Self-calibrating SOI-based MEMS three-dimensional force sensor and its fabrication process

Abstract

The present invention discloses a self-calibrating SOI-based MEMS three-dimensional force sensor and its preparation process. The present invention comprises a MEMS device layer and a substrate bonded to the MEMS device layer below; the MEMS device layer comprises a boss, an anchor point, an anchor area, a center plate, four L-shaped piezoelectric drive beams located around the center plate, and twelve pads; the substrate comprises a bonding area, a cavity, a conductive pad, and eight fixed metal electrodes. The present invention adds a drive module and a multi-capacitance detection module to the sensitive element, thereby ensuring the accuracy of the measurement results and saving additional calibration costs. While achieving self-calibration, the present invention adopts a three-dimensional integration approach, directly utilizing SOI wafers for external packaging.

Description

Technical Field

[0001] The present invention relates to the field of MEMS sensors, and in particular to a self-calibrating SOI-based MEMS three-dimensional force sensor. Background Art

[0002] Microelectromechanical systems (MEMS) devices, due to their miniaturization, integrated multi-functions, and high stability, play a key role in fields such as healthcare, robotics, and wearable devices. However, with long-term use or in complex operating environments, sensors inevitably experience device aging and performance degradation, making sensor calibration extremely important.

[0003] The key to achieving calibration is to provide driving excitation to simulate the external load. Currently, driving methods include electrostatic drive, piezoelectric drive, and electromagnetic drive, among which piezoelectric drive is more suitable for large-scale drive. In addition, most of the existing calibration research is carried out on inertial sensors. By integrating them on a piezoelectric vibration table, the piezoelectric vibration table drives the inertial sensor to vibrate, and finally the output of the piezoelectric vibration table and the inertial sensor is compared to achieve the calibration function. However, for traditional force sensors, their sensitive elements are encapsulated inside the sensor. When the sensor is not working, the inertial signal in the environment has almost no effect on its output. Therefore, the inertial excitation of the piezoelectric vibration table cannot drive the sensitive elements inside the force sensor, and it needs to be removed and sent to the factory for calibration.

[0004] In addition, the integration methods of MEMS sensors can be divided into single-chip integration, multi-chip integration and three-dimensional integration. The sensitive components in the first two integration methods are exposed to the outside. In order to protect the chip from environmental influences, additional packaging steps are required. Packaging is a more expensive step in the sensor manufacturing process. Summary of the Invention

[0005] The present invention aims to provide a self-calibrating SOI (Silicon-On-Insulator) based MEMS three-dimensional force sensor and its preparation process, so as to ensure the accuracy of measurement results and save additional calibration and packaging costs.

[0006] One aspect of the present invention provides a self-calibrating SOI-based MEMS three-dimensional force sensor, comprising a MEMS device layer and a substrate bonded below the MEMS device layer.

[0007] The MEMS device layer includes a boss, an anchor point, an anchor area, a center plate, four L-shaped piezoelectric driving beams located around the center plate, and twelve pads.

[0008] The boss is located above the anchor point and serves as a force-bearing structure during three-dimensional force detection.

[0009] The central plate and the anchor area are both located below the anchor point and on the same plane; the central plate serves as a movable upper plate for capacitance detection and is connected to the boss through the anchor point.

[0010] A single L-shaped piezoelectric drive beam includes, from top to bottom, an L-shaped silicon beam, a driving lower electrode Pt, a piezoelectric material PZT, and two distributed driving upper electrodes Pt. One end of the L-shaped piezoelectric drive beam is fixed at the center position of the center plate boundary, and the other end is fixed to the anchor area; the rest of the part is suspended in the air; the two distributed driving upper electrodes Pt are used to apply equal and opposite voltages to directly provide driving excitation for the sensitive elements inside the three-dimensional force sensor.

[0011] The twelve pads are located around the anchor area of ​​the MEMS device layer and are in the same plane as the distributed driving upper electrode Pt of the piezoelectric driving beam.

[0012] The substrate includes a bonding area, a cavity, a conductive pad, and eight fixed metal electrodes.

[0013] The bonding area is consistent with the pattern of the anchor area of ​​the MEMS device layer. The anchor area of ​​the flipped MEMS device layer is aligned and in contact with the bonding area of ​​the substrate below it, forming a closed space for capacitive sensing.

