A Comprehensive Quantitative Study of Low Pressure Inductively-Coupled Plasmas in Cl2, O2 and Mixtures
Résumé
and O2 are widely used in semi-conductor fabrication for gate etching and photo-resist stripping. Moreover they are an archetype for plasmas in simple electronegative gases with the advantage that techniques exist to measure the densities of nearly all their stable and reactive species. They provide an opportunity to benchmark models such as the Hybrid Plasma Equipment Model. We are then undertaking a comprehensive set of measurements in the pure gases and their mixtures. The plasma is sustained in a cylindrical chamber (55 cm diameter,10 cm height) by a 4-turn planar coil excited at 13.56 MHz above a dielectric window. Pure gases (Cl2 and O2) were studied over a range of pressure [5-80 mTorr] and RF power [50-550 W]. The effect of O2 addition (0-100%) to Cl2 plasmas was studied at 10 mTorr 500W. The electron density was determined by hairpin resonator probe. The Cl and O atom absolute densities and surface reaction coefficients were obtained by Two-Photon Absorption Laser- Induced Fluorescence. A novel ultra-broadband high sensitivity absorption bench was used to measure ground state and vibrationally excited Cl2 and vibrationally excited O2, as well as OCl, OClO and Cl- ions. In all gas mixtures, the electron density was observed to increase initially with gas pressure, pass through a maximum and then decrease at higher pressure. In O2 a broad maximum is observed around 40 mTorr, whereas for Cl2 the electron density peaks at 10 mTorr before dropping sharply. This difference can be attributed to rapid dissociative attachment of electrons in the case of Cl2. In all cases the electron density increases with RF power. In O2, highly vibrationally excited molecules O2(v) were observed ( with v up to 18), and analysis of the rotational structure of the O2 Schumann-Runge bands showed rotational temperatures (=translational temperatures) ranging from 400 K (10mTorr 100W) to 900 K (80 mTorr 500W). The non-equilibrium vibrational distribution may be attributed to electron impact excitation combined with low rates of V-T energy transfer processes in O2. In contrast, 51 Monday Afternoon, October 19, 2015 in pure Cl2 the vibrational temperature is equal to the gas translational temperature, (~1000K) due to efficient V-T transfers between Cl2 and Cl atoms. In Cl2/O2 mixtures the densities of electrons and Cl and O atoms all decreased when O2 or Cl2 is added to the pure gas (Cl2 or O2). This coincides with the formation of ClO and OClO, which consumes the atoms which are easier to ionize than molecules. We are currently working on modeling of these results to obtain a deeper understanding of the kinetics of Cl2/O2 plasmas.