CN111766404A

CN111766404A - Low-stress Z-axis MEMS accelerometer based on rigidity coupling

Info

Publication number: CN111766404A
Application number: CN202010748875.5A
Authority: CN
Inventors: 周铭; 鞠莉娜; 黄艳辉; 凤瑞
Original assignee: China North Industries Group Corp No 214 Research Institute Suzhou R&D Center
Current assignee: China North Industries Group Corp No 214 Research Institute Suzhou R&D Center
Priority date: 2020-07-30
Filing date: 2020-07-30
Publication date: 2020-10-13
Anticipated expiration: 2040-07-30
Also published as: CN111766404B

Abstract

The invention discloses a low-stress Z-axis MEMS accelerometer based on rigidity coupling, wherein a sensitive unit comprises a mass block, a support beam and fixed comb teeth, the main shaft direction angle of the support beam deviates 90 degrees, so that the mass block generates in-plane deflection under the action of external force, one end of the fixed comb teeth is fixed on a substrate through fixed tooth anchor points, the mass block is suspended on a central anchor point through the support beam, the central anchor point is fixed on the substrate, the mass block comprises movable comb teeth, the fixed comb teeth are inserted between the movable comb teeth to form a first comb tooth capacitor and a second comb tooth capacitor, and the comb tooth capacitors detect the in-plane deflection of the mass block. The main shaft direction angle of the supporting beam of the Z-axis accelerometer deviates 90 degrees, the mass blocks under the action of out-of-plane acceleration simultaneously generate in-plane deflection based on the rigidity coupling principle of the supporting beam, the in-plane deflection is detected through comb capacitors to finish the detection of the acceleration, and the Z-axis accelerometer has the advantages of good symmetry, low sensitivity to stress, low drift, small nonlinearity and the like.

Description

Low-stress Z-axis MEMS accelerometer based on rigidity coupling

Technical Field

The invention relates to the technical field of silicon micro mechanical sensors, in particular to a low-stress Z-axis MEMS accelerometer based on rigidity coupling.

Background

The MEMS silicon micro-accelerometer has the advantages of low cost, small volume, low power consumption, easy integration, high reliability and the like, and is widely applied to various fields of inertial measurement. The MEMS capacitive accelerometer has the excellent characteristics of high precision, good repeatability, low drift, simple manufacturing process, etc., and thus becomes one of the most developed and widely applied inertial devices. With the development of the technology, the precision requirement of the MEMS accelerometer is higher and higher, and the influence of the stress and the temperature on the precision of the MEMS capacitive accelerometer is particularly significant.

The MEMS accelerometer is divided into an X/Y axis and a Z axis according to a sensitive axial direction, the X/Y axis accelerometer senses the acceleration in a sensitive plane, and the Z axis accelerometer senses the acceleration out of the sensitive plane. With the miniaturization of devices and components, a single chip or board is required to realize the measurement of three-axis acceleration, which requires that the X/Y-axis accelerometer and the Z-axis accelerometer have considerable accuracy. However, in view of the current relevant MEMS capacitive accelerometer product indexes at home and abroad, generally, the temperature drift of the X/Y axis accelerometer is smaller than that of the Z axis accelerometer, i.e. the Z axis accelerometer is more sensitive to stress and temperature. The reason is the structural difference of the sensitive capacitor, the X/Y axis accelerometer usually adopts the comb structure sensitive capacitor, and the Z axis accelerometer more adopts the flat plate structure sensitive capacitor.

In fact, in addition to the change of the sensitive capacitance caused by the acceleration to be measured, the change of the sensitive capacitance caused by any other factors can be detected by the signal processing circuit to different degrees, thereby reducing the accuracy of the sensor. The sensitive capacitance changes most significantly due to changes in environmental factors such as external stress and temperature. These stresses are transmitted and act to ultimately manifest as deformation of the MEMS chip, such as warpage, bending, and the like. The acceleration sensitive unit arranged on the substrate of the MEMS chip is inevitably influenced by the deformation of the substrate, so that a false signal is output. The substrate deformation is mainly represented by Z-axis displacement, the Z-axis characteristic dimension of the comb capacitor is the comb height (i.e. the thickness of the mass layer, usually several tens of micrometers), and the Z-axis characteristic dimension of the plate capacitor is the plate gap (usually several micrometers). Therefore, when the substrate deformation causes the sensitive capacitance structure to generate relative displacement in the Z-axis direction, the ratio of the relative displacement to the height of the comb teeth is far smaller than the ratio to the plate gap, that is, the capacitance change caused by the relative displacement to the plate capacitance is far larger than the capacitance change caused by the comb teeth capacitance, so that the temperature drift of the Z-axis accelerometer based on the plate capacitance is larger than that of the X/Y-axis accelerometer based on the comb teeth capacitance, that is, the Z-axis accelerometer is more sensitive to stress and temperature.

