A Si-Si Bonding Sandwich Accelerometer With Precisely Controllable Capacitor Gap

Micro accelerometer is an important inertia sensor. Being one kind of accelerometers, capacitive accelerometer has many advantages, including high sensitivity, wide dynamic range and good temperature characteristics. As the result, capacitive accelerometer can be applied in many areas required high performance, such as inertia navigation, petrol exploration and seismic wave detecting.Based on former sensors designed in our team, a fully symmetrical capacitive accelerometer with precisely controllable capacitor gap is presented in this paper, which is used for detecting seismic waves. To get over the difficulty of wire-bonding-pad design by anodic bonding, we use four silicon wafers to build the structure of the senor. Through the silicon-silicon bonding under high temperature, we achieve the wafer-level vacuum package, while the mechanical noise is greatly reduced. The capacitor gap between the mass and the static plates is fabricated with multi-oxidation technology instead of KOH etching. By this way, we can precisely control much smaller capacitor gap which contributes greatly to the uniformity of the sensors.Firstly, the overall structure of the sensor is designed. According to the factors that influence the sensitivity, we fabricate a capacitor gap of 2.6|im and bumpers of 1μm by multi-oxidation simultaneously; 8 L-shape beams are attached to the corners of the proof mass in both the top and bottom plate, which helps a lot to decrease to the cross-axis sensitivity. The performance of the sensor is simulated by ANSYS, which agrees with the calculation very much. For sensor of 40μm thickness, the sensitivity of differential capacitance is 12.4pF/g, for those of 35μm thickness, the sensitivity is 19.7pF/g.Secondly, the detailed process of the sensor fabrication is presented. Specially, some necessary improvements are adopted in the process. A larger rectangle joint is set between the mass and cantilever beams instead of the former arc joint, which solves the problem of cracks in the beams. At the same time, a kind of dicing structure is introduced into the last process which reduced the asymmetric parasite capacitors.Finally, the performance of the sensors is tested. First, the bias voltage on one capacitor changes from OV to 6.3V, the pull-in voltage is got. It is 6.3V quite the same as the theoretical value 6.5V. Then, for two batches of sensors with different beam thickness, the sensitivities are 14.4pF/g and 19.9pF/g, respectively, which agrees with the theoretical computations. The quality factors are 74 and 198, respectively, which satisfy with the low-noise request.