CN117906801A - A MEMS three-dimensional force sensor and its preparation method - Google Patents

Abstract

The invention relates to a MEMS three-dimensional force sensor and a preparation method thereof, wherein the MEMS three-dimensional force sensor comprises: an elastic layer comprising a surface layer, a liquid metal layer and a first substrate; the liquid metal layer is arranged between the surface layer and the first substrate, and the liquid metal layer is provided with resistance wires; wherein, the Young modulus of the elastic layer is further controlled by controlling the phase state of the liquid metal layer; the force transmission layer comprises a second substrate which is attached to the first substrate, and the second substrate is provided with a plurality of microcontact units; the resistance wire is connected with the second substrate; the force sensitive layer comprises a third substrate which is attached to the second substrate, and the third substrate is provided with a plurality of cantilever beams; wherein the plurality of cantilevers Liang Zhishao and a portion of the microcontact units are disposed opposite to each other. The invention can be freely adjusted according to the requirements so as to adapt to different force measurement ranges and precision requirements.

Description

A MEMS three-dimensional force sensor and its preparation method

Technical Field

The present invention relates to the technical field of micro-electromechanical systems, and in particular to a MEMS three-dimensional force sensor and a preparation method thereof.

Background technique

Micro-Electro-Mechanical Systems (MEMS) is a technology that integrates mechanical and electronic components at the micron scale.

MEMS technology has been widely used in many fields, including sensors, measuring instruments, biomedicine and communications. In the field of force sensing, MEMS force sensors are composed of micromechanical devices and sensitive circuits, which can achieve accurate measurement of force.

However, different application scenarios may have different sensitivity requirements for force sensors. Traditional MEMS three-dimensional force sensors often have fixed sensitivity and cannot be adjusted according to actual needs.

Therefore, there is an urgent need to develop a MEMS three-dimensional force sensor with adjustable sensitivity.

Summary of the invention

In view of the deficiencies in the prior art, the present invention discloses a MEMS three-dimensional force sensor and a preparation method thereof.

The technical solution adopted by the present invention is as follows:

A MEMS three-dimensional force sensor, comprising:

The elastic layer comprises a surface layer, a liquid metal layer and a first substrate; the liquid metal layer is arranged between the surface layer and the first substrate, and the liquid metal layer is provided with a resistance wire; wherein the Young's modulus of the elastic layer is controlled by controlling the phase state of the liquid metal layer;

The force transmission layer comprises a second substrate bonded to the first substrate, wherein the second substrate is provided with a plurality of micro-contact units; and the resistance wire extends along a first direction and is connected to the second substrate;

The force-sensitive layer comprises a third substrate bonded to the second substrate, wherein the third substrate is provided with a plurality of cantilever beams; wherein the plurality of cantilever beams are arranged opposite to at least a portion of the micro-contact units;

When external force is applied to the elastic layers with different Young's moduli, the elastic layer with a high Young's modulus produces a relatively small deformation, and thus the phase change transmitted to the force-sensitive layer is smaller, causing the force-sensitive layer to produce a smaller signal output.

In one embodiment of the present invention, the surface layer of the elastic layer and the first substrate are made of polydimethylsiloxane.

In one embodiment of the present invention, the material of the second substrate of the force transfer layer is polydimethylsiloxane.

In one embodiment of the present invention, the plurality of micro-contact units all extend along the thickness direction of the force transmission layer, and the extension distances of the plurality of micro-contact units are the same.

In one embodiment of the present invention, the end of the micro-contact unit contacting the cantilever beam is a tip contact.

In one embodiment of the present invention, the third substrate defines a plurality of cavities for mounting the cantilever beam, one end of the cantilever beam abuts against an inner wall of the cavity, and the other end of the cantilever beam is suspended in the cavity.

In one embodiment of the present invention, the third substrate is further provided with a plurality of electrode micro-channels, the number of which is the same as that of the cantilever beams, and the electrode micro-channels are covered with a conductive metal material.

In one embodiment of the present invention, the second substrate is provided with a positioning column, the third substrate is provided with a positioning hole, and the positioning column is inserted into the positioning hole.

