Effect of the Si/ SiO2 Interface on Si diffusion in SiO2
Masashi Uematsu, Hiroyuki Kageshima, and Yasuo Takahashi
Device Physics Laboratory
With the scaling-down of Si devices, the bulk materials of interest are closer to material interfaces. Therefore, even phenomena in bulk materials, such as diffusion, are more likely to be affected by interfaces, such as the Si/SiO2 interface. In this report, we investigated the effect of the interface on Si self-diffusion in SiO2 by using isotopically enriched Si.
An isotopically enriched 28Si single crystal epi-layer was thermally oxidized to form 28SiO2 of thicknesses of 200, 300, and 650 nm. The samples were implanted with 30Si and diffusion-annealed at temperatures between 1150 and 1250 oC. Si self-diffusion in SiO2 was investigated using the implanted 30Si as makers. As shown in Fig. 1, we found that the Si self-diffusivity increases with decreasing 28SiO2 thickness, i.e., the distance between the diffusing 30Si species and 28Si/28SiO2 interface . In order to explain this thickness dependence, we constructed a model in which SiO molecules generated at the interface via Si + SiO2 → 2SiO and diffusing into SiO2 enhance Si self-diffusion in SiO2 . The thickness dependence arises because the SiO diffusion is so slow that the 28SiO concentration at the 30Si region critically depends on the distance from the interface, where the SiO is generated, as shown in Fig. 2. We propose a mechanism of Si self-diffusion in SiO2 enhanced by the existence of SiO via the reaction such that 30Si(s) + 28SiO(i) 28Si(s) + 30SiO(i), where substituted (s) Si atoms diffuse via the kick-out reaction with diffusing interstitial (i) SiO molecules. Based on this mechanism, we simulated and fitted the diffusion profiles of 30Si (simulation in Fig. 1).
Furthermore, the simulation predicts that the self-diffusivity would increase for a longer annealing time because more SiO molecules should be arriving from the interface. As predicted, the time-dependent diffusivity was experimentally observed, and this confirms our diffusion model. The diffusion length of SiO estimated from the present results is 0.4 nm for 10-second annealing at 1100 oC. The present study suggests that the effect of the Si/SiO2 interface may arise during silicon processes when the material thickness is down to 1 nm.
 S. Fukatsu et al., Appl. Phys. Lett. 83 (2003) 3897.
 M. Uematsu et al., Appl. Phys. Lett. 84 (2004) 876.
Fig. 1. Dependence of 28SiO2 thickness
on 30Si diffusion in 28SiO2
Fig. 2. Model.
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