT unit structure with a two-stage stepped vibration layer and a back load.

[0052] The upper and lower surfaces of the multi-gradient vibration layer (3) are connected to the top electrode (2-2) and the bottom electrode (2-1), respectively. The adhesion layer (2-3), the top electrode (2-2), and the bottom electrode (2-1) are deposited using magnetron sputtering technology and used to excite the multi-gradient vibration layer (3). The top electrode (2-2) can be circular, rectangular, square, annular, or polygonal. The coverage of the pMUT unit by the top electrode (2-2) is between 50% and 100%, effectively ensuring the emission sensitivity of the pMUT unit. In this embodiment, the coverage of the top electrode is 60%.

[0053] The materials for the top electrode (2-2) and bottom electrode (2-1) include metals such as Mo, Pt, Au, Ag, Cr, Ti, Al, and Cu. Preferably, in this embodiment, Au, a metal with good conductivity and corrosion resistance, is selected.

[0054] The adhesive layer (2-3) materials include metals Cr and Ti. Preferably, in this embodiment, Cr is selected because it has a better adhesion effect with metal Au.

[0055] The back cavity (1-2) and the back load (1-3) are formed using a deep silicon etching process. The purpose of the back load is to increase vibration damping, making the acoustic signal output by the pMUT unit smoother. Simultaneously, the multi-gradient vibration layer (3), under the combined action of the back load (1-3), couples multiple frequency bands into a single broadband band. The shape of the back load (1-3) includes one of the following: cylindrical, conical, ring-shaped, cubic, or cuboid. Preferably, a cylindrical back load is used in this embodiment.

[0056] The substrate layer (1) is made of SOI silicon wafer or oxide wafer. Preferably, to simplify the process and save on preparation costs, an oxide wafer is selected in this invention. The silicon dioxide insulating layer (1-4) is grown on the surface of the silicon wafer by a thermal growth process and is used for electrical isolation between the driving layer (2) and the substrate layer (1).

[0057] Reference Figure 2 A fabrication process for a broadband pMUT unit with a secondary non-uniform thickness vibration layer and a back load includes:

[0058] 1. For example Figure 2 As shown in (a), the spare part is a standard cleaned N-type. <111> A silicon wafer (1-1-1) with a crystal orientation growth of 500 μm is grown. A 500 nm thick silicon dioxide insulating layer (1-4) is oxidized on the surface of the silicon wafer by a thermal oxidation process.

[0059] 2. Figure 2 (b)-(c) Bottom electrode fabrication: spin-coating photoresist, photolithography using the first mask (2-1-1), followed by sputtering a 20nm thick metal Cr adhesion layer and a 100nm thick metal Au layer, and then removing excess metal electrodes by a lift-off process to complete the bottom electrode patterning.

[0060] 3. Figure 2 (d) Vibration layer deposition: An 800 nm thick piezoelectric film was grown on the bottom electrode by magnetron sputtering (3-1).

[0061] 4. Figure 2 (e) Preparation of the first-level vibration layer: Photoresist is spin-coated on the piezoelectric thin film, photolithography is performed using the second mask (3-2), and then the piezoelectric thin film (3-1) is dry-etched to remove the photoresist.

[0062] 5. Figure 2 (f)-(g) Preparation of the second-level vibration layer: Photoresist is spin-coated onto the first piezoelectric film (3-3) obtained in step 4, and photolithography is performed using the third mask (3-4). Then, the piezoelectric film (3-3) is dry-etched, and the second-level stepped vibration layer (3-5) is prepared after removing the photoresist.

[0063] 6. Figure 2 (h) Top electrode fabrication: Photoresist is spin-coated onto the device surface, and photolithography is performed using a fourth mask (2-2-1). Then, a 20 nm thick Cr metal adhesion layer and a 100 nm thick Au metal layer are sputtered onto the device surface using magnetron sputtering. Excess metal electrodes are removed using a lift-off process, completing the top electrode patterning. Finally, a layer of photoresist is spin-coated onto the upper surface of the device as a back-side etching protection layer.

