A CMUT packaging structure with focusing function and a preparation method thereof

Abstract

The application discloses a CMUT packaging structure with focusing function and a preparation method thereof, and mainly relates to a capacitive micromachined ultrasonic transducer. The packaging structure comprises, from top to bottom, a PDMS coupling layer, an acoustic metasurface, a metal support, a CMUT device and an epoxy resin layer. An outer shell made of metal is wrapped. By taking the shell as a base, the CMUT device and the acoustic metasurface are fixed in the shell, and the gap between the CMUT device and the shell is filled with epoxy resin through the bottom of the shell. The whole is placed on a spin coater, and the PDMS is injected by using a spin coating process. Then, the whole is placed in a vacuum box, heated and solidified to form a PDMS layer, and the packaging is completed. The CMUT packaging structure not only has high focusing performance, but also has the functions of water-tight packaging and impedance matching for the CMUT device. The CMUT packaging structure can be directly applied to the field of underwater sound or the field of medical ultrasound, such as underwater breast ultrasound imaging, underwater target detection and the like.

Description

Technical Field

[0001] This invention belongs to the field of micro-nano acoustic transducers, specifically relating to a CMUT packaging structure with focusing function and its fabrication method, which can be applied to the field of signal detection in underwater communication. Background Technology

[0002] Capacitive micromechanical ultrasonic transducers (CMUTs) are micromechanical devices manufactured using MEMS technology. They feature high conversion efficiency, high sensitivity, wide bandwidth, and good impedance matching with the working medium. They have broad application prospects in the field of underwater ultrasonics and offer advantages such as ease of manufacturing and stable performance, gradually replacing piezoelectric transducers in ultrasonic applications.

[0003] While CMUTs offer many advantages, their application in medical ultrasound testing has historically been limited to full immersion in aquatic environments, leaving many unresolved issues in underwater acoustics. Existing CMUTs, with their metallic surfaces, are prone to circuit aging and short circuits when directly submerged in water. Furthermore, because CMUTs excite a diffuse sound field, they cannot directly focus and utilize sound waves.

[0004] For example, CN patent 103418542A designs a CMUT component with an acoustic window. The CMUT emits sound waves, which are then focused and have their propagation direction changed by the acoustic window, which is shaped like a convex or concave lens. Alternatively, focusing can be achieved by modifying the CMUT to a concave lens shape. However, this design still has many shortcomings. For instance, the uneven surface of the acoustic window makes manufacturing difficult and hinders large-scale application. The CMUT surface achieves focusing by changing its shape, which increases manufacturing difficulty and cost. Furthermore, the coupling medium does not consider impedance matching, making it unsuitable for direct application in aquatic environments.

[0005] Based on this, the present invention proposes a planar CMUT packaging structure with focusing function and its preparation method. Summary of the Invention

[0006] The purpose of this invention is to achieve acoustic focusing underwater, and to provide a CMUT packaging structure with acoustic focusing function and its preparation method.

[0007] To achieve the above objectives, the technical solution of the present invention is as follows:

[0008] A CMUT packaging structure with focusing function comprises, from top to bottom, a PDMS layer, an acoustic metasurface, a metal support, a CMUT device, and an epoxy resin layer, with the outer layer fixed by a housing. The housing is made of a metal material or a high acoustic impedance material that holds the CMUT device and the acoustic metasurface. The PDMS layer encapsulates the acoustic metasurface.

[0009] The specific structure of this invention is as follows.

[0010] The packaging structure of this invention comprises: a CMUT device, the surface of which is a two-dimensional array composed of multiple CMUT micro-elements or a single circular CMUT element, which excites a sound field by the collective vibration of multiple CMUT micro-elements, and the frequency of the excited sound field can be adjusted according to voltage changes; an acoustic metasurface, which is an integral part composed of multiple annular structures with the same center but different radii, and has a flat surface. It is fixed to the CMUT device in a specifically designed housing, and different acoustic focusing effects can be achieved according to the design. The housing can be divided into a bottom and an outer part to facilitate the installation of the acoustic metasurface and the CMUT device. The bottom of the housing has a circular interface for the wires of the CMUT device to pass through. The size of the housing matches the acoustic metasurface and the CMUT device. The acoustic metasurface is fixed between the CMUT device and the acoustic metasurface by a metal bracket; a PDMS layer is located between the acoustic metasurface and the CMUT device, and covers the acoustic metasurface; and an epoxy resin layer is used to fill the gap between the back of the CMUT device and the housing.

[0011] The method for preparing the packaging device of the present invention includes the following steps:

[0012] The first step is to design the housing based on the dimensions of the CMUT device and the acoustic metasurface.

