CN-122017286-A - MEMS differential capacitive z-axis accelerometer without cross axis interference and preparation method thereof

CN122017286ACN 122017286 ACN122017286 ACN 122017286ACN-122017286-A

Abstract

The invention provides a MEMS differential capacitance z-axis accelerometer without cross axis interference and a preparation method thereof, relating to the technical field of sensors, wherein the accelerometer adopts two (111) crystal orientation silicon wafers to bond to form a sandwich structure, the device comprises a first silicon wafer provided with a first fixed electrode and a first cavity, and a second silicon wafer internally integrated with a movable mass block, an elastic beam, a second cavity, a movable electrode and a second fixed electrode. By designing the areas of the upper and lower fixed electrodes to be larger than the area of the middle movable electrode, the effective sensing area is constant, and cross axis interference is eliminated from the structural origin. The preparation method combines the key processes of deep reactive ion etching, TMAH anisotropic wet etching and gold-gold hot-pressing bonding. The invention has high sensitivity, excellent linearity and strong common mode rejection capability, and is suitable for the inertial measurement field with strict requirements on precision and stability.

Inventors

YANG PENG
CHEN XIAOPENG
Ling Sijia
TAN YIWEN
Yu Chenji
YIN JIAWEN
JIN QINGHUI

Assignees

宁波大学

Dates

Publication Date: 20260512
Application Date: 20251219

Claims (10)

1. A MEMS differential capacitive z-axis accelerometer free of cross-axis interference, comprising: The upper surface of the first silicon wafer is provided with a first fixed electrode, and the back surface of the first silicon wafer is provided with a first cavity; A second silicon wafer bonded to the first silicon wafer through a bonding layer; the second silicon wafer is internally provided with a movable mass block, a fixed frame surrounding the movable mass block and an elastic beam connecting the movable mass block and the fixed frame, wherein the movable mass block can move in a direction perpendicular to the plane of the second silicon wafer, and a second cavity for the movement of the movable mass block is also arranged in the second silicon wafer; the upper surface of the movable mass block is provided with a movable electrode, and the lower surface of the second silicon wafer is provided with a second fixed electrode; The first fixed electrode is opposite to the movable electrode up and down to form a first variable capacitor; the second fixed electrode is opposite to the movable electrode up and down to form a second variable capacitor; the first variable capacitor and the second variable capacitor form a differential capacitor pair.
2. The MEMS differential capacitive z-axis accelerometer of claim 1, wherein an area of both the first fixed electrode and the second fixed electrode is greater than an area of the movable electrode.
3. The MEMS differential capacitive z-axis accelerometer of claim 1, wherein the bonding layer is a metal bonding layer comprising a first metal layer formed on a lower surface of the first silicon wafer, a second metal layer formed on an upper surface of the second silicon wafer, and an intermetallic layer formed by solid state diffusion of the first metal layer and the second metal layer.
4. The preparation method of the MEMS differential capacitance type z-axis accelerometer is characterized by comprising the following steps of: S1, a first silicon wafer is provided, wherein a first fixed electrode is formed on the upper surface of the first silicon wafer, and a first cavity is formed on the back surface of the first silicon wafer through a micromachining process; S2, preparing a second silicon wafer, namely providing a second silicon wafer, forming a movable mass block, an elastic beam and a second cavity in the second silicon wafer through a micro-machining process, forming a movable electrode on the upper surface of the movable mass block, and forming a second fixed electrode on the lower surface of the second silicon wafer; and S3, bonding the back surface of the first silicon wafer and the upper surface of the second silicon wafer through a bonding layer.
5. The method of manufacturing as claimed in claim 4, wherein the first silicon wafer manufacturing step comprises: S11, providing a first silicon wafer, and preparing silicon oxide layers on the upper surface and the lower surface by adopting a plasma enhanced chemical vapor deposition method; S12, depositing metal on the front surface of the first silicon wafer through magnetron sputtering and patterning to form the first fixed electrode, the lead wire and the bonding pad; S13, depositing a silicon nitride layer on the front surface of the first silicon wafer for insulation protection, and exposing a bonding pad of the first fixed electrode by adopting a reactive ion etching process; S14, depositing metal on the back surface through magnetron sputtering and patterning to form a first metal layer for bonding; s15, removing the silicon oxide layer of the notch by adopting a reactive ion etching method on the back surface; S16, forming the first cavity on the back surface of the first silicon wafer through a deep reactive ion etching process.
6. The method of claim 4, wherein the second wafer preparation step comprises: s21, providing a second silicon wafer, and preparing silicon oxide layers on the upper surface and the lower surface by adopting a plasma enhanced chemical vapor deposition method; S22, patterning and etching the outline and the initial groove of the elastic beam and the movable mass block on the front surface of the second silicon wafer through a reactive ion etching and deep reactive ion etching process; s23, depositing a silicon oxide film on the upper surface and the lower surface of the second silicon wafer through a low-pressure chemical vapor deposition process; S24, removing silicon oxide at the bottom of the initial groove by adopting a reactive ion etching process, and then carrying out deepening etching on the initial groove by adopting a deep reactive ion etching process; S25, depositing and patterning a gold layer on the front side of the second silicon wafer through a magnetron sputtering process to form a movable electrode, a lead wire, a bonding pad and a second metal layer; s26, depositing a silicon nitride layer on the back surface of the second silicon wafer for insulation protection; and S27, carrying out anisotropic wet etching on the front surface of the second silicon wafer, and transversely etching to form the second cavity so as to release the movable mass block.
7. The method of manufacturing according to claim 6, wherein the anisotropic wet etching process for forming the second cavity uses TMAH etching solution.
8. The method of claim 4, wherein the bonding step is a gold-gold thermocompression bonding process, and the bonding layer forms a solid diffusion interface by thermocompression bonding a first metal layer formed on the back surface of the first silicon wafer and a second metal layer formed on the upper surface of the second silicon wafer.
9. The method of manufacturing according to claim 4, further comprising, after the bonding step, a via hole making step of: forming a through hole on the upper layer of the bonded device by an etching process to expose a bonding pad of the movable electrode; and exposing the bonding pad of the second fixed electrode at the lower layer of the bonded device.
10. The method of claim 4, wherein the first silicon wafer and the second silicon wafer are both (111) crystal orientation silicon wafers.

