CN114955980A - Flexible heart sound sensor based on PDMS-silicon nano-film and preparation method thereof - Google Patents

Abstract

The invention discloses a PDMS-silicon nano-film-based flexible heart sound sensor, and belongs to the technical field of semiconductors. The sensor consists of a silicon nano-film and a flexible substrate PDMS, wherein the silicon nano-film comprises a piezoresistive region, an ohmic contact region and an alloy region, and the alloy region is made of an alloy of chromium and gold. During preparation, an oxide layer is formed above an SOI sheet, an ohmic contact region is formed after diffusion of concentrated boron, a piezoresistive region is formed after etching of top silicon, an alloy region is formed after sputtering, metal corrosion and annealing, an eave structure is formed after etching and corrosion of an oxygen burying layer, a suspended silicon nano-film structure is formed after underetching of the oxygen burying layer, and finally a silicon nano-film is transferred to a flexible substrate PDMS. The sensor has high sensitivity, and meanwhile, due to the flexible substrate PDMS, the adaptability and the convenience of the sensor are improved.

Description

Flexible heart sound sensor based on PDMS-silicon nano-film and preparation method thereof

Technical Field

The invention belongs to the technical field of semiconductors, and particularly relates to a flexible heart sound sensor based on a PDMS-silicon nano film and a preparation method thereof.

Background

Heart sounds are a very important physiological signal of the human body, which is a reflection of the mechanical movement and the complex of the heart and the cardiovascular system. The sensor prepared based on the piezoresistive effect is widely applied to the pressure measurement fields of power machinery, biomedicine, aerospace and the like due to simple structure, good low-frequency characteristic and stable performance. Since the heart sound signal is weak, a sensor with higher sensitivity is required for detection. The sensitivity of the device based on piezoresistive effect is not very high due to the limited variation of the mobility of the multiple molecules, and the sensor prepared based on the silicon nano-film has huge piezoresistive effect and relies on the change of surface potential caused by strain so as to cause the variation of the concentration of the multiple molecules, so that the sensitivity of the device can be finally improved. Meanwhile, the most of the existing heart sound sensors are rigid, the adaptability is limited to a certain extent, and the devices are large in size and not portable enough.

Disclosure of Invention

The invention aims to solve the problems in the prior art, and provides a PDMS-silicon nano-film-based flexible heart sound sensor, which is manufactured by transferring top silicon of an SOI (silicon on insulator) sheet onto PDMS through a micro-nano processing technology.

The invention is realized by the following technical scheme:

a flexible heart sound sensor based on a PDMS-silicon nano film comprises the silicon nano film and a flexible substrate PDMS, wherein the silicon nano film is arranged at the center of the top surface of the flexible substrate PDMS; the silicon nano-film is of a rectangular sheet structure, the middle position of the silicon nano-film is a piezoresistive region, the positions on two sides of the silicon nano-film are ohmic contact regions respectively, and the middle position of the top surface of the ohmic contact region is provided with a blocky alloy region; the piezoresistive region and the ohmic contact region are top silicon parts of the SOI sheet, the ohmic contact region is subjected to concentrated boron diffusion, and the alloy region is made of an alloy of chromium and gold.

Further, the invention also provides a preparation method of the flexible heart sound sensor based on the PDMS-silicon nano film, which comprises the following steps: forming an oxide layer above the SOI sheet by PECVD (plasma enhanced chemical vapor deposition), forming an ohmic contact region after diffusion of concentrated boron, forming a piezoresistive region after etching top silicon, sputtering, corroding metal and annealing to form an alloy, forming an eave structure after etching and corroding the oxygen burying layer, forming a suspended silicon nano-film structure after underetching the oxygen burying layer, and finally transferring the silicon nano-film to the flexible substrate PDMS. The preparation process comprises the following steps:

1) carrying out standard cleaning on the SOI wafer, flushing with deionized water, and drying with nitrogen to ensure the cleanness of the wafer;

2) generating a silicon oxide layer on the surface of the top silicon of the SOI wafer;

3) setting a first layer of mask, covering and protecting the photoresist pattern, and etching the ohmic contact region;

4) heavily doping, and performing concentrated boron diffusion on the ohmic contact region;

5) corroding a silicon oxide layer and impurities formed on the surface of the top silicon layer after the heavy doping process;

6) setting a second layer of mask, covering and protecting the photoresist pattern, and etching the piezoresistive region;

