Thin film thickness monitoring using a doubly oscillating quartz crystal and measurement of growth rate

A single quartz crystal wafer of 5 MHz frequency and 20 mm*20 mm size, with two pairs of circular gold electrodes deposited on it, was used to measure the rate of growth and the thickness of thin films during deposition. This arrangement may be called a doubly oscillating quartz crystal monitor (DQM). The main limitations in the use of this arrangement arises from the coupling between the two electrode system. The coupling was minimised by depositing the gold electrodes at a suitable distance of about 4.2 mm. Further, the thin film whose thickness is to be measured was deposited on one side of one electrode pair while the other pair was kept covered from deposition. The resonance frequencies of the two electrode system were then processed in a conventional manner and the beat frequency was measured by a digital frequency meter. The latter's output was analysed in a frequency-to-voltage converter and the resulting output voltage plotted on a X-Y recorder or X-t chart recorder was proportional to the rate of growth of thin films during deposition.