THE THERMAL-STABILITY OF COSI2 ON POLYCRYSTALLINE SILICON - THE EFFECT OF SILICON GRAIN-SIZE AND METAL THICKNESS

The stability of CoSi2 on polycrystalline Si has been studied as a fun ction of both the initial Si grain size and the initial silicide thick ness. When the Si grain size is small (Si grain size << polycrystallin e Si thickness), CoSi2/polycrystalline Si structures are unstable due to grain growth in the polycrystalline Si. If initial silicide is thic k (silicide thickness approximate to polycrystalline Si thickness), ne arly 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 t o 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 growt h in the polycrystalline Si is mainly confined to the surface, with re latively few inverted grains. Fewer Si grains come in contact with the silicide due to the small volume fraction of the silicide, hence fewe r Si grains are converted to large grains, As the initial grain size o f the starting polycrystalline Si increases, the CoSi2/polycrystalline Si structures become more stable because the driving force for Si gra in growth decreases. Relatively few Si grains show enhanced grain grow th. As the metal thickness decreases, the silicide becomes less stable due to agglomeration of the silicide on the large grain polycrystalli ne Si. Hence, the driving force for the instability changes from grain growth in the Si to minimization of interface and surface energy of t he silicide.