Etching characteristics of crystal quartz by surface wave microwave induced plasma

Abstract

Plasma figuring technologies have been widely used in the processing of silicon-based materials at atmospheric pressure. Previous plasma figuring of silicon based optical surfaces has been undertaken using a radio frequency plasma jet through an Inductively Coupled Plasma (ICP) torch. Microwave plasma is suitable for processing those materials that cannot bear high temperature from the thermal plasma jet. For crystalline quartz (SiO4) processing, microwave plasma systems employ electrodes to couple the microwaves into the gas; however, the presence of reactive plasma interactions with any electrode surfaces, typically results in electrode degradation. To avoid this degradation, the Surface Wave Launched Microwave Induced Plasma (SWL-MIP) torch design was selected that uses the principal of surface wave launching. The electromagnetic frequency was set to 2.5 GHz for all the experiments. Argon is used as a main carrier gas. Carbon tetrafluoride (CF4) is used as a secondary gas for the creation of reactive species and consequently enables the material removal of silicon atoms from the substrates. Optical Emission Spectroscopy (OES) characterization confirmed that these parameters led to a plasma jet, which was stable both spatially and temporally. The optimum parameters were used for the material removal experiments of crystal quartz. Finally, a material removal rate of 0.18 mm3/min was achieved with substrate preheating to 200 °C. The maximum surface roughness at the bottom of a measured trench increased from an Sq of 1.5 nm up to a mean average Sq of 3.5 nm.