"Investigation of solid state reactions in Copper-Tin thin film systems"

Abstract

The problem of solid-state reactions in nanostructured thin film system with individual thicknesses of few nanometers is still a challenging subject. If the films are nanocrystalline, the mass transport along different grain boundaries (GBs) can have an important effect on the entire intermixing process. Solid state reactions between nanocrystalline Cu and Sn films are investigated at room temperature by depth profiling with secondary neutral mass spectrometry and by X-ray diffraction. A rapid diffusion intermixing is observed leading to the formation of homogeneous Cu6Sn5 layer. There is no indication of the appearance of Cu3Sn phase. This offers a way for solid phase soldering at room temperature, i.e. to produce homogeneous Cu6Sn5 intermetallic layer of several tens of nanometers during reasonable time (in the order of hours or less). The growth kinetics and phase formation mechanism of Cu6Sn5 are studied. From the detailed analysis of the growth of the planar reaction layer, formed at the initial interface, the value of the parabolic growth rate coefficient at room temperature is estimated. In addition, the overall increase of the composition near to the substrate inside the Cu film is interpreted by grain boundary diffusion induced solid state reaction: the new phase is formed along the grain boundaries and grows perpendicular to the boundary planes. From the initial slope of the composition versus time function, the interface velocity during this reaction is estimated. Copper- tin (Cu-Sn) layers are of great technological and scientific interest, and are frequently used to avoid health and environmental problems caused by the use of lead- based alloys. Sn thin films deposited on Cu- based substrate are often used for soldering of nanoelectronics devices. Keywords: Cu-Sn nanostructured thin films; growth kinetic studies; SNMS depth profiling; solid state reactions; soldering.