Stress measurement in thin films by geometrical optics

A simple variation of the Newton’s rings experiment is proposed for quick, relatively accurate stress measurements. The method involves depositing a thin film on a small circular thin substrate, in this case a common microscope cover slip (18 mm diam × 0.18 mm). The thin film then deforms the cover slip into roughly a paraboloid shape. The focal length of this ‘‘spherical mirror’’ is determined optically by reflecting parallel light from its surface onto a screen. Any magnification or contraction of the image of the film is indicative of distortion of the substrate due to film stress. Such magnification, whether greater or less than 1, immediately determines the sign of the stress. This method described for measuring stresses in the film is simple, fast and inexpensive, requiring only a laser, mirrors and a diverging lens. (AIP)