The present invention also provides a method for preparing a MEMS three-dimensional force sensor, and the method comprises the following steps:

S1, making an elastic layer;

S11, dripping PDMS solution into the first mold and curing to obtain a surface layer;

S12, pouring the liquid metal solution into the first mold in step S11, and solidifying it to obtain a liquid metal layer;

S13, placing a resistance wire on the surface of the liquid metal layer obtained in step S12;

S14, dripping PDMS solution into the second mold and curing to obtain a first PDMS unit;

S15, dripping PDMS solution into the third mold and curing to obtain a second PDMS unit;

S16, placing the liquid metal layer prepared in step S13, the first PDMS unit prepared in step S14, and the second PDMS unit prepared in step S15 in a fourth mold, dripping PDMS solution into it, and curing to obtain an elastic layer with a base;

S2, etching a micro-contact unit on the surface of the second substrate, coating the surface of the second substrate with a PDMS solution, and curing to obtain a force transmission layer;

S3. Etching a cantilever beam on the surface of the third substrate to obtain a force sensitive layer.

In one embodiment of the present invention, the first mold is a square with a bottom surface of 320μm×320μm and a groove depth of 200μm; the second mold is a square with a bottom surface of 400μm×400μm and a groove depth of 100μm; the third mold is a square with a bottom surface of 400μm×400μm and a groove depth of 300μm; the fourth mold is a square with a bottom surface of 400μm×400μm and a groove depth of 200μm.

The above technical solution of the present invention has the following advantages compared with the prior art:

The MEMS three-dimensional force sensor of the present invention can achieve more accurate measurement within different measurement ranges by adjusting the sensitivity of the sensor, making the sensitivity and range of the sensor adjustable and improving the measurement accuracy.

The MEMS three-dimensional force sensor described in the present invention can cope with a variety of working conditions. Different application environments may have different noise levels. The variable sensitivity sensor can adjust its sensitivity according to the actual working conditions to cope with different signal strengths and noise levels to ensure accurate and reliable measurement.

The MEMS three-dimensional force sensor of the present invention can avoid the need to purchase and maintain a variety of different sensors for different measurement ranges and requirements. By adjusting the sensitivity of the sensor, a variety of measurement requirements can be achieved on the same sensor, thereby saving costs and resources.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to make the contents of the present invention more clearly understood, the present invention is further described in detail below based on specific embodiments of the present invention in conjunction with the accompanying drawings.

FIG. 1 is a schematic structural diagram of a MEMS three-dimensional force sensor in the present invention.

FIG. 2 is an exploded view of the MEMS three-dimensional force sensor of the present invention from a first viewing angle.

FIG. 3 is an exploded view of the MEMS three-dimensional force sensor of the present invention from a second viewing angle.

FIG. 4 is an exploded view of the elastic layer in the present invention.

FIG. 5 is a schematic diagram of the structure of the force transmission layer in the present invention.

FIG6 is a schematic structural diagram of a force sensitive layer in the present invention from a first viewing angle.

FIG. 7 is a schematic structural diagram of a force sensitive layer in the present invention from a second viewing angle.

Explanation of the reference numerals in the specification: 100, elastic layer; 101, surface layer; 102, liquid metal layer; 103, resistance wire; 104, first substrate; 200, force transmission layer; 201, second substrate; 202, micro-contact unit; 203, positioning column; 300, force sensitive layer; 301, third substrate; 302, cantilever beam; 303, positioning hole.

Detailed ways

The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments so that those skilled in the art can better understand the present invention and implement it, but the embodiments are not intended to limit the present invention.

The above and other technical contents, features and effects of the present invention will be clearly presented in the following detailed description of the embodiments with reference to the accompanying drawings. The directional terms mentioned in the following embodiments, such as up, down, left, right, front or back, etc., are only reference directions of the accompanying drawings. Therefore, the directional terms used are used to illustrate and not to limit the present invention. In addition, in all embodiments, the same reference numerals represent the same elements.