Fig. 1 is a schematic diagram of a conventional seesaw-type Z-axis accelerometer affected by stress, in which two lower electrode plates are symmetrically distributed below a mass plate about a torsion axis to form a differential capacitor, an initial gap d0 of the capacitor is usually 1 to 3 μm, and the lower electrode plates are attached to a substrate. Under the influence of various stresses, the substrate generates bending deformation with certain curvature, the lower electrode plate deviates from the original position d, and the variation quantity caused by the plate capacitance is approximately delta C1 ≈ d/d0 ≈ C01.

At present, a seesaw structure and a sandwich structure are adopted in a Z-axis MEMS accelerometer, and the Z-axis MEMS accelerometer is specifically as follows:

US6841992, US6935175, US7146856 and US8079262 disclose a typical seesaw type Z-axis accelerometer, which uses a variable gap differential plate capacitor formed by a lower plate electrode and a mass plate which are symmetrically distributed about a torsion axis and are made on a substrate.

US9513310, CN201810650974.2, CN201710780370 disclose a kind of multi-mass-block coupled seesaw type Z-axis silicon micro-accelerometer, which sets the adjacent seesaw sensitive structures in opposite directions and adopts a variable gap flat plate to detect capacitance.

US7140250 and CN201911125779.9 disclose two seesaw type Z-axis accelerometers based on comb capacitance detection, wherein comb capacitances with different heights are adopted to replace plate capacitances, the sensitivity of sensitive capacitances to stress and temperature is reduced to a certain extent, and the defect is that the gain from mechanical displacement to capacitance change is lower.

CN201510114611.3 discloses an MEMS chip insensitive to package stress, which suspends the lower flat plate of a flat capacitor in a single point manner, reduces the influence of substrate deformation on the lower flat plate, and the scheme relates to multiple times of bulk silicon bonding and has a complex process.

US20180164339a1, EP3336559B1, CN200410029259.5 disclose a sandwich structure Z-axis accelerometer, which forms the displacement of the variable gap differential plate capacitive sensing mass by respectively arranging an upper plate electrode and a lower plate electrode above and below the movable mass.

EP1571454B1 discloses a Z-axis silicon micro accelerometer with a sandwich structure of chip-level stress isolation, which is provided with stress release beams around the middle layer mass block to reduce the stress transferred to the sandwich sensitive structure through stress release.

US6705166 discloses a Z-axis silicon microaccelerometer of sandwich structure, which adopts the out-of-plane linear motion of a variable area comb capacitance sensitive intermediate mass block, wherein the mass block is fixed on an annular anchor through a flexible ring, and the rigidity of the flexible ring is easily affected by stress.

In summary, most of the existing Z-axis MEMS accelerometers adopt variable-gap flat capacitors, and a small number of the existing Z-axis MEMS accelerometers adopt variable-area comb capacitors, which are sensitive to stress and temperature or have low sensitivity. Although the stress transmitted to the sensitive structure can be partially reduced by means of stress relief or isolation from the outside of the sensitive structure, the complexity of chip fabrication or package assembly is increased, and the stress cannot be completely eliminated all the time, so that the best method is to design the sensitive structure insensitive to the stress, and suppress and eliminate the influence of the stress and deformation from the source of the sensitive structure.

Disclosure of Invention

In view of the above technical problems, the present invention aims to: the low-stress Z-axis MEMS accelerometer based on rigidity coupling has the advantages that the main shaft direction angle of a supporting beam of the Z-axis MEMS accelerometer deviates 90 degrees, the mass blocks under the action of out-of-plane acceleration simultaneously generate in-plane deflection based on the rigidity coupling principle of the supporting beam, the in-plane deflection is detected through comb capacitors to finish the detection of the acceleration, and the low-stress Z-axis MEMS accelerometer based on the rigidity coupling principle of the supporting beam has the advantages of good symmetry, low sensitivity to stress, low drift, small nonlinearity and the like.

