Bulk micromachined electrostatic RMS-to-DC converter

Bulk micromachining in silicon and glass wafers and subsequent silicon-to-glass anodic bonding have been used for the realization of an electrostatic RMS-to-DC converter. A suspended membrane has been designed for: large dynamic operating range (detection limit by minimum mechanical-thermal noise and high value of the pull-in voltage), maximum bandwidth (low series resistance, high second harmonic suppression using squeeze film damping and suspension beam design), long-term stability, and a sufficient displacement-to-voltage sensitivity (membrane area and suspension arm length). Prototypes are typically composed of a 3 × 3 mm² perforated membrane area suspended by four beams of 200 μm length, 500 μm width, and 4 μm thickness micromachined out of silicon and aligned to a counter electrode on glass with 4-μm spacing in between. Measurements on realized devices show a 4.5 pF nominal capacitance. Static measurements indicate a sensitivity of 5 fF/V² and a voltage shift of 0.2 V. The nominal square relation is achieved within a 0.5% nonconformity error