p-Type Regrowth Layer Formed by Brazing of n-Type Si with Metal Support Disc

Abstract

In the construction of diodes, n-type surfaces of Si wafers are frequently brazed to metal contact discs (W or Mo) by using Al solder. After brazing, the diodes exhibit the foward voltage drop (FVD) because of the p-type regrowth layer formed on the n-type Si from the melt. It is considered that the modification of primary Si in a hyper-eutectic Al-Si alloy is effective in preventing the growth of regrowth layer. From this point of view, effects of thickness of Al, brazing temperature, holding time, cooling rate and P concentration in n-type Si on the regrowth layer have been investigated by microscopy. The following conclusions have been obtained.
(1) FVD in the diodes decreases with an increment of the discontinuity ratio of the regrowth layer.
(2) It is found that the modification in the hyper-eutectic Al-Si alloy and the prevention of growth of the regrowth layer on n-type Si have a close connection with each other. In the case of the Al-Si layer thinner than 4 μm, it is possible to get a large discontinuity ratio.
(3) Even if Al is thick (i.e. 15 μm), it is also possible to get a large discontintinuity ratio by using n-type Si containing about 1∼1.4 at%P.
(4) It is also found that the growth of the regrowth layer is affected greatly by the shape of the solid-liquid interface.