[0013] The acoustic metasurface is placed on the front of the CMUT device using a metal bracket to form a whole A. The whole A is then installed into the outer housing. The bottom housing is then bonded together, and the wires of the CMUT device are passed out from the wire outlet of the bottom housing. The bottom housing is then bonded together.

[0014] Epoxy resin is filled between the back of the CMUT device and the bottom of the housing through holes at the bottom of the housing. The entire assembly is then placed on a spin coater. Using the spin coating process, PDMS liquid and PDMS curing agent are simultaneously poured in through the gaps of the acoustic metasurface, allowing it to slowly cover the top of the acoustic metasurface from the front of the CMUT device. The entire assembly is then placed in a vacuum chamber with a specific vacuum level and temperature set to allow air bubbles in the PDMS mixture to escape and the entire assembly to dry, completing the third step of the encapsulation process.

[0015] The specific packaging method is as follows: The housing is designed according to the CMUT device and the acoustic metasurface. The gap distance between the acoustic metasurface and the CMUT device is designed, and the acoustic metasurface is connected to the CMUT device using a metal support to form a whole A.

[0016] Next, the entire assembly A is installed into the outer housing, the bottom housing is joined to the outer housing, and epoxy resin is filled between the lower surface of the CMUT device and the upper surface of the bottom housing through the holes in the bottom housing. The epoxy resin is used to reinforce the back of the CMUT device and to fix the entire assembly A inside the housing.

[0017] Using a spin coater, PDMS liquid and PDMS curing agent are injected into the gaps of the acoustic metasurface while the entire assembly is rotating. The PDMS liquid slowly and gradually covers the entire surface of the CMUT device, extending to the top of the acoustic metasurface. The assembly is then placed in a vacuum chamber with controlled vacuum and temperature. The vacuum environment removes air bubbles from the PDMS mixture and dries the assembly. The PDMS mixture then slowly solidifies, forming a PDMS layer that protects the CMUT device, completing the encapsulation process.

[0018] Preferably, the radius of the CMUT device is 30 mm to 42 mm.

[0019] Preferably, the acoustic field frequency excited by the CMUT device is between 900 kHz and 1100 kHz.

[0020] Preferably, the size of the CMUT device may be greater than or equal to the size of the acoustic metasurface.

[0021] Preferably, the acoustic metasurface is a ring structure with a radius of 25 mm to 40 mm.

[0022] Preferably, the thickness of the acoustic metasurface is less than the wavelength of the incident wave, and the thickness is preferably 0.9 mm to 1.5 mm.

[0023] Preferably, the material of the acoustic metasurface can be selected from high acoustic impedance materials such as aluminum, steel, copper, and nickel-chromium alloys.

[0024] Preferably, the gap thickness between the upper surface of the CMUT and the lower surface of the acoustic metasurface is 0.5 mm to 2 mm.

[0025] Preferably, the housing can be made of materials that can be selected through impedance matching, specifically materials with high acoustic impedance, such as steel, aluminum, nickel-chromium alloys, etc.

[0026] Preferably, the housing consists of two parts: a bottom and an outer part. Alternatively, it can be designed as a single unit, in which the acoustic metasurface and CMUT device are first fixed and installed into the housing before the PDMS liquid is added.

[0027] Preferably, if the housing adopts an integrated design, epoxy resin can be filled between the lower surface of the CMUT device and the housing while the entire A is installed inside the housing.

[0028] The beneficial effects of this invention are:

[0029] According to the present invention, acoustic focusing functionality can be achieved while simultaneously providing encapsulation effects. For the encapsulation of CMUT devices, the encapsulation structure not only possesses high-efficiency transmission performance but also provides watertight sealing and impedance matching functions for the CMUT devices. This makes the internal circuitry of the encapsulation structure less susceptible to water erosion or short circuits, extending the lifespan of the encapsulation structure. Because the present invention is small in size, it is convenient to carry during underwater operations and can also be used for acoustic detection in underwater areas. Attached Figure Description

[0030] Figure 1 This is a structural diagram of the packaging structure 1 of the present invention.

[0031] Figure 2 This is a quarter-section view of the three-dimensional structure 1 of the present invention.

[0032] Figure 3 This represents the COMSOL simulation model of Example 1.

[0033] Figure 4 This is a sound intensity diagram representing the CMUT device without an acoustic metasurface.

[0034] Figure 5 This indicates the simulated sound pressure focusing point and sound field intensity of Example 1.

[0035] Figure 6 This is a flowchart illustrating the process of fabricating the packaging structure.

[0036] Figure 7 It means Figure 6 A schematic diagram of the preparation process.

[0037] Figure 8 As shown Figure 6 A schematic diagram of the third step in the preparation method.

