Investigation of atomic oxygen density in a capacitively coupled O2/SF6 discharge using two-photon absorption laser-induced fluorescence spectroscopy and a Langmuir probe

Two-photon absorption laser-induced fluorescence (TALIF) spectroscopy was used for detection of absolute atomic oxygen density in a low-pressure capacitively coupled plasma source. We investigated the variation of atomic oxygen density for various mixtures of O-2/SF6 and report a significant five-fold increase of [O] when oxygen plasma was diluted with SF6 by only 5%. We attribute this increase in [O] to a combination of a change in surface conditions caused by constituents of SF6 plasma reacting with the reactor walls and also due to an increase in the electron temperature. Atomic oxygen production rates were determined using electron-energy distribution functions obtained with a cylindrical Langmuir probe. It was found that the effective electron temperature dramatically increased from approximately 1-8 eV as the SF6 content varied from 0% to 60% which consequently resulted in a three-fold increase in the atomic oxygen production rate. TALIF was also used to investigate the variation of [O] due to fluorination of the reactor walls and also after etching resist-coated wafers. It was found that [O] increased by over a factor of three after fluorinating the walls with SF6 plasma; on the other hand a coating formed on the reactor walls after a resist etch process resulted in a reduction of [O] by only 20%.