X-ray diffraction as a tool to study the mechanical behaviour of thin films

In this paper, we present results of in-situ X-ray strain measurements on Al and Cu thin films, which were mechanically strained by (i) thermal cycling or (ii) a dedicated micro-tensile tester. In the first case, continuous films or patterned lines were deposited on Si substrates and heated up to 500°C and strained as a result of thermal mismatch. For the tensile testing, films were deposited onto compliant polymer substrates. Then, film and substrate were strained simultaneously, while the film stress was determined by X-ray measurements. Both types of experiments show that, roughly speaking, the film strength increases with decreasing film thickness. A more detailed evaluation of the experimental results also provides information about effects, such as strain hardening and plastic anisotropy. Based on these observations, implications for a theoretical description of deformation in thin films are critically discussed.