Sensitivity of Residual Stress to Moisture Penetration on Optical Thin Films

Abstract

A Michelson-type, whole-field residual stress measurement method has been set up. From this method, two-dimensional out-of-plane partial slope contours of optical thin films were obtained. The linear and the rotational mismatch techniques in the experiment help to improve the sensitivity of measurement and determine the signs of residual stress. Whether aluminum film, the most popular material used in reflective optical components, is coated with protective layers (SiO or MgF2) or not, exposure to the background humidity is unavoidable because moisture penetrates into aluminum film. In this paper, we point out that the residual stress of aluminum film changes drastically because of moisture absorption. This highly sensitive residual stress measurement method also shows that protective layers cannot prevent moisture penetration.