Description

MEMS differential capacitive z-axis accelerometer without cross axis interference and preparation method thereof Technical Field The invention relates to the technical field of sensors, in particular to a MEMS differential capacitive z-axis accelerometer without cross-axis interference and a preparation method thereof. Background As an inertial sensor manufactured based on a semiconductor micromachining process, a micro-electromechanical system (MEMS) accelerometer has the remarkable advantages of small volume, low cost, high reliability, suitability for mass production and the like, and has become a core device for measuring physical parameters such as acceleration, inclination angle, vibration and the like in the fields of consumer electronics, automobile industry, internet of things, aerospace and the like. With the rapid development of unmanned aerial vehicles, autopilot technology and various portable intelligent devices, higher requirements are put forward on the performance of micro Inertial Measurement Units (IMUs). The three-axis MEMS accelerometer capable of realizing three-dimensional inertial sensing is an important research point, and the Z-axis accelerometer used for sensing acceleration vertical to the plane direction of the chip has higher design and manufacturing difficulty, and the performance directly determines the three-dimensional measurement precision, so that the three-axis MEMS accelerometer becomes an important research direction in the field of MEMS sensors. Currently, the mainstream Z-axis MEMS accelerometer mainly adopts a sandwich structure or a torsion pendulum structure based on parallel plate variable-pitch capacitance detection principle. The torsion type Z-axis accelerometer is paid attention to because the structure of the torsion type Z-axis accelerometer has good compatibility with the processing technology of the in-plane accelerometer. However, the torsional pendulum structure based on the single mass block has inherent defects that firstly, the common mode interference suppression capability of the torsional pendulum structure to environmental factors such as temperature change is poor, and secondly, in structural design, in order to improve the mechanical sensitivity of detecting Z-axis acceleration, the performance in other directions is always required to be sacrificed, so that the cross interference sensitivity of the torsional pendulum structure to X, Y axes is synchronously increased, and the measurement accuracy of the torsional pendulum structure in a complex motion environment is severely restricted. On the other hand, although the "sandwich" structure based on the parallel plate variable-pitch capacitance principle can theoretically obtain higher displacement sensitivity, many challenges are still faced in practical application: The cross axis interference problem is remarkable in that when the sensor receives in-plane (X/Y axis) acceleration, the movable mass block can generate in-plane displacement, so that the effective opposite area of the movable mass block and the upper and lower fixed polar plates is changed, parasitic capacitance signals are generated, and the measurement accuracy of Z axis acceleration is seriously interfered. The fringe field effect introduces nonlinearity, which limits the dynamic range and measurement linearity of the sensor because the fringe field of the parallel plate capacitor introduces significant nonlinearity errors. The traditional single-ended capacitance detection structure has limited capability of inhibiting common mode interference (such as temperature drift and environmental noise), and influences the long-term stability and the temperature stability of the sensor. Accordingly, there is a pressing need in the art for a Z-axis MEMS accelerometer solution that can fundamentally suppress cross-axis interference while having high sensitivity, high linearity, and excellent common-mode interference resistance. Disclosure of Invention The invention solves the key technical problems of low measurement precision and poor environmental adaptability of the traditional Z-axis accelerometer caused by serious cross axis interference and insufficient common mode inhibition capability by combining the unique differential capacitance design with constant effective area with the bonding process of vertical stacking. The invention provides a MEMS differential capacitive z-axis accelerometer without cross-axis interference, comprising: The upper surface of the first silicon wafer is provided with a first fixed electrode, and the back surface of the first silicon wafer is provided with a first cavity; A second silicon wafer bonded to the first silicon wafer through a bonding layer; the second silicon wafer is internally provided with a movable mass block, a fixed frame surrounding the movable mass block and an elastic beam connecting the movable mass block and the fixed frame, wherein the movable mass block can move in a direction perp