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

Abstract

The present invention is a flexible heart sound sensor based on PDMS-silicon nanofilm, belonging to the field of semiconductor technology. The sensor is composed of a silicon nanofilm and a flexible substrate PDMS, and the silicon nanofilm includes a piezoresistive area, an ohmic contact area and an alloy area, and the material of the alloy area is an alloy of chromium and gold. During preparation, an oxide layer is formed on the top of the SOI wafer, and an ohmic contact area is formed after concentrated boron diffusion. The piezoresistive area is formed after etching the top silicon layer, and the alloy area is formed after sputtering, corroding the metal and annealing. The eaves structure is formed after etching and corroding the buried oxide layer, and the suspended silicon nanofilm structure is formed after drilling and etching the buried oxide layer, and finally the silicon nanofilm is transferred to the flexible substrate PDMS. The sensor has high sensitivity, and at the same time, due to the flexible substrate PDMS, the adaptability and convenience of the sensor are increased.

Description

Technical Field

[0001] The invention belongs to the technical field of semiconductors, and in particular relates to a flexible heart sound sensor based on PDMS-silicon nano-membrane and a preparation method thereof. Background Art

[0002] Heart sounds are a very important physiological signal of the human body. They reflect the mechanical movement and comprehensive condition of the heart and cardiovascular system. Sensors prepared based on the piezoresistive effect are widely used in pressure measurement fields such as power machinery, biomedicine, and aerospace due to their simple structure, good low-frequency characteristics, and stable performance. Since the heart sound signal is relatively weak, a sensor with higher sensitivity is required to detect it. Due to the limited change in the multi-carrier mobility, the sensitivity of the device based on the piezoresistive effect is not very high. The sensor prepared based on silicon nanofilm has a giant piezoresistive effect. It relies on the change in surface potential caused by strain, thereby causing changes in multi-carrier concentration, which can ultimately improve the sensitivity of the device. At the same time, considering that most of the current heart sound sensors are rigid, their adaptability is limited, and the device size is large and not portable enough. Summary of the invention

[0003] The purpose of the present invention is to solve the problems existing in the above-mentioned technology, and proposes a flexible heart sound sensor based on PDMS-silicon nanofilm. The sensor is a flexible heart sound sensor made by transferring the top silicon of the SOI sheet to PDMS through micro-nano processing technology.

[0004] The present invention is achieved through the following technical solutions:

[0005] A flexible heart sound sensor based on PDMS-silicon nanomembrane comprises a silicon nanomembrane and a flexible substrate PDMS, wherein the silicon nanomembrane is arranged at the center of the top surface of the flexible substrate PDMS; the silicon nanomembrane is a rectangular sheet structure, wherein the middle position is a piezoresistive area, and the two sides are ohmic contact areas, respectively; a block-shaped alloy area is arranged at the middle position of the top surface of the ohmic contact area; wherein the piezoresistive area and the ohmic contact area are the top silicon parts of the SOI sheet, and the ohmic contact area is subjected to concentrated boron diffusion, and the material of the alloy area is an alloy of chromium and gold.

[0006] Furthermore, the present invention also provides a method for preparing the above-mentioned flexible heart sound sensor based on PDMS-silicon nanofilm, namely: forming an oxide layer on the top of the SOI wafer by PECVD, forming an ohmic contact area after concentrated boron diffusion, etching the top silicon to form a piezoresistive area, sputtering, corroding the metal and annealing to form an alloy, etching and corroding the buried oxide layer to form an eaves structure, drilling and etching the buried oxide layer to form a suspended silicon nanofilm structure, and finally transferring the silicon nanofilm to the flexible substrate PDMS. The specific preparation process steps are as follows:

[0007] 1) Take the SOI wafer for standard cleaning, rinse with deionized water and blow dry with nitrogen to ensure the cleanliness of the wafer;

[0008] 2) Generate a silicon oxide layer on the top silicon surface of the SOI wafer;

[0009] 3) Setting the first mask layer, covering the photoresist pattern for protection, and etching the ohmic contact area;

[0010] 4) Heavy doping, concentrated boron diffusion in the ohmic contact area;

[0011] 5) Etch away the silicon oxide layer and impurities formed on the top silicon surface after the heavy doping process;

