The dopant (B, P, and As) redistribution in a silicide on polycrystalline silicon structure after annealing at 800 and loOO ·C was studied. The distribution of boron was found to be quite different from these of phosphorus and arsenic. At 1000 ·C, the distribution coefficient for boron at the WSi 2 /polycrystalline silicon interface was found to be 2.7. The solubilities of phosphorus and arsenic in WSi 2 at 1000·C were estimated to be 6X lO19 and 1.6X 10 19 atoms/cm 3 , respectively. At 800 ·C, the diffusion coefficient for the dopants was found to be equal to, or greater than 3.3 X lO-12 cm 2 /s, which is at least three orders of magnitude larger than in silicon.
In order to increase the conductivity of polysilicon lines used in polysilicon VLSI technology, silicides (e:g., WSi2, MoSi2, and TaSi2) are presently being considered as overlays on the polysilicon lines. One advantage of a polysilicon/silicide structure is that, presumably, it is an oxidizable, self-passivating structure. Under certain oxidation conditions similar to those employed in polysilicon FET processing, voids may develop in the polysilicon layer and/or undesirable oxides lacking structural integrity may develop on the silicide surface. The oxidation mechanisms governing these phenomena are herein discussed.As sem}conductor device dimensions continue to decrease, the need to enhance the conductivity of doped polysilicon interconnections and gate electrodes in VLSI circuits becomes increasingly acute. Refractory metal silicides (e.g., TiSi2, WSi2, MoSi2, and TaSi3) are presently being considered as overlays to, or replacement for, polysilicon lines as a means of increasing the conductivity of these lines while still retaining an oxidizable structure for self-passivation (1). This structure, consisting of a metal silicide layer directly on top of a polysilicon film, is referred to as the "polycide" structure while the word "silicide" refers only to the metal silicide layer on top of the polysilicon film.The kinetics of the oxidation of the polysilicon/ silicide (polycide) structure have been previously reported (2, 3). It has been found that atoms from the underlying polysilicon layer diffuse through the silicide film to the top silicide surface, where the silicon is oxidized, while the silicide layer itself is not oxidized. We have found (4) that, under certain oxidizing conditions, voids may form in the polysilicon layer and/or undesirable oxides which lack structural integrity may grow on the top surface of the silicide layer.Geipel, et al. (5) have reported extensively on the microstructural, processing, and MOS characteristics of the polycide films. The purpose of this present article is to elaborate on the oxidation characteristics of the polycide structure and, in particular, to specify the conditions under which the aforementioned oxidation phenomena take place. Experimental ProcedureIn order to fully investigate the oxidation characteristics of the polycide structure, various silicon/silicide structures were formed. The standard procedure for forming the polycide structure consists of first depositing 400 nm of in situ phosphorus-doped polysilicon on 45 nm of thermal oxide grown on a <100> p-type silicon substrate. A 350 nm layer of WSi2 is then coevaporated on top of the polysilicon layer in a dual e-beam evaporator. The entire structure is then annealed at 1000~ (1273K) for 30 min in an inert ambient in order to homogenize and to lower the resistivity of the WSi2 layer. The blanket polycide structure is then patterned by CF4:02 plasma etching, using photoresist as an etch mask. The patterned polycide film is subsequently subjected to a dry-wet-dry oxidation process at 1009~ (1273K) so as to for...
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