An axial strain modulated double-ended tuning fork electrometer

This paper reports a micromechanical electrometer that uses a single-crystal silicon double-ended tuning fork resonator as the charge sensing element. Addition of a charge across an input capacitor induces a tensile axial strain in the resonating tines, resulting in a shift in the resonant frequency. The resonator has a nominal resonant frequency of 154 kHz and quality factor of 80,000 at room temperature and a pressure of just under 4 mTorr. The resonator has been embedded inside a closed loop circuit to realize a nonlinear oscillator with a short-term frequency deviation of 3.1 mHz (20 ppb). This yields a minimum measurable charge of 4 fC, equivalent to a noise force of 0.725 nN and strain resolution of 23.4 pɛ, set by the electronic noise of the oscillator circuit.