Planar copper-tin inter-metallic film formation on strained substrates

Abstract

This paper presents the results of four-point bending tests investigating the effects of substrate strain on the growth ɛ of interfacial Cu–Sn inter-metallic compounds (IMCs). Test specimens were cut into strips, 27.5mm in length and 5mm in width, from 4in. double polished silicon wafers. A very thin adhesion layer (Ta) was deposited on the silicon substrate by sputtering followed by a 10μm thick layer of copper using electroplating. Finally, a 30μm tin layer was deposited over the copper film also by electroplating. Samples were then placed in a furnace at 200°C to undergo bending in order to introduce in-plane strain under tension or compression. Control samples also underwent the same treatment without applied strain. Our aim was to investigate the influence of substrate strain and aging time on the formation of IMCs (1.54×10−4, 2.3×10−4 and 3.46×10−4). The thickness and separation of each phase (Cu3Sn) and η (Cu6Sn5) are clearly visible in scanning electron microscope images. Compressive strain and tensile strain both increased the thickness of the IMC layer during the aging process; however, the effects of compressive strain were more pronounced than those of tensile strain. We hypothesize that the increase in IMC thickness is related to the strain enhanced out-diffusion of Cu towards the solder as well as strain in the underlying lattice at the diffusion interface.