Research has found that traditional MEMS three-dimensional force sensors often have fixed sensitivity and cannot be adjusted according to actual needs. The sensor's measurement range will be limited, which may make the measurement range in some application scenarios too large or too small, and cannot meet the needs of accurate measurement. To change the sensitivity of the sensor, it is usually necessary to redesign the sensor structure and reassemble the various modules of the sensor. This process requires careful evaluation of the sensor's physical properties and measurement range to ensure that the sensor's sensitivity is appropriately adjusted while maintaining the stability of other performance indicators. However, the process of redesigning and adjusting the sensor structure brings many problems, such as the need for time and money investment, which may increase manufacturing costs and delay the product's time to market.

In order to solve the above problems, the present invention provides a MEMS three-dimensional force sensor and a preparation method thereof.

In conjunction with FIG. 1 to FIG. 3 , a MEMS three-dimensional force sensor includes:

The elastic layer 100 includes a surface layer 101, a liquid metal layer 102 and a first substrate 104; the liquid metal layer 102 is disposed between the surface layer 101 and the first substrate 104, and the liquid metal layer 102 is provided with a resistance wire 103; wherein the Young's modulus of the elastic layer 100 is controlled by controlling the phase state of the liquid metal layer 102;

The force transmission layer 200 includes a second substrate 201 attached to the first substrate 104, and the second substrate 201 is provided with a plurality of micro-contact units 202; and the resistance wire 103 extends along the first direction and is connected to the second substrate 201;

The force-sensitive layer 300 includes a third substrate 301 attached to the second substrate 201, and the third substrate 301 is provided with a plurality of cantilever beams 302; wherein the plurality of cantilever beams 302 are arranged opposite to at least part of the micro-contact units 202;

When external force is applied to the elastic layers 100 with different Young's moduli, the elastic layer 100 with a high Young's modulus produces a relatively small deformation, and thus transmits a smaller phase change to the force sensitive layer 300, causing the force sensitive layer 300 to produce a smaller signal output.

It should be noted that, as shown in FIG. 4 , the first direction specifically refers to the width direction of the liquid metal layer 102 .

In this embodiment, the surface layer 101 of the elastic layer 100 and the first substrate 104 are made of polydimethylsiloxane (PDMS).

In this embodiment, the material of the second substrate 201 of the force transfer layer 200 is polydimethylsiloxane (PDMS).

In this embodiment, the plurality of micro-contact units 202 extend along the thickness direction of the force transmission layer 200, and the extension distances of the plurality of micro-contact units 202 are the same. Specifically, the ends of the micro-contact units 202 contacting the cantilever beam 302 are tip contacts.

Further, the plurality of micro-contact units 202 are arranged in an M×N matrix, where M≥1 and N≥1. Specifically, eight micro-contact units 202 are arranged in an array as shown in FIG5 .

In this embodiment, in conjunction with Figures 6 and 7, the third substrate 301 has a plurality of cavities for mounting cantilever beams 302. One end of the cantilever beam 302 abuts against the inner wall of the cavity, and the other end of the cantilever beam 302 is suspended in the cavity. Specifically, there are four cantilever beams 302, and an angle of 90° is formed between two adjacent cantilever beams 302.

In this embodiment, the third substrate 301 is further provided with a plurality of electrode micro-channels having the same number as the cantilever beam 302 , and the electrode micro-channels are covered with a conductive metal material, wherein the conductive metal material is selected from one of copper, aluminum, silver and gold.

As a variation, in order to facilitate the subsequent rapid assembly and fitting of the force transmission layer 200 and the force sensitive layer 300, the second substrate 201 is provided with a positioning column 203, the third substrate 301 is provided with a positioning hole 303, and the positioning column 203 is inserted into the positioning hole 303. Preferably, the center of the positioning column 203 coincides with the center of the force transmission layer 200.

