About boron and arsenic diffusions in polycrystalline silicon under rapid thermal oxidation

This work provides an experimental insight into some physical mechanisms involved in the diffusion of arsenic and boron in polysilicon/monocrystalline Si bilayers, during the formation of shallow N+ emitters for the BiCMOS technology. The RTA-induced redistribution of As or B, so as As and B successively implanted in a 380 nm LPCVD polysilicon layer has been studied by SIMS measurements. The key idea in this paper is to perform annealings under an inert or an oxidizing ambient, in a way to put in evidence not only that arsenic diffuses along grain boundaries, but also that boron diffuses into the grain boundaries or in the grain and also in the neighbourhood of boundaries. As a consequence, in the underlying single crystal, arsenic and boron are deeper with a rapid thermal annealing under an inert atmosphere compared with a rapid thermal oxidation. A new model for impurities diffusivities have been initiated in order to take into account these phenomenon.