Dust grains as a diagnostic tools for sheath measurement in RF-discharge plasma

Abstract

Summary form only given, as follows. The space-charge sheath region adjacent to the electrode surfaces has been the subject of much investigation. As the ions required for etching of semiconductors receive their energy from the fields within the sheath, knowledge of the spatial variation of these fields is important for the design of manufacturing processes. Many sheath models have been proposed but their predictions are often contradictory and rarely tested by experiment. The location of the sheath edge is one of the most important parameters in many sheath models, and experimentally obtained data would allow better evaluation of existing sheath models. In this work the location of the sheath edge in a planar RF-discharge has been determined using test dust grains. The diagnostic technique is based on measurement of the equilibrium position of fine dust grains levitated above the powered electrode in an RF-discharge. Estimates shows that for grain with radius less than 500 nm the grain equilibrium position and sheath edge location differ by less than 5 percent, and this difference continues to decrease with grain radius. This suggests the possibility use test fine dust grains as a diagnostic tool for measurement of sheath edge location in an RF-discharge. 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. Additionally two independent techniques based on probe measurements and analysis of discharge optical emission have been used. All techniques provide values in a good agreement. Using the resulting value of sheath thickness the charge on dust particles in the sheath region has been estimated.