Temporal velocity distribution of positive and negative ions incident on a wafer in a pulsed two-frequency capacitively coupled plasma in CF ₄/Ar for SiO₂ etching

Two-frequency capacitively coupled plasmas (2f-CCP) are widely used for SiO₂ etching. As the size of the element of the ultralarge scale integrated circuit (ULSI) decreases, a number of problems during plasma etching have been reported. In particular, charging damage caused by the difference of the velocity distribution between electrons and positive ions incident on an etched wafer will become an increasingly important issue. A pulse-modulated plasma with continuous-wave (CW) biasing at low frequency will become one of the practical solutions to the reduction of charging damage. In this paper, we numerically investigate the properties of the energy and angular distributions of positive and negative ions incident on a SiO₂ wafer in a pulsed 2f-CCP system, which consists both of a very-high frequency (100 MHz) source for sustaining high density plasma and a low-frequency (1 MHz) bias for high-energy positive ions striking the wafer. The temporal behavior of the impact velocity distribution, which contributes to the reduction of the charges accumulated inside the hole/trench to be etched, are discussed for positive and negative ions.