Simple and inexpensive single-oscillation quartz crystal thin-film thickness monitor and growth-rate meter

Abstract

Commercially available quartz crystal thin-film thickness monitors have limitations due to: (i) their inability to indicate the absolute frequency of oscillation of the mass measuring crystal; and (ii) the instability of the variable frequency LC oscillator, if any, incorporated in the circuit. Recently it has been suggested that it is economical and simple to use a digital frequency meter for the direct measurement of the frequency shift due to film deposition of the measuring crystal. Such an arrangement gives better results than commercial film thickness monitors. A single oscillation quartz crystal monitor and growth rate meter has been designed using this idea and locally available electronic components. In this instrument a quartz crystal wafer having one pair of gold electrodes is used, while the shift in resonance frequency measured by a digital frequency meter is used for the estimation of thickness of thin films. To measure the rate of growth of the film the resonance frequency signal from the crystal is suitably processed to produce a DC signal, which may be recorded on an x-y/x-t recorder to obtain the rate of growth during the deposition of the film.