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Formation of C2 Radicals in High-Density C4F8 Plasmas Studied by Laser-Induced Fluorescence

Spatial and temporal variations of C2 radical density in high-density C4F8 plasmas have been investigated by laser-induced fluorescence (LIF) spectroscopy. Hollow-shaped spatial distributions have been observed for the C2 density, indicating that C2 radicals are produced from fluorocarbon film on the chamber wall and are lost in the plasma column by electron impact processes. The C2 density depends largely on the seasoning condition of the vacuum chamber, which supports the surface production of C2 radicals. Bombardment of positive ions onto fluorocarbon film is necessary for such surface production. The mechanism of this surface production is not the physical sputtering of fluorocarbon film but ion-enhanced chemical reactions with fluorocarbon film. Lifetime measurements of C2 radicals have also been carried out in the afterglow. The observed decay time constants (less than 0.8 ms) are shorter for high gas pressures, indicating the loss of C2 due to gas-phase reactions.