The thermal stability of CoSi2 on polycrystalline silicon: The effect of silicon grain size and metal thickness
Abstract
The stability of CoSi2 on polycrystalline Si has been studied as a function of both the initial Si grain size and the initial suicide thickness. When the Si grain size is small (Si grain size ≪ polycrystalline Si thickness), CoSi2/polycrystalline Si structures are unstable due to grain growth in the polycrystalline Si. If the initial silicide is thick (silicide thickness ≈ polycrystalline Si thickness), nearly all the Si grains are consumed by the silicide and converted into large Si grains. The conversion of the polycrystalline Si from small grains to large grains occurs first at the surface and then proceeds to the bottom interface, resulting in the inverted structures observed in earlier studies. Further annealing results in a "quilt" structure, with alternating grains of silicide and polycrystalline Si that run through the thickness of the film. As the silicide thickness decreases (silicide thickness ≪ polycrystalline Si thickness), the grain growth in the polycrystalline Si is mainly confined to the surface, with relatively few inverted grains. Fewer Si grains come in contact with the silicide due to the small volume fraction of the silicide, hence fewer Si grains are converted to large grains. As the initial grain size of the starting polycrystalline Si increases, the CoSi2/polycrystalline Si structures become more stable because the driving force for Si grain growth decreases. Relatively few Si grains show enhanced grain growth. As the metal thickness decreases, the silicide becomes less stable due to agglomeration of the silicide on the large grain polycrystalline Si. Hence, the driving force for the instability changes from grain growth in the Si to minimization of interface and surface energy of the silicide.