CN108793061B - A kind of preparation method of all-electrode relief structure CMUT device - Google Patents

Abstract

The present invention relates to the technical field of electronic science and technology, and more particularly, to a method for preparing a CMUT device with an all-electrode relief structure. An insulating layer, a cavity, a vibrating film, and a ring of relief structure are sequentially prepared on the substrate, and the top electrode, bottom electrode plate and wires of the vibrating film are fully covered to complete the preparation of a CMUT device with an all-electrode relief structure. The all-electrode embossed structure CMUT device prepared by the invention can effectively increase the output sound pressure in the collapse working mode to achieve the effect of increasing the ultrasonic transmission power; the preparation method has mature technology and is suitable for mass production. The invention can be used for the processing and manufacture of ultrasonic transducers in the fields of ultrasonic testing such as biomedical testing and industrial non-destructive testing, and will promote the development and application of ultrasonic probe technology based on CMUT devices, and has broad market application prospects.

Description

A kind of preparation method of all-electrode relief structure CMUT device

technical field

The present invention relates to the technical field of electronic science and technology, and more particularly, to a preparation method of a CMUT device with an all-electrode relief structure.

Background technique

In biomedical imaging, both high image resolution and real-time imaging are required. In addition, the equipment is also required to have low radiation to reduce damage to biological tissues. Ultrasound is a mechanical vibration wave that can propagate in air, liquid, solid and biological tissue. Compared with X-ray, CT and nuclear magnetic detection technology, it has no ionizing radiation and is more suitable for imaging of biological tissue. Ultrasonic detection technology is based on the echo principle, and the imaging speed is fast and the image resolution can reach the order of tens of microns, which is enough to meet the needs of biomedical detection. Therefore, ultrasonic detection technology has become the mainstream technology in the field of biomedical imaging. In addition to being widely used in biomedical imaging, ultrasonic testing technology is also widely used in the industrial non-destructive testing industry.

Ultrasonic transducer is the key component of ultrasonic testing technology, which realizes the mutual conversion between electric energy and ultrasonic energy. Among the ultrasonic transducers currently used, piezoelectric ultrasonic transducers are widely used, and a common piezoelectric material is piezoelectric ceramics. The traditional piezoelectric ultrasonic transducer has high acoustic impedance, which leads to complicated manufacturing process of ultrasonic probe and small bandwidth. In addition, piezoelectric materials cannot be integrated with circuits, and are not suitable for manufacturing high-integration ultrasound probes. Capacitive micromachined ultrasonic transducers (CMUTs) manufactured by microelectromechanical systems (MEMS) technology have low acoustic impedance and can be adjusted, so there is no need to add an impedance matching layer when manufacturing ultrasonic probes, which not only reduces the cost of probe manufacturing. The difficulty also increases the bandwidth. The CMUT device manufacturing process is compatible with the process of CMOS integrated circuits and uses photolithography technology. Therefore, CMUT is suitable for manufacturing high-integration, large-scale array ultrasound probes, which can meet the growing demand for information acquisition in the field of clinical medicine. In view of the above advantages, CMUT has been regarded as the next-generation ultrasonic transducer.

However, compared with the piezoelectric ceramic ultrasonic transducer, the output sound pressure of the existing CMUT is still relatively small. The attenuation coefficient of ultrasonic transmission in human tissue is large, and the amplitude of the signal will be significantly weakened after long-distance transmission. Ultrasonic detection technology is based on the reflected echo received by the ultrasonic probe to achieve target positioning. The small output sound pressure makes the weak echo signal easily interfered by external noise, which reduces the signal-to-noise ratio of the echo signal, thereby affecting the imaging quality. Therefore, improving the output sound pressure of the CMTU is one of the problems to be solved urgently in the current development of the CMUT technology.

At present, there are two ways to improve the output sound pressure of a CMUT. One is to modify the working mode of the CMUT to make it work in the collapsed mode; the other is to modify the structure of the vibrating membrane of the CMUT. The inventor proposes a preparation method of a CUMT device with an all-electrode relief structure that integrates the above two approaches.