[0012] 6) Set the second mask layer, cover the photoresist pattern for protection, and etch the piezoresistive area;

[0013] 7) After cleaning the SOI wafer, magnetron sputter metal chromium and gold, set the third mask, cover the photoresist pattern for protection, and photolithography to retain the metal in the alloy area;

[0014] 8) Wet-etch the metal in other areas, remove the glue, and then anneal the SOI wafer to form an alloy of chromium and gold in the alloy area;

[0015] 9) Set the fourth mask layer, cover the photoresist pattern for protection, and etch the buried oxide layer except for the piezoresistive area and the area directly below the ohmic contact area;

[0016] 10) Wet-etch the buried oxide layer directly below the piezoresistive area and the ohmic contact area, so that the side wall of this part of the buried oxide layer forms an arc-shaped concave surface, and an eaves structure is formed between the ohmic contact area, the arc-shaped concave surface of the buried oxide layer, and the underlying silicon;

[0017] 11) Spin-coat the photoresist and fully expose it, leaving the photoresist part inside the eaves structure;

[0018] 12) Drilling the buried oxide layer, and using a wet etching solution to completely etch away the remaining buried oxide layer to form a suspended silicon nanofilm;

[0019] 13) Prepare PDMS and then transfer the silicon nanofilm to the flexible substrate PDMS;

[0020] 14) Print electrodes on both sides of the silicon nanomembrane.

[0021] The present invention adopts a series of process methods such as photolithography, etching, and corrosion to prepare a silicon nanofilm and transfer it to a flexible substrate PDMS to form a device. Due to the giant piezoresistive effect of the silicon nanofilm, the sensitivity of the heart sound sensor is greatly improved. At the same time, the presence of the flexible substrate makes the heart sound sensor have better adaptability and portability.

[0022] Preferably, in step 1), the SOI wafer adopts a P-type SOI wafer with a crystal orientation of <100> From bottom to top, it consists of a 700μm thick bottom silicon layer, a 3μm thick buried oxide layer and a 340nm thick top silicon layer. The resistivity of the top silicon layer is 1~20Ω·cm.

[0023] Preferably, in step 2), the SOI wafer is placed in a plasma enhanced chemical vapor deposition apparatus to deposit a silicon oxide layer, and the thickness of the silicon oxide layer is 1 μm.

[0024] Preferably, in step 3), when etching the ohmic contact area, the SOI wafer is placed in a TATUNG vacuum oven to deposit HMDS adhesive at 130°C for 1800s, and the positive photoresist AZ6130 is evenly spin-coated, and the rotation speed is set to 3000rp / min. The photoresist pattern covers the area except the ohmic contact area. After pre-baking, exposure, development, removal of the base film, and hardening, the ohmic contact area is photoetched out, and then the SOI wafer is placed in a RIE-10NR etcher to dry-etch the silicon oxide in the ohmic contact area, and the etching depth of the ohmic contact area is 1μm.

[0025] Preferably, in step 4), when heavily doped, the SOI wafer is placed in a diffusion furnace (Qingdao Huaqi HQ100A), and concentrated boron is diffused in the ohmic contact region, with the temperature maintained at 1000° C. for 10 min.

[0026] Preferably, in step 5), when etching impurities, a buffered oxide etching solution is used at 40° C. to wet-etch the silicon oxide layer and impurities such as borosilicate glass formed on the surface of the top silicon.

[0027] Preferably, in step 6), when etching the piezoresistive area, a positive photoresist is evenly spin-coated on the SOI wafer, the photoresist pattern covers the piezoresistive area, pre-baked, exposed, developed and hardened, the piezoresistive area is retained, and the top silicon is etched by reactive ion etching (RIE), and the etched depth of the piezoresistive area is 340nm.

[0028] Preferably, in step 7), when sputtering the metal, the SOI wafer is placed in a magnetron sputtering coating machine, 30 nm thick chromium is sputtered first, then 300 nm thick gold is sputtered, and then photoresist is spin-coated, the photoresist pattern covers the alloy area, pre-baked, exposed, developed and hardened, and the alloy area is photolithographically formed.

