Solid-state interactions taking place at the interface region of bi-layer thin film of Sn(500 Angstrom)/Cu(500 Angstrom) is studied by depth dependent X-ray photoelectron spectroscopy (XPS). Composition variation across the cross-section is measured. The distribution of copper is found to be uniform over a distance from the Cu-Sn interface and decreases sharply near the surface region. On further annealing at higher temperature (200 degreesC) copper redistributes uniformly over larger distance to a lower concentration. XPS peak shape parameter is determined to characterize the nature of the interface. It is shown that for freshly prepared and also for annealed samples no sharp interface could be found. Microstructural characterization is done by X-ray diffraction (XRD) and SEM techniques. From a comparative study with thicker films (Sn(1000 Angstrom)/Cu(1000 Angstrom)), it has been concluded that the interface of thinner films are made of nano-sized crystallites. It has also been shown that the grains in thicker samples grow faster than the thinner samples. Formation of inter-metallic compounds and their transformations are found to be same in both films. (C) 2003 Elsevier Science B.V. All rights reserved.