SUMMARY OF THE INVENTION

In order to overcome at least one of the above-mentioned defects in the prior art, the present invention provides a method for preparing a CMUT device with an all-electrode relief structure, which realizes the advantages that the CMUT can be integrated with circuits, is easy to manufacture large-scale arrays, and is suitable for mass production. .

For solving the above-mentioned technical problems, the technical scheme of the present invention is as follows:

A preparation method of an all-electrode relief structure CMUT device, the preparation method comprises the following steps:

Step 1: Use a highly doped silicon wafer as a substrate to prepare a base layer;

Step 2: depositing an insulating layer on the substrate in step 1;

Step 3: deposit a polysilicon film on the insulating layer in step 2 to prepare a sacrificial layer, and remove the redundant polysilicon film layer;

Step 4: deposit a vibrating film on the CMUT unit prepared in step 3;

Step 5: further removing the polysilicon film layer retained in step 3 to form a closed cavity;

Step 6: depositing conductive layers for preparing the top and bottom electrode plates of the CMUT unit;

Step 7: Prepare a relief ring on the conductive layer on top of the vibrating membrane;

Step 8: Use the photolithography process to define the conductive layer to prepare the top electrode, the bottom electrode plate and the wires that fully cover the vibrating film, and remove the redundant conductive layer; complete the preparation of the all-electrode relief structure CMUT device.

Preferably, the step 7 specifically includes the following steps:

Step 71: define the size of the relief ring by using a positive photoresist on the conductive layer on top of the vibrating film and combining with a photolithography process;

Step 72: Define the size area of the relief ring at step 71 to prepare the relief ring using an electroplating process.

In the step 71, the height and width of the relief ring are determined by the height and width of the positive photoresist after development, and the height of the relief ring is 0.5-6 μm and the width is 0.5-3 μm; the step 72 The ribbed ring is nickel metal or other high-density metal.

Preferably, in the step 4, the vibrating film is a low residual stress silicon nitride film with a thickness of 0.3-2 μm; and the step 4 further includes a support layer integrally deposited with the vibrating film.

Preferably, the step 5 specifically includes the following steps:

Step 51 : using a dry etching process to etch an etching hole on the etching channel above the remaining polysilicon film layer, and then using a wet etching process to remove the remaining polysilicon film layer through the etching hole to form a cavity;

Step 52: use plasma enhanced chemical vapor deposition (PECVD) to deposit a silicon nitride layer to fill the etched holes in step 51;

Step 53 : use a dry etching process to etch the silicon nitride layer filled with the etched holes in step 52 , and reduce the thickness of the vibrating film to the film thickness of step 4 .

Preferably, the etchant used in the wet etching process in step 51 is potassium hydroxide solution.

Preferably, the insulating layer in the step 2 is a single-layer silicon nitride material deposited by a low pressure chemical vapor deposition (LPCVD) process or a material deposited by a dry oxidation process and a low pressure chemical vapor deposition (LPCVD) process. It is composed of a composite layer of silicon dioxide and silicon nitride.

Preferably, the step 6 specifically includes the following steps:

Step 61: use a dry etching process to remove the cover layer at one end of the substrate to expose the substrate;

Step 62: Deposit a conductive layer on the cell of Step 61 using an E-beam evaporation process.

Preferably, the conductive layer in the step 62 is a composite structure of chromium and gold.

When the all-electrode relief structure CMUT device works in the collapsed mode, the central part of the vibrating membrane collapses under the action of electrostatic force, and is attached to the surface of the insulating layer on the substrate to form a collapsed membrane part; The thin film is the part of the vibrating thin film when the CMUT device works. The relief structure is located above the vibrating membrane portion.

Compared with the prior art, the beneficial effects are:

(1) The present invention uses surface micromachining technology, which is compatible with the existing CMUT preparation process, and realizes the advantages that CMUT can be integrated with circuits, is easy to manufacture large-scale arrays, and is suitable for mass production;

(2) The present invention realizes the preparation of the relief structure CMUT device by using the surface micromachining technology. Nickel or other high-density metal is used as the material of the relief ring; the relief ring of larger density material can effectively reduce the influence of the ring size on the vibrating membrane;