[0029] Preferably, in step 8), the metal outside the alloy area is wet-etched using 22% (NH 4 ) 2 Ce(NO 3 ) 6 +8%HAC + 70% H 2 O corrosive solution corrodes metal chromium, using 5% I 2 + 10% KI +85% H2 O-containing etching solution corrodes metal gold; during annealing, the SOI wafer is placed in a vacuum annealing furnace, and the annealing temperature is set to 380°C and the time is set to 30 minutes.

[0030] Preferably, in step 9), the photoresist pattern covers the area except the ohmic contact area and the piezoresistive area, the SOI wafer is placed in a RIE etcher, the etching rate is 20 nm / min, the etching time is 150 min, and the etched buried oxide layer depth is 3 μm.

[0031] Preferably, in step 10), the buried oxide layer directly below the piezoresistive region and the ohmic contact region is wet-etched using 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.

[0032] Preferably, in step 11), when the photoresist is spin-coated for full exposure, the exposure dose is 20 mJ / cm 2 , and then develop the hard film to retain the photoresist under the eaves structure.

[0033] Preferably, in step 12), the wet etching solution is HF etching solution.

[0034] Preferably, in step 13), the silicon nanofilm is transferred twice to the flexible substrate PDMS.

[0035] Preferably, in step 14), electrodes are printed on both sides of the silicon nanomembrane using silver ink.

[0036] Preferably, the specific process of step 13) is as follows:

[0037] Step 1: Organically clean a new silicon wafer, then rinse it with deionized water, and then blow it dry with a nitrogen gun to ensure the cleanliness of the wafer surface;

[0038] Step 2: Place the clean silicon wafer into the Parylene deposition machine and deposit a Parylene film on the silicon wafer. The thickness of the Parylene film is 2μm. The purpose of depositing the Parylene film on the silicon wafer is to make it easier to peel off the PDMS from the silicon wafer when preparing the PDMS.

[0039] Step 3: Prepare PDMS with different viscosities and thicknesses, mix Sylgard184A (PDMS) and curing agent at a mass ratio of 10:1, pump out bubbles from the mixture and pour it evenly on the silicon wafer coated with Parylene film, with a uniform glue thickness of 300μm, then heat it on a heating table at 75℃ for 3h, prepare PDMS with a thickness of 300μm on the silicon wafer, peel off the 300μm PDMS from the silicon wafer, and cut it into 2cm×2cm square PDMS for standby use; mix Sylgard184A (PDMS) and curing agent at a mass ratio of 5:1, pump out bubbles from the mixture and pour it evenly on the silicon wafer coated with Parylene film, with a uniform glue thickness of 400μm, then heat it on a heating table at 75℃ for 3h, prepare PDMS with a thickness of 400μm on the silicon wafer, peel off the 400μm PDMS from the silicon wafer, and cut it into 1cm×5cm long strips of PDMS for standby use;

[0040] Step 4: Transfer the silicon nanomembrane. Place the SOI wafer carrying the silicon nanomembrane on a flat table, cover it with a long strip of PDMS, press it with your thumb and peel it off to complete the first transfer of the silicon nanomembrane. Place the long strip of PDMS with the side carrying the silicon nanomembrane facing up and place it on the silicon wafer together with the square PDMS, and put them together into a plasma degumming machine for oxygen plasma treatment. Then place the long strip of PDMS carrying the silicon nanomembrane in a horizontal position, stick the square PDMS to the long strip of PDMS, ensure that there is no gap, and press it with your thumb to complete the second transfer of the silicon nanomembrane. The square PDMS transferred for the second time is the flexible substrate PDMS.

[0041] Preferably, the specific process of step 14) is as follows:

[0042] The first step is to place the flexible substrate PDMS carrying the silicon nanofilm on the workbench of the inkjet printer, move the needle tip with silver ink to one side of the silicon nanofilm, start printing when the needle tip comes out of ink, and perform multiple printings. After printing the electrode on one side, print the electrode on the other side.

[0043] Step 2: After the electrodes are printed, the flexible PDMS substrate carrying the silicon nanofilm is immediately placed on a drying table at 120°C for 1 hour to solidify the silver ink.

[0044] Step 3: Place two leads on the silver electrodes on both sides of the silicon nanomembrane, apply conductive silver paste, heat and cure for 20 minutes, and then package it into shape.

