Z-axis mems accelerometer with discrete differential detection

Abstract

The application discloses a discrete differential detection Z-axis MEMS accelerometer in the field of micro mechanical sensors, comprising a substrate, a center anchor point arranged on the substrate, a first mass block and a second mass block, the middle parts of the first mass block and the second mass block being hingedly connected to the center anchor point, the same side ends of the first mass block and the second mass block being capable of crossing each other, discrete electrodes arranged on the substrate on the two sides of the center anchor point and corresponding to the first mass block and the second mass block, the discrete electrodes and the first mass block forming a first sensitive unit, and the discrete electrodes and the second mass block forming a second sensitive unit. The first mass block and the second mass block are hingedly connected to discretely arrange differential detection capacitors, each pair of differential detection capacitors is arranged in close proximity to a local part of the substrate as small as possible, so that the deformation of any local part on the substrate produces nearly the same capacitance change amount on the discretely arranged differential detection capacitors, thereby mutually offsetting and eliminating zero offset.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of micro-mechanical sensor, in particular to a Z-axis MEMS accelerometer with discrete differential detection. BACKGROUND

[0002] An accelerometer is a sensor that converts acceleration into an electrical signal. A capacitive MEMS accelerometer is a specific type of micro-mechanical accelerometer made on the basis of integrated circuit technology, which generally has a movable mass suspended above a substrate, and fixed electrodes forming a detection capacitance with the movable mass, to sense the movement or position change of the movable mass caused by external acceleration. The detection of the in-plane movement of the mass is generally referred to as an X-axis accelerometer, and the detection of the out-of-plane movement of the mass is generally referred to as a Z-axis accelerometer.

[0003] There is a type of Z-axis accelerometer that uses a seesaw structure, in which the movable mass is supported by the substrate, so that the movable mass deflects relative to the substrate under Z-axis acceleration. In many types of accelerometers, fixed detection electrodes located below and / or above the movable mass form a capacitive coupling with the movable mass to detect such deflection of the movable mass, thereby detecting the Z-axis acceleration. Other electrical elements, such as force feedback electrodes, can also be included below and / or above the movable mass.

[0004] One of the important indicators affecting the accuracy of the accelerometer is "zero offset", which refers to the output of a non-zero acceleration signal by the accelerometer when no external acceleration exists, and the accelerometer should output a zero acceleration signal. In the context of the present patent application, the zero offset can be caused by the deformation of the substrate supporting the accelerometer, including asymmetric deformation. The substrate deformation is mainly caused by the mechanical action between the MEMS accelerometer chip and the packaging carrier.

[0005] In various cases, when a specific part of the accelerometer used (such as the substrate) is subjected to internal thermal mechanical stress or absorbs external forces (such as vibration, impact), the substrate will deform in an uneven manner (e.g., asymmetrically), and the substrate deformation can change the nominal distance between the movable mass and one or more position sensing electrodes (the capacitance also changes accordingly), thereby introducing errors into the measurement of the accelerometer (herein referred to as "zero offset"). For example, even if no external acceleration exists, the accelerometer can output a non-zero acceleration signal according to the change in the nominal distance / capacitance caused by asymmetric deformation.

[0006] The seesaw type accelerometer determines the Z-axis acceleration according to the change in capacitance between the differential detection electrodes located above and / or below the movable mass. The existing technical solutions, US6841992, US6935175, US7146856, US8079262 disclose a typical seesaw accelerometer composed of sensing electrodes symmetrically arranged with respect to the supporting beam and a movable mass made on the substrate, US9513310, CN201810650974.2, CN201710780370 disclose a kind of double differential detection seesaw type accelerometer, adjacent seesaw structures move reversely.

[0007] Ideally, when the substrate is not deformed and no external Z-axis acceleration exists, the distance between the movable mass and each bottom differential detection electrode is equal, so the capacitance between the movable mass and each bottom differential detection electrode is equal. Since the capacitance of each set of differential detection electrodes is equal, they cancel each other out, so the accelerometer outputs a zero acceleration signal (i.e. zero offset is zero).

