Interdiffusion, phase formation, and stress development in Cu–Pd thin-film diffusion couples: Interface thermodynamics and mechanisms

Abstract

Cu–Pd thin-film diffusion couples (individual layer thicknesses of 50nm) have been prepared by dc-magnetron sputtering on silicon substrates coated with a thin amorphous Si3N4 layer. Stress evolution, microstructural development, and phase formation during interdiffusion have been investigated employing Auger-electron spectroscopy (in combination with sputter-depth profiling), x-ray diffraction, wafer curvature measurements and transmission electron microscopy. Upon annealing at relatively low temperatures (175–250°C) for durations up to 10h, considerable diffusional intermixing occurs. Interdiffusion is accompanied by sequential formation of new phases. First, Cu3Pd forms; subsequently, CuPd forms and grows at the expense of Cu3Pd, which has been interpreted as a consequence of interface thermodynamics. Annealing leads to a slight sharpening of the pre-existing {111}-fiber textures and a little increase in the average grain size. A combination of ex situ (x-ray diffraction) and in situ (wafer curvature) stress measurements revealed that tensile stresses are generated during annealing. The obtained stress results have been discussed in the light of possible mechanisms of stress generation, and the dominant sources of stress buildup and relaxation have been identified.

Low temperature homogenization in nanocrystalline PdCu thin film system

“…The XRD measurements confirmed the conclusions drawn from the SNMS profiles: depending on the initial film thicknesses the system arrived either into the CuPd or Cu 3 Pd phases or to their mixture (see also below). Indeed, in accordance with the XRD results of [6] the phases, called CuPd as well as the Cu 3 Pd above, correspond to the β (CuPd) and α′ (Cu 3 Pd) phases with average equilibrium compositions 40 at% Pd as well as 15 at%Pd, respectively in accordance with the phase diagram [17]. Furthermore the presence of super-lattice reflections (with mixed, even and odd, indexes) indicates that the β phase is at least partly ordered (figures 4 and 6(a)).…”

“…We have shown-in agreement with the experimental observations of [6] too-that the saturation of GBs in Cu was reached in very short times (in the early stages of the process), while we were able to determine the GB diffusion coefficient of Cu in Pd. In addition at 310°C after 1 h annealing time an almost complete homogenization with a composition of Cu 3 Pd was observed.…”

“…Figure 4 shows the XRD pattern of the 37.5 nm(Pd)/30 nm(Cu) film. The marked peaks of the XRD pattern were identified by the corresponding XRD patterns of Cu, Pd and CuPd [6]. The silicon substrate has two reflections, at 69.1°and 33°.…”

“…Furthermore the presence of super-lattice reflections (with mixed, even and odd, indexes) indicates that the β phase is at least partly ordered (figures 4 and 6(a)). Also, following the arguments in [6] it is likely, that the marked reflections (Cu 3 Pd) originate from the α′-Cu 3 Pd phase.…”

“…In DIGM the grain boundaries move perpendicular to the original boundary plane and leave an alloyed zone behind. Furthermore it was also illustrated [5][6][7][8][9][10][11][12] that complete homogenization of bilayered films can also happen by formations of reaction products along the grain boundaries, (grain boundary diffusion induced reaction, GBDIREAC [4]). The most important difference, as compared to the well-investigated cases of DIGM/DIR in systems forming solid solutions, lies in the formation of new phases.…”