Vibrationally Excited Fluxes to Wafers in Plasma Processing

Semiconductor plasma processing employs molecular gases to produce fluxes of radical and ions to wafers (for device fabrication) and reactor walls (for chamber cleaning). Although emphasis is typically on producing radicals and ions, the fluxes of vibrationally excited species to surfaces can exceed those of radicals and ions. These vibrationally excited species bring with them activation energy that can affect surface processes as well as deliver a power flux to the wall. In this work results from a computational investigation of inductively coupled plasmas (ICPs), of the type used for semiconductor processing, sustained in Ar/N₂/O₂ mixtures will be discussed from the perspective of quantifying vibrationally excited fluxes incident onto surfaces. The simulations were conducted with the Hybrid Plasma Equipment Model (HPEM). Ar/N₂/O₂ mixtures were used as a surrogate for actual plasma processing gas mixtures as reaction mechanisms are available for electron impact, V-V and V-T collisions for these gases. In low pressure systems as used in plasma etching (pressures of < tens mTorr) the vibrational distributions that arrive at surfaces are dominated by the initial electron impact excitation processes. The V-V and V-T collisions that shape these vibrational distributions have low frequencies at these pressures. At pressures typically used in plasma enhanced chemical vapor deposition (a few to 10 Torr), V-V and V-T collisions become more important in shaping the vibrational distributions incident onto surfaces.