[0008] Generally speaking, if the substrate experiences symmetric deformation, the capacitance between the movable mass and each detection electrode will experience the same amount of change. In this way, any zero offset caused by deformation will be cancelled out. However, in the case of asymmetric deformation of the substrate, or in the case of tilting of the movable mass relative to one side of the substrate, the differential detection capacitances on the opposite sides of the movable mass thus experience different amounts of change, so the measurement of the accelerometer will have a zero offset error.

[0009] How to reduce the effect of asymmetric deformation of the substrate on the zero offset of the seesaw type accelerometer, so that the differential detection capacitances on both sides of the movable mass exhibit the same amount of change to cancel out (e.g. reduce or eliminate) the zero offset, is the core of improving the accuracy of the Z-axis seesaw type accelerometer. SUMMARY

[0010] The purpose of the present application is to provide a discrete differential detection Z-axis MEMS accelerometer to solve the problem of large zero offset error and low accuracy when the accelerometer is used in the prior art.

[0011] To solve the above technical problems, the present application is implemented by using the following technical solutions:

[0012] The discrete differential detection Z-axis MEMS accelerometer comprises

[0013] a substrate, the substrate is provided with a central anchor point;

[0014] a first mass and a second mass, the first mass and the second mass are hingedly connected to the central anchor point, and the same side ends of the first mass and the second mass can cross each other.

[0015] a discrete electrode disposed on the substrate on both sides of the center anchor point and corresponding to the first mass and the second mass;

[0016] the discrete electrode and the first mass form a first sensing unit, and the discrete electrode and the second mass form a second sensing unit, and the first sensing unit and the second sensing unit interact to complete discrete differential detection.

[0017] In a further aspect of the application, the first mass and the second mass are arranged in a staggered manner along the Y axis and are hinged to the center anchor point through a deflection shaft.

[0018] In a further aspect of the application, the first mass is respectively provided with a left comb plate and a right comb plate on both sides, the second mass is respectively provided with a left comb plate and a right comb plate on both sides, and the combs on the left comb plate and the right comb plate are arranged in a staggered manner on the same side.

[0019] In a further aspect of the application, the center distance between the combs of the left comb plate and the center distance between the combs of the right comb plate are the same.

[0020] In a further aspect of the application, the width of the combs on the left comb plate and the right comb plate is equal and less than half of the center distance between the combs of the left comb plate.

[0021] In a further aspect of the application, the discrete electrode includes two groups of left comb electrodes and right comb electrodes arranged in a staggered manner, the two groups of left comb electrodes and right comb electrodes are respectively disposed on the substrate on both sides of the center anchor point, and the right comb electrodes and the right comb plate and the left comb electrodes and the left comb plate on the same side are one-to-one corresponding.

[0022] In a further aspect of the application, the width of the right comb electrode and the width of the comb on the right comb plate are equal and less than half of the center distance between the combs on the right comb plate.

[0023] In a further aspect of the application, the center of mass of the first mass and the second mass is offset from the hinge by a certain distance along the X axis.

[0024] Preferably, the combs on the left comb plate and the right comb plate are arranged along the X axis.

[0025] Preferably, the combs on the left comb plate and the right comb plate are arranged along the Y axis.

[0026] Compared with the prior art, the application has the following beneficial effects:

[0027] The application adopts a first mass block, a second mass block, i.e. two reverse deflection "seesaws" to discretely arrange the differential detection capacitors. The discrete arrangement refers to splitting a pair of differential detection capacitors with a larger area into multiple pairs of differential detection capacitors with a smaller area and a more compact layout. Each pair of differential detection capacitors is arranged in close proximity to a local part of the substrate as small as possible, so that the deformation of any local part of the substrate causes nearly the same amount of change in the discrete differential detection capacitors, which are mutually offset and eliminate zero offset. In other words, the Z-axis accelerometer adopting the solution reduces the influence of asymmetric substrate deformation, so that the change of the differential detection capacitors under zero acceleration input is more uniform and symmetrical, and therefore the zero offset is greatly reduced. BRIEF DESCRIPTION OF DRAWINGS

[0028] FIG. 1 is a schematic diagram of the overall structure of an embodiment of the application. Figure 1 FIG. 1 is a schematic diagram of the overall structure of an embodiment of the application.

