Application of the van der Pauw Structure as a Piezoresistive Pressure Sensor

PI: Ahsan Mian, Montana State University

The proposed  project aims at replacing the conventional filament type resistor piezoresistive sensor with a square conductor with four electrical contacts.  The four-contact conductor, popularly known as van der Pauw (VDP) structure, is a test structure widely used for measuring resistivity of arbitrarily shaped samples of constant thickness.  Over the years, many researchers have extended the original ideas to develop a variety of approaches for evaluating the resistivities of both isotropic and anisotropic materials using VDP type structures.  When used as pressure sensors, VDP devices have the potential to obviate some of the limitations of resistor based sensors.  As will be demonstrated in the project, they have potential to offer a greater sensitivity compared to that of resistors, and thus they also exhibit a corresponding reduction in sensitivity to thermal errors.  The classical VDP structure itself requires only one square of material plus room for four contacts, and the "x" and "y" sensing regions are physically merged.  Thus, these sensors have the potential to be made small enough to capture localized stress variations on a diaphragm surface without any loss of sensitivity.  In fact, it is the distributed nature of the VDP structure that leads to the enhanced sensitivity.

In the current investigation, the piezoresistive behavior of VDP structures fabricated on a circular silicon diaphragm surface will be considered.  The VDP resistance equations will be combined with the silicon piezoresistivity equations to yield relations for the change in resistance of a VDP sensor in terms of the applied state of stress.  In the second part, the sensitivity of the VDP sensors on circular diaphragm to applied pressure load will be studied analytically and numerically.  The numerical calculations will involve the use of anisotropic conduction-based simulations using finite difference technique.