[0038] Explanation of reference numerals in the attached figures: 1-Encapsulation structure, 2-Outer housing, 3-Bottom housing, 4-CMUT device, 5-PDMS layer, 6-Metal support, 7-Acoustic metasurface, 8-Epoxy resin layer, 12-Water area. Detailed Implementation

[0039] The CMUT packaging structure designed in this invention not only possesses high-efficiency focusing performance but also provides multiple functions for CMUT devices, including watertight packaging and impedance matching. It can be directly applied in the fields of underwater acoustics or medical ultrasound, such as underwater breast ultrasound imaging and underwater target detection.

[0040] The preferred embodiments of the packaging structure of the present invention and the preparation method using the packaging structure are described in detail below with reference to the accompanying drawings.

[0041] Elements with the same functional structure described in the accompanying drawings are illustrated by assigned numbers.

[0042] First, according to Figure 1 Explain the structure of encapsulation structure 1.

[0043] The packaging structure 1 of the present invention has: an outer shell 2, a bottom shell 3, a CMUT device 4, a PDMS layer 5, a metal support 6, an acoustic metasurface 7, and an epoxy resin layer 8.

[0044] The CMUT device 4 is used to excite the sound field. Ultrasonic waves are excited by the CMUT device 4 and focused by the acoustic metasurface 7. The CMUT device 4 is composed of a two-dimensional CMUT array or a single circular CMUT element. The two-dimensional CMUT array is composed of multiple CMUT micro-elements arranged together, and then combined from multiple CMUT elements. By exciting ultrasonic waves through the joint vibration of multiple micro-elements, sound fields of different frequencies can be generated. The acoustic metasurface 7 is used to focus the ultrasonic waves excited by the CMUT device 4. The acoustic metasurface 7 is a millimeter-sized acoustic grating-like structure composed of multiple ring structures of different diameters, which has a focusing effect. The PDMS layer 5 is used to encapsulate the CMUT device 4 to prevent it from short-circuiting upon contact with water or oxidizing upon contact with air, thus shortening its lifespan. The epoxy resin layer 8 is filled through the holes in the bottom shell 3 between the lower surface of the CMUT device 4 and the bottom shell 3 to protect the back of the CMUT device 4 and to make the encapsulation structure more stable.

[0045] Figure 2 The diagram shown is a three-dimensional cross-sectional view of the packaging structure, which includes: an outer shell 2, a bottom shell 3, a CMUT device 4, a PDMS layer 5, an acoustic metasurface 7, an epoxy resin layer 8, and a water layer 12.

[0046] like Figure 2 As shown, the acoustic metasurface 7 has gaps, and the PDMS liquid fills the gap between the CMUT device 4 and the acoustic metasurface 7 through the gaps in the acoustic metasurface 7.

[0047] Example 1

[0048] Example 1 uses Figure 3 , Figure 4 , Figure 5 This section describes the COMSOL simulation model used for the packaging structure and its simulation focusing effect. It also explains the case where the acoustic metasurface 7 is made of steel.

[0049] This structural model is a simplified simulation, such as Figure 3 As shown, the CMUT device 4, PDMS layer 5, acoustic metasurface 7, and the working area water 12 of the packaging structure are retained. The structural framework is simplified to cover the outside of the device.

[0050] The CMUT device 4 has a radius of 30 mm. Since the sound field excited by the square structure of the CMUT device is not significantly different from that excited by the circular structure in the simulation, the CMUT device 4 is designed as a cylinder with a radius of 42 mm and a thickness of 2 mm. Sound field excitation is performed by emitting ultrasonic waves through the lower end face. The frequency of the excited sound waves is between 900 kHz and 1100 kHz.

[0051] The gap between the acoustic metasurface 7 and the CMUT device 4 is 0.6 mm, and the simulated shape is a cylinder with a thickness of 1.1 mm and a radius of 30 mm. The parameters of the acoustic metasurface 7 are those of a conventional Fresnel zone plate lens, and the middle of the gap is a PDMS layer 5, whose acoustic impedance is comparable to that of the water body 12.

[0052] Figure 4 This is a graph showing the results of measuring the focusing effect of the packaging device in the long axis direction of Example 1. It can be seen that the packaging device can form a focal point at a distance of approximately 20 to 30 mm from the acoustic metasurface 7. Figure 4 Through Figure 3 The optimized structure resulted in a sound field of 1000 kHz excited by CMUT device 4, which is a cylinder with a radius of 42 mm and a thickness of 2 mm. Acoustic metasurface 7 is a traditional Fresnel zone lens with a radius of 30 mm and a thickness of 1.1 mm. The gap between acoustic metasurface 7 and CMUT device 4 is 0.6 mm. This structure results in a narrower focused sidelobe and a higher sound intensity.

[0053] If the acoustic metasurface is removed, and ultrasound is excited solely by CMUT device 4, its intensity diagram is as follows: Figure 5 As shown, there is no obvious focal point in the figure.

