Modeling, Simulation, and Design Guidelines for Piezoresistive Affinity Cantilevers

This paper presents design guidelines for piezoresistive affinity cantilevers for operation in liquid environments. For the first time, we consider the interdependence of various functional elements (such as biological, mechanical, and electrical) of the cantilever, their dependence on material choice, microfabrication processes, and geometry, and the resultant effects on the mechanical and electrical sensitivities of the cantilever. The cantilever design guidelines that include material selection as well as determination of geometrical dimensions are proposed. As an example, we have designed and simulated a multilayer piezoresistive silicon nitride affinity cantilever for performance in a liquid environment under constraints imposed by microfabrication and electrical and mechanical considerations. Systematic steps toward optimization of geometrical dimensions include initial analytical estimates of geometrical dimensions, followed by finite-element modeling and analysis of such cantilevers under the applied surface stress. Simulation studies brought forth the limitation on maximum obtainable ΔR/R as well as the nonlinear behavior of the cantilever which was not observed in analytical estimates.