7) cleaning an SOI (silicon on insulator) sheet, carrying out magnetron sputtering on chromium and gold, setting a third layer of mask, covering and protecting a photoresist pattern, and photoetching and retaining metal positioned in an alloy area;

8) corroding the metal in other areas by a wet method, and after removing the photoresist, annealing the SOI sheet to form an alloy of chromium and gold in the alloy area;

9) setting a fourth layer of mask, covering and protecting the photoresist pattern, and etching the buried oxide layer except the piezoresistance region and the ohmic contact region;

10) wet etching the oxygen burying layer right below the piezoresistive region and the ohmic contact region to form an arc concave surface on the side wall of the partial oxygen burying layer, and forming an eave structure among the ohmic contact region, the arc concave surface of the oxygen burying layer and the bottom layer silicon;

11) after spin-coating photoresist, fully exposing, and reserving the photoresist part in the eave structure;

12) etching the buried oxide layer, and completely etching off the residual buried oxide layer by using wet etching solution to form a suspended silicon nano film;

13) preparing PDMS, and then transferring the silicon nano film to a flexible substrate PDMS;

14) and printing electrodes on two sides of the silicon nano-film.

The invention adopts a series of processes such as photoetching, etching, corrosion and the like to prepare the silicon nano film and transfers the silicon nano film to the PDMS (polydimethylsiloxane) of the flexible substrate to form a device, the sensitivity of the heart sound sensor is greatly improved due to the giant piezoresistive effect of the silicon nano film, and meanwhile, the existence of the flexible substrate enables the heart sound sensor to have better adaptability and portability.

Preferably, in the step 1), the crystal orientation of the P-type SOI wafer is <100>, and the P-type SOI wafer sequentially comprises bottom silicon with the thickness of 700 microns, a buried oxide layer with the thickness of 3 microns and top silicon with the thickness of 340nm from bottom to top, wherein the resistivity of the top silicon is 1-20 omega cm.

Preferably, in the step 2), the SOI sheet is placed into a plasma enhanced chemical vapor deposition apparatus for deposition of a silicon oxide layer, and the thickness of the silicon oxide layer is 1 μm.

Preferably, in the step 3), when the ohmic contact region is etched, the SOI wafer is placed into a TATUNG vacuum oven to deposit an HMDS adhesive, the HMDS adhesive is continuously coated for 1800s at 130 ℃, the positive photoresist AZ6130 is uniformly and spin-coated, the rotating speed is set to 3000rp/min, the photoresist pattern covers the region except the ohmic contact region, the ohmic contact region is photoetched after pre-baking, exposure, development, base film removal and hardening, then the SOI wafer is placed into an RIE-10NR etching machine, silicon oxide of the ohmic contact region is dry-etched, and the etching depth of the ohmic contact region is 1 μm.

Preferably, in the step 4), during heavy doping, the SOI wafer is placed into HQ100A of Qingdao flag of a diffusion furnace, and the ohmic contact region is subjected to dense boron diffusion, the temperature is kept at 1000 ℃ and the time is 10 min.

Preferably, in the step 5), when the impurities are etched, impurities such as a silicon oxide layer and borosilicate glass formed on the surface of the top silicon layer are wet-etched at 40 ℃ using a buffered oxide etching solution.

Preferably, in step 6), when the piezoresistive region is etched, a positive photoresist is uniformly spin-coated on the SOI wafer, a photoresist pattern covers the piezoresistive region, the piezoresistive region is pre-baked, exposed, developed and hardened, the piezoresistive region is reserved, top silicon is etched by Reactive Ion Etching (RIE), and the depth of the etched piezoresistive region is 340 nm.

Preferably, in the step 7), when sputtering metal, the SOI wafer is placed into a magnetron sputtering coating machine, chromium with the thickness of 30nm is sputtered firstly, gold with the thickness of 300nm is sputtered secondly, then photoresist is coated in a spinning mode, the photoresist pattern covers the alloy area, and the alloy area is baked, exposed, developed and hardened to be photoetched.

Preferably, in step 8), the metal outside the alloy area is subjected to wet etching by using 22% (NH) ₄ ) ₂ Ce(NO ₃ ) ₆ +8% HAC + 70% H ₂ Corroding chromium metal with corrosive solution of O by using 5% of I ₂ + 10% KI +85% H ₂ Corroding the metal gold by the corrosive liquid of O; during annealing treatment, the SOI wafer is placed into a vacuum annealing furnace, and the annealing temperature is set to 380 ℃ for 30 min.

