A wide variety of techniques have previously been used in the measurement of mechanical properties of thin films. These techniques include miniature tension, bulge test, nano- or micro-indentation and microbeam deflection. Tensile tests measure in-plane properties of thin films. However, the in-plane properties can be different from out-of-plane properties, and it is the latter which are often more relevant to the performance of the films in their intended applications. In addition, tensile experiments impose tensile stresses on samples, which promote fracture failure earlier in the deformation, and may mask observations of plastic flow. In addition, areas supporting the load in tensile tests decrease as loads increase, leading to instability with necking and non-uniform straining. As a result, large-strain properties cannot be obtained front tensile tests.
Representative of previous tensile tests is bulge testing, which is a biaxial tensile test. There, both stresses and strains can only be roughly estimated through mathematical modeling because of the complex geometry of deformation.
Microindentation testing is a widely-used technique to study the mechanical properties of thin films. While simple in operation, expensive equipment has been required, and the non-uniform deformation results in complicated stress and strain fields, and also makes understanding and interpreting the measurements difficult. Even with the help of sophisticated computational models, no stress-strain relationships can be extracted from microindentation measurements.