[0014] The cavity is used to form an activity space for the piezoelectric driving beam and the center plate.

[0015] The eight fixed metal electrodes are distributed in the cavity, and the center plate serves as a common movable upper plate for capacitance detection, forming a capacitance pair with the eight fixed metal electrodes in the substrate cavity. The diagonal area of ​​the center plate corresponds to the four fixed metal electrodes A, B, C, and D on the substrate, forming a force detection capacitor C. A 、C B 、C C 、C D The symmetry axis area of ​​the center plate corresponds to the cross-distributed fixed metal electrodes E, F, G, H on the substrate, forming a self-calibration detection capacitor C E 、C F 、C G 、C H .

[0016] Another aspect of the present invention provides a process for preparing the self-calibrating SOI-based MEMS three-dimensional force sensor, comprising the following steps:

[0017] (a) Prepare an SOI low-resistivity silicon wafer and form an oxide layer on the front and back of the wafer through thermal oxidation.

[0018] (b) The trenches are deep reactive ion etched and the intermediate oxide layer is dry-removed; Cu is sputtered and electroplated to fill the trenches, and the surface is polished flat to expose the holes.

[0019] (c) A Pt electrode and a piezoelectric material PZT are sequentially grown on the top surface of the silicon wafer; the piezoelectric material PZT is simultaneously wet-etched to lead out a ground signal.

[0020] (d) A Pt electrode is deposited on the upper surface of the piezoelectric material PZT, and the upper metal electrode is patterned using ion beam etching to form a distributed upper electrode and pad for the L-shaped piezoelectric driving beam.

[0021] (e) The piezoelectric material PZT and the bottom Pt electrode are removed by IBE, and the top silicon is etched by DRIE process to open the center plate that needs to be released on the top, the four L-shaped piezoelectric drive beams, and the release hole to the oxide layer to stop automatically.

[0022] (f) Isotropic etching of the intermediate oxide layer using vapor hydrofluoric acid releases the center plate and the L-shaped piezoelectric drive beam.

[0023] (g) Etch the substrate silicon to form a protrusion.

[0024] (h) cutting the MEMS device layer wafers and bonding them to the substrate one by one, so that the flipped MEMS device layer pads are aligned and bonded to the conductive pads of the substrate, and the anchor area of ​​the MEMS device layer contacts the bonding area of ​​the substrate, forming a closed cavity for capacitance detection;

[0025] The capacitance signal, ground signal and piezoelectric drive voltage signal are all led to the back side of the substrate through silicon vias (TSVs).

[0026] Beneficial effects of the present invention:

[0027] 1. The present invention's tangential detection and calibration utilizes the principle of tilted capacitance detection. When a tangential force load is applied to the boss, driving the center plate or piezoelectric beam to torsionally swing the center plate about the x / y axis, the spacing between the center plate and its corresponding fixed metal electrode increases on one side, decreasing the capacitance; while the spacing on the other side decreases, increasing the capacitance. By subtracting these two capacitances, differential detection is achieved. Generally, capacitive sensors using differential detection have higher sensitivity than sensors using non-differential detection.

[0028] 2. The present invention utilizes the inverse piezoelectric effect of the piezoelectric material PZT to apply an equal and opposite voltage to the distributed upper electrode on the piezoelectric drive beam, directly providing driving excitation for the sensitive elements inside the force sensor, driving the common movable upper electrode (center plate) of the capacitance detection to twist around the x / y axis and translate along the z axis, thereby simulating the motion state when an external three-dimensional force is applied. Finally, the self-calibration function is realized by comparing the output of the detection capacitor pair with the calibration capacitor pair. It has the advantages of adjustable range, wide output range, low driving voltage, etc.

[0029] 3. The present invention adopts a three-dimensional integration method to bond the flipped MEMS device layer to the substrate to form a closed cavity, and directly uses silicon wafers for external packaging, avoiding the performance error problem introduced by the traditional sensor tape-out and packaging processes being separated, and at the same time greatly reducing the packaging cost. Compared with traditional sensors, the sensor of the present invention has stable performance, small error and low cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] FIG1( a ) is a schematic diagram of a flipped MEMS device layer of a sensor according to the present invention;

[0031] Figure 1(b) is a cross-sectional view of the overall structure of the sensor of the present invention;

[0032] FIG1( c ) is a top view of the sensor substrate of the present invention;

[0033] Figure 2 This is a process flow chart for preparing the sensor of the present invention.

