Electroless Deposition of Fe-Ni Alloy Thin Films for Power Semiconductor Package

Abstract

Next generation power semiconductor such as silicon carbide (SiC) and gallium nitride (GaN) are known to have an excellent operation characteristic at high temperatures. The high temperature operation needs the power semiconductor package capable of withstanding greater thermal stress. The package for these devices requires a metallization process producing films with low coefficients of thermal expansion (CTEs) matching with the semiconductor and insulating substrates such as ceramics. The metallization by a wet process is expected to be beneficial because of its higher throughput and lower cost than that by dry and high temperature processes. Especially, electroless plating process is of current interest because it is also possible to form films metallized on non-conductive material such as an insulating substrate. Pyrometallurgically produced Fe-Ni alloys with 55 to 70 wt% Fe contents exhibited low CTEs. Therefore, electroless Fe-Ni alloy deposits can be also expected to have low CTEs comparable with those of semiconductor and insulating substrates used in power semiconductor devices. Although the literature contains several reports[1-3] that address the Fe-Ni alloys electrodeposition, the electroless Fe-Ni alloy deposition have not been sufficiently explored. In this study, we preliminary investigated the effect of electroless deposition condition on the alloy compositions, deposition rate, and morphologies of the electoless Fe-Ni alloy deposits. The optimum bath composition for electroless Fe-Ni alloy deposition was as follows: FeSO4 (0~0.02 mol/L), NiSO4(0.03~0.05 mol/L), potassium citrate (0.1 mol/L), potassium pyrophosphate (0.005 mol/L), and dimethylamine borane (0.025 mol/L). The bath temperature was 343 K and the bath pH was adjusted to pH 10.0. In the case of a pretreatment with Pd catalyst, although Fe-Ni alloy deposits were able to be deposited on a Cu substrate, the deposition rate was not enough. Therefore, the Fe-Ni alloy deposits were deposited on the Cu substrate connected with Al sheets to accelerate the deposition rate[4]. Fig. 1 shows effect of Fe2+/ (Fe2++Ni2+) concentration on alloy composition of electroless Fe-Ni alloy deposits and deposition rate for 1 h. The alloy composition was measured by EPMA. With increasing Fe2+/ (Fe2++Ni2+) concentration, Fe contents linearly increased to approximately 70 wt% and Ni contents linearly decreased to approximately 30 wt%. The B contents decreased with increasing Fe2+ concentration and were less than 1 wt% at Fe2+/ (Fe2++Ni2+) concentration of 0.2 or above. The deposition rate gradually decreased from 1.1 to 0.4 μm/h with increasing Fe2+ concentration. The appearance of the all deposits was smooth and uniform. The electroless Fe-Ni alloy deposits with from 0 to 40 wt% Fe contents exhibited bright, whereas those with Fe contents of 60 wt% or above exhibited dull gray. Fig. 2 shows FE-SEM images of the electroless Fe-Ni alloy deposits obtained for 1 h. The all deposits did not have any cracks. The primary particle size increased from several nm to approximately 50 nm with increasing Fe contents. The secondary particle size of all deposits was approximately 100 nm. In conclusion, the Fe-rich Fe-Ni alloy deposits with maximum Fe contents 70 wt% were obtained using dimethylamine borane as a reducing agent by electroless plating process; the electroless Fe-Ni alloy plating process may become an attractive metallization process for power semiconductor package. Acknowledgement This work was partially supported by the Kyoto Technoscience Center and Industry-Academia Collaborative R&D Programs “Super Cluster Program” (JST).