A method for preparing a MEMS three-dimensional force sensor comprises the following steps:

S1, manufacturing an elastic layer 100;

S11. Design the first mold. The bottom surface of the first mold is a square of 320 μm×320 μm. The depth of the groove of the first mold is 200 μm. Prepare the PDMS solution according to the design requirements, drip an appropriate amount of the PDMS solution into the first mold, scrape it flat with a scraper, place it on a heating table, heat it at 80°C for 2 hours, let it stand until it solidifies, and demold it to obtain the surface layer 101;

S12, pouring the unsolidified liquid metal into the first mold in step S1, scraping the liquid metal flat with a blade, then cooling it down to wait for it to be formed, and demolding it to obtain a liquid metal layer 102 with a surface area of 320 μm×320 μm and a thickness of 200 μm;

S13, placing an appropriate amount of resistance wire 103 on the surface of the liquid metal layer 102 obtained in step S2;

S14, design a second mold, the bottom surface of the second mold is a 400μm×400μm square, and the depth of the groove of the second mold is 100μm. Drop an appropriate amount of the PDMS solution prepared in step S1 into the second mold, scrape it flat with a scraper, place it on a heating table, heat it at 80°C for 2h, let it stand until it solidifies, and demold it to obtain the first PDMS unit;

S15, design a third mold, the bottom surface of the third mold is a 400μm×400μm square, and the depth of the groove of the third mold is 300μm. Drop an appropriate amount of the PDMS solution prepared in step S1 into the third mold, scrape it flat with a scraper, place it on a heating table, heat it at 80°C for 2h, let it stand until it solidifies, and demold it to obtain a second PDMS unit;

S16. Place the liquid metal layer 102 prepared in step S3, the first PDMS unit prepared in step S4, and the second PDMS unit prepared in step S5 in a fourth mold with a bottom area of 400 μm×400 μm and a groove depth of 400 μm, drop an appropriate amount of the PDMS solution prepared in step S1, flatten it with a scraper, place it on a heating table, heat it at 80°C for 2 hours, let it stand until it solidifies, and demold it to obtain the elastic layer 100.

S2, manufacturing the force transfer layer 200; the processing is based on a 4-inch N-type SOI wafer, and the thicknesses of the device layer, buried oxide layer and substrate layer are 5 μm, 1 μm and 300 μm respectively;

S21, firstly, a 100nm silicon nitride layer is grown on the SOI surface by low pressure chemical vapor deposition (LPCVD) method for mask protection of wet etching; since the wet etching solution has a faster etching rate on the silicon oxide mask layer, this process uses silicon nitride as the mask layer of wet etching;

S22, after photolithography and development, the window to be etched is etched by reactive ion etching (RIE). This process requires etching LPCVD silicon nitride. Due to its high density, the etching rate is slow, so it is necessary to test its etching rate to prevent over-etching of silicon;

S23, wet etching to form micro-contact unit 202. Specifically, KOH wet etching solution (potassium hydroxide etching solution) is used to etch it, and the etching rate is related to the concentration of KOH solution and the temperature of the water bath. The concentration of KOH etching solution is 20%, and the etching is carried out in an 80°C water bath environment for about 3 hours to obtain a complete micro-contact unit 202;

S24, back photolithography development; deep reactive ion etching (DRIE) is used to etch out the circular through hole in the middle. Since the circular through hole needs to be etched through the wafer, the photoresist uses AZ4620 photoresist with higher viscosity. Considering the etching rate ratio of silicon to the etching rate of the photoresist, the photoresist spreading speed is determined to keep the photoresist at a reasonable thickness. Before deep silicon etching, the silicon nitride of LPCVD in step S21 needs to be completely etched;

S25. After the mold of the force transfer layer 200 is prepared, the force transfer layer 200 is prepared by molding and demolding; a PDMS solution is poured into the mold for preparing the force transfer layer 200, and the force transfer layer 200 is obtained by solidifying; wherein, a molding tool is required as an auxiliary when heating and molding the PDMS solution to form an external support area of the force transfer layer 200. In addition, the thickness of the force transfer layer 200 can be controlled by controlling the depth of the molding tool.

