Simulation of variations in the negative ion density in time-modulated Cl₂ plasmas

The variations in the negative ion density in a time-modulated (TM) Cl₂plasma with pulse duration have been investigated. The behavior of electrons in the plasma is simulated by a model based on the Monte Carlo method, and the variations in the negative ion density are deduced from attachment and detachment by electron impact and recombination with positive ions. It is found that the negative ion density increases significantly while electron density and temperature decrease when the microwave power is turned off. The time-averaged negative ion density during the power-off period varies with the pulse duration, and this dependence of the negative ion density on pulse duration is qualitatively the same as that of the etch rate obtained by the experiment. The present results suggest that the impingement of negative ions on the substrate affects the etching performance in a TM Cl₂ plasma.