Langmuir probe and Laser Photodetachment Study of Afterglow Phase in Dual RF Frequency Pulsed Plasma Etching Processes Operated with Synchronized DC Bias

The study of the fundamental physical properties that govern the plasmas becomes more crucial for the control of etch profile and etch rate for the development of the high aspect ratio etching (HARE) process. However, despite the importance of radio frequency (RF) discharges using fluorocarbon gases for plasma etching in microelectronics, the connection between fundamental plasma properties, such as the electron temperature (T_e), electron density (n_e), and the kinetics in such plasmas is not yet fully understood. We combinedly used RF compensated Langmuir probe (LP) and laser photodetachment techniques to study the dynamics of plasma parameters and negative ions, respectively, in (40.68 MHz/3 MHz) frequency pulsed RF O₂/C₄F₈/Ar plasmas with synchronized DC bias. Result show that during RF on phase, the plasma remains electropositive with a characteristic high n_e and T_e. The electronegativity (α) enhanced significantly in the afterglow phase with a very small T_e and n_e due to electron attachment processes with a very high decay rate of n_e. The enhanced electronegativity and a high DC bias condition associated with secondary electron emissions are attributed to generating a separate warm electron population (T_w) with a low T_e in the afterglow phase in DC synchronized condition.