Wall effects on the chemistry in a pulsed oxygen/silane radiofrequency helicon plasma

Abstract

A pulsed oxygen/silane radiofrequency (13.56 MHz) plasma is created in a helicon diffusion reactor used for silicon dioxide deposition. An energy selective mass spectrometer is used to measure the time-averaged ion energy distribution function of the four predominant positive ions (H3+, H3O+, SiH3+, and SiOH+) and the positive ion mass spectra during the breakdown phase and the steady-state phase of the pulse. Charging of the silica-covered chamber wall by ∼55 V is observed both for continuous and pulsed excitations. All positive ions with mass greater than 62 (as well as H2O+ and SiOH+) are mostly detected during the plasma breakdown phase. This could result from some physical sputtering of the chamber walls at the beginning of each pulse (when the plasma potential and the ion bombardment energy reach their maximum) or from an increase in the density of heavy radicals during the post-discharge (as a result of the electron density and temperature decrease) which can be ionized during plasma breakdown of the oncoming pulse. During the plasma breakdown an ion bombardment energy of ∼70 V is measured which is larger than the physical sputtering threshold (25 V) of thermally grown SiO2 (P-etch ∼2 Å s−1) in an oxygen plasma. Consequently, for inadequate pulsing conditions, physical sputtering of the soft and porous suboxides (P-etch >30 Å s−1) deposited on the chamber sidewalls will occur, thereby allowing hydrogen-rich radicals and positive ions to reenter the plasma chemistry.