A Highly Sensitive MEMS Capacitive Accelerometer

Abstract

The present invention discloses a high-sensitivity MEMS capacitive acceleration sensor, which belongs to the technical field of micro-electromechanical systems; it comprises an upper plate layer, an intermediate plate layer and a lower plate layer stacked in sequence; the main body of the plate layer is an upper plate, and the upper surface of the upper plate is provided with an upper electrode lead-out site; the lower surface of the upper plate is provided with an upper annular groove; the area between the upper annular grooves is an upper electrode surface; the upper electrode surface has an upper anti-overload convex point; the main body of the intermediate plate layer comprises an upper intermediate plate and a lower intermediate plate closely attached, the center of the upper intermediate plate is provided with an intermediate electrode surface connected by an annular beam, and one side of the upper surface of the upper intermediate plate is provided with an intermediate electrode lead-out site; the upper surface of the lower intermediate plate is provided with an annular cavity and an air guide groove, and the area between the annular cavities is a mass block. The present invention improves the sensitivity, stability and consistency of the sensor and can effectively reduce the processing difficulty of the sensor.

Description

Technical Field

[0001] The invention relates to the technical field of micro-electromechanical systems, and in particular to a high-sensitivity MEMS capacitive acceleration sensor. Background Art

[0002] Inertial devices refer to inertial instruments and the inertial measurement devices and inertial measurement systems composed of these instruments. As one of the most important inertial instruments, the accelerometer is a sensor that can measure the acceleration of an object. Since its launch, it has been used in vibration monitoring, seismic detection, oil exploration, automotive electronics, attitude control, hydroacoustic measurement, inertial navigation and inertial guidance systems. With the vigorous development of micro-mechanical manufacturing technology and the demand for various military and civilian products, the field of inertial technology is undergoing a profound change. New accelerometers represented by MEMS technology Micro-Electro-Mechanical System and micro-electromechanical system have been continuously developed and improved. With its many advantages such as small size, light weight, low cost, strong environmental adaptability, high reliability and long life, they have been widely used in military, civilian and commercial fields.

[0003] With the in-depth study of MEMS processing technology, the structure and form of MEMS accelerometers have become more and more diversified, and there are many ways to classify MEMS accelerometers. According to the different sensing methods, they can be divided into piezoresistive accelerometers, piezoelectric accelerometers, capacitive accelerometers, resonant accelerometers, tunneling accelerometers, thermal convection accelerometers, etc. Among them, capacitive accelerometers have become the most developed and more mature type of micro-electromechanical accelerometers at home and abroad due to their good repeatability, high sensitivity, small temperature coefficient, and high-precision closed-loop servo control.

[0004] MEMS capacitive accelerometers mainly include two structures, comb-tooth type and sandwich type. The comb-tooth type is named because its electrodes, including fixed electrodes and movable electrodes, are shaped like comb teeth, also known as interdigital type. The comb-tooth type capacitive accelerometer has a simple process, but the detection accuracy is low. The sandwich type, also known as the cantilever beam type, is a sandwich structure. It is named because the movable electrode is sandwiched between the fixed electrode and looks like a sandwich. When the mass block is stimulated by external acceleration and moves, its capacitor spacing will change, resulting in a change in the size of the capacitor. When the displacement of the mass block is small, the size of the capacitor is approximately linearly proportional to the external acceleration.

[0005] In the "sandwich" structure capacitive acceleration sensor, in order to increase the sensitivity of the sensor, it is generally necessary to increase the area facing the detection capacitor or increase the weight of the inertial mass block. However, due to the thickness of the silicon wafer and the processing technology, a bonding process is often used to connect two silicon wafers with the same structure and size in a "back-to-back" form to achieve the production of a large mass block. At the same time, in order to ensure the stability of the sensor and reduce cross-sensitivity, a fully symmetrical beam-mass block structure design is generally adopted. Therefore, the manufacturing method and structure of the beam-mass block structure become the key to sensor design and processing, and directly determine the performance of the acceleration sensor.