The technical scheme of the invention is as follows:

the utility model provides a low stress Z axle MEMS accelerometer based on rigidity coupling, includes sensitive unit and substrate, sensitive unit includes quality piece, a supporting beam and fixed broach, the main shaft direction angle of a supporting beam deviates 90 degrees, makes quality piece under the exogenic action produce the in-plane deflection, the one end of fixed broach is fixed in the substrate through deciding the tooth anchor point, the quality piece hangs in central anchor point through a supporting beam, central anchor point is fixed in the substrate, the quality piece includes movable broach, fixed broach is inserted and is located between the movable broach, forms first broach electric capacity and second broach electric capacity, first broach electric capacity and second broach electric capacity detection quality piece deflect in-plane.

In the preferred technical scheme, the support beam is a single straight beam, a double-section folding beam or a three-section folding beam.

In a preferable technical scheme, a groove is arranged on the left side or the right side of the support beam.

In a preferred technical scheme, a groove is respectively arranged at the diagonal position of the supporting beam.

In a preferred technical scheme, the support beams are provided with a plurality of groups, and the groups of support beams are circumferentially and rotationally symmetrical at intervals of 90 degrees with respect to a central axis of the sensitive unit.

In the preferred technical scheme, the fixed tooth anchor points are arranged around the central anchor point, and the fixed tooth anchor points and the central anchor point are arranged in the circular anchor area.

In the preferred technical scheme, the area of the circular anchor area is far smaller than the area of the chip occupied by the mass block.

In an optimal technical scheme, the first comb-tooth capacitor and the second comb-tooth capacitor form a differential capacitor pair based on the in-plane deflection motion of the mass block.

In an optimal technical scheme, the first comb-tooth capacitor and the second comb-tooth capacitor are symmetrically arranged around an X axis and a Y axis simultaneously.

In the preferred technical scheme, the movable comb teeth and the fixed comb teeth are both arc-shaped comb tooth structures.

Compared with the prior art, the invention has the advantages that:

the main shaft direction angle of the supporting beam of the Z-axis MEMS accelerometer deviates 90 degrees, the main shaft direction angle can be changed by changing the shape of the unfilled corner, so that rigidity coupling is formed between the out-of-plane bending rigidity and the in-plane bending rigidity of the supporting beam (the convex oblique beam), the mass blocks under the action of out-of-plane acceleration simultaneously generate in-plane deflection based on the rigidity coupling principle of the supporting beam (the convex oblique beam), and the in-plane deflection is detected through comb capacitors to finish acceleration detection. The method has the advantages of good symmetry, low sensitivity to stress, low drift, small nonlinearity and the like.

Drawings

The invention is further described with reference to the following figures and examples:

FIG. 1 is a schematic diagram of the effect of substrate deformation on the variable gap plate capacitance of a typical prior art seesaw accelerometer;

FIG. 2 is a general schematic diagram of a low stress Z-axis MEMS accelerometer based on stiffness coupling;

FIG. 3 is a schematic side view of a low stress Z-axis MEMS accelerometer based on stiffness coupling;

FIG. 4 is a schematic diagram of a sensitive unit of a low-stress Z-axis MEMS accelerometer based on rigid coupling;

FIG. 5 is a schematic diagram of a support beam structure of a stiffness-coupled low stress Z-axis MEMS accelerometer;

FIG. 6 is a cross-sectional view of a support beam with a missing corner for a low stress Z-axis MEMS accelerometer based on stiffness coupling;

FIG. 7 is a cross-sectional view of a support beam with two notches for a low stress Z-axis MEMS accelerometer based on stiffness coupling;

FIG. 8 is a schematic diagram of out-of-plane displacement of a stiffness-coupled low stress Z-axis MEMS accelerometer under the effect of out-of-plane acceleration;

FIG. 9 is a schematic diagram of in-plane deflection of a low stress Z-axis MEMS accelerometer under out-of-plane acceleration based on stiffness coupling;

FIG. 10 is a schematic diagram of a low stress Z-axis MEMS accelerometer based on stiffness coupling insensitive to substrate deformation and stress.