(3) The use of positive photoresist and photolithography technology can accurately control the size of the relief ring; the relief ring is prepared by combining with the electroplating process, and the electroplating process is suitable for the preparation of rings with a large height; while using traditional electron beam evaporation (E-beamevaporation), thermal evaporation (thermal evaporation), magnetron sputtering (Sputtering) and other metal film deposition processes to deposit metal layers with a height of more than 1μm, not only time-consuming, but also very expensive;

(4) Prepare a top electrode structure that fully covers the vibrating membrane. The full coverage of the top electrode ensures the density uniformity of the structure (membrane and top electrode) below the relief structure, and improves the adjustment of the vibration center of the vibrating membrane to the position of the relief structure. accuracy;

(5) Compared with the existing CMUT devices, the CMUT with all-electrode relief structure working in the collapsed mode can significantly improve the effective output sound pressure; this invention solves the problem of low output sound pressure of the existing CMUT, and will promote the The development and application of ultrasonic probe technology of CMUT.

Description of drawings

FIG. 1 is a schematic structural diagram of the present invention in a collapsed working state.

Fig. 2 (a)～(m) is the preparation process flow chart of the present invention.

Detailed ways

The accompanying drawings are for illustrative purposes only, and should not be construed as limitations on this patent; in order to better illustrate the present embodiment, some parts of the accompanying drawings may be omitted, enlarged or reduced, and do not represent the size of the actual product; for those skilled in the art It is understandable to the artisan that certain well-known structures and descriptions thereof may be omitted from the drawings. The positional relationships described in the drawings are only for exemplary illustration, and should not be construed as a limitation on the present patent.

As shown in FIG. 2 (m), the preparation method provided by the present invention to prepare an all-electrode relief structure CMUT device includes a relief structure, a top electrode 61, a vibrating film 1, a cavity 2, an insulating layer 4 and a substrate arranged in sequence. 5, also includes a support layer 3 supporting the cavity 2; a bottom electrode plate 62 is provided at one end of the top surface of the base 5; the support layer 3 is provided with a cavity 2, and the cavity 2 is arranged between the vibrating membrane 1 and the Between the insulating layers 4; a top electrode 61 fully covering the vibrating film 1 is arranged on the top of the vibrating film 1; the top surface of the top electrode 61 is also provided with a relief structure; the relief structure is a relief ring 7. Its center is located on the same vertical line as the center of the vibrating membrane 1 and the cavity 2, and the bottom end of the relief ring 7 is connected to the top surface of the top electrode 61. As shown in FIG. 1, it is a schematic structural diagram of the all-electrode relief structure CMUT device working in the collapsed mode. When working in the collapsed mode, the central part of the vibrating membrane 1 collapses under the action of electrostatic force, and is attached to the The surface of the insulating layer 4 on the substrate becomes the collapsed membrane part 11 ; the vibration membrane 1 of the non-collapsed part is the vibration membrane part 12 when the CMUT device operates. By adjusting the DC bias voltage during operation, the vibration center of the vibrating membrane portion 12 is moved to the position of the relief ring 7, thereby achieving the effect of increasing the output sound pressure.

A preparation method of an all-electrode relief structure CMUT device, specifically comprising the following steps:

Step 1: Use a highly doped silicon wafer as the substrate 5 to prepare the base layer; use a highly doped silicon wafer (conductivity: 0.1-1 ohm-cm, thickness 525 μm) as the substrate 5, the above-mentioned substrate 5 It can also be used as the common bottom electrode of the CMUT unit;

Step 2: In step 1, an insulating layer 4 is deposited on the substrate 5; the insulating layer 4 in step 2 is a single-layer silicon nitride material deposited by a low pressure chemical vapor deposition (LPCVD) process or a dry type It is composed of a composite layer of silicon dioxide and silicon nitride deposited by an oxidation process and a low pressure chemical vapor deposition (LPCVD) process; this embodiment is a composite insulating layer of silicon dioxide and silicon nitride;

Step 3: Use a low pressure chemical vapor deposition (LPCVD) process to deposit a polysilicon thin film layer as a sacrificial layer 81 on the insulating layer 4 prepared in step 2, and remove the excess polysilicon thin film layer; the thickness of the polysilicon thin film layer determines the CMUT device hollow the thickness of cavity 2;