[0034] FIG3( a ) is a schematic diagram of the operation of the sensor of the present invention under the action of shear force + Fx;

[0035] FIG3( b ) is a schematic diagram of the operation of the sensor of the present invention under the action of the normal force -Fz;

[0036] FIG3( c ) is a schematic diagram of the operation of the sensor of the present invention to achieve X-direction calibration;

[0037] FIG3( d ) is a schematic diagram of the working state of the sensor of the present invention for realizing -Z direction calibration. DETAILED DESCRIPTION

[0038] The present invention will be further explained below in conjunction with the examples and drawings. The following examples are only used to illustrate the present invention, but are not intended to limit the scope of the present invention.

[0039] The self-calibrating SOI-based MEMS three-dimensional force sensor includes a MEMS device layer and a substrate bonded below the MEMS device layer (the following description of directions is based on a front view).

[0040] The MEMS device layer includes a boss, an anchor point, an anchor area, four L-shaped piezoelectric driving beams, a center plate and 12 pads.

[0041] The boss is located above the anchor point and serves as a force-bearing structure during three-dimensional force detection.

[0042] The central plate and the anchor area are both located below the anchor point and on the same plane. The central plate serves as a movable upper plate for capacitance detection and is connected to the boss through the anchor point.

[0043] The L-shaped piezoelectric driving beam includes an L-shaped silicon beam, a driving lower electrode Pt, a piezoelectric material PZT and two distributed driving upper electrodes Pt from top to bottom. Its two ends are respectively fixed at the center position and anchor area of ​​the center plate boundary, and the rest of the parts are suspended. The four L-shaped piezoelectric beams are arranged clockwise or counterclockwise around the center plate.

[0044] The 12 pads are located around the anchor area of ​​the MEMS device layer, coplanar with the distributed top electrode Pt of the piezoelectric drive beam. Three pads are distributed around each anchor area to route the voltage and ground signals from the two distributed top electrodes on a piezoelectric drive beam to the surrounding area, where they are bonded to the conductive pads on the substrate for electrical connection.

[0045] The substrate includes a bonding area, 12 conductive pads, a cavity, 20 through-silicon vias, and 8 fixed metal electrodes.

[0046] The bonding area is consistent with the pattern of the anchor area of ​​the MEMS device layer. The anchor area of ​​the flipped MEMS device layer is aligned and in contact with the bonding area of ​​the substrate below it, forming a closed space for capacitive sensing.

[0047] The 12 conductive pads are distributed around the substrate bonding area and bonded to the corresponding 12 pads on the flipped MEMS device layer to achieve electrical connection. Three conductive pads are set up around each bonding area, one for electrical connection to the ground signal and the other two for electrical connection to the voltage signals of the two distributed upper electrodes on the piezoelectric beam.

[0048] The cavity is used to form an active space for the piezoelectric beam and the center plate, and the depth of the cavity is the initial spacing of the capacitive sensor and is determined by the depth of the substrate groove.

[0049] The 20 through-silicon vias are located inside the substrate, 12 of which are located between the 12 conductive pads and the back of the substrate, and are used to lead the ground signals of the center plate and the piezoelectric drive beam, as well as the voltage signals of the distributed upper electrode to the back of the substrate; the other 8 are located between the 8 fixed metal electrodes in the cavity of the substrate and the back, and are used to lead the capacitance signals of the fixed metal electrodes to the back of the substrate.

[0050] The eight fixed metal electrodes are distributed in the cavity. The center plate serves as a common movable upper plate for capacitance detection, forming a capacitance pair with the eight fixed metal electrodes in the substrate cavity. To ensure that only the spacing changes during detection, the total area of ​​the eight fixed metal electrodes is less than the area of ​​the center plate. The diagonal area of ​​the center plate corresponds to the four fixed metal electrodes A, B, C, and D on the substrate, forming a force detection capacitor C. A 、C B 、C C 、C DThe symmetry axis area of ​​the center plate corresponds to the cross-distributed fixed metal electrodes E, F, G, H on the substrate, forming a self-calibration detection capacitor C E 、C F 、C G 、C H ; and the capacitance signals of the 8 fixed metal electrodes are led to the back side of the substrate through the silicon vias underneath.

