Stress in Alkali Halide Films

A detailed investigation has been made of the stress in alkali halide films evaporated onto glass, and it has been shown that this can be done satisfactorily by measuring the end deflection of a substrate using a surface probe. Initial difficulties arising from residual stress in the substrates have been largely overcome by carefully annealing the glass. The stress has been derived from the curve of observed deflection, plotted in terms of the force factor, against film thickness. A survey of the various models for explaining stress has been made and it has been shown that there are at least two, different, stress-producing mechanisms. In the early stages of growth, a misft model is suggested, in which the oxygen sites in the glass surface act as preferred positions for the metal ions of the halide. The situation is not as clear in thicker films, but either a thermal expansion or a density model could provide a partial explanation.