Stress Determination in Textured Thin Films Using X-Ray Diffraction

Thin film stresses are important in many areas of technology. In the semiconductor industry, metal interconnects are prone to stress voiding and hillock formation. Stresses in passivation layers can lead to excessive substrate curvature which can cause alignment difficulty in subsequent lithographic processing. In other thin film applications, stresses can cause peeling from mechanical failure at the film-substrate interface. Beyond these issues of reliability, stress and the resulting strain can be used to tune the properties of thin film materials. For instance, strain, coupled with the magnetostrictive effect, can be utilized to induce the preferred magnetization direction. Also, epitaxial strains can be used to adjust the bandgap of semiconductors. Finally, the anomalous mechanical properties of multilayered materials are thought to be partially due to the extreme strain states in the constituents of these materials. To fully optimize thin film performance, a fundamental understanding of the causes and effects of thin film stress is needed. These studies in turn rely on detailed characterization of the stress and strain state of thin films. X-ray diffraction and the elastic response of materials provide a powerful method for determining stresses. Stresses alter the spacing of crystallographic planes in crystals by amounts easily measured by x-ray diffraction. Each set of crystal planes can act as an in-situ strain gauge, which can be probed by x-ray diffraction in the appropriate geometry. Hence it is not surprising that x-ray diffraction is one of the most widely used techniques for determining stress and strain in materials. (For reviews of this topic, see References 5–7.) This article is a tutorial on the use of x-ray diffraction to extract the stress state and the unstrained lattice parameter from thin films. We present a handbook of useful results that can be widely applied and should be mastered by anyone seriously interested in stresses in crystalline thin films with a crystallographic growth texture.