[0029] FIG. 1 is a schematic diagram of the overall structure of an embodiment of the application. Figure 2 FIG. 1 is a schematic diagram of the overall structure of an embodiment of the application.

[0030] FIG. 1 is a schematic diagram of the overall structure of an embodiment of the application. Figure 3 FIG. 1 is a schematic diagram of the overall structure of an embodiment of the application. Figure 2 FIG. 1 is a schematic diagram of the overall structure of an embodiment of the application.

[0031] FIG. 1 is a schematic diagram of the overall structure of an embodiment of the application. Figure 4 FIG. 1 is a schematic diagram of the overall structure of an embodiment of the application.

[0032] FIG. 1 is a schematic diagram of the overall structure of an embodiment of the application. Figure 5 FIG. 1 is a schematic diagram of the overall structure of an embodiment of the application.

[0033] FIG. 1 is a schematic diagram of the overall structure of an embodiment of the application. Figure 6 FIG. 1 is a schematic diagram of the overall structure of an embodiment of the application.

[0034] FIG. 1 is a schematic diagram of the overall structure of an embodiment of the application. Figure 7 FIG. 1 is a schematic diagram of the overall structure of an embodiment of the application.

[0035] FIG. 1 is a schematic diagram of the overall structure of an embodiment of the application. Figure 8 FIG. 1 is a schematic diagram of the overall structure of an embodiment of the application.

[0036] FIG. 1 is a schematic diagram of the overall structure of an embodiment of the application. Figure 9 FIG. 1 is a schematic diagram of the overall structure of an embodiment of the application. Figure 7 FIG. 1 is a schematic diagram of the overall structure of an embodiment of the application.

[0037] FIG. 1 is a schematic diagram of the overall structure of an embodiment of the application.

[0038] 10 is a substrate; 20 is a center anchor point; 30a is a first sensitive unit; 30b is a second sensitive unit; 3011 is a first mass; 3012 is a second mass; 302a is a left comb tooth plate; 302b is a right comb tooth plate; 308a is a left comb tooth electrode; 308b is a right comb tooth electrode; 307 is a deflection axis; 308 is a discrete electrode; 309 is a capacitance gap; 40 is a discrete differential capacitance. DETAILED DESCRIPTION

[0039] The technical solutions in the embodiments of the present application will be clearly and completely described with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments of the present application. The following description of at least one exemplary embodiment is merely illustrative in nature and in no way limits the present application and its applications or uses.

[0040] To make the MEMS accelerometer sensitive unit common mode the capacitance change caused by the deformation of the uneven substrate 10, the solution proposed in the present application is to realize the symmetry of the differential detection capacitance in a smaller local part on the substrate 10, where the smaller local part refers to a region on the substrate 10 with an area much smaller than the area of the substrate 10. The design idea of this solution is that any two very close micro-differential electrodes on the substrate 10 will in principle produce common mode capacitance changes caused by the deformation of the substrate 10 at that place. The present application describes this as discrete differential detection. The discrete differential capacitance 40 still maintains local symmetry under the deformation of the uneven substrate 10, so that the overall differential capacitance is symmetrical, thereby reducing the zero offset. The present application will be further described in detail through two specific embodiments Embodiment One

[0041] The present embodiment discloses a discrete differential detection Z-axis MEMS accelerometer, which comprises a substrate 10, wherein the substrate 10 is provided with a center anchor point 20; a first mass 3011 and a second mass 3012, wherein the first mass 3011 and the second mass 3012 are hingedly connected in the middle of the center anchor point 20, and the same side ends of the first mass 3011 and the second mass 3012 can cross each other.

[0042] A discrete electrode 308 is arranged on the substrate 10 on both sides of the center anchor point 20 and is correspondingly arranged with the first mass 3011 and the second mass 3012; the discrete electrode 308 and the first mass 3011 constitute a first sensitive unit 30a.