[0045] The present invention discloses a flexible heart sound sensor based on PDMS-silicon nanofilm and a preparation method thereof, wherein a silicon nanofilm of a certain size is successfully transferred to a flexible substrate PDMS through a series of micro-nano processing processes. Compared with the existing technology, the present invention has the beneficial effects of forming a flexible heart sound sensor with a silicon nanofilm as a piezoresistor, and because the silicon nanofilm has a giant piezoresistive effect, the sensitivity of the heart sound sensor can be improved, and at the same time, PDMS is a flexible substrate, which increases the adaptability and convenience of the sensor. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] The drawings herein are used to provide further explanation of the present invention and constitute a part of this application. The illustrative embodiments of the present invention and their description are used to explain the present invention and do not constitute improper limitations on the present invention.

[0047] Figure 1 It is the structural front view of the sensor of the present invention.

[0048] Figure 2 It is a top view of the structure of the sensor of the present invention.

[0049] Figure 3 The present invention is a flowchart of the method for preparing the sensor.

[0050] Figure 4 Schematic diagram of the device structure formed at different stages in the sensor preparation process of the present invention.

[0051] In the figure: 1- flexible substrate PDMS, 2- piezoresistive area, 3- ohmic contact area, 4- alloy area;

[0052] A-bottom silicon, B-buried oxide layer, C-top silicon, D-silicon oxide layer, E-eaves structure. DETAILED DESCRIPTION

[0053] In order to make those skilled in the art better understand the present invention, the present invention is further clearly and completely described below in conjunction with the accompanying drawings and embodiments. It should be noted that the embodiments and features in the embodiments of the present application can be combined with each other without conflict.

[0054] This embodiment provides a flexible heart sound sensor based on PDMS-silicon nanofilm, such as Figure 1 and Figure 2As shown, it includes a silicon nanomembrane and a flexible substrate PDMS 1, wherein the silicon nanomembrane is arranged at the center of the top surface of the flexible substrate PDMS 1; the silicon nanomembrane is a rectangular sheet structure, wherein the middle position is a piezoresistive region 2, and the two sides are respectively located at ohmic contact regions 3, and a block-shaped alloy region 4 is arranged in the middle position of the top surface of the ohmic contact region 3; wherein, the piezoresistive region 2 and the ohmic contact region 3 are the top silicon parts of the SOI sheet, and the ohmic contact region 3 is subjected to concentrated boron diffusion, and the material of the alloy region 4 is an alloy of chromium and gold.

[0055] The preparation method of the above-mentioned flexible heart sound sensor based on PDMS-silicon nanofilm has the following preparation process: Figure 3 As shown in the block diagram, the specific steps include:

[0056] 1) Take the SOI wafer for standard cleaning, rinse with deionized water and blow dry with nitrogen to ensure the cleanliness of the wafer;

[0057] The SOI wafer adopts P-type SOI wafer with crystal orientation of <100> From bottom to top, it consists of a 700μm thick bottom silicon A, a 3μm thick buried oxide layer B and a 340nm thick top silicon C. The resistivity of the top silicon C is 1~20Ω·cm.

[0058] 2) Generate a silicon oxide layer D on the top silicon C surface of the SOI wafer, such as Figure 4 a in;

[0059] The SOI wafer is placed in a plasma enhanced chemical vapor deposition apparatus to deposit a silicon oxide layer D, and the thickness of the silicon oxide layer D is 1 μm.

[0060] 3) Setting the first mask layer, covering the photoresist pattern for protection, and etching the ohmic contact area 3;

[0061] When etching the ohmic contact area 3, the SOI wafer is placed in a TATUNG vacuum oven to deposit HMDS adhesive at 130°C for 1800s, and the positive photoresist AZ6130 is evenly spin-coated. The rotation speed is set to 3000rp / min. The photoresist pattern covers the area except the ohmic contact area 3. After pre-baking, exposure, development, primer removal, and hardening, the ohmic contact area 3 is photoetched out, and then the SOI wafer is placed in a RIE-10NR etcher to dry-etch the silicon oxide in the ohmic contact area 3. The etching depth of the ohmic contact area 3 is 1μm.

