Tensile testing of thin films supported on compliant substrates

Elastic properties of thin films are routinely measured by straining them on a flexible carrier substrate. Stress–strain characteristics for the film/substrate composite strip are obtained and a simple mixture theory is used to extract the elastic properties of the film by assuming a uniform one-dimensional tensile stress distribution within the film thickness. In this paper, we examine the validity of using this simple theory by determining the three-dimensional elastic stress field in a composite strip. Three sample geometries and loading configurations are considered. In case A, the film is coated on one side of the carrier substrate while in case B, the carrier substrate is coated on both sides. For both case, the external load is applied only on the substrate. In case C, the geometry was the same as case A, but the external load was applied to both the film and the substrate. For film to substrate moduli ratios less than 5, the mixture theory provided adequate results with errors less than 10% for all cases considered. However, for case A, the rule of mixtures theory tends to significantly overestimate the tensile stress in the film for moduli ratio exceeding 5. An error analysis is also presented which provides guidance regarding the minimum film thickness necessary to attain results with a given accuracy. Overall the results presented in this paper provide researchers with rules to design the sample geometry and film thickness to accurately determine the elastic properties of thin films using the composite film/strip experiment. These rules are exemplified by measuring the elastic properties of a thin sputtered Al film and a hexagonal honeycomb structured Si/polymer composite film produced through an evaporation induced self-assembly process. In both cases, films are deposited on a flexible Kapton substrate and strained using a home-built film tester.