A low-pressure encapsulated deep reactive ion etched resonant pressure sensor electrically excited and detected using 'burst' technology

A purely silicon resonant pressure sensor fabricated using deep reactive io n etching (DRIE) and encapsulated at low pressure by two glass lids is pres ented. The sensor consists of a vibrating dual-diaphragm capsule suspended at four points in a fixed frame. The support beams an hollow and act as pre ssure inlet ports. As the ambient gas pressure changes, the resonator shape changes, thereby changing its resonance frequency. The sensor integrates c orner holes and is encapsulated at low pressure to reduce squeezed-film dam ping effects between the resonating structure and the glass lid. The sensor is electrostatically excited into a balanced mode of oscillation and capac itively detected using a novel 'burst' technology. This technique is based on independently exciting the structure and detecting the resulting output frequency at separate periods in time. Several sizes and design variations of the sensor have been fabricated and evaluated. Measurements show the sma llest structure (5 mm membrane diameter width) to have a Q factor of 14 000 after low-pressure encapsulation, pressure sensitivity of 15 ppm/mbar(-1) over the range 0.1-1500 mbar, and expected temperature sensitivity of -34 p pm degrees C-1. The structure had a resonance frequency of 35 078 Hz in atm ospheric air pressure. If higher sensitivity is desired, a larger sensor ca n be chosen (140 ppm/mbar(-1) for a sensor with a 10 mm wide membrane), how ever, at the expense of a lower Q factor.