Bulge testing of single- and dual-layer thin films

Abstract

The bulge testing technique determines the mechanical properties of solid thin films by measuring the deformation that forms in response to the application of a controlled differential pressure to a thin film window. By comparing the pressure-displacement relation with a mechanical model, the elastic modulus and residual stress in the film can be measured. While the bulge testing technique can be quite effective, the technique is not routinely used because of difficulties that often arise with using this technique. The difficulties include specimen preparation and mounting, automated bulge height measurement and the correlation of bulge deformation with the mechanical properties of the thin film. This paper describes developments in the bulge testing technique that alleviate many of these difficulties, as well as presenting results from the testing of single and dual layer thin films. Single film tests were conducted on samples of B-doped-Si, SiC, and diamond-like carbon. A total of 135 windows with three different window aspect ratios and two different thicknesses were investigated. In a preliminary study to determine the feasibility of extending the technique to the testing of multilayer films, the mechanics of a dual layer system were measured. The dual layer system was an Al layer on top of B-doped-Si. The results from the single film test were that the elastic moduli of the B-doped-Si were close to nominal bulk values and the diamond-like carbon was about half that of diamond. The SiC elastic moduli measurements were inconclusive because of the large prestress. Elastic moduli measurements from nanoindentation were about 50% higher. It should be noted that neither the variation of the aspect ratio nor the variation of the film thickness led to different results. The measured prestresses agreed quite well with wafer curvature measurement. The dual-layer measurements yielded values for the elastic modulus of thin film Al that were within 5% of the nominal bulk values.