A search for strain gradients in gold thin films on substrates using x-raydiffraction

The high strengths of gold thin films on silicon substrates have been studi ed with particular reference to the possible effect of strain gradients. Wa fer curvature/thermal cycling measurements have been used to study the stre ngths of unpassivated, oxide-free gold films ranging in thickness from 0.1 to 2.5 mu m. Films thinner than about 1 mu m in thickness appear to be weak ened by diffusional relaxation effects near the free surface and are not go od candidates for the study of strain gradient plasticity. Our search for p lastically induced strain gradients was thus limited to thicker films with correspondingly larger grain sizes. Three related x-ray diffraction techniq ues have been used to investigate the elastic strains in these films. The s tandard d(hkl) vs sin(2) Psi technique has been used to find the average st rain through the thickness of the films. The results are consistent with wa fer curvature measurements. We have also measured a number of d(hkl)'s as a function of penetration depth to construct depth-dependent d(hkl) vs sin(2 ) Psi plots. These data show that the residual elastic strain is essentiall y independent of depth in the film. Finally, a new technique for sample rot ation has been used to measure the d(hkl)'s for a fixed set of grains in th e film as a function of penetration depth. Again, no detectable gradient in strain has been observed. These results show that the high strengths of un passivated gold films relative to the strength of bulk gold cannot be ratio nalized on the basis of strain gradients through the film thickness. Howeve r, a sharp gradient in strain close to the film substrate interface cannot be ruled out. (C) 2000 American Institute of Physics. [S0021-8979(00)03815- 9].