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BORON DIFFUSION MECHANISM IN SILICON OXIDE USING AB INITIO METHODS V. Zubkov*, J. P. Senosiain**, S. Aronowitz*, V. Sukharev*, C. B. Musgrave*** *LSI Logic Santa Clara **Department of Materials Science & Engineering, Stanford University ***Department of Chemical Engineering, Stanford University

ABSTRACT Density functional theory was employed to explore the diffusion mechanism of boron in amorphous silicon oxide. The oxide was modeled with clusters of various sizes, and both neutral boron atoms and cations were considered. Three stable structures were found where B (or B+) was inserted into oxide: one in which B (or B+) is divalent and two in which B (or B+) is trivalent. Boron diffusion through silicon oxide proceeds as a sequence of B hops from one inserted position to another. For neutral boron the rate limiting step is B hop from one of trivalent structures to a divalent one with activation energies (Ea) in the range of 2.0-3.1 eV, depending on the model cluster. In the case of a cation the rate-limiting step is the B+ hop over the O atom in a divalent structure Si-B+-O-Si with calculated Ea of 2.4-2.8 eV. Experimental activation energies for B diffusion in silicon oxide are in the 2.3 – 4.2 eV range. Our results suggest that both neutral and cation boron can participate in B diffusion in oxide. INTRODUCTION Quantum chemical calculations [1,2] have shown that B atoms are easily inserted into the siloxane bond resulting in Si-O-B-Si link where B is divalent. It has been suggested that dopant diffusion in SiO2 proceeds as a sequence of hops from one inserted position to another. This mechanism is consistent with the observed decrease of B diffusivity by nitridation. Nitridation of the gate oxide is widely used [2,3] to suppress B diffusion through the gate oxide since it causes a positive threshhold voltage shift [3-6] and even a source-drain shortcircuit [6]. It is highly probable that nitridation leads to formation of N containing siloxane bonds such as Si-ON-Si [1,2]. Calculated energy of B insertion into nitridized bonds reaches up to 10 eV compared with ≈ 3 eV for a divalent boron insertion into regular siloxane bond [1,2]. This might hinder B hops. However, the B atom is normally trivalent and it has been suggested that B in SiO2 is bonded to three O atoms and diffusion proceeds as B exchanges with Si accompanied by a corresponding motion of O atoms [7]. Recently it has been suggested that B atoms diffuse in SiO2 as a cation B+ [8]. In this work we found stable structures with inserted B where B can be divalent or trivalent. Activation energies, Ea, for B hops between possible inserted structures were calculated and the rate limiting hops were determined. Similar calculations but on a smaller scale were performed for B+. MODELS AND METHOD The structure of amorphous silica is a network of siloxane rings containing from 2 to 6 or more silicon atoms per ring [9]. Several open chain and ring models were used to investigate B

B5.11.1

insertion and hopping. Open chain models I-IV contain one or two Si atoms. Ri