[0062] 4) Heavy doping, concentrated boron diffusion is performed on the ohmic contact region 3;

[0063] When heavily doped, the SOI wafer is placed in a diffusion furnace (Qingdao Huaqi HQ100A) to diffuse concentrated boron into the ohmic contact region 3, with the temperature maintained at 1000°C for 10 minutes.

[0064] 5) Etch away the silicon oxide layer D and impurities formed on the surface of the top silicon C after the heavy doping process, such as Figure 4 b in;

[0065] When etching impurities, a buffered oxide etching solution is used at 40° C. to wet-etch the silicon oxide layer D and borosilicate glass impurities formed on the surface of the top silicon C.

[0066] 6) Set the second mask layer, cover the photoresist pattern with protection, and etch the piezoresistive area 2, such as Figure 4 c in;

[0067] When etching the piezoresistive area 2, a positive photoresist is evenly spin-coated on the SOI wafer, and the photoresist pattern covers the piezoresistive area 2. Pre-baking, exposure, development and hardening are performed to retain the piezoresistive area 2. Reactive ion etching (RIE) is used to etch the top silicon C. The etched depth of the piezoresistive area 2 is 340nm.

[0068] 7) After cleaning the SOI wafer, magnetron sputtering of metal chromium and gold is performed, a third mask is set, a photoresist pattern is covered for protection, and the metal located in alloy area 4 is retained by photolithography;

[0069] When sputtering metal, the SOI wafer is placed in a magnetron sputtering coating machine, first sputtering 30nm thick chromium, then sputtering 300nm thick gold, and then spin-coating photoresist. The photoresist pattern covers the alloy area 4 area, pre-baking, exposure, development and hardening, and photolithography out the alloy area 4 area.

[0070] 8) Wet-etch away the metal in other areas, remove the glue, and then anneal the SOI wafer to form an alloy of chromium and gold in alloy area 4, such as Figure 4 d in;

[0071] Wet etching of metals outside alloy zone 4, using 22% (NH 4 ) 2 Ce(NO 3 ) 6 +8% HAC + 70% H 2 O corrosive solution corrodes metal chromium, using 5% I 2 + 10% KI +85% H 2 O-containing etching solution corrodes metal gold; during annealing, the SOI wafer is placed in a vacuum annealing furnace, and the annealing temperature is set to 380°C and the time is set to 30 minutes.

[0072] 9) Set the fourth mask layer, cover the photoresist pattern for protection, and etch the buried oxide layer B except for the piezoresistive area 2 and the ohmic contact area 3 directly below;

[0073] The photoresist pattern covers the area except the ohmic contact area 3 and the piezoresistive area 2. The SOI wafer is placed in a RIE etcher with an etching rate of 20 nm / min and an etching time of 150 min. The depth of the etched buried oxide layer B is 3 μm.

[0074] 10) Wet-etch the buried oxide layer B directly below the piezoresistive region 2 and the ohmic contact region 3, so that the sidewall of this part of the buried oxide layer B forms an arc-shaped concave surface, and an eaves structure E is formed between the ohmic contact region 3, the arc-shaped concave surface of the buried oxide layer B, and the bottom silicon A, as shown in FIG. Figure 4 e in;

[0075] The buried oxide layer B directly below the piezoresistive region 2 and the ohmic contact region 3 is wet-etched using 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.

[0076] 11) Spin-coat the photoresist and fully expose it, leaving the photoresist part inside the eaves structure;

[0077] When the spin-coated photoresist is fully exposed, the exposure dose is 20mJ / cm 2 , and then develop the hard film to retain the photoresist under the eaves structure.

[0078] 12) Drill and etch the buried oxide layer B, and use a wet etching solution to completely etch away the remaining buried oxide layer B to form a suspended silicon nanofilm, such as Figure 4 f in;

[0079] The buried oxide layer B is wet-etched with HF etching solution, and the etching depth is 3μm.

