Application of Laser Interferometry for Transient Film Thickness Measurements

A laser interferometry technique for making transient measurements of film thickness of the order of 10 to more than 1000 μm is described. The basis for these measurements was reported previously [1] but the technique was applied to solid glass slides and a slowly thinning silicone oil film. The current work describes an adaptation of the technique for the measurement of rapidly changing film thickness, as occurs with evaporating films. A beam from a helium-neon laser is focused on the film at an oblique angle. Some of the laser light is reflected off of the top surface of the film and some is reflected off of the bottom surface. The light reflected from the two surfaces forms a concentric interference fringe pattern which is projected onto a screen and recorded by a high-speed camera. The film thickness is directly related to the spacing of the fringes. To demonstrate the technique, measurements of the time-varying thickness of three evaporating films are presented and experimental considerations are discussed.