Au (metal) induced layer exchange and crystallization of amorphous Si (a-Si) on corning glass (CG) in a-Si/Au/CG thin film specimen upon vacuum annealing (~ 10^− 8 mbar) is reported. The crystallization characteristics of these specimens at temperatures ranging on either side of the Au-Si eutectic temperature T_e(Au-Si) are investigated. Our results shows that solid state diffusion assisted layer exchange of the a-Si and Au layer, followed by crystallization of a-Si into crystalline Si (c-Si) occur at 350 °C (below the T_e). The upper limit to this crystallization mechanism was observed to be above the T_e(Au-Si) and this continues till ~ 400 °C in the present study. Above this limit, the layer exchange phenomenon ceases and eutectic mixing reaction of Au-Si takes over to form diffusion limited crystalline aggregates of Si with different microstructural attributes.