Detailed Description

In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention will be described in further detail with reference to the accompanying drawings in conjunction with the following detailed description. It should be understood that the description is intended to be exemplary only, and is not intended to limit the scope of the present invention. Moreover, in the following description, descriptions of well-known structures and techniques are omitted so as to not unnecessarily obscure the concepts of the present invention.

The low-stress Z-axis MEMS accelerometer based on rigid coupling has an overall structure as shown in FIG. 2, and comprises a sensitive unit 1, a circular anchor area 2 and a substrate 3 from top to bottom in sequence, and specifically comprises a mass block 11, a support beam 13, fixed comb teeth 146, a central anchor point 12, a fixed tooth anchor point 145 and the substrate 3. The mass block 11 is suspended on a unique central anchor point 12 through a plurality of groups of support beams 13, the central anchor point 12 is fixed on the substrate 3, the mass block 11 and the support beams 13 are positioned on the same layer, and a gap is formed between the mass block 11 and the substrate 3, as shown in fig. 3, the whole sensitive unit 1 is of a single-pivot quasi-suspension structure.

As shown in fig. 4, the mass block 11 includes a plurality of arc-shaped movable comb teeth 147, a plurality of similarly arc-shaped fixed comb teeth 146 are correspondingly inserted between the movable comb teeth 147 to finally form a first comb-tooth capacitor 14a and a second comb-tooth capacitor 14b, the first comb-tooth capacitor 14a and the second comb-tooth capacitor 14b are symmetrically arranged about the X axis and the Y axis, and two sets of the first comb-tooth capacitor and the second comb-tooth capacitor are arranged in fig. 4.

By changing the overlapping angle theta of arc-shaped comb teeth₀The change in the overlapping area of the comb teeth is achieved as shown in fig. 5, thereby producing a change in capacitance, which has the natural advantage of linearity, with the first comb capacitor 14a and the second comb capacitor 14b forming a differential capacitance pair based on the in-plane deflection motion of the mass, i.e., either one increases while the other decreases by an equal amount. The roots of the fixed combs 146 are fixed to the substrate 3 by fixed-tooth anchors 145, all fixed-tooth anchors 145 are arranged next to the central anchor 12, the central anchor 12 and the plurality of fixed-tooth anchors 145 are arranged in a circular anchor region 2, and the area of the circular anchor region 2 should be much smaller than the area of the sensitive unit.

Fig. 5 is a schematic structural diagram of a group of support beams 13, in which a three-section folded beam structure, which may be a single straight beam or a two-section folded beam structure, is adopted in the embodiment, and the difference is that the multi-section folded beam has relatively lower out-of-plane stiffness and in-plane stiffness, and is suitable for low-range and high-sensitivity design. The main axis direction angle 137 of the support beam 13 is deviated by 90 degrees, and the support beam 13 can be referred to as a camber beam 13. In fig. 5, the cross-sectional shape of each section of the convex oblique beam is the same, and one or two unfilled corners 136 are arranged on the upper side or the lower side of each section of the convex oblique beam 13.

Fig. 6 is a schematic cross-sectional view of the oblique convex beam 13 with a notch, where the existence of the notch 136 causes the main shaft direction angle 137 (θ p) of the oblique convex beam 13 to deviate from 90 degrees, and the main shaft direction angle 137 can be changed by changing the shape of the notch 136, so that a stiffness coupling is formed between the out-plane bending stiffness and the in-plane bending stiffness of the oblique convex beam 13, that is, the oblique convex beam 13 will generate a bending moment in the direction of the main shaft 138 while being subjected to the out-plane bending by the external force, and the mass 11 subjected to the bending moment will be subjected to the in-plane deflection.

Fig. 7 is a schematic cross-sectional view of a convex oblique beam 13 with two notches, wherein the two notches 136 are arranged diagonally, and the two notches arranged diagonally have smaller main shaft direction angles 137 when the sizes of the notches 136 are the same, i.e. the stiffness coupling is larger, and the sensitivity of the corresponding sensitive unit is also larger.

Fig. 6 and 7 show only two structures of the support beam, but it is also possible to adopt other structures that make the angle of the main axis of the support beam deviate from 90 degrees, such as grooves near the corners, etc., typically grooves with a constant slope made by a semiconductor wet etching process (such as KOH).

