THIN METAL-FILM CHARACTERIZATION AND CHEMICAL SENSORS - MONITORING ELECTRONIC CONDUCTIVITY, MASS LOADING AND MECHANICAL-PROPERTIES WITH SURFACE ACOUSTIC-WAVE DEVICES

Acoustoelectric coupling between charge carriers in a thin metal film and the evanescent electric field associated with a surface acoustic wave (SAW) propagating along the surface of a piezoelectric substrate provides a unique interaction mechanism with which to study the film's electronic charge transport properties, as well as changes in these properties resulting from chemisorption from the gas phase. We utilized 97 MHz ST-cut quartz- and 32 MHz Y-cut LiNbO3-based SAW devices to follow ( 1) the process of vacuum deposition of thin metal films (5-500 angstrom), (2) the (room temperature) relaxation of such films after their deposition, and (3) the chemisorption-induced changes in both the mass density and electrical conductivity of such films. Changes in mass density were followed with a resolution exceeding ]% of a monolayer. Sheet conductivity measurements made with the 97 MHz SAW device correlated well with simultaneous measurements made with a d.c. electrometer. In addition, changes in the mechanical properties of freshly deposited thin films were observed during relaxation and when gaseous species interact with them; we believe this is a result of adsorption predominantly along grain boundaries. The combination of responses to three different physical parameters can be utilized in the design of chemical sensors.