DUAL QUARTZ-CRYSTAL MICROBALANCE

Construction and performance of a dual quartz crystal microbalance is described. The final probe has a dipstick configuration that is partic ularly suitable for sensing and monitoring applications in viscous and /or conducting liquids. The differential (heterodyned) frequency measu rement substantially eliminates the deleterious effects of viscosity, temperature, and conductivity. The corresponding performance coefficie nts are temperature df/dT = 1.5 Hz/degrees C, viscosity df/d eta(L) = 103 Hz/cP, and conductivity df/dM = 108 Hz/M, where conductivity is ex pressed in terms of molarity of sodium chloride. As an example, the et ching of a 2000-Angstrom-thick layer of aluminum has been monitored as a function of time.