Investigations of Surface Reactions in Neutral Loop Discharge Plasma for High-Aspect-Ratio SiO 2 Etching

The relationship between fine etching and gas structure in magnetic neutral loop discharge (NLD) plasma has been investigated using C 4 F 8 , C 3 F 8 , and CF 3 FOC=CF 2 (HFE-216) gases. It was found that CF 3 + ions were effectively generated in the HFE-216 plasma compared with those in the C 4 F 8 or C 3 F 8 plasma under the same conditions. Hydrofluorocarbon (HFC) gases such as CH 2 F 2 (HFC-32) and CH 3 CHF 2 (HFC-152a) were also employed to realize highly selective etching for SiO 2 to the photoresist. C 1s X-ray photoelectron spectra showed a prevalence of C–C and C–CF x bonds in the films deposited on the surface in the HFC plasma. This implies that the deposited film was mainly composed of carbon atoms. It was also found from X-ray photoelectron spectroscopy (XPS) and Fourier transformed infrared (FTIR) analyses that the chemical state of fluorine in this film was not C–F, but H–C–F. This may lead to the realization of microloading free etching with high resist selectivity in the HFE-216/HFC-152a mixture plasma. Studies on the relationship between etch performance and thin H–C–F polymer film formation were carried out in the HFE-216/HFC-152a mixture plasma. It can be thought that the interaction of the H–C–F film on the surface and C x F y species from the plasma is very low compared with that of a C–F film. The H–C–F film on the sidewall may play serve as a lubricant and may transport etchants to the bottom of the etched pattern. As a result, 50-nm-diameter holes and 40 nm space patterns with aspect ratios of 18 and 22.5, respectively, were successfully fabricated in the HFE-216 + HFC-152a + O 2 plasma. The H–C–F film formation on the etched surface enables high-aspect-ratio etching with high selectivity.