A servo-controlled capacitive pressure sensor using a capped-cylinder structure microfabricated by a three-mask process

A silicon-micromachined servo-controlled capacitive pressure sensor is described. The use of a capped-cylinder shape with pick-off electrodes external to a sealed cavity permits this device to be fabricated in only three masking steps. Device behavior is evaluated experimentally and by finite element analysis. A fabricated device with 2 mm diameter, 9.7 mum structural thickness and 10 mum cavity height provides a measured sensitivity of 0.516 V/kPa over a dynamic range of 20-100 kPa gauge pressure, with a nonlinearity of <3.22% of full scale. The open-loop sensitivity of this device averaged over a dynamic range of 0-250 kPa is -408 ppm/kPa. A voltage bias applied to the servo-electrode can be used to tune both the open-loop and servo-controlled sensitivity by more than 30%. An alternative design in which the Si electrode is segmented to relieve residual stress provides 10-20% more open-loop sensitivity with similar structural dimensions. Fabricated devices are sealed within a metal package filled with an inert dielectric liquid. This enhanced open-loop sensitivity by a factor of about 1.7, and in servo-controlled operation, reduced restoring voltage by a similar factor. Measurements and analysis of temperature responses of these devices are presented.