A new apparatus for measuring the ultimate strain of thin films

Abstract

A four-point bending apparatus was built and tested that imparts controlled strain to thin inorganic films deposited onto a substrate and enables that strain to be measured quantitatively at the onset of film cracking. The measured quantity is the net ultimate strain, the sum of the residual strain and the material's intrinsic ultimate strain. With this apparatus, the strain being inflicted on the film does not have to be measured directly but can be calculated from the applied force, the Young's modulus, and the geometry of the substrate, as long as the film is much thinner than the substrate. The onset of cracking was detected with an electrochemical method that gave sharp and reproducible indications of film failure. The apparatus was tested with plasma-enhanced chemical vapor deposited silicon nitride films of thicknesses from 0.2 to 1.5 μm on 0.90 mm and 0.45 mm thick stainless steel substrates. The ultimate strain of these films was found to be constant over this range of thicknesses at about 0.20%. This apparatus can be used for any insulating film that can be deposited onto a conductive substrate, as long as the ultimate strain of the film is less than the limit of tensile elastic strain of the substrate.