MOSFETs-based Pressure Sensors in Thin Film SOI Technology

In this paper, novel intrinsically digital pressure sensors concept is presented and successfully demonstrated. The sensors principle consists in integrating active devices, used as pressure transducers, directly on very thin dielectric membranes. A uniform thickness of 1.2-1.5 µm is obtained. Two original architecture types were designed and fabricated. The first solution implements a single NMOSFET acting as a current source which is sensitive to the sum of both components of stress (σxx+σyy) and located at the center of a membrane. Between 0.1 and 4.8 bar, the output frequency and the N-source transducer current are raised respectively of +12 % and +20 %. The sensor power consumption is 2.1 µW. The second solution is based on PMOS mirror cells. PMOSFETs are placed in opposite orientation, i.e. perpendicularly to each other, taking advantage of the opposite sign and the high value of their piezoresistive coefficients. These cells are sensitive to stress components difference (σxx-σyy). Therefore, the sensors are suspended on rectangular membranes, which provide a good stress asymmetry. Simple and double mirror cells based sensors do show a frequency output variation of +21 % and +44 %, respectively, between 0.1 and 4.9 bars.