INTERFEROMETRIC MEASUREMENT OF THICKNESS OF THIN FILMS OF FORMVAR DEPOSITED ON GLASS PLATES

An experimental set-up for measuring the thickness of Formvar films deposited on a glass plate is developed. Formvar film is deposited on a plane glass plate to form a step using a dipping technique. The glass plate containing the film step is coated with a thin film of aluminium to give a reflectivity of about 70% in a vacuum evaporation unit. Multiple beam Fizeau fringes in transmission mode are obtained by an interference of a partially reflecting optically plane surface with the partially reflecting glass plate containing the step of Formvar. There is a fringe shift at the step due to abrupt change or gradual change in the air gap. The fringe spacings and fringe shifts at the step are measured for three wavelengths by a distance measuring device and the step height is calculated. The method provides thickness measurement of an accuracy of +-100 A. The method can be very useful for thickness measurement of thin plastic films for extreme ultraviolet (XUV) soft X-ray research applications.