Preferably, in step 9), the photoresist pattern covers the region except the ohmic contact region and the piezoresistive region, the SOI wafer is placed into an RIE etching machine, the etching rate is 20nm/min, the etching time is 150min, and the depth of the etched buried oxide layer is 3 μm.

Preferably, in step 10), the buried oxide layer right below the piezoresistive region and the ohmic contact region is wet-etched by using an HF etching solution, and the lateral depth of the etching is 3 μm, that is, the depth of the arc-shaped concave surface is 3 μm.

Preferably, in step 11), the exposure dose is 20mJ/cm when the spin-on photoresist is subjected to full exposure ² And developing the hardened film to retain the photoresist under the eave structure.

Preferably, in step 12), the wet etching solution is an HF etching solution.

Preferably, in step 13), the silicon nanomembrane is transferred to the flexible substrate PDMS by two transfers.

Preferably, in step 14), electrodes are printed with silver ink on both sides of the silicon nanomembrane.

Preferably, the specific process of step 13) is as follows:

the first step is as follows: organically cleaning a new silicon wafer, then washing the silicon wafer clean by using deionized water, and drying the silicon wafer by using a nitrogen gun to ensure the cleanness of the surface of the wafer;

the second step: putting the clean silicon wafer into a Parylene deposition machine, and plating a Parylene film on the silicon wafer, wherein the thickness of the Parylene film is 2 mu m; the purpose of plating the Parylene film on the silicon chip is that the PDMS can be more easily stripped from the silicon chip when the PDMS is prepared;

the third step: preparing PDMS with different viscosities and thicknesses, mixing Sylgard184A (PDMS) and a curing agent according to a mass ratio of 10:1, uniformly pouring the mixed solution on a silicon wafer plated with a Parylene film after air bubbles are pumped, uniformly coating the mixed solution to a thickness of 300 micrometers, then heating the mixed solution on a heating table at 75 ℃ for 3 hours to prepare PDMS with a thickness of 300 micrometers on the silicon wafer, removing the PDMS with a thickness of 300 micrometers from the silicon wafer, and cutting the PDMS into square PDMS with a size of 2cm multiplied by 2cm for later use; mixing Sylgard184A (PDMS) and a curing agent according to a mass ratio of 5:1, uniformly pouring the mixed solution on a silicon wafer plated with a Parylene film after air bubbles are pumped, homogenizing the mixed solution to a thickness of 400 microns, then heating the mixed solution on a heating table at 75 ℃ for 3 hours to prepare PDMS with a thickness of 400 microns on the silicon wafer, removing the PDMS with the thickness of 400 microns from the silicon wafer, and cutting the PDMS into long PDMS strips with a thickness of 1cm multiplied by 5cm for later use;

the fourth step: transferring the silicon nano film, namely putting the SOI sheet loaded with the silicon nano film on a flat table, covering the SOI sheet with a long PDMS strip, and removing the SOI sheet after pressing with a thumb to finish the primary transfer of the silicon nano film; placing the strip PDMS with the surface carrying the silicon nano film facing upwards and on a silicon chip together with the square PDMS, and placing the strip PDMS and the square PDMS in a plasma degumming machine for oxygen plasma treatment; and then placing the strip PDMS carrying the silicon nano film at a horizontal position, attaching a square PDMS on the strip PDMS, and pressing with a thumb after ensuring no gap to complete the second transfer printing of the silicon nano film, wherein the square PDMS of the second transfer printing is the flexible substrate PDMS.

Preferably, the specific process in step 14) is as follows:

the first step is as follows: placing the flexible substrate PDMS whole body loaded with the silicon nano-film on a workbench of an ink-jet printer, moving the needle point absorbed with silver ink to one side of the silicon nano-film, starting printing when the needle point discharges the ink, performing multiple times of printing, and printing the electrode on the other side after printing the electrode on one side;

the second step is that: after the electrode is printed, the flexible substrate PDMS whole body loaded with the silicon nano-film is immediately placed on a baking table with the temperature of 120 ℃ to be heated for 1h, so that the silver ink is cured;

the third step: and placing the two leads on the silver electrodes on the two sides of the silicon nano film, coating conductive silver paste, heating and curing for 20min, and then packaging and forming the silicon nano film.

