Dust grains as a diagnostic tool for rf-discharge plasma

Abstract

Dust particles can be a useful plasma diagnostic tool. Sufficiently dust particles can be used as test grains for the visualization of the plasma potential distribution; larger dust particles can be used as specific probes to determine electron temperature. Here we report on diagnostic measurements carried out in a capacitively coupled planar rf discharge. The location of the sheath edge has been determined using test dust grains. Our diagnostic technique is based on measuring the equilibrium position of fine grains levitated above the powered electrode in an rf-discharge. Estimates show that for grains with radii less than 500 mm the grain equilibrium position and sheath edge location differ by less than 5 percent, and this difference continues to decrease with decreasing investigate grain radius. We use this technique to diagnose the sheath in an argon plasma which was generated at pressures in the range 20-100 mTorr by applying a 15 MHz signal to the power electrode. The shape of the potential well above the confining electrode was also visualized using even smaller dust grains that were generated in the discharge. The well shape was found to depend strongly on the confining potential.