Step 4: Deposit vibrating film 1 on the CMUT unit of step 3; use a low pressure chemical vapor deposition (LPCVD) process to deposit a low residual stress silicon nitride film layer on the unit of step 3, vibrate in step 4 The thickness of the film 1 is 0.3-2 μm; step 4 further includes a support layer 3 integrally deposited with the vibration film 1 ;

Step 5: further remove the polysilicon thin film layer 8 retained in step 3 to form a closed cavity 2;

Step 6: depositing the conductive layer 6 for preparing the top electrode 61 and bottom electrode plate 62 of the CMUT unit;

Step 7: prepare a relief ring 7 on the conductive layer 6 on top of the vibrating membrane 1;

Step 8: Use the photolithography process to define the conductive layer 6 to prepare the top electrode 61, the bottom electrode plate 62 and the wires fully covering the vibration film 1, and remove the redundant conductive layer; complete the preparation of the all-electrode relief structure CMUT device. The bottom electrode can be realized by a high-concentration doped substrate 5, or a metal layer can be used at one end of the insulating layer 4 close to the substrate layer 5. In this embodiment, the bottom electrode plate 62 is a gold and metal layer covering one end of the substrate 5. Conductive layer 6 of chromium composite metal.

Further, the described step 5 specifically includes the following steps:

Step 51: Use a dry etching process to etch the etching hole 9 on the etching channel above the remaining polysilicon film layer 8, and then use a wet etching process to remove the remaining polysilicon film layer through the etching hole 9 with a potassium hydroxide solution 8. Form cavity 2;

Step 52 : depositing a silicon nitride layer using plasma enhanced chemical vapor deposition (PECVD) to fill the etched hole 9 in step 51 ;

Step 53 : use a dry etching process to etch the silicon nitride layer after filling the etching hole 9 in step 52 , and reduce the thickness of the vibration film to the film thickness of step 4 .

Further, the step 6 specifically includes the following steps:

Step 61: Using a dry etching process, remove the cover layer at one end of the substrate 5 to expose the substrate 5;

Step 62 : deposit the conductive layer 6 of the chromium, gold composite structure on the unit of step 61 using an E-beam evaporation process.

Further, the step 7 specifically includes the following steps:

Step 71: Use a positive photoresist on the conductive layer 6 on the top of the vibrating film 1 and combine the photolithography process to define the size of the relief ring 7; the height and width of the relief ring 7 are determined by the positive photoresist. The height and width after development are determined, and the height of the relief ring 7 is 0.5-6 μm and the width is 0.5-3 μm;

Step 72 : Define the size area of the relief ring 7 in step 71 and use an electroplating process to prepare the relief ring 7 , the relief ring 7 is metal nickel or other high-density metal, and this embodiment is metal nickel.

In this embodiment, as shown in Figure 2 (a) ~ (m), the specific implementation steps are as follows:

Step 1: As shown in Figure 2 (a), use a highly doped silicon wafer (conductivity: 0.1-1 ohm-cm, thickness 525 μm) as the substrate 5;

Step 2: After cleaning the silicon wafer substrate 5 in step 1, put it into an oxidation furnace tube, and use a dry oxidation process to deposit silicon dioxide 41 with a thickness of 150 nm on the silicon wafer substrate; The pressure chemical vapor deposition (LPCVD) process deposits 150 nm silicon nitride 42 on the above-mentioned silicon dioxide 41, as shown in Figure 2 (b). So far, a composite of silicon dioxide 41 and silicon nitride 42 has been prepared. insulating layer 4;

Step 3: First use a low pressure chemical vapor deposition (LPCVD) process to deposit a 300 nm polysilicon thin film layer as a sacrificial layer 81 on the insulating layer 4 prepared in step 2. The thickness of the polysilicon thin film layer determines the thickness of the cavity 2 of the CMUT device. As shown in Fig. 2 (c); the shape of each CMUT unit, including cavity 2 and etching channels, etc., is then defined using a photolithography process. In this embodiment, the CMUT unit is circular, the radius is 20 μm, the width of the etching channel is 2 μm, and the length is 12 μm. Then use dry etching technology to etch away the polysilicon film layer outside the CMUT unit, and the remaining area is the outline of the CMUT unit. Cavity 2, after this step, the residual photoresist needs to be removed;

