Simulation of a Pulse Time-Modulated Bulk Plasma in Cl2

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Abstract
A simulation of a time-modulated (TM) ECR plasma in Cl2 is presented. The behavior of electrons in the bulk plasma is simulated by taking account of diffusion of electrons from the ECR region and confinement of electrons by the sheath electric field. The positive and negative ion densities are deduced from the total number of collisions of ionization, attachment, and detachment by electrons. A simulation is performed for a plasma with a pressure of 2 mTorr, microwave power of 500 W and a pulse duration time of 100 µ s. The calculated time constants of the electron density and temperature, and the saturation current ratio between the positive ions and electrons agree well with the results of probe measurements. The positive ion density shows little time variation. The negative ion density decreases in the power-on period because electron detachment from negative ions increase due to high energy electrons diffusing from the ECR region, but in the power-off period, the negative ion density significantly increases since attachment collisions increase as the electron temperature decreases. This variation in the negative ion density corresponds to the variation in etching rate of Si with pulse interval observed in a previous experiment. This indicates that negative ions contributes to the etching reaction in a TM Cl2 plasma.