Polymer coated MEMS resonator for room temperature NH3 sensing

This work presents a feasibility study of using a polymer coated MEMS doubly-clamped beam resonator for low power sensing of NH3 at sub-ppm level in air at room temperature. Upon NH3 absorption, the polymer swells leading to an increase in the effective mass of the beam and buildup of stress in the polymer layer, that result in a decrease in the resonance frequency. The normalized frequency shift (Delta f/f(0)) of such beams is larger than that of conventional cantilever type resonators. Poly(acrylic acid) (PAA) and polypyrrole (PPY) were compared using a quartz crystal microbalance and PAA was selected for inkjet printing onto the resonators thanks to its high NH3 uptake. The sensor with 12 printed PAA layers shows a NH3 sensitivity (Delta f/f(0) per ppm) of 0.7 x 10(-3) ppm(-1) at 40% relative humidity and a detection limit below 100 ppb. The sensor has a response time of 5 minutes, which scales linearly with the number of the printed polymer layers. The sensitivity was found to increase with RH.