STUDY OF Co/Sn MULTILAYER SYSTEM WITH VARIOUS TIN LAYER THICKNESS AND REFLOW TEMPERATURES
Abstract
Co–Sn–Co multi-layer films with various Sn-layer thicknesses have been deposited by using the electrodeposition technique. The deposition was performed at room temperature on gold coated silicon substrates. The thickness of Sn layer was kept ∼4.5, 2.5, 1.5 and 1μm, while the thickness of each Co layer was fixed to ∼0.75μm. The Sn layer was sandwiched between two Co layers. The total thickness of the films was ∼6, 4, 3, 2.5μm, respectively. The intended ratio of Sn was about 25 at.% while Co was about 75 at.%. The separation inside the Sn-layer has been observed at higher Sn-layer thicknesses with increasing reflow temperature. It may be due to the combined effect of thermal stress arising during cooling from elevated reflow temperatures and solidification shrinkage in the thick Sn layer due to the formation of IMC CoSn2. However, 1μm has been found to be a critical thickness, which remains intact when sandwiched between two cobalt layers. The structure of these multi-layer films was studied as a function of temperature. It has been observed from X-ray diffraction (XRD) that the as-deposited films exhibit strong peaks belonging to elemental Co and Sn. At temperatures of 270–290∘C, CoSn2 begins to appear and grow at the interfaces and in the middle of Sn layer with the decrease in elemental Sn. However, sufficient amount of Co is still present in pure form as is evident by the XRD and field emission scanning electron microscopy images at all temperatures. This study confirms that only one reaction product, viz. CoSn2, formed in as-deposited during reflow at temperatures 270–290∘C for a fixed time of 10min, although several other stable IMCs, e.g. Co3Sn2, CoSn, CoSn2, CoSn3 exist in the Co–Sn system at 250∘C according to phase diagram. It is not uncommon that all the thermodynamically stable IMCs do not form in the system due to kinetic reasons. Accordingly, the formation of IMCs through interfacial reaction has been discussed in this paper.
