Reduction Mechanism of Surface Roughness on ArF-Photoresist Using C$_{5}$HF$_{7}$ Gas Plasma

Fluorocarbon (FC) plasmas have been used in reactive ion etch processes for the fabrication of high-aspect-ratio-contact-hole on SiO$_{2}$. There are some needs, such as high selectivity over photoresist (PR), Si$_{3}$N$_{4}$ and Si, avoiding surface roughness formation on ArF- PR. C$_{5}$F$_{8}$ gas is known to improve the SiO$_{2}$ selectivity compared with C$_{4}$F$_{8}$ and conventional gas chemistries. Recently, we achieved that highly selective etching of SiO$_{2}$ against PR, Si$_{3}$N$_{4}$ and Si using a newly-designed gas, C$_{5}$HF$_{7}$, and O$_{2}$, Ar gas mixture. So far, we have investigated the etch performances and its mechanism using C$_{5}$HF$_{7}$ gas chemistry through comparison with C$_{5}$F$_{8}$ gas. In this study, we focused on the mechanism of reducing the surface roughness formation on ArF-PR during SiO$_{2}$ etching in the C$_{5}$HF$_{7}$ gas chemistry. The plasma etching time dependency of surface morphology on ArF-PR was compared with the case of C$_{5}$F$_{8}$. For C$_{5}$F$_{8}$/O$_{2}$/Ar plasma, surface roughness increased. For C$_{5}$HF$_{7}$/O$_{2}$/Ar plasma, RMS roughness about 2 nm was formed on the PR surface at 5 sec. As the time elapsing, surface roughness stayed constant. The RMS roughness caused by C$_{5}$HF$_{7}$ gas chemistry was lower than that of C$_{5}$F$_{8}$. Since F atoms in FC film were reduced by hydrofluorocarbon species, CxHFy, the FC polymerization was enhanced selectively on PR to form a thicker FC film that protects the PR surface from ion bombardments, while keeping the high etch rate for SiO$_{2}$.