[0054] Example 2

[0055] use Figure 6 , Figure 7 , Figure 8 An example illustrating the method for preparing the packaging structure of the present invention.

[0056] Figure 6 This is a flowchart of the process for fabricating the packaging structure. Figure 7 , Figure 8 It means Figure 6 A diagram illustrating the steps of the preparation method. Figure 7 (A) represents the process after the first process (P1) is completed. Figure 7 (B) indicates the process after the second process (P2) is completed. Figure 8 This indicates the process after the third process (P3) is completed, and... Figure 1 The structures are the same.

[0057] The method for preparing the packaging structure of the present invention will be described according to each of the following steps.

[0058] First process (P1): such as Figure 7 As shown in (A), the acoustic metasurface 7 is supported on the upper part of the CMUT device 4 by a metal bracket, forming a whole A. The metal bracket may consist of at least 3 fixing points.

[0059] Second process (P2): such as Figure 7 As shown in (B), first, the integral A formed by the CMUT device 4, the metal bracket 6, and the acoustic metasurface 7 is installed onto the outer housing, so that the wires of the CMUT device pass through the bottom housing 3 of the housing, and the outer housing 2 and the bottom housing 3 are bonded together. Then, the integral A is installed into the designed housing interior to form integral B.

[0060] Third process (P3): such as Figure 8 As shown, epoxy resin is filled between the lower surface of the CMUT device 4 and the bottom housing 3 through the holes in the bottom housing 3 to form an epoxy resin layer 8, thus fixing the entire assembly A within the housing. The assembly A is then placed on a spin coater, and while rotating, PDMS liquid and PDMS curing agent are slowly poured in through the gaps in the acoustic metasurface 7, allowing the PDMS mixture to gradually cover the surface of the CMUT device 4 and finally fill the upper part of the acoustic metasurface. The assembly filled with the PDMS mixture is then placed in a vacuum chamber. The vacuum chamber is set to a specific vacuum level and temperature to remove air bubbles from the PDMS mixture and dry the assembly, preventing air bubbles from remaining within the PDMS layer during curing. The assembly is then removed, and the PDMS mixture slowly cures, forming a PDMS layer that protects the CMUT device, completing the encapsulation process.

[0061] According to Embodiment 2 described above, the CMUT device 4 can be packaged in a housing through a third process.

[0062] The above description, with reference to the accompanying drawings, illustrates the CMUT packaging structure and fabrication method for underwater applications according to the present invention, as well as preferred embodiments of the packaging structure and simulation results obtained using COMSOL simulation software. The purpose is to enable those skilled in the art to understand the content of the present invention and implement it accordingly.

[0063] It should be noted that the directional indicators such as up, down, left, and right used in this invention are not intended to limit the structure of this invention, but are only for the convenience of description.

[0064] The detailed descriptions listed above are merely specific descriptions of feasible embodiments of the present invention, and are not intended to limit the scope of protection of the present invention. All equivalent methods or modifications that do not depart from the technology of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a CMUT package structure with focusing function, characterized in that, The application relates to a CMUT device (4) and an acoustic metasurface (7) which are fixed together and are installed in a shell (2). The first step is to design the shell according to the size of the CMUT device (4) and the acoustic metasurface (7); the second step is to design the gap distance between the acoustic metasurface (7) and the CMUT device (4), connect the acoustic metasurface (7) with the CMUT device (4) by a metal support (6) to form a whole A, install the whole A into an external shell (2), bond a bottom shell (3), and make the lead wire of the CMUT device (4) pass through the lead hole of the bottom shell; the third step is to fill epoxy resin between the back of the CMUT device and the bottom of the shell through the hole of the bottom of the shell, fix the whole A in the shell, place the whole on a spin coater, fill PDMS liquid and PDMS curing agent through the gap of the acoustic metasurface by using a spin coating process, make the PDMS liquid slowly cover the acoustic metasurface from the front of the CMUT device to the upper part of the acoustic metasurface, place the whole in a vacuum box, set the vacuum degree and temperature, make the bubbles in the PDMS mixed liquid discharge, and dry the whole. The shell is designed as two parts of a bottom and an external part or as an integrated part; when the shell is designed as two parts, the whole fixed with the acoustic metasurface (7) and the CMUT device (4) is installed into the external shell (2) first, then the bottom shell (3) is bonded and filled with epoxy resin; when the shell is designed as an integrated part, the acoustic metasurface and the CMUT device are installed at the same time, and the epoxy resin is filled. ​ 2. The method of claim 1, wherein the CMUT package structure having a focusing function is prepared by the steps of: forming a CMUT cell on a substrate; forming a cavity in the substrate; and forming a focusing lens on the substrate. ​

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