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The use of a surface-acoustic-wave (SAW) device to monitor etching of thin films is described. The device is a delay-line-stabilized SAW oscillator in which the propagation path is coated with a thin film of the material to be etched. Removal of material decreases the mass loading on the delay line and this increases the frequency of the oscillator. The frequency of a 75-MHz oscillator is found to increase by more than 690 KHz for 1-µm decrease in film thickness. Using dual-oscillator arrangement, one can simultaneously monitor substrate temperature as well as thickness of material removed.