A digital thickness monitor which measures the thickness of a film deposited on a quartz crystal by measuring the change of frequency of a single oscillator controlled by the resonance of the crystal is described. Conventional beathyphen;frequency techniques involving two precision oscillators are avoided by means of circuits including a presettable memory and an uphyphen;down counter. The monitor is easily reset at the start of a deposition, can display either frequency change or frequency, and can be easily modified to display film thickness directly. Film thickness determined by the monitor is compared favorably to thickness values determined interferometrically.
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