Effectiveness of polycrystalline silicon diffusion sources

Abstract

Boron and arsenic concentration profiles, diffused from polycrystalline silicon (polysilicon) into the underlying-single crystalline silicon (mono) substrate, were analyzed by Rutherford backscattering spectrometry and secondary ion mass spectrometry for various levels of oxygen concentration at the poly/mono interface. In contrast with previous reports it was found that chemically grown interfacial oxide layers of about 1.4-nm thickness provide more effective diffusion sources than oxygen-free interfaces. This surprising phenomenon is caused by the strong correlation between the crystalline structure of the polysilicon layer and the diffusion rates of dopant species in that layer. It is shown that the small amount of oxide at the poly/mono interface prevents the epitaxial realignment of the polysilicon, thereby maintaining high diffusion rates in the polysilicon, without offering a significant barrier to the diffusion of boron and arsenic across the interface.