Radial distributions of ion velocity, temperature, and density in ultrahigh-frequency, inductively coupled, and electron cyclotron resonance plasmas

Abstract

Spatially resolved velocity distribution functions of metastable chlorine ions are measured in ultrahigh-frequency (UHF) plasma and compared with those in electron cyclotron resonance (ECR) plasma and inductively coupled plasma (ICP). In UHF plasma, ion velocity distribution function (IVDF) perpendicular to the surface normal of a wafer exhibits approximately no acceleration of ions to the side wall of a plasma reactor as the measuring point becomes radially far from the center of the reactor, even at 14 cm from the center where IVDF’s are shifted evidently in ECR plasma and ICP. This indicates that an almost flat profile of plasma potential is realized along the radius of the reactor in UHF plasma. As a consequence, metastable chlorine ion temperature remains low and almost constant over a radius of 14 cm in UHF plasma. Relative density of metastable chlorine ions in UHF plasma increases toward the side wall of the reactor. In contrast to this, the density exhibits a maximum in the center of the reactor and decreases towards the side wall in ECR plasma and ICP. The increased densities near the side wall in UHF plasma imply increased plasma production off the symmetry axis of the reactor, which makes UHF plasma uniform in a wider region than in ECR plasma and ICP. The effect of the discharge frequency on radial distribution of metastable chlorine ion density is also discussed.