Miniature piezoresistive sensor for detecting volatile organic components

Abstract

This paper reports a piezoresistive gas sensor, which detects the stress of polymer swollen due to sorption of trace gas molecules. The sensor consists of a Si-polymer composite and a Si cantilever with a piezoresistor at the support. The stress is transformed via the cantilever lateral deflection to the strain at the piezoresistor at the cantilever support. The saturation time for water signal is ca 440 s, and the response is reversible. The response rate, d(∆R/R)/dt, exhibits a maximum at 15.5 s. The ethanol and acetone signals show maxima of lower magnitude. The Fickian diffusion model with variable surface concentration of analyte represents very well the sensor response. The predicted maximum response rate is [d(∆R/R)/dt]max∝Sε∞D1/2b1/2, where S is device sensitivity, ε∞, is equilibrium strain of swollen polymer, D is analyte diffusivity, and b is the rate of change of surface concentration of analyte. The response time corresponding to this maximum is tmax∝1/b. For low humidity, the observed sensitivity is S=2.2. An array of sensors, with properly selected polymer sorbents is expected to function as detector of volatile gas components in human breath and/or in atmosphere.