Electroless nickel underlayer deposited between copper filler and silicon substrate: effect of bath pH

Abstract

This research proposed intermediate materials between Cu filler and Si wafer to reduce the coefficient of thermal expansion (CTE) mismatch in the through silicon via (TSV) structure. The proposed material is Ni underlayer because it has CTE value between Cu and SiO2 which is 13 ppm/°C (Cu = 17 ppm/°C, SiO2 = 0.6 ppm/°C). The deposition process of nickel underlayer was conducted using electroless deposition process at various pH value (4.5, 5.5 and 6.5) of plating bath and deposition time (20, 40 and 60 minutes). The deposition behaviour was studied through surface and cross-sectional analysis under scanning electron microscopy (SEM). The Ni thickness was measured using cross-sectional analysis under SEM. The chemical composition of Ni deposits was measured using energy dispersive spectroscopy equipped on SEM. The performance test analysis was conducted using Standard ASTM D3359 (Cross Hatch Tape-Test) for coating adhesion test and 4-Point Probe Test for resistivity measurement. It was found that the optimum pH of the Ni bath is 6.5 due to high stability of plating bath to form uniform Ni deposited on silicon wafer. These optimum coating parameters showed homogeneous and uniform distribution of Nickel deposited on the surface of silicon with excellent adhesion properties for all pH value.