Measuring the Elastic Modulus of Soft Thin Films on Substrates

The use of instrumented indentation to measure the mechanical properties of thin films supported on substrates where the Young's modulus of the film (E1) is substantially less than that of the Young's modulus of the substrate (E2) with modulus ratios from E1/E2 = 0.0001 to 1 is important for investigating materials such as soft polymers, cellulosic sheets, and biological materials. Most existing models for determining the elastic properties of films or sheets on substrates from indentation measurements were developed for the analysis of metal and dielectric films on semiconductor substrates and thus have been used in cases where E1/E2 is ~0.01 to ~10. In the present work, flat punch indentation of systems with E1/E2 = 0.0001 to 1 is investigated via finite element (FE) modeling and experiments. A FE parametric study in which E1/E2 was varied from 0.0001 to 1 was performed to quantify the effect of substrate stiffness on the measurement of the elastic film properties. A semi-analytical model that treats the thin film and substrate as two springs in series was fit to the FE results to allow for use of the results presented. Preliminary experiments, in which a series of film/substrate systems with various modulus mismatch (E1/E2 from ~0.0005 to ~1) were characterized using instrumented indentation, were performed to evaluate the effectiveness of the model for extracting films properties from indentation measurements. The results of the parametric FE study show that for very stiff substrates, the measured stiffness becomes insensitive to changes in substrate modulus. The analytical model and FE model agree to within 7% for E1/E2 values between 0.0001 to 1 and a/t ratios from 1 to 100. Comparison of the preliminary experimental results and FE model show reasonable agreement, but further investigation is required to obtain better correlation.