A film thickness monitor in vacuum evaporation by the transmission of beta particles

During the evaporation process, the film thickness is continuously monitored by means of the transmission of beta particles ejected from the Carbon-14 source of 1.85 MBq. As the detector of the particles, an end-window Geiger-Mueller tube is used and connected with a linear counting rate meter with a time constant of 3 sec. The advantages of this method are that the sensing device requires no protection against temperature change and minimum operator skill is required. In this experiment, the calibration curve of counting rate versus film thickness is represented by a simple exponential curve and is not influenced by the thickness of the mica substrate on which the evaporant condenses. The resolution obtained is +-0.055 mg/cm2, i.e., +-480 A for the evaporation of silver. (author)