[0006] Among the existing capacitive "sandwich" structure acceleration sensor processing methods, single crystal silicon and KOH wet etching methods are generally used to make mass block structures, and then the cantilever beam structure is released by dry etching. Since the silicon wafers generally used are thicker and the etching rate of wet etching is lower, the etching time of a single wafer takes several or even dozens of hours, which can easily cause the mask to be damaged and drilled, resulting in defects on the surface of the silicon wafer, affecting the subsequent bonding process and even causing the silicon wafer to break. At the same time, due to the lack of a self-stop layer or a method to accurately control the etching thickness, the thickness of the beam cannot be accurately controlled, so that the sensor cannot achieve the expected design effect, affecting indicators such as sensitivity and stability. Summary of the invention

[0007] In view of the problems existing in the above-mentioned background technology, the present invention proposes a high-sensitivity MEMS capacitive acceleration sensor; the sensor adopts a "sandwich" structure of four-layer silicon wafer bonding, the middle mass block adopts a circular design, and the cantilever beam adopts a spiral structure, which can provide a larger capacitance area in a limited space and improve the sensitivity of the sensor. At the same time, the middle beam-mass block structure is made of SOI silicon wafers, and the thickness of the cantilever beam can be accurately controlled by the self-stop process using the middle silicon dioxide box layer; while improving the sensitivity, stability and consistency of the sensor, it can effectively reduce the processing difficulty of the sensor.

[0008] In order to achieve the above object, the technical solution adopted by the present invention is:

[0009] A high-sensitivity MEMS capacitive acceleration sensor comprises an upper plate layer, a middle plate layer and a lower plate layer stacked in sequence;

[0010] The main body of the upper electrode layer is the upper plate, the upper surface of the upper plate is provided with an upper electrode lead-out site; the lower surface of the upper plate is provided with an upper annular groove; the area between the upper annular grooves is the upper electrode surface; the upper electrode surface has an upper overload protection convex point;

[0011] The main body of the intermediate electrode layer includes an upper intermediate electrode plate and a lower intermediate electrode plate which are closely attached, wherein the upper intermediate electrode plate is located on the lower surface of the upper plate, and the lower intermediate electrode plate is located on the upper surface of the lower plate; an intermediate electrode surface connected by an annular beam is provided at the center of the upper intermediate electrode plate, and an intermediate electrode lead-out site is provided on one side of the upper surface of the upper intermediate electrode plate; an annular cavity and an air guide groove are provided on the upper surface of the lower intermediate electrode plate, and the area between the annular cavities is a mass block; one end of the air guide groove is connected to the annular cavity, and the other end is connected to the outside;

[0012] The main body of the lower plate layer is the lower plate, the upper surface of the lower plate is provided with a lower annular groove; the area between the lower annular grooves is the lower electrode surface; the lower electrode surface has a lower overload protection convex point; one of the two opposite sides of the lower surface of the lower electrode plate are provided with a lower electrode lead-out site;

[0013] The upper electrode surface, the middle electrode surface, the mass block and the lower electrode surface are directly opposite to each other.

[0014] Furthermore, the upper electrode layer, the middle electrode layer and the lower electrode layer are connected by a bonding process.

[0015] Furthermore, the upper plate and the lower plate are both made of a single crystal silicon wafer;

[0016] Furthermore, the intermediate electrode layer is made of two SOI silicon wafers through a bonding process.

[0017] Furthermore, the upper anti-overload bump and the lower anti-overload bump are both formed based on thermal oxidation, photolithography and dry etching.

[0018] Furthermore, the lower surface of the upper electrode plate and the upper surface of the lower electrode plate both have a frame layer manufactured based on thermal oxidation, photolithography and dry etching.

[0019] Furthermore, the anti-overload bumps and the frame layer are made of silicon dioxide.

