Critical Film Thickness Phenomenon in a Polymer-Coated Surface Acoustic Wave Resonator Chemical Gas Sensors

Theoretical calculation results are described to illustrate the experimentally observed phenomenon of critical film thickness in a polymer-coated surface acoustic wave (SAW) resonator sensor. Based on coupling-of-modes (COM) model incorporating the perturbation theory, the polymer coating characteristics of the device were calculated showing a close agreement with the experimental results in terms of the electrical response and its occurrence of critical film thickness situation. The situation is qualitatively explained by identifying and analyzing the symmetric and antisymmetric modes in standing waves across the resonance cavity. An analytic symmetric and antisymmetric admittance relationship that supports the resonance modes within the passband helps tracking the coating behavior exhibiting critical thickness condition. The calculation results for coatings below, close to, at or above the critical thickness supports not only the experimental findings that the sensor be operated below the critical thickness for an optimum sensor performance but also the existence of an optimum film thickness range at which the sensor provides maximum sensitivity at low loss degradation as required for stable sensor oscillator operation. This study would help identify and estimate an optimum polymer film thickness range which provides low electrical performance degradation for SAW resonator sensors.