Investigation of Low Temperature Brazing between n+ Type Si and W Electrode using Al-Cu Solder

When an n+ type surface of a diode is bonded to an electrode using Al solder, a thin p-type Si layer, i.e. a regrowth layer is formed on the n+ type Si after bonding. Because of the layer formation, a considerably high forward voltage drop (FVD) is observed in the regrowth layer. In order to reduce the FVD, a brazing mechanism between n+ type Si evaporated with Al and W electrode evaporated with Cu has been studied in the temperature range from 800 to 873 K. The Cu content was varied from 12.4 to 62.3 wt% by varying the thickness of Cu film. The FVD and the regrowth layer are compared with the previous case of Al solder without Cu film. The brazing temperature between n+ type Si and W electrode was reduced to about 800 K. The temperature reduction was caused by the formation of ternary eutectic melt of the Al-Cu-Si system. The FVD was substantially lowered in comparison with the case of Al solder. In order to prevent the crack formation in the brazed layer, it is necessary to keep the Cu content up to 29 wt%, corresponding to the eutectic composition in the Al-Cu-Si system.