The invention discloses a flexible heart sound sensor based on a PDMS-silicon nano film and a preparation method thereof, wherein the silicon nano film with a certain size is successfully transferred to a flexible substrate PDMS through a series of micro-nano processing technologies. Compared with the prior art, the invention has the beneficial effects that: the flexible heart sound sensor with the silicon nano-film as the piezoresistance is formed, the sensitivity of the heart sound sensor can be improved due to the giant piezoresistance effect of the silicon nano-film, and meanwhile, the adaptability and the convenience of the sensor are improved as the PDMS is the flexible substrate.

Drawings

The accompanying drawings, which are incorporated in and constitute a part of this application, illustrate exemplary embodiments of the invention and, together with the description, serve to explain the invention and not to limit the invention.

FIG. 1 is a front view of the structure of the sensor of the present invention.

Fig. 2 is a top view of the structure of the sensor of the present invention.

FIG. 3 is a flow chart of a method for manufacturing a sensor according to the present invention.

FIG. 4 is a schematic diagram of the structure of a device formed at various stages during the fabrication of a sensor according to the present invention.

In the figure: 1-flexible substrate PDMS, 2-piezoresistive region, 3-ohm contact region and 4-alloy region;

a-bottom silicon, B-buried oxide layer, C-top silicon, D-silicon oxide layer and E-eave structure.

Detailed Description

In order that those skilled in the art will better understand the present invention, a more complete and complete description of the present invention is provided below in conjunction with the accompanying drawings and embodiments. It should be noted that the embodiments and features of the embodiments in the present application may be combined with each other without conflict.

The embodiment provides a flexible heart sound sensor based on a PDMS-silicon nano-film, as shown in fig. 1 and fig. 2, which includes a silicon nano-film and a flexible substrate PDMS 1, wherein the silicon nano-film is disposed at a central position of a top surface of the flexible substrate PDMS 1; the silicon nano-film is of a rectangular sheet structure, the middle position of the silicon nano-film is a piezoresistive region 2, the positions on two sides of the silicon nano-film are ohmic contact regions 3 respectively, and the middle position of the top surface of each ohmic contact region 3 is provided with a blocky alloy region 4; the piezoresistive region 2 and the ohmic contact region 3 are top silicon parts of the SOI sheet, the ohmic contact region 3 is subjected to concentrated boron diffusion, and the alloy region 4 is made of an alloy of chromium and gold.

The preparation method of the flexible heart sound sensor based on the PDMS-silicon nano film has the preparation flow shown in the block diagram of FIG. 3, and specifically comprises the following steps:

1) carrying out standard cleaning on the SOI wafer, flushing with deionized water, and drying with nitrogen to ensure the cleanness of the wafer;

the SOI sheet adopts a P-type SOI sheet with the crystal orientation of <100>, and comprises a bottom layer silicon A with the thickness of 700 mu m, a buried oxide layer B with the thickness of 3 mu m and a top layer silicon C with the thickness of 340nm from bottom to top in sequence, wherein the resistivity of the top layer silicon C is 1-20 omega cm.

2) Generating a silicon oxide layer D on the surface of the top silicon C of the SOI wafer, as shown in a in FIG. 4;

and putting the SOI wafer into a plasma enhanced chemical vapor deposition instrument to deposit a silicon oxide layer D, wherein the thickness of the silicon oxide layer D is 1 mu m.

3) Setting a first layer of mask, covering and protecting the photoresist pattern, and etching the ohmic contact region 3;

when the ohmic contact region 3 is etched, the SOI wafer is placed in a TATUNG vacuum oven to deposit HMDS adhesive, the temperature is kept for 1800s at 130 ℃, positive photoresist AZ6130 is uniformly and spin-coated, the rotating speed is set to be 3000rp/min, the photoresist pattern covers the region except the ohmic contact region 3, the ohmic contact region 3 is photoetched after pre-baking, exposure, development, bottom film removal and film hardening, then the SOI wafer is placed in an RIE-10NR etching machine, silicon oxide of the ohmic contact region 3 is dry-etched, and the etching depth of the ohmic contact region 3 is 1 mu m.

4) Heavily doping, and performing concentrated boron diffusion on the ohmic contact region 3;

when heavily doped, the SOI wafer is placed into HQ100A in Qingdao island of diffusion furnace, and the ohmic contact region 3 is subjected to dense boron diffusion, the temperature is kept at 1000 ℃, and the time is 10 min.

