Particle detection using an integrated capacitance sensor

Capacitance sensors, for detecting small particles, have been integrated on to a custom CMOS VLSI chip. The sensors employ a charge measuring circuit a nd differential amplifier which depend strongly on close matching of compon ents for correct operation. Finite element modelling has revealed the magni tude of anticipated capacitance changes which, for particles on the surface of the chip, may be of the order of a femtofarad. Simulations using comput ational fluid dynamics (CFD) suggest that, if the sensor is inclined to the flow, the particles can be expected to hit the surface. Consideration of t he surface of the chips suggest that surface roughness is not significant. Static tests reveal output voltage changes of several hundred millivolts in the presence of small particles on the surface. This drops dramatically as particles are moved vertically away from the surface. Dynamic tests confir m that the sensor is able to detect plastic particles as small as 20 mum th at are carried at velocities up to about 10 m/s with a time resolution of 2 mus. Recent work has involved the capture of high speed video images of pa rticles passing the chip in order that we can verify correct operation of t he sensor. Representative video images are presented. (C) 2001 Elsevier Sci ence B.V. All rights reserved.