Vertical comb-finger capacitive actuation and sensing for CMOS-MEMS

A new method for out-of-plane (vertical) electrostatic actuation and capacitive displacement-sensing that utilizes sidewall capacitance change of multiconductor comb fingers is analyzed and experimentally verified. Combining the inherited in-plane (lateral) actuation and sensing capacities of comb fingers, three-dimensional actuation/sensing can be realized. A maskless post-CMOS micromachining process is employed and the fabrication is compatible with standard CMOS processes. Applications include an three-axis microstage, a z-axis accelerometer and a lateral-axis gyroscope that use the proposed vertical comb-finger actuation/sensing method. The measured maximum vertical displacement of the microstage is 3.5 μm with a resonant frequency of 6.17 kHz. Measured sensitivity of the z-axis accelerometer is 0.5 mV/g with less than -40 dB cross-axis sensitivity, noise floor 6 mg/ √Hz, and linear range from -27 to 27 g. The lateral-axis gyroscope design uses integrated comb drives for out-of-plane actuation, and is motivated by the desire to integrate three-axis gyroscopes on a single chip. The packaged gyroscope operates at atmospheric pressure with a sensitivity of 0.12 mV/°/s and the resonant frequency of the drive mode is thermomechanically tuned between 4.2-5.1 kHz. Resonant frequency matching between the drive and sense modes is realized by integrating a polysilicon heater inside the spring beams.