[0043] The specific structure of this embodiment will be described in detail below with reference to the accompanying drawings Figure 2

[0044] A center anchor 20 is arranged in the center region of the surface of the substrate 10, the area of the center anchor 20 is much smaller than the area of the substrate 10, two first mass blocks 3011 and second mass blocks 3012 are hung on the center anchor 20, the first mass blocks 3011 and the second mass blocks 3012 are in-plane rotation 180° symmetrical about the center axis (Z axis) of the center anchor 20, discrete electrodes 308 are arranged on both sides of the substrate 10, the discrete electrodes 308 and the second mass blocks 3012 form a second sensitive unit 30b, the first sensitive unit 30a and the second sensitive unit 30b interact to complete discrete differential detection.

[0045] Reference is made to the accompanying drawings Figure 2 、 4 , 5, the first mass blocks 3011 and the second mass blocks 3012 in the embodiment each include a left comb-tooth plate 302a and a right comb-tooth plate 302b, the left comb-tooth plate 302a includes N combs, N should be greater than or equal to 2, in the embodiment, N = 6, the combs are arranged along the X axis direction (perpendicular to the deflection axis 307), and are arrayed in N along the Y direction, the center distance between adjacent combs is represented as Fc, and the comb width is represented as WF, which satisfy WF < Fc / 2 in size, the right comb-tooth plate 302b includes N combs, the combs are arranged along the X axis direction, and are arrayed in N along the Y direction, the center distance between adjacent combs is represented as Fc, and the comb width is represented as WF, which satisfy WF < Fc / 2 in size. When designing the left comb-tooth plate 302a and the right comb-tooth plate 302b, the left comb-tooth plate 302a and the right comb-tooth plate 302b of the first sensitive unit 30a and the right comb-tooth plate 302b and the left comb-tooth plate 302a of the second sensitive unit 30b should not overlap in the projection plane, so that the first mass blocks 3011 and the second mass blocks 3012 independently move, the left comb-tooth plate 302a of the first mass blocks 3011 and the right comb-tooth plate 302b of the second mass blocks 3012 are alternately arranged along the Y axis direction on the same side of the center anchor 20, and the right comb-tooth plate 302b of the first mass blocks 3011 and the left comb-tooth plate 302a of the second mass blocks 3012 are arranged in the same way, forming an interdigital structure. When designing the center anchor 20, the shape used in the embodiment can be used for the common deflection axis 307 of the first mass blocks 3011 and the second mass blocks 3012.

[0046] Preferably, the accompanying drawings are observed again Figure 2 and 5 , the left comb-tooth plate 302a and the right comb-tooth plate 302b in the embodiment are not equal in rotational inertia about the deflection axis 307, so that the mass centers of the two mass blocks are offset from the deflection axis 307 by a distance Xc, in the design, under the action of acceleration perpendicular to the plane of the mass blocks 301 (Z axis), the inertial torque generated by the left comb-tooth plate 302a is greater than that generated by the right comb-tooth plate 302b, the mass blocks 301 deflect around the deflection axis 307 under the action of the inertial torque, until the elastic torsional torque of the support beam (the structure on both sides of the deflection axis 307) balances the inertial torque, according to Hooke's law, the deflection angles of the two mass blocks are proportional to the acceleration

[0047] Figure 1 shows a schematic diagram of a MEMS accelerometer according to the present application; Figure 3 and 4 In the embodiment, the discrete electrodes 308 include two groups of left and right comb electrodes 308a and 308b arranged in parallel and spaced apart, and the two groups of left and right comb electrodes 308a and 308b are arranged on the substrate 10 on the two sides of the center anchor point 20, and the right comb electrodes 308b and the right comb plate 302b and the left comb electrodes 308a and the left comb plate 302a on the same side correspond to each other one by one; the width of the right comb electrodes 308b is equal to the width of the comb teeth of the right comb plate 302b and is less than half of the center distance of the comb teeth on the right comb plate 302b, i.e., less than Fc / 2.

