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Etching of smoothing/without undercutting deep trench in silicon with SF6/O2 containing plasmas

In this article, the effects of He and Ar which are added into SF6/O2 containing plasmas were investigated in deep Si etching. External controllable parameters such as the He or Ar gas flow were used to study the changes of Si etch rate, Si undercut, oxide selectivity and feature shape correlation. The opening width of feature shape is from 0.22 μm to 0.42 μm. The result shows that the Si undercut decreases and the profile of Si trench is more vertical with the increasing He gas flow in all structures. But the Si etch rate and oxide selectivity will decrease. It is noteworthy that Ar and He have similar effects in reducing undercut. After detailed data analysis, it was found that there were some correlations between Si undercut and normalized residence time in both plasmas. The mechanism of this phenomenon is that the F radicals are diluted due to the addition of a large amount of He and Ar. With the decrease of the F radicals, the lateral etching ability decreases and undercut decreases. The concentration of O and F radicals decreases at almost the same rate when He or Ar gas is mixed, the sidewall profile of trench has no change. Due to the additional vertical bombardment of He+ and Ar+ which produces in the plasma with the addition of the He and Ar, the sidewalls of the Si trench become more vertical. Based on this, an optimized trench etching process is proposed and the etch rate is 1.25 um min−1, the selectivity for oxide mask is 9.5, and the undercut is less than 50 nm.