[0080] 13) Prepare a flexible substrate PDMS 1, and then transfer the silicon nanofilm to the flexible substrate PDMS 1 by two transfers, as shown in FIG. Figure 4 The g in the formula is as follows:

[0081] Step 1: Organically clean a new silicon wafer, then rinse it with deionized water, and then blow it dry with a nitrogen gun to ensure the cleanliness of the wafer surface;

[0082] Step 2: Place the clean silicon wafer into the Parylene deposition machine and deposit a Parylene film on the silicon wafer. The thickness of the Parylene film is 2μm. The purpose of depositing the Parylene film on the silicon wafer is to make it easier to peel off the PDMS from the silicon wafer when preparing the PDMS.

[0083] Step 3: Prepare PDMS with different viscosities and thicknesses, mix Sylgard184A and curing agent at a mass ratio of 10:1, pump out bubbles from the mixture and pour it evenly on the silicon wafer coated with Parylene film, with a uniform glue thickness of 300μm, then heat it on a heating table at 75℃ for 3h, prepare PDMS with a thickness of 300μm on the silicon wafer, peel off the 300μm PDMS from the silicon wafer, and cut it into 2cm×2cm square PDMS for standby use; mix Sylgard184A and curing agent at a mass ratio of 5:1, pump out bubbles from the mixture and pour it evenly on the silicon wafer coated with Parylene film, with a uniform glue thickness of 400μm, then heat it on a heating table at 75℃ for 3h, prepare PDMS with a thickness of 400μm on the silicon wafer, peel off the 400μm PDMS from the silicon wafer, and cut it into 1cm×5cm long strips of PDMS for standby use;

[0084] Step 4: Transfer the silicon nanomembrane. Place the SOI wafer carrying the silicon nanomembrane on a flat table, cover it with a long strip of PDMS, press it with your thumb and peel it off to complete the first transfer of the silicon nanomembrane. Place the long strip of PDMS with the side carrying the silicon nanomembrane facing up and place it on the silicon wafer together with the square PDMS, and put them together into a plasma degumming machine for oxygen plasma treatment. Then place the long strip of PDMS carrying the silicon nanomembrane in a horizontal position, stick the square PDMS to the long strip of PDMS, ensure that there is no gap, and press it with your thumb to complete the second transfer of the silicon nanomembrane. The square PDMS transferred for the second time is the flexible substrate PDMS.

[0085] 14) Printing electrodes on both sides of the silicon nanomembrane with silver ink as follows;

[0086] Step 1: Place the flexible substrate PDMS 1 carrying the silicon nanomembrane on the workbench of the inkjet printer, move the needle tip with silver ink to one side of the silicon nanomembrane, start printing when the needle tip comes out of ink, and perform multiple printings. After printing the electrode on one side, print the electrode on the other side.

[0087] Step 2: After the electrode is printed, the flexible substrate PDMS 1 carrying the silicon nanofilm is immediately placed on a drying table at 120°C for 1 hour to solidify the silver ink;

[0088] Step 3: Place two leads on the silver electrodes on both sides of the silicon nanomembrane, apply conductive silver paste, heat and cure for 20 minutes, and then package it into shape.

[0089] The above is a clear and complete description of the technical solutions in the embodiments of the present invention. The described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

Claims

1. A flexible heart sound sensor based on PDMS-silicon nanofilm, characterized in that: It includes a silicon nanomembrane and a flexible substrate PDMS, wherein the silicon nanomembrane is arranged at the center of the top surface of the flexible substrate PDMS; the silicon nanomembrane is a rectangular sheet structure, wherein the middle position is a piezoresistive area, and the two sides are ohmic contact areas, and a block alloy area is arranged in the middle position of the top surface of the ohmic contact area; wherein the piezoresistive area and the ohmic contact area are the top silicon parts of the SOI sheet, and the ohmic contact area is diffused with concentrated boron, and the material of the alloy area is an alloy of chromium and gold.

