Coupling High Force Sensitivity and High Stiffness in Piezoresistive Cantilevers with Embedded Si-Nanowires

Using a single-mask add-on process module, two usually non-coexistent properties of cantilevers, large stiffness and large force sensitivity, are summoned in a single device. To achieve this, a stress-concentrating (SC) principle is used: instead of decreasing the cantilever thickness, which is the conventional approach in increasing its force sensitivity, the stress is locally concentrated in piezoresistive beams or wires. Hence, with a negligible decrease in stiffness, a higher sensitivity is obtained. Results of finite element simulations and experimental data confirm the feasibility of the proposed design. Fabricated cantilevers with implemented stress-concentrating silicon nanowires show a more than 5-fold improvement in force sensitivity compared to a conventional cantilever with the same dimensions, while maintaining the same stiffness. With thinner wires, the sensitivity increases 8-fold at the expense of a 15% decrease in stiffness.