5) Etching off the silicon oxide layer D and impurities formed on the surface of the top silicon C after the heavy doping process, as shown in b in FIG. 4;

when the impurities are corroded, the buffering oxide etching solution is used for corroding the silicon oxide layer D and the borosilicate glass impurities formed on the surface of the top silicon C by a wet method at the temperature of 40 ℃.

6) Setting a second layer of mask, covering and protecting the photoresist pattern, and etching the piezoresistive region 2, as shown in c in fig. 4;

when the piezoresistive region 2 is etched, positive photoresist is uniformly coated on the SOI wafer in a spin mode, the photoresist pattern covers the piezoresistive region 2 region, the piezoresistive region 2 region is subjected to pre-baking, exposure, development and hardening, the piezoresistive region 2 region is reserved, the top silicon C is etched by adopting Reactive Ion Etching (RIE), and the depth of the etched piezoresistive region 2 is 340 nm.

7) Cleaning the SOI sheet, carrying out magnetron sputtering on chromium and gold, setting a third layer of mask, covering and protecting a photoresist pattern, and photoetching and retaining the metal positioned in the alloy area 4;

when sputtering metal, the SOI wafer is put into a magnetron sputtering film plating machine, firstly, chromium with the thickness of 30nm is sputtered, then gold with the thickness of 300nm is sputtered, then, photoresist is coated in a spinning mode, a photoresist pattern covers the area of the alloy area 4, and the area of the alloy area 4 is subjected to prebaking, exposure, development and hardening to be photoetched.

8) Etching off the metal in other areas by a wet method, and after photoresist is removed, carrying out annealing treatment on the SOI sheet to enable the alloy area 4 to form an alloy of chromium and gold, as shown in d in figure 4;

wet etching the metal outside the alloy region 4 by 22% (NH) ₄ ) ₂ Ce(NO ₃ ) ₆ +8% HAC + 70% H ₂ The corrosion solution of O is used for corroding chromium metal, and 5 percent of I is adopted ₂ + 10% KI +85% H ₂ Corroding the metal gold by using the corrosive liquid of O; during annealing treatment, the SOI wafer is placed into a vacuum annealing furnace, and the annealing temperature is set to 380 ℃ for 30 min.

9) Setting a fourth layer of mask, covering and protecting photoresist patterns, and etching the buried oxide layer B except the part right below the piezoresistive region 2 and the ohmic contact region 3;

and covering the region except the ohmic contact region 3 and the piezoresistive region 2 with a photoresist pattern, placing the SOI wafer into an RIE etching machine, wherein the etching rate is 20nm/min, the etching time is 150min, and the depth of the etched buried oxide layer B is 3 mu m.

10) Wet etching the oxygen burying layer B right below the piezoresistive region 2 and the ohmic contact region 3 to form an arc concave surface on the side wall of the partial oxygen burying layer B, and forming an eave structure E among the ohmic contact region 3, the arc concave surface of the oxygen burying layer B and the bottom layer silicon A, as shown in E in FIG. 4;

and (3) corroding the buried oxide layer B right below the piezoresistive region 2 and the ohmic contact region 3 by adopting a HF corrosive liquid wet method, wherein the transverse depth of corrosion is 3 microns, namely the depth of the arc concave surface is 3 microns.

11) Fully exposing after spin-coating the photoresist, and reserving the photoresist part in the eave structure;

when the spin-coating photoresist is subjected to full exposure, the exposure dose is 20mJ/cm ² And developing the hardened film to retain the photoresist under the eave structure.

12) Etching the buried oxide layer B, and completely etching away the rest part of the buried oxide layer B by using wet etching liquid to form a suspended silicon nano film, as shown in f in fig. 4;

and (3) etching the buried oxide layer B by using an HF etching solution with the etching depth of 3 mu m.

13) Preparing a flexible substrate PDMS 1, and then transferring the silicon nano film onto the flexible substrate PDMS 1 by two times of transfer printing, as shown in fig. 4 g, specifically as follows:

the first step is as follows: organically cleaning a new silicon wafer, then washing the silicon wafer clean by using deionized water, and drying the silicon wafer by using a nitrogen gun to ensure the cleanness of the surface of the wafer;

the second step is that: putting the clean silicon wafer into a Parylene deposition machine, and plating a Parylene film on the silicon wafer, wherein the thickness of the Parylene film is 2 mu m; the purpose of plating the Parylene film on the silicon chip is that the PDMS can be more easily stripped from the silicon chip when the PDMS is prepared;