From the above, it can be seen that to make the force transfer layer 200, it is necessary to first prepare a reverse mold, and mainly etch the micro-contact unit 202 and the positioning column 203 on the front surface of the silicon wafer by wet etching/dry etching. The approximate shape of the micro-contact unit 202 is a pyramid structure, and a certain thickness of PDMS solution is spin-coated on its surface by a rotary coating method. After the PDMS is solidified, it is removed to form the force transfer layer 200.

S3, manufacturing the force sensitive layer 300; selecting a 4-inch n-type (100 crystal phase) SOI silicon wafer, the thickness of the device layer, buried oxide layer and substrate layer of which are 5 μm, 1 μm and 300 μm respectively, and the processing process is as follows:

S31, formed by thermal oxidation on the positive surface of SOI silicon wafer The silicon dioxide layer is used as a hard mask for the first ion implantation, and photolithographic marks are made on the silicon dioxide layer through an etching process;

S32, the positive surface of the SOI silicon wafer is spin-coated with photoresist. The photoresist selected is AZ5214 photoresist, which is used as a positive photoresist. After HDMS wafer pretreatment system, spin coating, and pre-baking, a hard mask protection is formed. The thickness of the formed photoresist is about 2μm.

S33, photolithography, development, patterning of photoresist on the positive surface of the SOI silicon wafer; through the photolithography process, the positive photoresist undergoes a cross-linking reaction under ultraviolet rays of a certain intensity, providing a pattern for etching holes in silicon dioxide;

S34, etching (RIE) the surface silicon oxide, injecting boron ions to form lightly doped B+; etching the surface silicon dioxide, the thickness of the etched silicon oxide is 300nm, and after the ion implantation is completed, ultrasonically cleaning the surface photoresist;

S35, rapid annealing (RTA); after annealing, the wafer resistance is tested and the parameters of ion implantation dose, implantation energy and annealing temperature are obtained. After annealing, a thin oxide layer will be formed during the annealing process, which needs to be rinsed with BOE solution;

S36, spin-coating photoresist on the front surface, after photolithography and development, etching the silicon dioxide layer to form a window for the second ion implantation; then performing the second boron ion implantation, completing rapid annealing, testing the companion wafer resistance, and determining the parameters of the second boron ion implantation;

S37, sputter 700nm aluminum, pattern and perform metallization treatment (annealing); sputter 700nm aluminum on the wafer surface by magnetron sputtering (FHR). Photolithography development, pattern the surface electrode, form electrode micro-channels, use aluminum etching solution to corrode the aluminum on the surface that is not protected by photoresist during the patterning process, wash off the surface photoresist after the etching, and metallize the aluminum to form an ohmic contact. Test the varistor using a manual probe station;

S38, plasma enhanced chemical vapor deposition (PECVD) 1μm silicon nitride; chemical vapor deposition generates 1μm thick silicon nitride (SiNx) for surface protection;

S39, etching out wire holes; photolithography develops the surface wire hole windows and etches silicon nitride, and checks whether the silicon nitride is etched completely. If the etching is not completed, the lead cannot be completed;

S310, etching silicon nitride, silicon oxide, and silicon on the positive surface, and stopping at the buried oxide layer; surface patterning, it is necessary to completely etch the thin film grown in the above process and the device layer of the SOI wafer, and it is necessary to use the silicon nitride in the etching step S31 and the silicon oxide in the step S32, and use deep silicon to etch the silicon in the middle layer of the SOI wafer;

S311, etching silicon oxide on the back, deep silicon etching of single crystal silicon, and stopping at the buried oxide layer; in deep silicon etching, it is necessary to control the etching time according to the actual etching situation and stop at the silicon dioxide layer;

S312, etching the buried oxide layer in the middle of the SOI wafer to release the device. After the release is completed, the sensor is cut along the reserved cutting path by laser cutting.

