Poly-silicon films with low impurity concentration made by hot wire chemical vapour deposition

Abstract

Analyses of various impurities (O, C, W, Fe, Ni and Cr) in the poly-Si material (both in layers as well as in cell configurations) made by HWCVD have been carried out to judge the quality of this material for application in devices. SIMS analysis showed that the oxygen concentration in the bulk of a poly-Si film made at a low hydrogen dilution (Poly2) is 3×10 18 cm −3 and the oxygen content drops to this value within a depth of only 50 nm from the surface. On the other hand, a poly-Si film made at a high hydrogen dilution (Poly1) has a high and homogeneous oxygen content of more than 2×10 21 cm −3. However, in a double-layer structure (Poly2 on top of Poly1), the oxygen content of the bottom layer (Poly1) is significantly smaller than the bare Poly1 film, though this oxygen concentration is still much higher than that in the top Poly2 layer. We attribute this behaviour to the structural difference between these two films (2000 cm −1 Si–H IR vibration in low-dilution material and 2100 cm −1 vibration in the high-dilution material). We propose that the oxygen penetration in Poly1 occurs by two processes: (1) oxygen incorporation during growth: (2) post-deposition oxygen intrusion. The first process occurs at a low deposition rate and is dependent on the type of growth process. The second process is due to the intrusion of water vapour into the film through the voids, which increases the conductivity of the film depending on the amount of intrusion. We have shown that our device quality compact poly-Si : H (Poly2) resists oxygen incorporation even when deposited in an oxygen-rich atmosphere.