In the embodiment, the convex oblique beam 13 is manufactured by adopting an MEMS deep silicon etching process, the convex oblique beams 13 with two unfilled corners 136 relate to front and back surface alignment and etching, and the manufacturing difficulty is higher, in the preferred embodiment, the convex oblique beam with one unfilled corner is adopted, the manufacturing of the convex oblique beam is completed by performing deep silicon etching on the front surface and the back surface twice based on an SOI silicon chip bonding technology, the manufacturing of the mass block 11 and the comb capacitors (14 a and 14 b) is completed simultaneously in the process, and the manufacturing method is different from the problems of multilayer bonding registration errors, interlayer gap errors and the like related in the manufacturing process of the existing seesaw or sandwich-structured Z-axis accelerometer.

The number of the inclined convex beams 13 is at least four, in a preferred embodiment, four groups of the inclined convex beams 13 are adopted to support the movable mass block 11, the root of each group of the inclined convex beams 13 is connected with the central anchor point 12, the end parts of the inclined convex beams 13 are connected with the mass block 11, the four groups of the inclined convex beams 13 are circumferentially and rotationally symmetrically arranged at intervals of 90 degrees relative to the central axis of the sensitive unit, namely, the main axes of the inclined convex beams 13 are in the same circumferential direction, so that the directions of the Z-axis bending moments generated by each inclined convex beam 13 due to rigidity coupling are the same, and the mass block 11 is forced to generate Z-axis (in-plane) deflection under the. The four groups of convex oblique beams 13 enable the sensitive unit to have high linear motion stiffness in the directions of the X axis and the Y axis in the plane, namely the linear motion modal frequency of the sensitive unit along the X axis and the Y axis is high, so that the displacement response of the sensitive unit to the acceleration of the X axis and the Y axis is very small, and the in-plane deviation belongs to common mode variation for the first comb capacitors 14a and the second comb capacitors 14b, so that the sensitive unit 1 is insensitive to the in-plane acceleration.

Fig. 8 and 9 are schematic diagrams of the operation of the Z-axis MEMS accelerometer based on stiffness coupling, as shown in the side view of fig. 8, the mass 11 is subject to out-of-plane (Z-axis) acceleration to generate out-of-plane deflection Z, which is proportional to the acceleration and inversely proportional to the out-of-plane stiffness of the oblique beam 13, the out-of-plane stiffness is mainly determined by the thickness t of the mass, which is usually several tens of micrometers, and the number of folding sections of the beam, and Z is a few micrometers<<t, the capacitance change of the first comb capacitor 14a and the second comb capacitor 14b due to z is very small, and the two capacitance changes to a common modulus, so z does not participate in the final output of the accelerometer. Meanwhile, Z causes the convex oblique beam 13 to generate a bending moment in the Z-axis direction, so that the mass block 11 is forced to simultaneously generate deflection θ Z around the Z-axis, and the capacitance of the first comb tooth is increased (+ Δ C), and the capacitance of the second comb tooth is decreased by the same amount (- Δ C), as shown in fig. 9, the change of the differential capacitance is converted into final voltage output through the signal processing circuit. Initial comb teeth 146 and movable comb teeth 147 of comb capacitors (14 a, 14 b)The angle of overlap (theta in FIG. 5)₀) The basic capacitance C0 of the differential capacitance is determined, the larger the value of the delta C/C0 is, the higher the signal-to-noise ratio of the accelerometer is, the initial overlapping angle of the comb teeth in the invention can be set according to the optimal capacitance design, the influence of the material, the processing and other factors is avoided, the larger displacement to capacitance change sensitivity can be realized, and the advantage of the sensitive unit is also realized.

