Investigation of air damping effect in two kinds of capacitive MEMS accelerometers

In this study, the air damping ratios of an unsealed chip of a capacitive accelerometer subjected to different pressures were measured by a special designed circuit in a static state test. There are total eight trapezoid grooves on the top and bottom electrode wafers aiming at reducing air damping effect. Finite element method based on energy balance method and transient blade row model is used to simulate in ANSYS/CFX with a new formula of effective viscosity of the air derived from simulation and validated by experimental results. The numerical results are in good agreement with the experimental results. With the same method and the new formula of effective viscosity, the air damping ratio of a new chip without trapezoid grooves is simulated as well. The comparison of the two kind of chips shows that the air damping ratio decreases 63.4% after four trapezoid grooves were engrooved on the top and bottom electrode wafers. As a result, engrooving on the top and bottom electrode wafers provides a new way to reduce air damping effect.

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