Step 4: Use a low pressure chemical vapor deposition (LPCVD) process to deposit a low residual stress silicon nitride thin film layer on the CMUT unit of step 3. The part of the deposited silicon nitride film layer on the top surface of the remaining polysilicon film layer 8 is used as the vibration film 1 of the CMUT unit, and the parts on both sides of the remaining polysilicon film layer 8 are used as the supporting layer 3 of the device for supporting the The cavity 2; the support layer 3 and the vibration film 1 are integrally deposited, as shown in Figure 2 (e). The thickness of the vibration film 1 is 0.3-2 μm, and the thickness of the vibration film 1 in this embodiment is 700 nm;

Step 5: Specifically include the following steps:

Step 51: First, use the photolithography technology to define the etching hole area on the etching channel covered by the silicon nitride in step 4 of the CMUT unit, and the diameter of the etching hole 9 is 2 μm; The silicon nitride layer in the area of the hole 9 is etched away, so that the etched hole 9 leads directly to the remaining polysilicon thin film layer 8, as shown in FIG. 2(f). After this process is completed, the residual photoresist needs to be removed;

Step 52: Immerse the whole silicon wafer after the above-mentioned etching and etching holes in the potassium hydroxide solution, perform wet etching at room temperature to retain the polysilicon thin film layer 8, and the polysilicon is completely removed from the etching holes 9 through the etching channel , so that a cavity 2 is formed under the vibration film 1 of silicon nitride, and the height of the cavity 2 is determined by the height of the polysilicon film layer 8 remaining, which is 300 nm in this embodiment, as shown in Fig. 2(g) The etched holes were filled with silicon nitride with a thickness of 1.2 μm deposited by plasma-enhanced chemical vapor deposition (PECVD), thereby forming a closed cavity 2, as shown in Fig. 2(h); because the plasma The gas pressure in the enhanced chemical vapor deposition (PECVD) furnace tube is very low, and it can be considered that the cavity 2 is vacuum closed;

Step 53: After filling the etching hole 9 in step 52, the silicon nitride vibration film of the CMUT device is thickened, so the thickness of the film needs to be reduced to 700 nm. The 1.2 μm silicon nitride layer deposited on the thin film in step 52 is removed by a dry etching process, and the thinned CMUT unit is shown in FIG. 2(i). After completing this step process, the residual photoresist needs to be removed;

Step 6: Specifically include the following steps:

Step 61: After the area of the bottom electrode plate 62 of the CMUT device is defined by photolithography, the insulating layer 4 and the silicon nitride layer above the area of the bottom electrode plate next to the array element of the CMUT device are then separated by a dry etching process. Removing, exposing the substrate 5, that is, the silicon nitride 41, silicon dioxide 42 of the insulating layer 4, and the silicon nitride deposited in step 4 are all etched away, exposing the substrate 5 doped with silicon. After this process, the exposure of the highly doped silicon substrate in the bottom electrode pad region is achieved, as shown in Figure 2(j). After this step, the residual photoresist needs to be removed;

Step 62: Using an E-beam evaporation process, deposit 20 nm of chromium and 180 nm of gold on the silicon wafer including the CMUT unit after step 61 to form a composite metal layer as a conductive layer 6, as shown in the figure 2(k). Chromium serves as the adhesion layer between gold and silicon nitride, the silicon substrate, and gold serves as the conductive layer 6 . This conductive layer 6 is used to prepare the top electrode 61 and the bottom electrode plate 62 of the CMUT unit;

Step 7: Specifically include the following steps:

Step 71 : Define relief rings 7 on the conductive layer 6 in Step 62 using positive photoresist and a photolithography process. First, the photo-etching technology is used to define the area of the relief ring 7 on the conductive layer 6 by using a positive photoresist. Each CMUT unit has a concentric relief ring 7; after the exposure and development steps, the relief ring 7 The conductive layer 6 of the area is exposed, and the remaining areas are covered by photoresist. The width of the ring is 2 μm, the inner diameter is 12 μm, the outer diameter is 14 μm, and the thickness of the photoresist is 2 μm. The width of the relief ring 7 is determined by the width of the positive photoresist area after development, and the maximum height of the relief ring 7 is determined by the thickness of the photoresist, that is, the width of the relief ring 7 in this embodiment is 2 μm , the inner diameter is 12 μm, and the outer diameter is 14 μm;

