MEMS Electrometer With Femtoampere Resolution for Aerosol Particulate Measurements

Electrostatic charge measurements are at the base of chemical, physical and biological experiments. In this paper, we present an electrometer based on the vibrating capacitance of a microelectromechanical systems (MEMS) resonator for the detection of small currents from ionized particles in an aerosol particle detection system. We use a porous sensing-electrode coupled to a MEMS resonating electrometer. Operating at resonance, charge is collected on the MEMS electrometer and modulated at the resonant frequency and its harmonics. Induced voltage is read with a low-leakage very high-input impedance feedback amplifier. Because of the specific readout technique, a switched-reset is used to prevent charge saturation. Sensitivity improvements are achieved by modifying the low noise-readout amplifier by reducing input-referred noise and parasitic capacitance. The electrometer achieves a noise floor <;1 fA produced by 10 nm diameter particles within an airflow of 1.0 L/min. At this flow rate, the minimum detectable current (1 fA) corresponds to a minimum measureable particle density of 400 cm^-3. The MEMS electrometer is compared with and calibrated against commercial electrometer and a particle counter, respectively.