Laser-induced fluorescence and Langmuir probe determination of Cl2+ and Cl+ absolute densities in transformer-coupled chlorine plasmas

Abstract

Laser induced fluorescence detection of Cl2+ has been used to track their relative concentration in a high-density inductively (transformer) coupled (TCP) 10 mTorr chlorine plasma as a function of the 13.56 MHz radio frequency (rf) power. This relative Cl2+ number density was compared to the total absolute positive ion density (ni+=nCl++nCl2+) obtained with a Langmuir probe. Both nCl2+ and ni+ doubled with increasing rf power from 8 to ∼55 W in the capacitively coupled (dim) mode. Above the transition from a capacitively to inductively coupled (bright-mode) plasma at ∼85±35 W, nCl2+ decayed with rf power while ni+ continued to increase. Consequently, Cl2+ is the dominant ion in dim-mode operation and Cl+ is the dominant ion in bright-mode operation, at ni+⩾6×1010 cm−3. With the plasma operating in the reaction ion etch (RIE) mode (the stage powered at 14.56 MHz, and no TCP power) nCl2+ tracked ni+ over the entire range of powers (2–150 W). Thus, Cl2+ is the dominant ion during capacitively coupled RIE operation.