Thin plasma-polymerized layers of hexamethyldisiloxane for acoustoelectronic humidity sensors

Plasma-polymerized layers are obtained from hexamethyldisiloxane (HMDSO). Using scanning electron microscopy (SEM), the effect of glow discharge duration and modification with ammonia on polymer surface and morphology is studied. It was shown that polymer layers were uniform and without defects. The effect of some plasma polymerization parameters on the humidity-sorptive properties of the polymers are investigated by measuring the mass changes as a result of water sorption. For this purpose, a system consisting of a sensor array of quartz crystal microbalances coated with plasma polymer films is used. The resonant frequency of the coated quartz resonators decreases with increasing relative humidity. The results showed that the increase in the glow discharge current density, gas phase pressure, layer thickness and modification with ammonia in plasma, increases the sensitivity of the polymers to humidity. The study shows that plasma-polymerized HMDSO layers may be used as a hygroscopic element for sensing the humidity of ambient air and therefore, for the development of quartz resonator sensors. (C) 1997 Elsevier Science S.A.