Plasma-based pseudo-wet mechanism for cryogenic SiO₂ etching using hydrogen-contained fluorocarbon gases with an in-situ surface analysis

In the manufacturing of semiconductor devices, plasma etching techniques are wide used. The cryogenic etching has recently drawn much attentions for high aspect ratio contact etching. However, the mystery for the dramatic enhancement of SiO₂ using CHF₃ plasma at cryogenic temperature, which has been published over 30 years ago [Ohiwa et al., JJAP 31, 405 1991], still remains unsolved. In this work, we investigated the effect of substrate temperature (T_s) on etch rate (ER) of the SiO₂ films using CF₄ plasma with different concentrations of H₂ additive (20 – 60 %). The in situ spectroscopic ellipsometry data shows that the ER with H₂ = 33 %/CF₄ plasma significantly increased by 65 %, as T_s was decreased from 20 to –60 °C. Concerning the H₂ additive, the ER at 20 ^oC monotonically decreased as expected from 2.59 to 1.40 nm/s as the H₂ additive was increased from 20 to 60 %. Contrarily, the ER reached a maximal value of 3.36 at H₂ = 33 % and then decreased to 2.64 nm/s at H₂ = 60 %. The F radical density decreased with increasing H₂ concentration using the actinometry method, which explains the ER decrease at 20 °C, however, not for that at –60 °C. The variation of HF neutrals generated from the CF₄/H₂ plasma corresponds with the results of ER at –60 °C. Combined with the results of surface structure using in-situ FTIR, a “pseudo-wet” etching model for cryogenic etching of SiO₂ with the HF absorption is proposed.