[0020] Furthermore, the annular beam of the intermediate plate layer is an annular elastic beam formed by dry etching the SOI silicon wafer device layer, and the etching stops at the buried oxide layer.

[0021] Furthermore, the mass block of the intermediate plate layer is a cylindrical structure formed by deep dry etching of the bottom silicon of the SOI silicon wafer, and the etching stops at the buried oxide layer.

[0022] Furthermore, the upper electrode surface, the middle electrode surface and the lower electrode surface are all circular, and have the same area.

[0023] Furthermore, the upper electrode lead-out site, the middle electrode lead-out site and the lower electrode lead-out site are rectangular metal strips formed by sputtering.

[0024] Furthermore, there are gaps between the upper electrode layer and the middle electrode layer, and between the lower electrode layer and the middle electrode layer, and the width of the gap is the same as the height of the frame layer.

[0025] The beneficial effects of the above technical solution of the present invention are:

[0026] 1 The present invention uses two SOI silicon wafers bonded together to form an intermediate electrode. The beam is etched from the device layer of the SOI silicon wafer, and the mass block is etched from the bottom silicon of the SOI silicon wafer. Both can achieve self-stopping through the middle buried oxide layer of the SOI silicon wafer, which is beneficial to the precise control of the sensor structure size and improves consistency;

[0027] 2 The beam-mass block structure of the present invention adopts a circular design, which can effectively increase the area facing the capacitor plates within the effective space, thereby increasing the capacitance value generated by the mass block per unit movement distance; on the other hand, it increases the mass of the mass block, which can effectively improve the sensitivity of the sensor;

[0028] 3 All structures in the present invention are prepared by dry etching method, which can avoid damage to the device caused by long-term wet etching. The target structure can be obtained without using convex corner protection structure, and the etching depth can be accurately controlled, which greatly reduces the difficulty of processing.

[0029] The present invention can achieve the adjustment of different resonant frequencies, different ranges and different sensitivities by changing the thickness of the SOI silicon wafer device layer and the bottom silicon, adjusting the parameters of the beam and the mass block, and adjusting the spacing between the electrodes.

[0030] 5 The present invention adopts a symmetrical structural design, which can increase the stability of the sensor. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 It is a schematic diagram of the overall structure of an embodiment of the present invention.

[0032] Figure 2 for Figure 1 Exploded diagram.

[0033] Figure 3 for Figure 1 Schematic diagram of the upper surface structure of the middle upper electrode layer.

[0034] Figure 4 yes Figure 1 Schematic diagram of the lower surface structure of the middle and upper electrode layer.

[0035] Figure 5 yes Figure 1 Schematic diagram of the structure of the intermediate plate layer.

[0036] Figure 6 yes Figure 5Schematic diagram of the structure of the upper surface of the middle and lower intermediate plates.

[0037] Figure 7 yes Figure 1 Schematic diagram of the lower surface of the middle and lower level boards.

[0038] Figure 8 for Figure 1 Schematic diagram of the upper surface of the middle and lower plates. DETAILED DESCRIPTION

[0039] The present invention is further described below in conjunction with the accompanying drawings and specific embodiments.

[0040] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0041] like Figure 1 As shown, the acceleration sensor includes an upper plate layer 11, an intermediate plate layer 12 and a lower plate layer 13. The upper plate layer 14 and the lower plate layer 13 are respectively made of a single piece of low-resistance single crystal silicon material; the intermediate plate layer includes two pieces of the same parameters and low-resistance SOI silicon wafers, which are formed by "back-to-back" bonding. The upper plate layer, the intermediate plate layer and the lower plate layer are connected together by a bonding process.