[0051] Example:

[0052] Figure 1(a) is a schematic diagram of the flipped MEMS device layer of the sensor of the present invention, comprising an anchor region 1, 12 pads (2a, 2b, 2c, 2d, 2e, 2f, 2g, 2h, 2i, 2j, 2k, 2l), four L-shaped piezoelectric drive beams (3a, 3b, 3c, 3d), and a center plate 4. Center plate 4 serves as a shared movable top plate for capacitance detection, forming eight capacitance pairs with eight fixed metal electrodes within the substrate cavity. Center plate 4 is located at the center of the bottom surface of the MEMS device layer, connected to the bosses via anchor points, and is coplanar with anchor region 1.

[0053] Four L-shaped piezoelectric drive beams surround the center plate in a clockwise / counterclockwise arrangement. After the MEMS device layer is flipped, each beam consists of an L-shaped silicon beam 15, a driving lower electrode Pt16, a piezoelectric material PZT17, and a distributed driving upper electrode Pt5. One end of each beam is fixed to the center of the center plate's edge, the other end is fixed to the anchor area, and the rest of the beam is suspended in the air. By applying equal and opposite voltages to the distributed upper electrodes (5a, 5b, 5c, 5d, 5e, 5f, 5g, 5h) on the four L-shaped piezoelectric drive beams (3a, 3b, 3c, 3d), the center plate 4 is driven to achieve three degrees of freedom (torsion about the x / y axes and translation along the z axis), thereby simulating the motion state when a three-dimensional force is applied.

[0054] Twelve pads are distributed around the anchor area 1, with three pads distributed around each week. Pads (2a, 2b, 2c, 2d) are used to lead the ground signal to the surrounding areas, and pads (2e, 2f, 2g, 2h, 2i, 2j, 2k, 2l) are used to lead the voltage signals of the two distributed upper electrodes on each piezoelectric drive beam to the surrounding areas.

[0055] Figure 1(b) shows a cross-sectional view of the sensor's overall structure, which consists of an inverted MEMS device layer 6 bonded to a substrate 7. The MEMS device layer's anchor area 1 is in close contact with the substrate's bonding area 12, forming a closed space for capacitive sensing. The initial capacitance spacing is determined by the depth of the substrate cavity 11. When a three-dimensional force is applied to the boss 8, it drives the center plate 4 through the anchor point 9, causing the spacing between the center plate and the fixed metal electrode to change, resulting in a change in capacitance.

[0056] At the same time, pad 2 of the MEMS device layer is bonded to the conductive pad 13 of the substrate in a one-to-one correspondence to achieve electrical connection, and the ground signals of the center plate and piezoelectric drive beam, as well as the voltage signal of the distributed upper electrode, are led to the back side of the substrate through silicon vias (10i, 10j, 10k, 10l, 10m, 10n, 10o, 10p, 10q, 10r, 10s, 10t) below the conductive pad. Since this figure is a cross-sectional view, the silicon vias (10o, 10p, 10q, 10r, 10s, 10t) are not shown. In addition, eight silicon vias (10a, 10b, 10c, 10d, 10e, 10f, 10g, 10h) below the eight fixed metal electrodes in the substrate cavity 11 are used to lead the capacitive signals of the fixed metal electrodes to the back side of the substrate.