Fig. 10 is a schematic diagram of the stress suppression principle of the accelerometer according to the embodiment, first, the sensing unit 1 is supported by a single point at the central anchor point 12, and the stress and deformation of the substrate cannot be transmitted to the convex oblique beam, so that the convex oblique beam 13 has free mechanical characteristics, and the convex oblique beam 13 and the mass block 11 are hardly affected by the stress. Then, the influence of the substrate stress and deformation on the comb capacitance is discussed, assuming that the fixed tooth anchor point 145 is far from the central anchor point L ', the thickness of the mass block 11 is t, the edge of the mass block 11 is far from the central anchor point L, and since the area of the circular anchor region 2 is far smaller than that of the sensitive unit 1, L'/L is usually less than 0.1. When substrate 3 is bent with the same curvature as in fig. 1, the outer end of fixed comb 146 will deviate from its original position d ', approximately d ' ≈ L '/L × d according to the geometric relationship, see d in fig. 1, and the capacitance variation caused to the comb capacitance (14 a, 14 b) is approximately Δ C1 ' ≈ d '/t × C01, with a thickness t of typically several tens of micrometers. Comparing Δ C1 ' with Δ C1 in fig. 1, Δ C1 '/Δ C1 ≈ L '/L × d0/t is approximated, and usually d0/t <0.1 and L '/L <0.1, and finally Δ C1 ' < [ < Δ C1 can be obtained. Under ideal conditions, Δ C1 'and Δ C2', Δ C1 and Δ C2 are common mode variables, but in practice, complete symmetry is not possible, and under the same asymmetry, (Δ C1 '- Δ C2') < (Δ C1- Δ C2), i.e., the accelerometer of the embodiment is less sensitive to stress.

In summary, the sensitivity of the Z-axis MEMS accelerometer based on the stiffness coupling to the stress is much smaller than that of the existing Z-axis accelerometer based on the variable gap plate capacitance detection, and as described above, the characteristic is derived from the combination of the convex oblique beam design, the circular anchor area design and the comb capacitance design.

It is to be understood that the above-described embodiments of the present invention are merely illustrative of or explaining the principles of the invention and are not to be construed as limiting the invention. Therefore, any modification, equivalent replacement, improvement and the like made without departing from the spirit and scope of the present invention should be included in the protection scope of the present invention. Further, it is intended that the appended claims cover all such variations and modifications as fall within the scope and boundaries of the appended claims or the equivalents of such scope and boundaries.

Claims

1. The utility model provides a low stress Z axle MEMS accelerometer based on rigidity coupling, includes sensitive unit and substrate, its characterized in that, sensitive unit includes quality piece, supporting beam and fixed broach, supporting beam's main shaft direction angle deviates 90 degrees, makes quality piece under the exogenic action produce the in-plane deflection, the one end of fixed broach is fixed in the substrate through deciding the tooth anchor point, the quality piece hangs in central anchor point through supporting beam, the central anchor point is fixed in the substrate, the quality piece includes movable broach, fixed broach is inserted and is located between the movable broach, forms first broach electric capacity and second broach electric capacity, first broach electric capacity and second broach electric capacity detection quality piece deflect in-plane.

2. The stiffness-coupling-based low stress Z-axis MEMS accelerometer according to claim 1, wherein the support beam is a single straight beam, a two-section folded beam, or a three-section folded beam.

3. The stiffness coupling based low stress Z-axis MEMS accelerometer according to claim 1, wherein the support beam is provided with a groove on the left or right side.

4. The low stress Z-axis MEMS accelerometer based on rigid coupling of claim 1, wherein a groove is provided at each diagonal position of the support beam.

5. The stiffness coupling based low stress Z-axis MEMS accelerometer according to claim 1, wherein the supporting beams are provided with multiple sets of supporting beams having circumferential rotational symmetry spaced 90 degrees apart about a central axis of the sensitive unit.

6. The stiffness coupling based low stress Z-axis MEMS accelerometer of claim 1, wherein the fixed-tooth anchor points are disposed around a central anchor point, the fixed-tooth anchor points and the central anchor point being disposed within a circular anchor region.

7. The stiffness coupling based low stress Z-axis MEMS accelerometer according to claim 6, wherein the area of the circular anchor region is much smaller than the chip area occupied by the proof mass.

8. The stiffness coupling based low stress Z-axis MEMS accelerometer according to claim 1, wherein the first and second comb capacitors form a differential capacitor pair based on in-plane deflection motion of the mass.

9. The stiffness coupling based low stress Z-axis MEMS accelerometer according to claim 1, wherein the first comb capacitors and the second comb capacitors are symmetrically arranged about both the X-axis and the Y-axis.

10. The stiffness coupling based low stress Z-axis MEMS accelerometer according to claim 1, wherein the movable comb teeth and the fixed comb teeth are both arc-shaped comb tooth structures.