[0048] In use, as Figure 5 , the first mass 3011 and the second mass 3012 are deflected in opposite directions around the deflection axis 307, and the left comb electrodes 308a and the right comb electrodes 308b of the first mass 3011 form differential detection on the two sides of the deflection axis 307, and the left comb electrodes 308a and the right comb electrodes 308b of the second mass 3012 also form differential detection on the two sides of the deflection axis 307, and the two pairs of differential detection capacitors need to be strictly symmetrical about the deflection axis 307 to completely eliminate the zero offset, and obviously such strict symmetry does not conform to the actual situation. Therefore, the left comb electrodes 308a of the first mass 3011 and the right comb electrodes 308b of the second mass 3012 form discrete differential detection on the same side of the deflection axis 307, and the right comb electrodes 308b of the first mass 3011 and the left comb electrodes 308a of the second mass 3012 form discrete differential detection on the other side of the deflection axis 307, as Figure 1 , “+ΔC” and “-ΔC” form 2N (=12) pairs of discrete differential capacitors 40.

[0049] Referring to Figure 3 and 4 , the gap between the left comb plate 302a and the left comb electrodes 308a is d, and the gap d is usually realized by a wet etching or dry etching process in a MEMS bulk silicon process, and is usually realized by a sacrificial layer release process in a MEMS surface silicon process, and the gap d produced is usually 1 μm-10 μm, which should be optimized by comprehensively considering factors such as capacitive sensitivity, pull-in voltage, and gas damping. The micromechanical structures such as the two masses and the center anchor point 20 are of silicon material, and are usually released and formed at one time by a deep silicon etching (Bosch) process, and the width WF of the comb teeth of the left / right comb plate can be 10 μm-200 μm, which is easy to realize for the deep silicon etching process

[0050] Continuing to observe Figure 1, Figure 3 ,

[0051] The first mass 3011 and the second mass 3012 are both suspended by the center anchor 20. According to the principle that the single-point support structure is insensitive to the stress / deformation of the substrate 10, the two masses will not deviate from the initial position with the deformation of the substrate 10, that is Figure 3 The left comb finger plate 302a of the first sensing unit 30a and the right comb finger plate 302b of the second sensing unit 30b will not produce Z-direction deformation under zero acceleration input. Therefore, the main reason for the zero offset is that the detection electrodes arranged on the surface of the substrate 10 follow the deformation of the substrate 10. Figure 3 The curvature radius along the Y-axis is asymmetric about the Z-axis (ρL≠ρR).

[0052] Any two adjacent comb fingers of the left comb finger plate 302a and the right comb finger plate 302b and the discrete electrode 308 below them form a pair of discrete differential capacitors 40. Figure 4 The discrete differential capacitors 40 shown are respectively denoted as CL(n) and CL(n+1). Assuming that the substrate 10 is deformed with a curvature radius ρ, the difference between the gaps of a pair of discrete differential capacitors 40 with a center distance of Fc / 2 is δd, which causes the discrete differential capacitors 40 to be asymmetric and produce a zero offset. According to the geometric relationship, it is not difficult to obtain δd∝Fc / 2, so reducing the center distance Fc / 2 of the discrete differential capacitors 40 will proportionally reduce δd. In the extreme case, Fc / 2 is small enough and tends to zero, that is, the area of the discrete differential capacitors 40 on the substrate 10 is small enough, and δd also tends to zero. At this time, any pair of discrete differential capacitors 40 can be considered to remain locally highly symmetric under any deformation of the substrate 10. The deformation of the substrate 10 mainly produces common-mode capacitance changes, so the zero offset caused by the deformation of the substrate 10 is greatly reduced.

[0053] In the simulation calculation, as shown in Figure 6

[0054] ​The relationship between the zero offset caused by the deformation of the substrate 10 and the number 2N of discrete differential capacitors 40 in the seesaw type Z-axis accelerometer. 2N=1 corresponds to the existing single differential detection seesaw structure, 2N=2 corresponds to the existing double differential detection seesaw structure, and 2N=4, 6, 8, 10, 12, 14, 16, 18, 20… correspond to the Z-axis accelerometer with discrete differential capacitors 40 in the embodiment. The simulation condition is that the curvature radii of the four quadrants in the XY coordinate system on the substrate 10 are all asymmetric, and the total area of the differential detection electrodes is basically equal. The zero offset of single differential detection (2N=1) is 100%, and the zero offset of double differential detection (2N=2) is about 35%. The zero offset of the discrete differential detection (2N≥4) in the embodiment is ≤25%, and decreases inversely with the increase of N. In example 1, 2N=12, the zero offset is reduced to 9%, and if 2N=20, the zero offset is further reduced to 5%, which can be reduced by an order of magnitude compared with the existing double differential detection. It can be seen that the Z-axis MEMS accelerometer with discrete differential detection can greatly reduce the zero offset error caused by the deformation of the substrate 10, and is less sensitive to manufacturing errors, packaging errors and various stress changes, and has better manufacturability. Example Two

