The plasma of a CF4-quartz etching system is investigated with optical methods. The presence of CF2, CF, F, and CO is demonstrated from ultraviolet and visible emission. Using infrared absorption the absolute densities of the reaction products SiF4and CO are found to be 2.5 ± 0.4 X 1020and3 ± 2x 1020m-3. CF2 radicals are not found with infrared absorption which means that their density is below 1019m-3. The results indicate that quartz is etched through the reaction Si02 + 2CF2—â–º SiF4+ 2CO.