2. The method for preparing a flexible heart sound sensor based on PDMS-silicon nanofilm according to claim 1, characterized in that: The steps include: 1) Take the SOI wafer for standard cleaning, rinse with deionized water and blow dry with nitrogen to ensure the cleanliness of the wafer; 2) Generate a silicon oxide layer on the top silicon surface of the SOI wafer; 3) Setting the first mask layer, covering the photoresist pattern for protection, and etching the ohmic contact area; 4) Heavy doping, concentrated boron diffusion in the ohmic contact area; 5) Etch away the silicon oxide layer and impurities formed on the top silicon surface after the heavy doping process; 6) Set the second mask layer, cover the photoresist pattern for protection, and etch the piezoresistive area; 7) After cleaning the SOI wafer, magnetron sputter metal chromium and gold, set the third mask, cover the photoresist pattern for protection, and photolithography to retain the metal in the alloy area; 8) Wet-etch the metal in other areas, remove the glue, and then anneal the SOI wafer to form an alloy of chromium and gold in the alloy area; 9) Set the fourth mask layer, cover the photoresist pattern for protection, and etch the buried oxide layer except for the piezoresistive area and the area directly below the ohmic contact area; 10) Wet-etch the buried oxide layer directly below the piezoresistive area and the ohmic contact area, so that the side wall of this part of the buried oxide layer forms an arc-shaped concave surface, and an eaves structure is formed between the ohmic contact area, the arc-shaped concave surface of the buried oxide layer, and the underlying silicon; 11) Spin-coat the photoresist and fully expose it, leaving the photoresist part inside the eaves structure; 12) Drilling the buried oxide layer, and using a wet etching solution to completely etch away the remaining buried oxide layer to form a suspended silicon nanofilm; 13) Prepare PDMS and then transfer the silicon nanofilm to the flexible substrate PDMS; 14) Print electrodes on both sides of the silicon nanomembrane.

3. The method for preparing a flexible heart sound sensor based on PDMS-silicon nanofilm according to claim 2, characterized in that: In step 1), the SOI wafer adopts a P-type SOI wafer with a crystal orientation of <100> , which consists of a 700μm thick bottom silicon, a 3μm thick buried oxide layer and a 340nm thick top silicon from bottom to top. The resistivity of the top silicon is 1~20Ω·cm; In step 2), the SOI wafer is placed in a plasma enhanced chemical vapor deposition apparatus to deposit a silicon oxide layer, and the thickness of the silicon oxide layer is 1 μm.

4. The method for preparing a flexible heart sound sensor based on PDMS-silicon nanofilm according to claim 2, characterized in that: In step 3), when etching the ohmic contact area, the SOI wafer is placed in a TATUNG vacuum oven to deposit HMDS adhesive, and the temperature is maintained at 130°C for 1800s. The positive photoresist AZ6130 is evenly spin-coated, and the rotation speed is set to 3000rp / min. The photoresist pattern covers the area except the ohmic contact area. After pre-baking, exposure, development, removal of the base film, and hardening, the ohmic contact area is photoetched. Then, the SOI wafer is placed in a RIE-10NR etcher, and the silicon oxide in the ohmic contact area is dry-etched. The etching depth of the ohmic contact area is 1μm. In step 4), when heavily doped, the SOI wafer is placed in a diffusion furnace (Qingdao Huaqi HQ100A) to diffuse concentrated boron in the ohmic contact area, with the temperature maintained at 1000°C for 10 minutes; In step 5), when etching impurities, a buffered oxide etchant is used at 40° C. to wet-etch the silicon oxide layer and borosilicate glass impurities formed on the top silicon surface.

5. The method for preparing a flexible heart sound sensor based on PDMS-silicon nanofilm according to claim 2, characterized in that: In step 6), when etching the piezoresistive area, a positive photoresist is evenly spin-coated on the SOI wafer, and the photoresist pattern covers the piezoresistive area. The piezoresistive area is pre-baked, exposed, developed and hardened, and the piezoresistive area is retained. The top silicon is etched by reactive ion etching (RIE), and the etched piezoresistive area has a depth of 340nm. In step 7), when sputtering metal, the SOI wafer is placed in a magnetron sputtering coating machine, 30nm thick chromium is sputtered first, and then 300nm thick gold is sputtered, and then photoresist is spin-coated, the photoresist pattern covers the alloy area, pre-baked, exposed, developed and hardened, and the alloy area is photolithographically carved out; In step 8), the metal outside the alloy area is wet-etched, and the metal chromium is corroded by an etching solution of 22% (NH4)2Ce(NO3)6 +8% HAC + 70% H2O, and the metal gold is corroded by an etching solution of 5% I2 + 10% KI +85% H2O; during annealing, the SOI wafer is placed in a vacuum annealing furnace, and the annealing temperature is set to 380°C and the time is set to 30 minutes.

