Effects of film thickness on sensitivity of SAW mercury sensors

An ST cut quartz 262 MHz dual delay line surface acoustic wave (SAW) mercury vapor sensor has been designed, fabricated, and rested. A gold sensing film is deposited on one delay path while the second delay path remains bare and acts as a reference to cancel out frequency amalgamations due to variations in temperature, gas flow rate, etc. The sensor's ability to detect mercury is due to the strong interaction between gold and mercury, known as amalgamation. In the present work a large number of gold films of various thicknesses have been exposed to mercury concentrations from 20 to 100 parts per billion (ppb) in dry nitrogen. It is shown that the steady-stale response magnitude is highest for a 75 Å gold film with decreasing sensitivity for films which are thinner of thicker. A model is presented which explains this behavior.