Mechanical properties of TiN thin films investigated using biaxial tensile testing

Abstract

Mechanical properties of TiN thin films were investigated using bulge test measurements performed on square freestanding membranes. For this purpose, both sides of n type Si(1OO) wafers were first coated by a layer of low stress SiN thin films (thickness t = 465 nm). The square windows of side 2a = 4 mm were then made using standard micromachining techniques to obtain freestanding SiN membranes. The edges of the windows were usually aligned with the [110] directions of the underlying Si wafer in order to make perfect squares bounded by (111) planes. These specimens were then coated by a layer of TiNx (0·84 < x < 1·3), using the rf magnetron sputtering method. Bulge testing was first conducted on an SiN film to determine its proper residual stress and Youngs modulus, which were found to be (σi = 227 ± 15 MPa and E = 225 ± lO GPa respectively. Then the composite membrane made of TiNx together with underlying SiNy was bulged and the related load-displacement variation was measured. Finally, using a simple rule of mixtures formula, the elastic mechanical properties of TiNx coatings were determined. Both the Young s modulus and residual stress increased with bias voltage, N/Ti ratio, and coating density. In contrast, the effect of substrate temperature below 600°C was found to be less significant than that of other parameters. Scanning electron microscopy of cross-sectioned specimens showed that coating growth occurs by the formation of equiaxial nanocrystallites, 10-30 nm in size, leading to columnar morphology beyond a thickness of 100-150 nm. The columns are nearly perpendicular to the film surface. The bulge test results are discussed in terms of coating microstructure and chemical composition determined by means of electron probe microscopy.