Limits of validity of the crystallite group method in stress determination of thin film structures

Abstract

The mechanical state of stress of thin films and multilayers is of great importance because of its influence on many physical properties: magnetic, dielectric, elastic, etc. For these reasons, the determination of stresses in these materials is becoming of increasing importance. Among the methods available for stress or strain determination, the X-ray technique, through the sin 2 ψ method, is of great interest because of the low mean depth penetration of X-ray and because it allows the strain measurements in different phases of a thin film, leading thus, for instance in a multilayer, to a complete knowledge of the state of stress. Usually, the thin films exhibit strong textures inducing some difficulties in the stress calculus. One easy way to solve the problem is to use the crystallite group method. It is a pseudo sin 2 ψ analysis based on the assumptions done in single crystal analysis. Of course, the stronger the texture, the better the validity of the measurements. In this work, we calculate using simulations and modelling (based on the Voigt and the Reuss hypotheses) the limit of this technique vs. the mosaicity of the texture around an ideal orientation for both fibre texture and single crystal-like texture. It is shown that the crystallite group method can be employed even for rather weak texture, allowing its use in a wide range of thin polycrystalline coatings.