We have developed highly sensitive electrometers and electrostatic fieldmeters (EFMs) that make use of micromechanical variable capacitors. Our systems operate using the same basic principle as a chopper-stabilized amplifier or vibrating-reed electrometer. We use micromechanical resonators with specialized electrodes that implement variable capacitors. Modulating the input capacitance of the sensor generates an ac sense voltage proportional to the input charge or field that is detected using a lock-in amplifier. If the motional frequency is sufficiently high, 1/f noise does not significantly interfere with the measurement and the noise floor is thereby dramatically reduced.
We designed resonators having conventional comb-drive actuation and suspension elements and specialized electrodes to implement the variable capacitors in each sensor. Each resonator uses differential actuation and sensing to reduce the feedthrough of drive signals to the sense network. In the electrometer, we used a balanced comb structure to implement second-harmonic sensing. The electrometer systems also include mechanical switches to reset the voltage and charge on the input electrode. In the EFMs, we designed resonators that use two different electrode geometries to alternately shield and then expose a detection electrode. All devices operate in ambient air at room temperature.
We have used two fabrication methods, the first a hybrid technology utilizing fluidically self-assembled JFETs and SOI microstructures, and the second an integrated process combining 0.8-µm CMOS and 6-µm-thick polysilicon microstructures. Custom linear circuits were designed for each system to amplify signals for lock-in detection.
We present measured data from an electrometer that exhibits a charge resolution of 4.5 aC rms (28 electrons) in a 0.3-Hz bandwidth with an input capacitance of 0.7 pF. The resolution of this electrometer is unequaled by any known ambient-air-operated instrument over a wide range of operating conditions. The EFM has a resolution of 630 V/m, the best reported figure for a MEMS device. We analyze these results using analytical models and simulations and discuss their primary sources of error.