[0042] like Figure 2 As shown, the upper electrode layer includes an external frame 21, an upper electrode lead-out site 22, an internal frame 23, an anti-overload convex point 24, an annular groove 25 and an upper electrode surface 26. The main material of the upper electrode layer is low-resistance single crystal silicon; the external frame 21 is a rectangular structure formed after cutting; the upper electrode lead-out site 22 is a conductive metal strip formed based on photolithography, etching and sputtering; the internal frame 23 is a frame structure formed based on thermal oxidation, photolithography and dry etching; the anti-overload convex point 24 is a protruding structure formed based on thermal oxidation, photolithography and dry etching; the annular groove 25 is a sunken structure formed based on photolithography and dry etching; and the upper electrode surface 26 is circular.

[0043] Preferably, the thickness of the upper electrode layer is 400 microns;

[0044] Preferably, the material of the upper electrode lead-out site 22 is Au;

[0045] Preferably, the height of the internal frame 23 is 2 to 3 microns;

[0046] Preferably, the number of the anti-overload protrusions 24 is greater than 4, square or circular, 1 micron in height, and symmetrically distributed on the surface of the circular electrode surface 26;

[0047] Preferably, the internal frame 23 and the anti-overload protrusion 24 are made of silicon dioxide;

[0048] Preferably, the circular electrode surface 26 and the annular groove 25 are concentric.

[0049] like Figure 3 As shown, the intermediate plate mainly includes an external frame 31, an intermediate electrode surface 32, an annular beam 33, a mass block 34, an annular cavity 35, an air guide groove 36, an internal frame 37, a fixed anchor point 38 and an intermediate electrode lead-out site 39. The intermediate plate is formed by two SOI silicon wafers (including a device layer, a buried oxide layer and a bottom silicon) bonded "back to back"; the external frame 31 is made of the SOI silicon wafer device layer after photolithography, dry etching and cutting; the intermediate electrode surface 32 is a cylindrical structure made of the SOI silicon wafer device layer by photolithography and dry etching; the annular beam 33 is a cantilever beam structure made of the SOI silicon wafer device layer by photolithography and dry etching, and the etching stops at the buried oxide layer; the annular beam 33 is connected to the intermediate electrode surface 32 and the external frame 31 through a fixed anchor point 38; the mass block 34 is a cylindrical structure made of the bottom silicon layer of the SOI silicon wafer ... intermediate plate is formed by two SOI silicon wafers (including a device layer, a buried oxide layer and a bottom silicon layer) bonded "back to back"; the external frame 31 is formed by two SOI silicon wafers (including a device layer, a buried oxide layer and a bottom silicon layer) bonded "back to back"; the It stops at the buried oxide layer; the middle electrode surface 32 and the mass block 34 are connected through the buried oxide layer; the annular cavity 35 is a cylindrical cavity made of the bottom silicon of the SOI silicon wafer by photolithography and dry etching; the gas guide groove 36 is a rectangular groove formed by the bottom silicon of the SOI silicon wafer by photolithography and dry etching; the internal frame 37 is made of the lower silicon of the SOI silicon wafer by photolithography, dry etching and cutting; the external frame 31 and the internal frame 37 are connected through the buried oxide layer; the fixed anchor point 38 connects the annular cantilever beam 33 with the middle electrode surface 32 and the external frame 31; the middle electrode lead-out site 39 is a metal strip made of metal material by sputtering process.

[0050] Preferably, the SOI silicon wafer device layer has a thickness of 50 microns, the buried oxide layer has a thickness of 2 microns, and the bottom silicon layer has a thickness of 400 microns;

[0051] Preferably, the mass block 34, the annular cavity 35 and the intermediate electrode surface 32 are concentric;

[0052] Preferably, the intermediate electrode surface 32 and the mass block 34 have the same area;

[0053] Preferably, the number of the annular beams 33 is 4 pairs, which are symmetrically distributed;

[0054] Preferably, the annular beam 33 is an elastic beam with a certain stiffness coefficient;

[0055] Preferably, the metal material of the intermediate electrode lead-out site 39 is Au;

[0056] Furthermore, the air guide groove 36 is used to discharge the internal air during the sensor packaging process;