[0057] FIG1( c ) is a top view of the sensor substrate of the present invention, including a bonding area 12, 12 conductive pads (13 a, 13 b, 13 c, 13 d, 13 e, 13 f, 13 g, 13 h, 13 i, 13 j, 13 k, 13 l), a cavity 11 and fixed metal electrodes A, B, C, D, E, F, G, H (14 a, 14 b, 14 c, 14 d, 14 e, 14 f, 14 g, 14 h). The bonding area 12 is consistent with the anchor area pattern of the flipped MEMS device layer and is in corresponding contact; 12 conductive pads are distributed around the bonding area, 3 per week, and are bonded to the 12 pads of the flipped MEMS device layer, of which 4 conductive pads (13a, 13b, 13c, 13d) are used to realize the electrical connection of the ground signal of the center plate and the piezoelectric drive beam, and the other 8 conductive pads (13e, 13f, 13g, 13h, 13i, 13j, 13k, 13l) are used to realize the electrical connection of the voltage signal of the distributed upper electrode; the fixed metal electrodes are distributed in the cavity, and the center plate serves as a common upper plate for capacitance detection, forming a capacitor pair with the 8 fixed metal electrodes in the substrate cavity, wherein the diagonal area of ​​the center plate corresponds to the 4 fixed metal electrodes A, B, C, D (14a, 14b, 14c, 14d) on the substrate, forming a force detection capacitor C A 、C B 、C C 、C D The symmetry axis area of ​​the center plate corresponds to the four fixed metal electrodes E, F, G, H (14e, 14f, 14g, 14h) distributed in a cross on the substrate, forming a calibration detection capacitor C E 、C F 、C G 、C H To ensure that only the spacing changes during capacitance detection, the sum of the areas of the eight fixed metal electrodes is less than the center plate area. The capacitance signal is led to the back of the substrate through silicon vias (10a, 10b, 10c, 10d, 10e, 10f, 10g, and 10h) on the back of the fixed metal electrodes.

[0058] Figure 2 This is a process flow chart for preparing the sensor of the present invention. The specific process flow is as follows:

[0059] (i) Prepare a 4-inch SOI low-resistivity silicon wafer (upper silicon layer 50 μm thick, lower silicon layer 300 μm thick, and middle oxide layer 4 μm thick) and form an oxide layer on both sides of the wafer by thermal oxidation.

[0060] (j) A 50 μm deep trench is created by deep reactive ion etching (DRIE), the intermediate oxide layer is dry-removed, Cu is sputtered and electroplated to fill the trench, and the surface is polished to expose the hole. This step is to achieve electrical connection between the upper and lower silicon layers of SOI.

[0061] (k) A 200nm thick Pt electrode and a 1µm thick PZT piezoelectric material are grown on the top surface of the silicon wafer. The PZT is wet-etched and grounded. This step preserves the PZT on the center plate to increase the dielectric constant of the capacitor.

[0062] (l) A 200 nm thick Pt electrode is deposited on the upper surface of the PZT, and the upper metal electrode is patterned using ion beam etching to form the distributed upper electrode and pad of the L-shaped piezoelectric driving beam.

[0063] (m) The PZT and bottom Pt are removed by IBE, and the top silicon is etched by DRIE. The center plate that needs to be released on the top is opened, and the four L-shaped piezoelectric drive beams are released and the release hole stops at the oxide layer.

[0064] (n) Isotropic etching of the middle oxide layer using steam hydrofluoric acid releases the center plate and the L-shaped piezoelectric drive beam. In this process, the backside thermal oxide layer is also removed simultaneously.

[0065] (o) Etching the substrate silicon to form a protrusion.

[0066] (p) The MEMS device layer wafers are first cut and then bonded to the substrate one by one, so that the pads of the flipped MEMS device layer are aligned and bonded to the conductive pads of the substrate. The anchor area of ​​the MEMS device layer contacts the bonding area of ​​the substrate, forming a closed cavity for capacitance detection. The capacitance signal, ground signal, and piezoelectric drive voltage signal are all led to the back side of the substrate through TSV.

[0067] Figure 3 (a) is a schematic diagram of the working of the sensor of the present invention under the action of shear force +Fx. X When the boss transfers the load to the center plate 4 through the anchor point 9, the center plate 4 is driven to twist around the y-axis. The boundary in the +X direction drops, and the distance between the boss and the corresponding fixed metal electrodes (14b, 14d) becomes smaller, and the capacitance value C is detected. B 、CD becomes larger to 2C+2ΔC; the boundary in the -X direction rises, and the distance between the corresponding fixed metal electrodes (14a, 14c) becomes larger, and the detection capacitance value C A 、C C It becomes smaller to 2C-2ΔC, so the tangential differential capacitance is calculated to be 4ΔC. The detection method in the Y direction is the same.

[0068] Figure 3(b) is a schematic diagram of the working state of the sensor of the present invention under the action of normal force -Fz. Z When the boss drives the center plate 4 to move in the -Z direction through the anchor point 9, the distance between the boss and the fixed metal electrodes (11a, 11b, 11c, 11d) is reduced, and the capacitance value C is detected. A 、C B 、C C 、C D They all become larger to C+ΔC, and the change value of each capacitance is almost the same.

