Abstract
For polycrystalline films, X-Ray diffraction (XRD) is the most common nondestructive technique for measuring residual stress because of its excellent phase selective capability and precise interplanar distance determination. However, the measurement of accurate stress and stress gradient on thin films is challenging and critical mainly due to confined thickness and unique microstructure. In this study, we proposed an average X-ray strain (AXS) method by using cos2sin2 XRD technique at several rotational () angles to improve the accuracy of the measurement of X-ray stress (XRS) or X-ray elastic constants (XECs). The major concept was to increase sampling volume by measuring X-ray strain at multiple rotational angles. In addition, by adjusting the grazing incident angle of X-ray the variation of stress with thickness could be measured. Since the resultant stress was an integrated magnitude from the surface to the penetration depth, a layer-by-layer method was adopted to resolve the real layer stress at different penetration depth. TiN hard coating on Si (100) substrate was selected as the model system, where the residual stress was determined by laser curvature, sin2, cos2sin2 and high energy XRD methods, from which the influence of sampling volume was revealed and the corresponding statistical meanings of each method were discussed. When sufficient sampling volume was obtained, the accuracy of AXS method could be reduced to 3% comparing to macrostress acquired from laser curvature method. The thickness limitation of AXS methods was about 350 nm by comparing the XRS and XECs to other techniques and literature data. For ultra-thin film less than 160 nm, a new multiple (hkl) diffraction AXS method was developed. With the correct XRS’s measured at different penetration depths, the stress gradients on TiN coatings with thickness 1.5, 2.7 and 3.9 μm were successfully acquired and the results were correlated to the film structure and fracture morphology in our previous study.