DEFORMATION IN THIN-FILMS BY THERMAL STRAIN

Stresses and strains are always present in thin films deposited on substrates. When these strains are larger than the elastic limit, they are relaxed by various mechanisms, deform the films, and are detrimental to the stability or performance of thin-film devices. The relaxation processes are strongly affected by the film microstructure (such as grain size and film thickness), the shear modulus of the substrate, the surface oxide and misfit dislocations often observed in the layered-structural films. The strain relaxation causes large film microstructure changes. In contrast, although the strain is supported elastically, small microstructure changes are observed. Nonuniform distributions of the strain are observed in the film in directions both parallel and normal to the film surface. Different shrinkages are measured in grains with different crystal orientations due to strong anisotropy in the elastic constants of the film and the strain gradient from the top to bottom of the film is observed. In this paper, deformation of thin films caused by thermal strain is reviewed using illustrative data of Pb films deposited on Si substrates.