[0069] Figure 3(c) is a schematic diagram of the working state of the sensor of the present invention to achieve +X tangential self-calibration. In the self-calibration mode, equal and opposite voltages as shown in the figure are applied to the distributed upper electrodes (5a, 5b, 5c, 5d, 5e, 5f, 5g, 5h), wherein the red part is applied with a positive voltage, the green part is applied with a negative voltage, and the black part is grounded. The inverse piezoelectric effect of PZT will cause the piezoelectric drive beam (3a, 3b, 3c, 3d) to drive the center plate 4 to twist around the y-axis, thereby simulating the motion state when +Fx is applied, and the greater the applied voltage, the greater the range of calibrated tangential force. At this time, the boundary of the center plate in the +X direction drops, and the distance from the fixed metal electrode (14g) becomes smaller, and the calibration capacitance value C G The boundary in the -X direction rises, and the distance from the fixed metal electrode (14f) increases, and the calibration capacitance value C F It is reduced to C-ΔC, and the tangential differential capacitance is calculated to be 2ΔC. Due to the symmetry of the structure, the calibration capacitor C E C H It is almost unchanged. Finally, the tangential self-calibration function is realized by comparing the proportional relationship between the detection capacitance and the calibration capacitance. The tangential self-calibration method in the Y direction is the same.

[0070] Figure 3 (d) is a schematic diagram of the sensor of the present invention implementing -Z normal self-calibration. In the self-calibration mode, the distributed upper electrodes (5a, 5b, 5c, 5d, 5e, 5f, 5g, 5h) are applied with equal and opposite voltages as shown in the figure, wherein the red portion is applied with a positive voltage, the green portion is applied with a negative voltage, and the black portion is grounded. The piezoelectric drive beams (3a, 3b, 3c, 3d) drive the center plate 4 to move horizontally along the -Z axis to simulate the motion state when -Fz is applied. The greater the applied voltage, the greater the range of calibrated normal force. At this time, the center plate as a whole moves horizontally in the -Z direction, and the spacing with the fixed metal electrodes EFGH (14e, 14f, 14g, 14h) is all reduced. Each detection capacitance value becomes larger than C+ΔC, and each capacitance change value is almost the same. The normal self-calibration function is achieved by comparing the detection capacitance with the calibration capacitance.

[0071] In summary, the present invention provides a self-calibrating SOI-based MEMS three-dimensional force sensor. At the beginning of the structural design, a driving module and a multi-capacitor detection module are added to the sensitive element to ensure the accuracy of the measurement results and save additional calibration costs. While achieving self-calibration, the present invention adopts a three-dimensional integration method, directly using the SOI wafer for external packaging, and the electrical signal is led to the back of the substrate through the silicon via, which saves additional packaging costs.

Claims

1. A self-calibrating SOI-based MEMS three-dimensional force sensor, characterized by: including a MEMS device layer and a substrate bonded below the MEMS device layer; The MEMS device layer includes a boss, an anchor point, an anchor area, a center plate, four L-shaped piezoelectric driving beams located around the center plate, and twelve pads; The boss is located above the anchor point and serves as a force-bearing structure during three-dimensional force detection; The central plate and the anchor area are both located below the anchor point and on the same plane; the central plate serves as a movable upper plate for capacitance detection and is connected to the boss through the anchor point; A single L-shaped piezoelectric drive beam comprises, from top to bottom, an L-shaped silicon beam, a driving lower electrode Pt, a piezoelectric material PZT, and two distributed driving upper electrodes Pt. One end of the L-shaped piezoelectric drive beam is fixed to the center of the center plate boundary, and the other end is fixed to the anchor area; the rest of the beam is suspended in the air. The two distributed driving upper electrodes Pt are used to apply equal and opposite voltages to directly provide driving excitation for the sensitive elements within the three-dimensional force sensor. The twelve pads are located around the anchor area of ​​the MEMS device layer and are in the same plane as the distributed driving upper electrode Pt of the piezoelectric driving beam; The substrate includes a bonding area, a cavity, a conductive pad, and eight fixed metal electrodes; The bonding area is consistent with the pattern of the anchor area of ​​the MEMS device layer, and the anchor area of ​​the flipped MEMS device layer is aligned and in contact with the bonding area of ​​the substrate below it, forming a closed space for capacitive sensing; The cavity is used to form an activity space for the piezoelectric driving beam and the center plate; The eight fixed metal electrodes are distributed in the cavity, and the center plate serves as a common movable upper plate for capacitance detection, forming a capacitance pair with the eight fixed metal electrodes in the substrate cavity. The diagonal area of ​​the center plate corresponds to the four fixed metal electrodes A, B, C, and D on the substrate, forming a force detection capacitor C. A 、C B 、C C 、C D The symmetry axis area of ​​the center plate corresponds to the cross-distributed fixed metal electrodes E, F, G, H on the substrate, forming a self-calibration detection capacitor C E 、C F 、C G 、C H .