[0057] Furthermore, the internal frame 23 in the upper electrode layer 11 is connected to the external frame 31 in the middle electrode layer 12 by a bonding process;

[0058] Furthermore, there is a gap between the internal frame 23 in the upper electrode layer 11 and the middle electrode surface 32 of the middle electrode layer 12 through a bonding process, and the gap width is the same as the height of the internal frame 23 in the upper electrode layer 11;

[0059] like Figure 4 As shown, the lower electrode layer includes an external frame 41, a lower electrode lead-out site 42, an internal frame 43, an overload protection bump 44, an annular groove 45 and a lower electrode surface 46. The lower electrode layer material is low-resistance single crystal silicon; the external frame 41 is rectangular; the lower electrode lead-out site 42 is a conductive metal strip formed based on photolithography and sputtering; the internal frame 43 is a rectangular structure formed based on thermal oxidation, photolithography and dry etching; the overload protection bump 44 is a convex structure formed based on thermal oxidation, photolithography and dry etching; the annular groove 45 is a sunken structure formed based on photolithography and dry etching; and the lower electrode surface 46 is circular.

[0060] Preferably, the thickness of the lower electrode layer is 400 microns;

[0061] Preferably, the material of the lower electrode lead-out site 42 is Au, and is symmetrically distributed up and down;

[0062] Preferably, the height of the internal frame 43 is 2 to 3 microns;

[0063] Preferably, the number of the anti-overload protrusions 44 is greater than 4, square or circular, 1 micron in height, and symmetrically distributed on the surface of the circular electrode surface 46;

[0064] Preferably, the internal frame 43 and the anti-overload convex material 44 are silicon dioxide;

[0065] Preferably, the circular electrode surface 46 and the annular groove 45 are concentric.

[0066] Furthermore, the inner frame 43 in the lower electrode layer 41 is connected to the outer frame 31 in the middle electrode layer 12 by a bonding process;

[0067] Furthermore, there is a gap between the internal frame 43 in the lower electrode layer 41 and the middle electrode surface 32 of the middle electrode layer 12 through a bonding process, and the gap width is the same as the height of the internal frame 43 in the lower electrode layer 41;

[0068] The present invention has an adjustable working frequency and has the advantages of large effective area, high reliability, easy manufacture, high sensitivity, etc. In addition, the present invention adopts an annular beam structure, and the number of beams can be designed according to needs, so as to obtain different system elastic coefficients and system characteristic frequencies. By adjusting the number of cantilever circular ring beams, it can adapt to different use environments. For example, when the number of annular beams is adjusted to 3 to 6, the characteristic frequency of the acceleration sensor system decreases, thereby obtaining a small-range, high-sensitivity MEMS capacitive acceleration sensor.

[0069] The working principle of the present invention is as follows:

[0070] The MEMS capacitive acceleration sensor consists of an upper plate layer, a lower plate layer and an intermediate plate layer, which serve as the upper electrode, the lower electrode and the intermediate electrode respectively. The intermediate electrode contains a movable beam-mass block structure. The intermediate electrode mass block senses acceleration and transmits the acceleration signal through the differential capacitance formed by the upper and lower electrodes. The displacement change caused by acceleration causes the capacitance between the intermediate mass block and the upper and lower fixed plates to change. Ideally, the mass block is located in the middle of the two fixed plates. In the absence of acceleration, the mass layer does not move, and the expression of the upper and lower capacitance values ​​is:

[0071]

[0072] Where C1 and C2 are the capacitance values ​​of the upper and lower parts, ε is the relative dielectric constant, ε0 is the vacuum dielectric constant, S is the area facing the two electrodes, and d0 is the initial distance between the electrodes. Since the mass layer does not move, the capacitances C1 and C2 are equal, and ΔC = 0, which means that no capacitance change occurs.

