Residual stresses and fracture properties of magnetron sputtered Ti films on Si microelements

The influence of different deposition parameters on the microstructure, the residual stress, and the mechanical properties of thin Ti films has been investigated. Ti films were deposited onto Si (001) wafers with a native oxide layer, held at a temperature <250 °C, by dc magnetron sputtering. The film thickness was 1 μm. The microstructure of the films, as determined by x-ray diffraction (XRD) and transmission electron microscopy (TEM), was varied by changing the Ar sputtering pressure PAr between 0.8 and 8.3 mTorr, and by applying a negative substrate bias Vs between 0 and 300 V. The residual stresses in the films were determined by both a beam curvature technique and the XRD sin2 ψ method. For Vs=0, the residual stress in the films changed abruptly from tensile to compressive as PAr decreased below 2.5 mTorr. A maximum tensile stress of 0.4 GPa was obtained at 3 mTorr. The compressive stress reached at PAr=0.8 mTorr was 0.4 GPa. TEM showed accompanying structural changes from columnar structure with a high amount of voids in the grain boundaries to a dense structure at PAr=0.8 mTorr. For PAr=3 mTorr, where the largest tensile stress was obtained, the application of a negative substrate bias resulted in Ar ion bombardment that caused compressive stresses to develop for Vs≳75 V and a stress saturation value of 0.6 GPa was reached for Vs≳100 V. TEM showed a dense film structure for Vs≳75 V. The primary energetic particles bombarding the surface were, Ti atoms in the absence of an applied bias and Ar+ ions for sputtering with an applied bias. The energy of the energetic particle in each case was a function of the deposition condition and for both stress transitions, the energy was shown to be of approximately the same magnitude, ∼15 eV/deposited Ti atom. The Young’s modulus of the Ti film was experimentally estimated, and found in fair agreement with previously reported bulk data. Fracture tests were performed on Ti-coated Si cantilever beams in situ in a scanning electron microscope. The fracture strength of coated cantilever beams were typically 10%–15% lower than that of uncoated beams.