the third step: preparing PDMS with different viscosities and thicknesses, mixing Sylgard184A and a curing agent according to a mass ratio of 10:1, uniformly pouring the mixed solution on a silicon wafer plated with a Parylene film after air bubbles are pumped, uniformly coating the mixed solution to a thickness of 300 micrometers, then heating the mixed solution on a heating table at 75 ℃ for 3 hours to prepare PDMS with a thickness of 300 micrometers on the silicon wafer, removing the PDMS with the thickness of 300 micrometers from the silicon wafer, and cutting the PDMS into square PDMS with the thickness of 2cm multiplied by 2cm for later use; mixing Sylgard184A and a curing agent according to a mass ratio of 5:1, uniformly pouring the mixed solution after air pumping on a silicon wafer plated with a Parylene film, homogenizing the mixed solution to a thickness of 400 microns, then heating the mixed solution on a heating table at 75 ℃ for 3 hours to prepare PDMS with a thickness of 400 microns on the silicon wafer, removing the PDMS with the thickness of 400 microns from the silicon wafer, and cutting the PDMS into long PDMS strips with the thickness of 1cm multiplied by 5cm for later use;

the fourth step: transferring the silicon nano film, namely putting the SOI sheet loaded with the silicon nano film on a flat table, covering the SOI sheet with a long PDMS strip, and removing the SOI sheet after pressing with a thumb to finish the primary transfer of the silicon nano film; placing the strip PDMS with the surface carrying the silicon nano film facing upwards and on a silicon chip together with the square PDMS, and placing the strip PDMS and the square PDMS in a plasma degumming machine for oxygen plasma treatment; and then placing the strip PDMS carrying the silicon nano film at a horizontal position, attaching a square PDMS on the strip PDMS, and pressing with a thumb after ensuring no gap to complete the second transfer printing of the silicon nano film, wherein the square PDMS of the second transfer printing is the flexible substrate PDMS.

14) Printing electrodes on two sides of the silicon nano film by using silver ink, specifically as follows;

the first step is as follows: placing the whole flexible substrate PDMS 1 loaded with the silicon nano-film on a workbench of an ink-jet printer, moving the needle point absorbed with silver ink to one side of the silicon nano-film, starting printing when the needle point discharges the ink, performing multiple times of printing, and printing the electrode on the other side after printing the electrode on one side;

the second step is that: after the electrode is printed, the whole flexible substrate PDMS 1 loaded with the silicon nano-film is immediately placed on a baking table with the temperature of 120 ℃ to be heated for 1h, so that the silver ink is cured;

the third step: and placing the two leads on the silver electrodes on the two sides of the silicon nano film, coating conductive silver paste, heating and curing for 20min, and then packaging and forming the silicon nano film.

The technical solutions in the embodiments of the present invention are clearly and completely described above, and the described embodiments are only a part of the embodiments of the present invention, but not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

Claims (10)

1. A flexible heart sound sensor based on a PDMS-silicon nanometer film is characterized in that: the flexible substrate PDMS is arranged on the surface of the substrate PDMS; the silicon nano-film is of a rectangular sheet structure, the middle position of the silicon nano-film is a piezoresistive region, the positions on two sides of the silicon nano-film are ohmic contact regions respectively, and the middle position of the top surface of the ohmic contact region is provided with a blocky alloy region; the piezoresistive region and the ohmic contact region are top silicon parts of the SOI sheet, the ohmic contact region is subjected to concentrated boron diffusion, and the alloy region is made of an alloy of chromium and gold.

2. The method of preparing a PDMS-silicon nanomembrane-based flexible heart sound sensor of claim 1, comprising the steps of:

1) taking an SOI (silicon on insulator) wafer for standard cleaning, flushing with deionized water, and drying with nitrogen to ensure the cleanness of the wafer;

2) generating a silicon oxide layer on the surface of the top silicon of the SOI wafer;

3) setting a first layer of mask, covering and protecting the photoresist pattern, and etching the ohmic contact region;

4) heavily doping, and performing concentrated boron diffusion on the ohmic contact region;

5) corroding a silicon oxide layer and impurities formed on the surface of the top silicon layer after the heavy doping process;

6) setting a second layer of mask, covering and protecting the photoresist pattern, and etching the piezoresistive region;

7) cleaning an SOI (silicon on insulator) sheet, carrying out magnetron sputtering on chromium and gold, setting a third layer of mask, covering and protecting a photoresist pattern, and photoetching and retaining metal positioned in an alloy area;

