Design issues in SOI-based high-sensitivity piezoresistive cantilever devices

In this work, the mechanical design and optimization of high-sensitivity piezoresistive cantilevers used for detecting changes in surface stresses due to binding and hybridization of biomolecules on the surface of the cantilever is investigated. The silicon-based cantilevers are typically of a micron order thickness doped with boron to introduce piezoresistivity. Microcantilever beams can be built as micro-mechanical arrays which could provide a basis for developing devices capable of performing multi-plexed, low-cost genomic and proteomic analyses. This paper provides several design solutions in optimizing the cantilever mechanical design to address the sensitivity required when approaching recognition of single base pairing of DNA molecules. The sensitivity of such piezoresistive cantilevers to the chemo-mechanical stress induced currents depends not only on the cantilever geometric properties, such as depth and width but also on the depth of the piezo layer (dopant) and its doping characteristics. It is often an expensive exercise to determine the optimum design parameters for increased sensitivity, particularly the dopant characteristics for such MEMS devices. A managed solution or parametric solution algorithm based on a finite element simulation is used to help determine optimum location and depth of this piezoresistive layer in the cantilever that maximizes the piezoresistor signals. Further, novel approaches for increasing the sensitivity of piezoresistive cantilevers through selected structural discontinuities are discussed.