6. The method for preparing a flexible heart sound sensor based on PDMS-silicon nanofilm according to claim 2, characterized in that: In step 9), the photoresist pattern covers the area except the ohmic contact area and the piezoresistive area, and the SOI wafer is placed in the RIE etcher, the etching rate is 20nm / min, the etching time is 150min, and the etched buried oxide layer depth is 3μm; In step 10), the buried oxide layer directly below the piezoresistive region and the ohmic contact region is wet-etched using HF etching solution, and the lateral depth of the etching is 3 μm; In step 11), when the photoresist is fully exposed by spin coating, the exposure dose is 20mJ / cm 2 , and then develop the hard film to retain the photoresist under the eaves structure.

7. The method for preparing a flexible heart sound sensor based on PDMS-silicon nanofilm according to claim 2, characterized in that: In step 12), the wet etching solution is HF etching solution.

8. The method for preparing a flexible heart sound sensor based on PDMS-silicon nanofilm according to claim 2, characterized in that: In step 13), the silicon nanofilm is transferred twice to the flexible substrate PDMS; In step 14), electrodes are printed on both sides of the silicon nanomembrane using silver ink.

9. The method for preparing a flexible heart sound sensor based on PDMS-silicon nanofilm according to claim 8, characterized in that: Step 13) The specific process is as follows: Step 1: Organically clean a new silicon wafer, then rinse it with deionized water, and then blow it dry with a nitrogen gun to ensure the cleanliness of the wafer surface; Step 2: Place the clean silicon wafer into the Parylene deposition machine and deposit a Parylene film on the silicon wafer. The thickness of the Parylene film is 2μm. Step 3: Prepare PDMS with different viscosities and thicknesses, mix Sylgard184A and curing agent at a mass ratio of 10:1, pump out bubbles from the mixture and pour it evenly on the silicon wafer coated with Parylene film, with a uniform glue thickness of 300μm, then heat it on a heating table at 75℃ for 3h, prepare PDMS with a thickness of 300μm on the silicon wafer, peel off the 300μm PDMS from the silicon wafer, and cut it into 2cm×2cm square PDMS for standby use; mix Sylgard184A and curing agent at a mass ratio of 5:1, pump out bubbles from the mixture and pour it evenly on the silicon wafer coated with Parylene film, with a uniform glue thickness of 400μm, then heat it on a heating table at 75℃ for 3h, prepare PDMS with a thickness of 400μm on the silicon wafer, peel off the 400μm PDMS from the silicon wafer, and cut it into 1cm×5cm long strips of PDMS for standby use; Step 4: Transfer the silicon nanomembrane. Place the SOI wafer carrying the silicon nanomembrane on a flat table, cover it with a long strip of PDMS, press it with your thumb and peel it off to complete the first transfer of the silicon nanomembrane. Place the long strip of PDMS with the side carrying the silicon nanomembrane facing up and place it on the silicon wafer together with the square PDMS, and put them together into a plasma degumming machine for oxygen plasma treatment. Then place the long strip of PDMS carrying the silicon nanomembrane in a horizontal position, stick the square PDMS to the long strip of PDMS, ensure that there is no gap, and press it with your thumb to complete the second transfer of the silicon nanomembrane. The square PDMS transferred for the second time is the flexible substrate PDMS.

10. The method for preparing a flexible heart sound sensor based on PDMS-silicon nanofilm according to claim 8, characterized in that: Step 14) The specific process is as follows: The first step is to place the flexible substrate PDMS carrying the silicon nanofilm on the workbench of the inkjet printer, move the needle tip with silver ink to one side of the silicon nanofilm, start printing when the needle tip comes out of ink, and perform multiple printings. After printing the electrode on one side, print the electrode on the other side. Step 2: After the electrode is printed, the flexible substrate PDMS carrying the silicon nanofilm is immediately placed on a drying table at 120°C for 1 hour to solidify the silver ink; Step 3: Place two leads on the silver electrodes on both sides of the silicon nanomembrane, apply conductive silver paste, heat and cure for 20 minutes, and then package it into shape.

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