8) corroding the metal in other areas by a wet method, and after removing the photoresist, annealing the SOI sheet to form an alloy of chromium and gold in the alloy area;

9) setting a fourth layer of mask, covering and protecting the photoresist pattern, and etching the buried oxide layer except the piezoresistance region and the ohmic contact region;

10) wet etching the buried oxide layer right below the piezoresistive region and the ohmic contact region to form an arc-shaped concave surface on the side wall of the buried oxide layer, and forming an eave structure among the ohmic contact region, the arc-shaped concave surface of the buried oxide layer and the bottom layer silicon;

11) after spin-coating photoresist, fully exposing, and reserving the photoresist part in the eave structure;

12) etching the buried oxide layer, and completely etching off the residual buried oxide layer by using wet etching solution to form a suspended silicon nano film;

13) preparing PDMS, and then transferring the silicon nano film to a flexible substrate PDMS;

14) and printing electrodes on two sides of the silicon nano-film.

3. The method of claim 2, wherein the PDMS-silicon nanomembrane-based flexible heart sound sensor comprises:

in the step 1), the SOI sheet adopts a P-type SOI sheet with the crystal orientation of <100>, and comprises bottom silicon with the thickness of 700 mu m, an oxygen buried layer with the thickness of 3 mu m and top silicon with the thickness of 340nm from bottom to top in sequence, wherein the resistivity of the top silicon is 1-20 omega-cm;

and in the step 2), the SOI sheet is placed into a plasma enhanced chemical vapor deposition instrument for deposition of a silicon oxide layer, wherein the thickness of the silicon oxide layer is 1 micrometer.

4. The method of claim 2, wherein the PDMS-silicon nanomembrane-based flexible heart sound sensor comprises:

in the step 3), when an ohmic contact region is etched, the SOI sheet is placed in a TATUNG vacuum oven to deposit an HMDS adhesive, the HMDS adhesive is continuously coated for 1800s at 130 ℃, a positive photoresist AZ6130 is uniformly and spin-coated, the rotating speed is set to 3000rp/min, the photoresist pattern covers the region except the ohmic contact region, the ohmic contact region is photoetched after pre-drying, exposure, development, bottom film removal and film hardening, then the SOI sheet is placed in an RIE-10NR etching machine, silicon oxide of the ohmic contact region is etched by a dry method, and the etching depth of the ohmic contact region is 1 mu m;

in the step 4), when heavy doping is carried out, the SOI wafer is placed into a Qingdao flag HQ100A of a diffusion furnace, and dense boron diffusion is carried out on an ohmic contact region, wherein the temperature is kept at 1000 ℃ for 10 min;

and 5) when the impurities are corroded, wet-etching a silicon oxide layer and borosilicate glass impurities formed on the surface of the top silicon layer at 40 ℃ by using a buffered oxide etching solution.

5. The method of claim 2, wherein the PDMS-silicon nanomembrane-based flexible heart sound sensor comprises:

step 6), when the piezoresistive region is etched, uniformly spin-coating a positive photoresist on the SOI wafer, covering the piezoresistive region by a photoresist pattern, prebaking, exposing, developing and hardening, reserving the piezoresistive region, and etching the top silicon layer by adopting Reactive Ion Etching (RIE), wherein the etched piezoresistive region is 340nm in depth;

in the step 7), when metal is sputtered, the SOI wafer is placed into a magnetron sputtering coating machine, chromium with the thickness of 30nm is sputtered, gold with the thickness of 300nm is sputtered, photoresist is coated in a rotating mode, a photoresist pattern covers an alloy area, and the alloy area is subjected to prebaking, exposure, development and hardening to be photoetched;

in the step 8), metal outside the alloy area is subjected to wet etching, and 22% (NH) is adopted ₄ ) ₂ Ce(NO ₃ ) ₆ +8% HAC + 70% H ₂ Corroding chromium metal with corrosive solution of O by using 5% of I ₂ + 10% KI +85% H ₂ Corroding the metal gold by the corrosive liquid of O; during annealing treatment, the SOI wafer is put into a vacuum annealing furnace,the annealing temperature is set to 380 ℃ and the time is set to 30 min.