[0073] When the MEMS capacitive accelerometer senses external acceleration, due to the inertial force, the mass layer will shift in the opposite direction of the acceleration, resulting in a displacement of x. At this time, the expression of the upper and lower capacitance values ​​is:

[0074]

[0075]

[0076] At this time, the difference between the upper and lower capacitances is:

[0077]

[0078] Through the conversion output of the differential capacitor, the acceleration value a can be calculated as:

[0079]

[0080] Where m is the mass of the mass layer mass block, and k is the total elastic stiffness of the system.

[0081] It should be noted that the above description is only a specific example of the present invention and does not constitute any limitation to the present invention. Obviously, for professionals in this field, after understanding the content and principles of the present invention, it is possible to make various modifications and changes in form and details without departing from the principles and structures of the present invention. However, these modifications and changes based on the ideas of the present invention are still within the scope of protection of the claims of the present invention.

Claims

1. A high-sensitivity MEMS capacitive acceleration sensor, characterized in that: It comprises an upper plate layer, an intermediate plate layer and a lower plate layer stacked in sequence; The main body of the upper electrode layer is the upper plate, the upper surface of the upper plate is provided with an upper electrode lead-out site; the lower surface of the upper plate is provided with an upper annular groove; the area between the upper annular grooves is the upper electrode surface; the upper electrode surface has an upper overload protection convex point; The main body of the intermediate electrode layer includes an upper intermediate electrode plate and a lower intermediate electrode plate which are closely attached, wherein the upper intermediate electrode plate is located on the lower surface of the upper plate, and the lower intermediate electrode plate is located on the upper surface of the lower plate; an intermediate electrode surface connected by an annular beam is provided at the center of the upper intermediate electrode plate, and an intermediate electrode lead-out site is provided on one side of the upper surface of the upper intermediate electrode plate; an annular cavity and an air guide groove are provided on the upper surface of the lower intermediate electrode plate, and the area between the annular cavities is a mass block; one end of the air guide groove is connected to the annular cavity, and the other end is connected to the outside; The main body of the lower plate layer is the lower plate, the upper surface of the lower plate is provided with a lower annular groove; the area between the lower annular grooves is the lower electrode surface; the lower electrode surface has a lower overload protection convex point; one of the two opposite sides of the lower surface of the lower electrode plate are provided with a lower electrode lead-out site; The upper electrode surface, the middle electrode surface, the mass block and the lower electrode surface are directly opposite; the upper electrode plate layer, the middle electrode plate layer and the lower electrode plate layer are connected by a bonding process; the upper electrode plate and the lower electrode plate are both made of a single crystal silicon wafer; the middle electrode plate layer is made of two SOI silicon wafers by a bonding process; the upper anti-overload bumps and the lower anti-overload bumps are both formed based on thermal oxidation, photolithography and dry etching; the lower surface of the upper electrode plate and the upper surface of the lower electrode plate both have a frame layer made based on thermal oxidation, photolithography and dry etching; the anti-overload bumps and the frame layer are silicon dioxide; the annular beam of the middle electrode plate layer is an annular elastic beam formed by dry etching the SOI silicon wafer device layer, and the etching stops at the buried oxide layer; the mass block of the middle electrode plate layer is a cylindrical structure formed by deep dry etching of the bottom silicon of the SOI silicon wafer, and the etching stops at the buried oxide layer.

2. A high-sensitivity MEMS capacitive sensor according to claim 1, characterized in that: The upper electrode surface, the middle electrode surface and the lower electrode surface are all circular, and the areas of the three are the same.

3. A high-sensitivity MEMS capacitive sensor according to claim 1, characterized in that: The upper electrode lead-out site, the middle electrode lead-out site and the lower electrode lead-out site are rectangular metal strips formed by sputtering.

4. The high-sensitivity MEMS capacitive sensor according to claim 1, characterized in that: There are gaps between the upper electrode layer and the middle electrode layer, and between the lower electrode layer and the middle electrode layer, and the width of the gap is the same as the height of the frame layer.

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