Evaluation of plasma density in RF CCP discharges from ion current to Langmuir probe: experiment and numerical simulation

Experimental measurements of current-voltage relationship in RF CCP discharge in argon at 81 MHz have been performed by cylindrical Langmuir probes technique. Two different probe radii have been used: 50 and 250 mu m. The high plasma density 10(10)-10(11) cm(-3) has been estimated at specific input power under study. The experimental data on nonmonotonic behavior of probe current with pressure were observed firstly for conditions of RF discharge plasmas. To analyze the probe measurements the fast numerical model for ion current collected by a cylindrical probe has been developed. This model is based on the particle-in-cell with Monte-Carlo collision method for ions motion and Boltzmann relation for electrons. The features of probe data at studied conditions were discussed. The comparative analysis of different collisionless approaches for plasma density calculation from ion probe current is done. It is shown that in general collisionless theories underestimate the plasma density value. For correct evaluation of plasma density experimental I-V probe measurement should be supplied by the numerical simulation. It was demonstrated that the collisionless analytical theory of orbital motion can formally give correct results on plasma density at some plasma conditions even when ion collisions take place. The physical reasons of this accidental validity are explained.