Investigation of Pulse-Modulation Effect in Electron Beam Excited Plasma with Time Resolved Optical Emission Spectroscopy

Micromachining of optical devices attracts much attention. In the process, the fast atomic-beam etching or the ion-beam etching has been employed, since the conventional reactive plasma etching can't be employed as the radio-frequency self-biasing is not efficiently supplied to the thick dielectric materials. However, a pulse-modulated electron-beam- excited plasma (EBEP) has a potential to realize the high etching rate without any additional bias power supply. Therefore, we have investigated the effect of pulse-modulation of EBEP using time resolved optical emission spectroscopy. Plasma was generated at a pressure of 0.27Pa and the fed gases were C$_{4}$F$_{8}$/Ar, a discharge current of 25A and an electron acceleration-voltage of 65V with a pulse-modulation frequency of 50kHz. It was found that CF$_{2}$ optical emission intensity at the 50{\%}-duty ratio was involved in two lifetimes of $\tau _{1}$=3 and $\tau _{2} $=19.8$\mu $s compared with Ar optical emission intensity. Moreover, CF$_ {2}$ radical density was evaluated by using Actinometry in order to compare with F atom density. F atom density increased with the increase of duty ratio, but CF$_{2}$ radical density decreased. Therefore, the dissociation degree of C$_{4}$F$_{8}$ was controlled by the duty ratio of electron acceleration voltage.