A NEW INTERPRETATION OF BULGE TEST MEASUREMENTS USING NUMERICAL-SIMULATION

Analysis of the deflection of a circular membrane under differential p ressure (bulge test) is a well-known method of determining the elastic properties of thin films. However, analytical models always suffer fr om simplifying hypotheses. In this study we present a new approach, ba sed on numerical modeling, to interpret pressure-deflection curves. By adjusting Young's modulus and Poisson's ratio in the simulation progr am, it is possible to reproduce the experimental curves faithfully. Th e method was successfully tested with two different materials (silicon and aluminium) with known elastic properties and was then used to det ermine biaxial Young's moduli of CVD diamond thin films for three diff erent microstructures. The values of E varied from 565 to 620 GPa (ass umin,g a Poisson ratio of 0.1). Grain boundaries are thought to be res ponsible for the relatively low values of Young's moduli. Uncertaintie s in E are relatively large (10%-15%) because the method is highly sen sitive to experimental parameters such as thickness or membrane diamet er and to the initial residual stress state which is known only approx imately.