Design, fabrication, and testing of the van der Paw piezoresistive structure for pressure sensing
Cassel, Robert Douglas
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The project characterizes a piezoresistive sensor under variations of both size and orientation with respect to the silicon crystal lattice for its application to MEMS pressure sensing. The sensor to be studied is a four-terminal piezoresistive sensor commonly referred to as a van der Pauw (VDP) structure. The VDP sensor is used primarily in sheet resistance measurements, but has also been determined to be useful in determining the stress components at a point on (100) and (111) silicon wafer surfaces. In a previous study, our team has determined the relation between the biaxial stress state at a point and the piezoresistive response of the VDP by combining the VDP resistance equations with the equations governing silicon piezoresistivity. It was found that the theoretical sensitivity of the VDP sensor is over three times higher than the conventional filament type resistor. With MEMS devices being used in applications which continually necessitate smaller size, understanding the effect of size on VDP performance is important. In order to test the validity of the theoretical calculations which were done by our group, appropriate devices were manufactured on a (100) silicon test wafer. The wafer was designed to have numerous pressure sensitive diaphragms which can reliably sustain a pressure difference of approximately 50kPa. Each diaphragm was doped with a VDP or other sensor designed to test the sensitivity of the VDP vs. a certain parameter. These parameters include size, misalignment, and diaphragm position, in addition to the comparison of sensitivity to conventional sensor types. A test strip was also included in the design in order to determine an empirical relationship between stress and resistance. In testing the VDP devices for comparison with conventional sensor types, it was found that the VDP devices had a linear response as expected, were the most sensitive, and provided a number of additional advantages. Specifically, the VDP device allows for significant miniaturization, because its resistance value is independent of size, and the measurement technique is independent of line resistance. The simple geometry of the VDP also simplifies fabrication.