The working principle of the present invention is as follows:

The sensing principle of the MEMS three-dimensional force sensor designed by the present invention with adjustable force sensitivity is based on the silicon-based piezoresistive effect and cantilever beam structure. The piezoresistive effect is a physical phenomenon, which refers to the phenomenon that when a semiconductor is subjected to stress, the energy band changes due to stress, the energy of the energy valley moves, and its resistivity changes. Within the microelastic range, the piezoresistive effect of semiconductor materials is reversible. Based on this reversible piezoresistive effect, the present invention builds a signal conversion bridge between mechanical signals and electrical signals. When an external force is applied to the sensor, the cantilever beam 302 of the force-sensitive layer 300 is deformed, and stress concentration occurs at the end of the cantilever beam 302, that is, the end away from the center of the force-sensitive layer 300, resulting in a change in the resistance value of the piezoresistive area at the end of the cantilever beam 302. By measuring the change in resistance value, pressure information can be obtained. A Wheatstone bridge circuit is used as a detection circuit, and the magnitude of the force is determined by detecting the output voltage signal of the bridge. When the MEMS three-dimensional force sensor is subjected to a vertical load, that is, when the MEMS three-dimensional force sensor is subjected to a normal force, the normal force has a uniform effect on the four cantilever beams 302, which can be detected by the average resistance of the cantilever beams 302; when the MEMS three-dimensional force sensor is subjected to a horizontal load, that is, when the MEMS three-dimensional force sensor is subjected to a tangential force, the tangential force causes the resistance of the cantilever beam 302 to change inconsistently, which can be detected by the difference in the change in resistance of the cantilever beams 302 on both sides.

The principle of adjustable force sensitivity of the MEMS three-dimensional force sensor is as follows: the elastic layer 100 is composed of PDMS and liquid metal. At different temperatures, the phase of the liquid metal in the elastic layer 100 is different, resulting in a change in the overall equivalent Young's modulus of the elastic layer 100. The deformation generated when subjected to force is different, which in turn leads to different deformations of the cantilever beam 302 of the force sensitive layer 300, thereby achieving adjustment of the sensor output signal. In the production of the elastic layer 100, a resistance wire is wound on the surface of the liquid metal layer 102 to control the phase of the liquid metal, further achieving adjustable sensor sensitivity.

From the above, it can be seen that the elastic layer 100 in the MEMS three-dimensional force sensor provided by the present invention is an important component for sensing force, and the change of its Young's modulus will directly affect the conduction and deformation of force. By controlling the Young's modulus, the stiffness of the elastic layer 100 can be adjusted, thereby changing the sensitivity of the sensor to force. When the Young's modulus of the elastic layer 100 is higher, the sensor responds more sensitively to external forces and can detect smaller force changes; and when the Young's modulus of the elastic layer 100 is lower, the sensor is less sensitive to force and can adapt to processing larger force signals.

When MEMS three-dimensional force sensors are used in palpation surgery, surgeons need to use highly sensitive sensors to identify early tumors or suspicious lesions in a small area, and need to use sensors with a large range to identify nodules deep under the skin. Large range and high sensitivity cannot be achieved at the same time. Therefore, the sensitivity and range of the probe need to be intelligently adjustable. The sensitivity-adjustable MEMS three-dimensional force sensor is integrated into the palpation probe. By adjusting the sensitivity of the sensor, the probe sensitivity and range can be adjusted. The adjustable sensitivity of surgical clamps is crucial to enhancing the safety of tissues. When surgical clamps are used for surgical suture needles, a large range is preferred. When surgical clamps are used to clamp tissues, a high sensitivity and small range is preferred. Similarly, the sensitivity-adjustable MEMS three-dimensional force sensor is integrated into the clamp. By adjusting the sensitivity and range of the sensor, the sensitivity of the surgical clamp can be adjusted.

In the description of the embodiments of the present invention, it is also necessary to explain that, unless otherwise clearly specified and limited, the terms "set" and "connection" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be indirectly connected through an intermediate medium, or it can be the internal communication of two components. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

Obviously, the above embodiments are merely examples for clear explanation and are not intended to limit the implementation methods. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the implementation methods here. The obvious changes or modifications derived from these are still within the protection scope of the invention.

