Paper PS1-TuA9
Optical Emission and Langmuir Probe Diagnostics of CH₃F-O₂ Inductively Coupled Plasmas

Tuesday, October 29, 2013, 4:40 pm, Room 102 B

CH₃F plasmas, mostly with added O₂, are used in selective Si₃N₄ etching over Si or SiO₂. Despite their use, fundamental plasma studies in these gas mixtures are very scarce. In this work, optical and Langmuir probe diagnostics were employed to study inductively couple plasmas in CH₃F/O₂ gas mixtures. In 50% CH₃F/50% O₂ plasmas, the electron density increased linearly (0.7x10¹¹→2.7x10¹¹ cm^-3) as power was increased 150-400W at 10 mTorr, but only weakly (1.7x10¹¹→2.7x10¹¹ cm^-3) within the pressure range of 10-40 mTorr at 300W. The effective electron temperature representing the high energy tail of bi-Maxwellian EEPFs was nearly independent of power and pressure. The gas temperature increased from 400-900 K as a function of inductive mode power between 75 and 400 W at 10 mTorr. For a constant feed gas flow rate and composition, the absolute H, F and O atom densities, estimated by optical emission rare gas actinometry, increased linearly with power. The feedstock gas was highly dissociated and most of the fluorine and oxygen was contained in reaction products HF, CO, CO₂, H₂O and OH. Reaction mechanisms were proposed to explain the observed behavior of the relative density of F and HF vs. power and pressure. Measured relative densities as a function of O₂ addition to CH₃F/O₂ changed abruptly for H, O, and particularly F atoms (factor of 4) at 48% O₂. A corresponding transition was also observed in electron density, effective electron temperature and gas temperature, as well as in C, CF and CH optical emission. These abrupt transitions were attributed to the reactor wall reactivity, changing from a polymer-coated surface to a polymer-free surface, and vice-versa, as the O₂ content in the feed gas crossed 48%.

*Work supported by Lam Research Corp.