Laser-Induced Fluorescence and Langmuir Probe Determination of Cl sub 2 sup + and CL sup + Absolute Densities in Transformer-Coupled Chlorine Plasmas

Laser induced fluorescence (LIF) detection of Cl sub 2 sup + ions 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 Cl sub 2 sup + number density was compared to the total absolute positive ion density (n sub 1 sup + = n sub (Cl+) + n sub (Cl 2) sup +) that was obtained using Langmuir probe analysis under the same plasma conditions. Both n sub (Cl2) sup + and n sub i sup + exhibited a two-fold increase with increasing rf power from 8 to ~55 W with the discharge in the capacitively coupled (dim) mode. Above the transition from a capacitively to inductively coupled (bright-mode) plasma at an rf power of ~85 +- 35 W, n sub (Cl2) sup + decayed with rf power while n sub i sup + continued to increase. Consequently, Cl sub 2 sup + is the dominant ion in dim-mode operation and Cl sup + is the dominant ion in bright-mode operation, for which n sub i sup + >= 6x10 sup (10) cm sup (-3). With the plasma operating in the reactive ion etch (RIE) mode (the stage powered at 14.56 MHz, and no TCP power), n sub (Cl2) sup+ tracked n sub i sup + over the entire range of powers investigated (2 to 150 W). Thus, Cl sub 2 sup + is the dominant ion during capacitively-coupled RIE operation.