Stress Evolution in Thin Films; Diffusion and Reactions

After a brief discussion of possible mechanisms of stress generation in thin film diffusion/reaction couples, two recent experimental examples are reviewed: (i) Thin film diffusion couples (Pd-Cu, individual layer thicknesses: 50nm) prepared by DC-magnetron sputtering on silicon substrates. The microstructural development, phase formation and the stress evolution during diffusion annealing have been investigated employing Auger-electron spectroscopy in combination with sputter depth profiling, transmission electron microscopy, in-situ wafer-curvature measurements and ex-situ and, in particular, in-situ X-ray diffraction measurements. (ii) Tin layers on copper substrates (layer thicknesses of some microns) prepared by electrodeposition. Upon storage at ambient temperatures, Cu diffuses into the Sn layer and forms the intermetallic phase η’- Cu6Sn5. The phase formation is accompanied by a volume expansion and as a consequence, compressive residual stresses can be generated in the Sn layers. These compressive residual stresses may drive the formation of Sn whiskers on the Sn surface. The microstructural development, phase formation and the stress evolution during diffusion annealing have been investigated employing scanning electron and focused ion beam microscopy, metallography and ex-situ and, in particular, in-situ X-ray diffraction measurements.