Use of the bulge test in measuring the mechanical properties of thin films.

摘要

Since its first application to thin films in the 1950's the bulge test has become a standard technique for measuring thin film mechanical properties. While the apparatus required for the test is simple, interpretation of the data is not. Failure to recognize this fact has led to inconsistencies in the reported values of properties obtained using the bulge test. We have used the finite element method to model the deformation behavior of a thin film in a bulge test for a variety of initial conditions and material properties and have analyzed the standard bulge test model in light of these results. We find that specimen geometry and initial conditions have a tremendous influence on apparent stiffness of a material. The findings lead to changes in both data analysis and sample preparation that make the bulge test an accurate and reliable technique.; Bulge test experiments conducted on polyimide and polycarbonate thin films confirm that the analysis techniques are effective and allow one to measure and distinguish between the elastic stiffnesses of very compliant materials. A new method of sample preparation that involves depositing films on already freestanding substrates is discussed. This composite technique was tested by comparing the biaxial modulus of Ag extracted from bulge tests of Ag on SiN{dollar}sb{lcub}rm x{rcub}{dollar} with known bulk values. The method produces highly reproducible results when the metal film contributes more than half of the stiffness of the composite. the same method was applied to Ag-Pd multilayer films. This system had previously shown enormous enhancements in biaxial modulus at bilayer periods around 2 nm when tested with the bulge test (i.e., the "supermodulus effect"). The moduli we measure using better analysis and sample preparation techniques show no enhancement and seriously call into question the previous finding.; Results from both Ag and Ag-Pd multilayers indicate that crystallographic texture can have a measurable effect on the film biaxial modulus. The variation in biaxial modulus with distribution of {dollar}langle 111rangle{dollar} plane normals about the film normal was modeled using the average single crystal elastic constants of Ag and Pd.