Gas sensitivity comparison of polymer-coated SAW and stw resonators operating at the same acoustic wave length

Results from systematic gas sensing experiments on polymer coated surface-transverse-wave- (STW) and surface-acoustic-wave- (SAW) based two-port resonators on rotated Y-cut quartz, operating at the same acoustic wave length of 7.22 μm, are presented. The acoustic devices are coated with chemosensitive films of different viscoelastic properties and thicknesses, as solid hexamethyldisiloxane (HMDSO), semisolid styrene (ST) and soft allyl alcohol (AA). The sensor sensitivities to vapors of different chemical analytes are automatically measured in a sensor head, evaluated and compared. It is shown that thin HMDSO- and ST-coated STW sensors are up to 3.8 times more sensitive than their SAW counterparts, while SAW devices coated with thick soft AA-films are up to 3.6 times more sensitive than the STW ones. The results show that SAWs are more suitable for operation with soft coatings while STWs perform better with solid and semisolid films. A close-to-carrier phase noise evaluation shows that the vapor flow homogeneity, the concentration of the analyte, its sorption dynamics and the design of the sensor oscillator are the major limiting factors to the sensor noise and its resolution. A well designed ST-coated 700 MHz STW sensor provides a 178 kHz sensor signal at a 630 ppm concentration of tetra-chloro ethylene and demonstrates a short-term stability of 3×10^-9/s which results in a sensor resolution of about 7 parts per billion (ppb)