The design, fabrication and testing of electrostatically drive, high-sensitivity dc voltmeters (ESVs), which have demonstrated sensitivity to 10 mV signals is presented in this report. The ESVs have a micromachined test electrode (third layer of polysilicon) 5 um above the shutter, which provides testing repeatability, increased sensitviy to the input and most importantly the ability to accurately model ESV performance. The theoretical first- and second-harmonic output currents are derived. The results of predicted and experimentally meausred signals are compared and show the validity of modeling equations when microstructure parameters such as resonant frequency, lateral spring constant, quality factor, (dC/dx)comb and (dC/dx)sh are known. FastCap, a three-dimensional capacitance-extraction program, has accurately simulated the capacitance between the input and detector electrodes (compared to values measured by an electromechanical amplitude modulation technique) and the other parameters can be derived from the microstructure characteristics.
Probe-to-probe and substrate feedthrough produce a current that is synchronous with the sensed signal and therefore limit the performance of the ESV. Geometrically contoured shutters have been designed to take advantage of mechanical modulation to produce signals at higher harmonic frequencies. Differential detection has shown increased sensitivity to the input voltage by doubling the output signal and also by nearly eliminating the common-mode-parasitic-drive feedthrough signal. Parasitic feedthrough can also be minimized through actuation of the device in vacuum. Dissipative processes in vacuum are significantly diminished, reducing the ac drive-voltage (and therefore feedthrough) required to resonate the microstructure to a certain amplitude of oscillation.
Future ESVs will be micro packaged in a silicon nitride shell and integrated with signal processing circuitry making today's hybrid systems obsolete. Future directions and possible designs for the next generation of ESVs will also be introduced in this report.