Claims (10)

1. A MEMS three-dimensional force sensor, comprising: An elastic layer (100) comprises a surface layer (101), a liquid metal layer (102) and a first substrate (104); the liquid metal layer (102) is arranged between the surface layer (101) and the first substrate (104), and the liquid metal layer (102) is provided with a resistance wire (103); wherein the Young's modulus of the elastic layer (100) is controlled by controlling the phase state of the liquid metal layer (102); The force transmission layer (200) comprises a second substrate (201) bonded to the first substrate (104), wherein the second substrate (201) is provided with a plurality of micro-contact units (202); and the resistance wire (103) extends along a first direction and is connected to the second substrate (201); A force-sensitive layer (300) comprises a third substrate (301) bonded to the second substrate (201), wherein the third substrate (301) is provided with a plurality of cantilever beams (302); wherein the plurality of cantilever beams (302) are arranged opposite to at least a portion of the micro-contact units (202); When external force is applied to the elastic layers (100) having different Young's moduli, the elastic layer (100) having a higher Young's modulus produces a relatively smaller deformation, and thus transmits a smaller phase change to the force-sensitive layer (300), causing the force-sensitive layer (300) to produce a smaller signal output. 2. The MEMS three-dimensional force sensor according to claim 1, characterized in that the surface layer (101) of the elastic layer (100) and the first substrate (104) are made of polydimethylsiloxane. 3. The MEMS three-dimensional force sensor according to claim 1, characterized in that the material of the second substrate (201) of the force transfer layer (200) is polydimethylsiloxane. 4. The MEMS three-dimensional force sensor according to claim 1 is characterized in that the plurality of micro-contact units (202) extend along the thickness direction of the force transfer layer (200), and the extension distances of the plurality of micro-contact units (202) are the same. 5. The MEMS three-dimensional force sensor according to claim 1, characterized in that the end of the micro-contact unit (202) contacting the cantilever beam (302) is a tip contact. 6. The MEMS three-dimensional force sensor according to claim 1 is characterized in that the third substrate (301) is provided with a plurality of cavities for mounting the cantilever beam (302), one end of the cantilever beam (302) abuts against the inner wall of the cavity, and the other end of the cantilever beam (302) is suspended in the cavity. 7. The MEMS three-dimensional force sensor according to claim 6 is characterized in that the third substrate (301) is also provided with a plurality of electrode micro-channels having the same number as the cantilever beam (302), and the electrode micro-channels are covered with conductive metal material. 8. The MEMS three-dimensional force sensor according to claim 1, characterized in that the second substrate (201) is provided with a positioning column (203), the third substrate (301) is provided with a positioning hole (303), and the positioning column (203) is inserted into the positioning hole (303). 9. A method for preparing a MEMS three-dimensional force sensor, comprising the steps of: S1, manufacturing an elastic layer (100); S11, dripping PDMS solution into the first mold and curing to obtain a surface layer (101); S12, pouring the liquid metal solution into the first mold in step S11, and solidifying to obtain a liquid metal layer (102); S13, placing a resistance wire (103) on the surface of the liquid metal layer (102) obtained in step S12; S14, dripping PDMS solution into the second mold and curing to obtain a first PDMS unit; S15, dripping PDMS solution into the third mold and curing to obtain a second PDMS unit; S16, placing the liquid metal layer (102) prepared in step S13, the first PDMS unit prepared in step S14, and the second PDMS unit prepared in step S15 in a fourth mold, dripping PDMS solution into it, and curing to obtain an elastic layer (100) having a base (104); S2, etching a micro-contact unit (202) on the surface of the second substrate (201), coating the surface of the second substrate (201) with a PDMS solution, and curing the solution to obtain a force transmission layer (200); S3. Etching a cantilever beam (302) on the surface of the third substrate (301) to obtain a force sensitive layer (300). 10. The method for preparing a MEMS three-dimensional force sensor according to claim 9 is characterized in that the first mold is a square with a bottom surface of 320μm×320μm and a groove depth of 200μm; the second mold is a square with a bottom surface of 400μm×400μm and a groove depth of 100μm; the third mold is a square with a bottom surface of 400μm×400μm and a groove depth of 300μm; the fourth mold is a square with a bottom surface of 400μm×400μm and a groove depth of 200μm.