High-mode resonant piezoresistive cantilever sensors for tens-femtogram resoluble mass sensing in air

Abstract

According to the demand for an ultrasensitive mass sensor for bio/chemical molecular detection, resonant cantilever sensors are developed for detection in an air environment. Both a piezoresistive bridge and a metal coil are integrated in the cantilever for signal sensing and Lorentz-force resonance excitation, respectively. Compared with conventional first flexure mode resonance, measurement results for the second mode resonance show an improved mass-sensing resolution from 0.17 pg to 0.06 pg due to the higher quality factor. For further improving the resolution, an optimized electromagnetic excitation method specifically for the second resonant mode is proposed and developed. The optimized method provides a two-point excitation that matches the second mode shape function of the cantilever deflection and excites the second mode more efficiently. Compared to a cantilever with a conventional excitation method, the optimally excited cantilever improves the quality factor from 307 to 857. Based on the experimental results for the optimally excited second mode resonant sensor, 29 × 10−15 g resolution for in-air mass sensing is achieved. The developed second mode resonant cantilever sensor with piezoresistive sensing element integrated on-chip is promising for use in high-performance portable biological/chemical sensing applications.