tips: results and discussions

1996, Tuda et al.

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Figure 3 shows effects of surface re-emission on the transport of neutral reactants in a microstructure. In the figure the incident neutral flux normalized by its value in open space, Γn /Γn0, is plotted along the surface position in a twodimensional rectangular trench with its aspect ratio of unity (the same depth and width). Note that calculations were made both with and without surface reemission of neutrals, where their sticking probability was Sn = Sn0(1 - a) and the sticking coefficients on bare surfaces (a = 0) were taken to be Sn0 = 0.1 and 0.5. In addition, the neutral-to-ion flux ratio toward the substrate was Γn0 /Γi0 = 1, and the ratio of sheath voltage to ion temperature was R = eVs /kTi = 100.

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As can be seen in the figure, the incident flux of neutrals at the bottom surface is reduced to Γn /Γn0 〜 0.5 without reemission, because of the shadowing effect by the mask; in this case, most of incoming neutrals stick on the sidewall surfaces. On the other hand, in the presence of the effects of surface reemission, the incident fluxes at the bottom are Γn /Γn0 〜 0.7 for Sn0 = 0.5 and Γn /Γn0 〜 0.9 for Sn0 = 0.1. This implies that neutrals can be effectively transported into the bottom of the trench by the surface reemission. In Refs. 23 and 24 the transport of ions and neutrals was modeled taking account of the surface reemission, but their sticking probabilities were assumed to be independent of the surface coverage. In this article the neutral sticking probability was given by Sn = Sn0(1 - a); most of incoming neutrals fail to stick on sidewall surfaces even if Sn0 〜 1, because the surface coverage is a;1 on the sidewalls, as is shown later. In particular, the effects of reemission on sidewall surfaces may be important in LPHD plasma etching environments.