[0055] Reference to the accompanying drawings Figure 7 、 8 , 9, the Z-axis MEMS accelerometer with discrete differential detection in the embodiment is changed in the arrangement direction of the comb teeth on the discrete electrodes 308 and the left and right comb tooth plates in the two mass blocks compared with example 1. The comb teeth on the left comb tooth plate 302a and the right comb tooth plate 302b in the embodiment are arranged along the Y axis, and the discrete electrodes 308 are also arranged along the Y axis.

[0056] From the technical common sense, the application can be realized through other embodiments without departing from the spirit or essential characteristics thereof. Therefore, the above disclosed embodiments are only examples in all aspects, and are not the only ones. All changes within the scope of the application or within the scope equivalent to the application are included in the application.

Claims

1. A Z-axis MEMS accelerometer with discrete differential detection, characterized in that, Comprising a substrate (10) provided with a central anchor point (20); a first mass (3011) and a second mass (3012) hingedly connected to the central anchor point (20) at the middle portion thereof, and having opposite ends capable of crossing each other; the first mass (3011) is provided with a left comb plate (302a) and a right comb plate (302b) on both sides thereof, the second mass (3012) is provided with a left comb plate (302a) and a right comb plate (302b) on both sides thereof, and the combs on the left comb plate (302a) and the right comb plate (302b) are alternately and staggeringly arranged on the same side; a discrete electrode (308) provided on the substrate (10) on both sides of the central anchor point (20) and corresponding to the first mass (3011) and the second mass (3012); the discrete electrode (308) and the first mass (3011) form a first sensitive unit (30a), and the discrete electrode (308) and the second mass (3012) form a second sensitive unit (30b), and the first sensitive unit (30a) and the second sensitive unit (30b) interact to complete the discrete differential detection.

2. The discrete differential detected Z-axis MEMS accelerometer of claim 1, wherein, the first mass (3011) and the second mass (3012) are staggeringly arranged along the Y axis and are hingedly connected to the central anchor point (20) through a deflection shaft (307).

3. The discrete differential detected Z-axis MEMS accelerometer of claim 1, wherein, the center distance between the combs of the left comb plate (302a) on the same side is the same as the center distance between the combs of the right comb plate (302b) on the same side.

4. The discrete differential detected Z-axis MEMS accelerometer of claim 3, wherein, the width of the combs on the left comb plate (302a) and the right comb plate (302b) is equal and less than half of the center distance between the combs of the left comb plate (302a) on the same side.

5. The discrete differential detected Z-axis MEMS accelerometer of claim 3, wherein, the discrete electrode (308) includes two groups of left comb electrodes (308a) and right comb electrodes (308b) arranged in parallel, and the two groups of left comb electrodes (308a) and right comb electrodes (308b) are provided on the substrate (10) on both sides of the central anchor point (20), and the right comb electrode (308b) and the right comb plate (302b) and the left comb electrode (308a) and the left comb plate (302a) on the same side correspond to each other.

6. The discrete differential detected Z-axis MEMS accelerometer of claim 5, wherein, the width of the right comb electrode (308b) is equal to the width of the comb on the right comb plate (302b) and is less than half of the center distance between the combs on the right comb plate (302b).

7. The discrete differential detection Z-axis MEMS accelerometer according to claim 1, wherein the first mass (3011) and the second mass (3012) have different rotational inertia around the hinge.

8. The discrete differential detected Z-axis MEMS accelerometer of any of claims 1-7, wherein, the combs on the left comb plate (302a) and the right comb plate (302b) are arranged along the X axis.

9. The discrete differential detected Z-axis MEMS accelerometer of any of claims 1-7, wherein, the combs on the left comb plate (302a) and the right comb plate (302b) are arranged along the Y axis.

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