6. The method of claim 2, wherein the PDMS-silicon nanomembrane-based flexible heart sound sensor comprises:

step 9), the photoresist pattern covers the area except the ohmic contact area and the piezoresistive area, the SOI wafer is placed into an RIE etching machine, the etching rate is 20nm/min, the etching time is 150min, and the depth of an etched buried oxide layer is 3 microns;

in the step 10), wet etching is carried out on the buried oxide layer right below the piezoresistive region and the ohmic contact region by adopting an HF etching solution, and the transverse depth of etching is 3 microns;

in the step 11), when the spin-coating photoresist is subjected to full exposure, the exposure dose is 20mJ/cm ² And developing the hardened film to retain the photoresist under the eave structure.

7. The method of claim 2, wherein the PDMS-silicon nanomembrane-based flexible heart sound sensor comprises:

in the step 12), the wet etching solution is HF etching solution.

8. The method of claim 2, wherein the PDMS-silicon nanomembrane-based flexible heart sound sensor comprises:

in the step 13), the silicon nano film is subjected to two times of transfer printing, so that the silicon nano film is transferred to the flexible substrate PDMS;

in step 14), electrodes are printed on both sides of the silicon nano-film with silver ink.

9. The method of claim 8, wherein the PDMS-silicon nanomembrane-based flexible heart sound sensor comprises: the specific process of the step 13) is as follows:

the first step is as follows: organically cleaning a new silicon wafer, then washing the silicon wafer clean by using deionized water, and drying the silicon wafer by using a nitrogen gun to ensure the cleanness of the surface of the wafer;

the second step is that: putting the clean silicon wafer into a Parylene deposition machine, and plating a Parylene film on the silicon wafer, wherein the thickness of the Parylene film is 2 mu m;

the third step: preparing PDMS with different viscosities and thicknesses, mixing Sylgard184A and a curing agent according to a mass ratio of 10:1, uniformly pouring the mixed solution on a silicon wafer plated with a Parylene film after air bubbles are pumped, uniformly coating the mixed solution to a thickness of 300 micrometers, then heating the mixed solution on a heating table at 75 ℃ for 3 hours to prepare PDMS with a thickness of 300 micrometers on the silicon wafer, removing the PDMS with the thickness of 300 micrometers from the silicon wafer, and cutting the PDMS into square PDMS with the thickness of 2cm multiplied by 2cm for later use; mixing Sylgard184A with a curing agent according to a mass ratio of 5:1, uniformly pouring the mixed solution on a silicon wafer plated with a Parylene film after air bubbles are pumped, uniformly coating the mixture to a thickness of 400 micrometers, then heating the mixture on a heating table at 75 ℃ for 3 hours, preparing PDMS with a thickness of 400 micrometers on the silicon wafer, removing the PDMS with a thickness of 400 micrometers from the silicon wafer, and cutting the PDMS into long PDMS strips with a thickness of 1cm multiplied by 5cm for later use;

the fourth step: transferring the silicon nano film, namely putting the SOI sheet loaded with the silicon nano film on a flat table, covering the SOI sheet with a long PDMS strip, pressing the SOI sheet with a thumb and then removing the PDMS strip to finish the first transfer of the silicon nano film; placing the strip PDMS with the surface carrying the silicon nano film facing upwards and on a silicon chip together with the square PDMS, and placing the strip PDMS and the square PDMS in a plasma degumming machine for oxygen plasma treatment; and then placing the strip PDMS carrying the silicon nano film at a horizontal position, attaching a square PDMS on the strip PDMS, and pressing with a thumb after ensuring no gap to complete the second transfer printing of the silicon nano film, wherein the square PDMS of the second transfer printing is the flexible substrate PDMS.

10. The method of claim 8, wherein the PDMS-silicon nanomembrane-based flexible heart sound sensor comprises: step 14) the specific process is as follows:

the first step is as follows: placing the flexible substrate PDMS whole body loaded with the silicon nano-film on a workbench of an ink-jet printer, moving the needle point absorbed with silver ink to one side of the silicon nano-film, starting printing when the needle point discharges the ink, performing multiple times of printing, and printing the electrode on the other side after printing the electrode on one side;

the second step is that: after the electrode is printed, the flexible substrate PDMS whole body loaded with the silicon nano-film is immediately placed on a baking table with the temperature of 120 ℃ to be heated for 1h, so that the silver ink is cured;

the third step: and placing the two leads on the silver electrodes on the two sides of the silicon nano film, coating conductive silver paste, heating and curing for 20min, and then packaging and forming the silicon nano film.

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