Dual-mode piezo-on-silicon resonant temperature and humidity sensor for portable air quality monitoring systems

This paper demonstrates a dual-mode, micromachined temperature and relative humidity (RH) sensor platform using two ZnO-on-silicon, lateral-mode bulk acoustic wave (BAW) resonators. A moisture-insensitive reference resonator is utilized in conjunction with an identical, humidity functionalized resonator to yield a thermally compensated RH sensor. The reference sensor exploits the dual-mode response of fundamental and higher order in-plane resonance modes in the range of 30-100 MHz to eliminate the RH sensor dependence on temperature. Hygroscopic polyethyleneimine (PEI) is deposited on annexed sensing platforms of the BAW resonator to enable operation from 20 to 92 %RH with high Q in air. A humidity sensitivity of 1.2 kHz/%RH from 20 to 60%RH and 14 kHz/%RH from 60 to 92%RH is measured at room temperature for the RH sensor which, combined with the reference sensor, occupies a total area of .053 mm².