SPATIAL PROFILING OF ION AND NEUTRAL EXCITATION IN NOBLE-GAS ELECTRON-CYCLOTRON-RESONANCE PLASMAS

Optical emission from neutrals and ions of several noble gases has bee n profiled in an electron cyclotron resonance plasma system. In argon plasmas with a net microwave power of 750 W, the neutral (696.5-nm) an d ion (488-nm) emission profiles are slightly center peaked at 0.32 mT orr and gradually shift to a hollow appearance at 2.5 mTorr. Neon prof iles show a similar trend from 2.5 to 10.0 mTorr. For the noble gases, transition pressure scales with the ionization potential of the gas, which is consistent with neutral depletion. Studies of noble gas mixtu res, however, indicate that neutral depletion is not always dominant i n the formation of hollow profiles. For Kr/Ar, Ar/Ne, and Ne/He plasma s, the majority gas tends to set the overall shape of the profile at a ny given pressure. For the conditions of the current system, plasma de nsity appears to be more dominant than electron temperature in the for mation of hollow profiles. The general method described is also a stra ightforward, inexpensive technique for measuring the spatial distribut ion of power deposited in plasmas, particularly where absolute scale c an be calibrated by some other means. (C) 1994 American Institute of P hysics.