Step 72: Electroplating using the Watts Nickel (Nickel Sulfate, Nickel Chloride, and Boric Acid) method. The electroplating anode is a pure nickel plate, and the cathode is the CMUT unit to be plated with nickel rings. By choosing the appropriate plating conditions (concentration, current, temperature and time, etc.), accurate control of the height of the nickel ring can be achieved. In this example, Watt's nickel electroplating formula (nickel sulfate, nickel chloride and boric acid is used, and the reference ratio is NiSO ₄ .6-7H ₂ O: NiCl ₂ .6H ₂ O: H ₃ BO ₃ : H ₂ O=676: 114 : 96: 2400 (g)), the solution temperature was set at 50 °C, the DC current was 80 mA, and the plating time was 50 s. The thickness of the electroplated nickel is 2 μm, so the height of the nickel ring is 2 μm, as shown in Figure 2(l). After the electroplating is completed, the photoresist on the silicon wafer is removed with acetone, that is, the processing flow of the relief ring 7 is completed. The conductive layer 6 of the composite metal in step 62 is used as the base surface of electroplating nickel to improve the electroplating bonding force of the relief ring 7;

Step 8: Use the photo-etching process to define the electrodes of the CMUT device (fully covering the top electrode, the bottom electrode plate) and the wires between the CMUT cells. The above-mentioned top electrode 61 area is a full-electrode structure that fully covers the vibrating film 1. The bottom electrode The polar plate 62 is arranged at one end of the substrate 5; the silicon wafer containing the CMUT unit is immersed in the gold etchant, and the gold in the non-electrode and wire areas is removed; then the silicon wafer is immersed in the chromium etchant, and the non-electrode and wire are immersed in the chrome etchant. The chrome removal of the area, as shown in Figure 2(m), after this step is completed, the residual photoresist needs to be removed. The top electrode 61 of the composite metal covers the entire area of the vibrating membrane 1 to form an all-electrode structure. Using the E-beam evaporation process in step 62, the conductive layer 6 with uniform density can be prepared to obtain a top electrode 61 with uniform density distribution on the vibrating membrane 1, and the top electrode 61 with uniform density can improve the vibrating membrane. Vibration stability, it is also easy to move the vibration center of the vibrating membrane part to the relief ring by adjusting the DC bias voltage;

For the all-electrode relief structure CMUT device completed in the above steps 1-8, a wafer dicing machine is used to divide and scribe the CMUT array on the silicon wafer. The cut CMUT array is then mounted on a printed circuit board (PCB) using epoxy. The common top electrode connected to the top electrode 61 and the bottom electrode plate 62 are connected to the corresponding electrodes on the PCB board through a wire bonding machine with aluminum wires. After completing the wire connections for all electrodes, epoxy the connected aluminum wires to protect them.

When the CMUT device operates in collapse mode, the central portion of the film is attached to the base insulating layer. In a specific voltage range, the relief structure is located near the vibration center of the non-collapsed annular membrane, which can increase the output sound pressure. The finite element simulation shows that the ridge structure can increase the maximum output sound pressure of the CMUT in collapse mode by about 88.1% ^[1] ([1] Yu, Y., Pun, SH, Mak, PU, Cheng, CH, Wang, J., Mak, PI, andVai, M. I, Design of a Collapse-Mode CMUT with an Embossed Membrane forImproving Output Pressure, IEEE Transactions on Ultrasonics, Ferroelectrics,and Frequency Control, 2016, 63(6):854- 863).

Obviously, the above-mentioned embodiments of the present invention are only examples for clearly illustrating the present invention, and are not intended to limit the embodiments of the present invention. For those of ordinary skill in the art, changes or modifications in other different forms can also be made on the basis of the above description. There is no need and cannot be exhaustive of all implementations here. Any modifications, equivalent replacements and improvements made within the spirit and principle of the present invention shall be included within the protection scope of the claims of the present invention.