[0064] 7. Figure 2(i)-(k) Fabrication of back cavity and back load: Spin-coat photoresist on the lower surface of the device, sputter a metal Al protective layer (1-1-3), use the fifth mask (1-1-2) to pattern the metal Al layer and use it as the hard film for etching and remove the photoresist, and etch a silicon layer of 50um-200um thickness by deep ion reaction, and then stop etching; To fabricate the back load (1-3), spin-coat photoresist on the lower surface of the device, then sputter a second metal Al protective layer (1-1-3), use the sixth mask (1-1-4) to pattern the metal Al layer and remove the photoresist, and etch a silicon layer of 300um-450um thickness by deep ion reaction, and then stop etching.

[0065] 8. Remove the photoresist on the upper surface of the device and the Al metal protective layer on the lower surface of the device. This completes the fabrication of a broadband pMUT unit with a secondary non-uniform thickness vibration layer and a back load in this embodiment.

[0066] See attached document Figure 3 This is a frequency band distribution diagram of pMUT units with different sizes of back loads in this embodiment. In this embodiment, it has been verified that a broadband pMUT unit with a two-stage non-uniform thickness resonator and a back load has an 85% bandwidth (-6dB) at 3.9MHz, and an absolute bandwidth of 2-3.5MHz. Finally, it should be noted that the above embodiments are merely preferred examples for clearly illustrating the present invention, but they are not intended to limit the implementation of the present invention. Those skilled in the art should understand that the technical features in the above solutions can be arbitrarily combined, and other modifications or equivalent substitutions can be made based on the above specific implementation methods. It is impossible to exhaustively list all implementation methods here. Therefore, any modifications, improvements, equivalent substitutions, etc., derived from the technical solutions of the present invention within the spirit and principles of the present invention should be within the scope of protection claimed by the present invention.

Claims

1. A novel wideband pMUT cell characterized by, The pMUT unit structure comprises, from bottom to top, a substrate layer (1), a driving layer (2), and a multi-gradient vibration layer (3). The substrate layer (1) comprises a support (1-1), a back cavity (1-2), a back load (1-3), and a silicon dioxide insulation layer (1-4). The driving layer (2) comprises a bottom electrode (2-1), a top electrode (2-2), and an adhesive layer (2-3) for fixing the bottom electrode (2-1) and the top electrode (2-2). The multi-gradient vibration layer (3) comprises a multi-gradient piezoelectric film. The pMUT unit realizes frequency band coupling by cooperatively controlling the multi-gradient vibration layer (3) and the back load (1-3), and further widens the working frequency band, wherein the back load (1-3) is located inside the back cavity (1-2) and is prepared by a deep silicon etching process.

2. A novel wideband pMUT cell according to claim 1, wherein, The shape of the pMUT unit includes but is not limited to a cylinder, a ring, a square, and a cuboid.

3. The novel wideband pMUT cell of claim 1, wherein, The distribution form of the back load (1-3) includes symmetric distribution and asymmetric distribution.

4. The novel wideband pMUT cell of claim 3, wherein, The shape of the back load (1-3) is one of a cylinder, a ring, a square, a cuboid, and the like.

5. The novel wideband pMUT cell of claim 1, wherein, The material of the multi-gradient piezoelectric film includes piezoelectric ceramic, piezoelectric crystal, composite piezoelectric material, PVDF, AlN, and ZnO.

6. The novel wideband pMUT cell of claim 5, wherein, The gradient of the multi-gradient piezoelectric film is guided by thickness variation, and the thickness of the multi-gradient piezoelectric film is 0.1-10 μm.

7. The novel wideband pMUT cell of claim 5, wherein, The shape of the multi-gradient piezoelectric film includes a multi-stage non-equal-thickness vibration layer and a concentric ring-shaped equal-thickness or non-equal-thickness vibration layer.

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