Microelectronic capacitance transducer for particle detection

This paper describes the CMOS circuit design of a sensor for detecting changes of capacitance due, for instance, to the incidence of particles or bubbles on the electrodes. The circuit is based on a simple design originating at the University of California, Berkeley, for measuring crosstalk on integrated circuits. The basic front-end sensor circuit comprises eight MOSFETs and has a sensitivity of 40 mV/fF. A differential amplifier receives the outputs from two sensor circuits each having 20-μm square inter-digitated electrodes. The resulting sensitivity of the fabricated sensor is 1 V/fF with a noise level equivalent to 10 aF. Monte Carlo circuit simulations have been used to identify transistor dimensions to yield acceptable yield, and prototype custom silicon chips have been fabricated using a 0.8-μm CMOS process. Static and dynamic tests, using polyamide particles as small as 10-μm diameter, verify correct operation of the sensors. The sensor is now being developed for application in miniature electrical tomography systems.