Claims (9)

1. a preparation method of all-electrode relief structure CMUT device, is characterized in that, described preparation method comprises the following steps: Step 1: using a highly doped silicon wafer as a substrate (5) to prepare a base layer; Step 2: depositing an insulating layer (4) on the substrate (5) in step 1; Step 3: depositing a polysilicon film on the insulating layer (4) in step 2 to prepare a sacrificial layer (81), and removing the redundant polysilicon film layer to define a CMUT unit; Step 4: depositing a vibrating film (1) on the CMUT unit prepared in step 3; Step 5: removing the polysilicon thin film layer (8) retained in step 3 to form a closed cavity (2); Step 6: depositing a conductive layer (6) for preparing the top electrode (61) and bottom electrode plate (62) of the CMUT unit; Step 7: preparing a relief ring (7) on the conductive layer (6) on top of the vibrating membrane (1); Step 8: using a photolithography process to define the conductive layer (6), preparing the top electrode (61), the bottom electrode plate (62) and the wires fully covering the vibrating membrane (1), and removing the redundant conductive layer (6); The preparation of the all-electrode relief structure CMUT device is completed; the step 5 specifically includes the following steps: Step 51: Use a dry etching process to etch an etching hole (9) on the etching channel on the top of the remaining polysilicon film layer (8), and then use a wet etching process to remove the remaining polysilicon film through the etching hole (9). layer (8), forming a closed cavity (2); Step 52: use plasma enhanced chemical vapor deposition (PECVD) to deposit a silicon nitride layer to fill the etched hole (9) in step 51; Step 53: Using a dry etching process, the silicon nitride layer after filling the etching hole (9) in step 52 is etched, and the thickness of the vibrating film is reduced by the thickness of the step 4. 2. the preparation method of a kind of all-electrode relief structure CMUT device according to claim 1, is characterized in that, described step 7 specifically comprises the following steps: Step 71: use positive photoresist on the conductive layer (6) on top of the vibrating film (1) and combine the photolithography process to define the size of the relief ring (7); Step 72: Define the size area of the relief ring (7) at step 71 to prepare the relief ring (7) using an electroplating process. 3 . The method for preparing a CMUT device with an all-electrode relief structure according to claim 2 , wherein the height and width of the relief ring ( 7 ) in the step 71 are determined by the positive photoresist after developing. 4 . The height and width of the relief ring (7) are determined by the height and width of 0.5-6 μm, and the width is 0.5-3 μm; in the step 72, the relief ring (7) is metal nickel. 4 . The method for preparing a CMUT device with an all-electrode relief structure according to claim 1 , wherein in the step 4, the vibrating film (1) is a low residual stress silicon nitride film with a thickness of 0.3-2 μm. 5 . ; Step 4 includes a supporting layer (3) deposited integrally with the vibrating membrane (1). 5 . The method for preparing a CMUT device with an all-electrode relief structure according to claim 4 , wherein the etchant in the wet etching process in the step 51 is potassium hydroxide solution. 6 . 6. The method for preparing an all-electrode relief structure CMUT device according to claim 1, wherein the insulating layer (4) in the step 2 is deposited by a low pressure chemical vapor deposition (LPCVD) process. A single-layer silicon nitride material or a composite layer of silicon dioxide and silicon nitride deposited by dry oxidation and low pressure chemical vapor deposition (LPCVD) processes. 7. the preparation method of a kind of all-electrode relief structure CMUT device according to claim 1, is characterized in that, described step 6 specifically comprises the following steps: Step 61: use a dry etching process to remove the cover layer at one end of the substrate (5) to expose the substrate (5); Step 62: Deposit a conductive layer (6) on the CMUT cell prepared in Step 61 using an E-beam evaporation process. 8 . The method for preparing a CMUT device with an all-electrode relief structure according to claim 7 , wherein the conductive layer ( 6 ) in the step 62 is a composite structure of chromium and gold. 9 . 9 . The method for preparing an all-electrode relief structure CMUT device according to claim 1 , wherein when the all-electrode relief structure CMUT device operates in a collapsed mode, the vibration film (1) has a The central part collapses under the action of electrostatic force, and is attached to the surface of the insulating layer (4) on the substrate (5) to form a collapsed membrane part (11); the membrane of the uncollapsed part is the vibrating membrane part (12) when the CMUT device works ), the relief ring (7) is located in the vibrating membrane part (12).

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