2. The self-calibrating SOI-based MEMS three-dimensional force sensor according to claim 1, characterized in that: The four L-shaped piezoelectric driving beams are arranged clockwise or counterclockwise around the central plate.

3. The self-calibrating SOI-based MEMS three-dimensional force sensor according to claim 1, characterized in that: The anchor area has three pads distributed around it, which are used to lead the voltage signals and ground signals of two distributed upper electrodes on a piezoelectric driving beam to the surrounding areas and bond with the conductive pads of the substrate to achieve electrical connection.

4. The self-calibrating SOI-based MEMS three-dimensional force sensor according to claim 3, characterized in that: There are twelve conductive pads distributed around the substrate bonding area, and bonded to the twelve pads of the flipped MEMS device layer to achieve electrical connection; three conductive pads are arranged every week in the bonding area, one of which is used to achieve electrical connection of the ground signal, and the other two are used to achieve electrical connection of the voltage signals of the two distributed upper electrodes on the piezoelectric drive beam.

5. The self-calibrating SOI-based MEMS three-dimensional force sensor according to claim 4, characterized in that: There are twenty through-silicon vias inside the substrate, twelve of which are located between the twelve conductive pads and the back of the substrate, and are used to lead the ground signals of the center plate and the piezoelectric drive beam, as well as the voltage signals of the distributed upper electrode to the back of the substrate; the other eight are located between the eight fixed metal electrodes in the cavity of the substrate and the back, and are used to lead the capacitance signals of the fixed metal electrodes to the back of the substrate.

6. The self-calibrating SOI-based MEMS three-dimensional force sensor according to claim 1, characterized in that: The depth of the cavity is the initial spacing of the capacitive sensor and is determined by the depth of the substrate groove.

7. The self-calibrating SOI-based MEMS three-dimensional force sensor according to claim 1, characterized in that: The total area of ​​the eight fixed metal electrodes is smaller than the area of ​​the central plate.

8. A process for preparing the self-calibrating SOI-based MEMS three-dimensional force sensor according to claim 1, characterized in that: (a) Prepare a SOI low-resistivity silicon wafer and form an oxide layer on the front and back of the wafer by thermal oxidation; (b) Deep reactive ion etching trenches are used to dry-remove the intermediate oxide layer; Cu is sputtered and electroplated to fill the trenches, and the surface is polished to expose the holes; (c) sequentially growing a Pt electrode and a piezoelectric material PZT on the top surface of a silicon wafer; simultaneously, wet-etching the piezoelectric material PZT to extract a ground signal; (d) Depositing a Pt electrode on the upper surface of the piezoelectric material PZT, and patterning the upper metal electrode using ion beam etching to form a distributed upper electrode and pad for the L-shaped piezoelectric drive beam; (e) The piezoelectric material PZT and the bottom Pt electrode are removed by IBE, and the top silicon is etched by DRIE. The center plate and four L-shaped piezoelectric drive beams that need to be released are opened, and the release holes are automatically stopped at the oxide layer. (f) Using vapor hydrofluoric acid to isotropically etch the intermediate oxide layer, releasing the center plate and the L-shaped piezoelectric drive beam; (g) etching the substrate silicon to form a protrusion; (h) cutting the MEMS device layer wafers and bonding them to the substrate one by one, so that the flipped MEMS device layer pads are aligned and bonded to the conductive pads of the substrate, and the anchor area of ​​the MEMS device layer contacts the bonding area of ​​the substrate, forming a closed cavity for capacitance detection; The capacitance signal, ground signal and piezoelectric drive voltage signal are all led to the back side of the substrate through silicon vias.

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