A thin film that is deposited on a substrate or is part of multifilm stack structure will exhibit residual stress that depends on the film thickness. In general, residual stress of the film is defined as the stress that the film will experience when it is in standalone condition or not in contact with any other surface. Film stress is usually classified into two classes:
Tensile and Compressive Stress
In case of PVD films, residual stress arises because of (a) difference in coefficient of thermal expansion of the film and the substrate at the time of cool down from deposition temperature, and (b) adsorbed atoms not condensing into a state of low energy or equilibrium with one another. Such residual stresses are like stored energy that can be released during subsequent processing steps or with time. For materials with high elastic modulus like Cr, W or metal oxides, very high stresses can be attained with large amount of stored energy. For low elastic modulus materials like Au or Ag, plastic deformation will occur to relieve stress before high levels of stress are attained.
The distribution of stress along the thickness of the film is important factor in determining the behavior of the film. If the stress is not uniform throughout the thickness of the film, the film will curl up when detached from the substrate. If it is uniform, the film will lie flat. Film under tensile stress will try to contract, bowing the substrate in the process such that the film is on the concave side. If the film is under compressive stress, it will try to expand bowing the substrate such that film is on the convex side. Tensile stress relieves itself through formation of micro-cracks and subsequent peeling of the cracked surface from the substrate. Compressive stress relieves itself by buckling visible in the form of “worm tracks” or “blisters” on the film surface (see graphic below).
Film stress can be tuned from compressive regime into tensile regime by changing key process parameters such as Power, process gas flow (Ar), substrate temperature etc. Below is an example of such process tuning achieved for TiN (Titanium Nitride).