INTERNAL-STRESS COMPENSATION AND SCALING IN ULTRASENSITIVE SILICON PRESSURE SENSORS

The pressure sensitivity of boron-doped silicon membranes has been characterized as a function of diaphragm dimensions and internal membrane stress. Using an electrostatic technique based on silicon microbridges, the internal stress for p++ silicon (on glass), LPCVD silicon dioxide, and LPCVD silicon nitride was measured; typical values are 40, -300, and 950 MPa, respectively. Silicon membranes having several different edge lengths and deposited oxide and/or nitride coatings were characterized for sensitivity. The results confirm a previously reported analytical scaling theory for these structures. While the pressure sensitivity can be reduced by more than a factor of twenty in these silicon membranes due to boron-induced internal stress, the use of stress-compensating dielectrics can improve this sensitivity by a factor of six or more. Based on this theory and the measured material parameters, scaled experimental devices show typical sensitivities within 10-20% of the theoretical design targets. Pressure sensitivities as high as 2900 ppm/Pa have been achieved.