Modulation of the plasma uniformity by coil and dielectric window structures in an inductively coupled plasma

Abstract

The effects of coil and dielectric window structures on the plasma distribution are examined in a cylindrically symmetric planar inductively coupled plasma (ICP). A two-dimensional (2D) fluid model is employed to investigate the design issues of ICP source for etching. When the gradient coil structure is applied at 400 W and 20 mTorr, the ionization rate caused by the power deposition decreases at the reactor center as compared to that in a reactor with a planar coil above the planar dielectric window, and a rather uniform plasma is obtained. However, for the vertical coil geometry, all the coils move to the position of the outermost coil, and the peaks of the power deposition and ionization rate appear at the radial edge of the substrate. In this case, the plasma density is characterized by an edge-high profile. Further, it is observed that the plasma uniformity is improved by increasing the source power under a gas pressure of 20 mTorr and becomes better when the gas pressure increases to 30 mTorr with the source power being fixed at 400 W in the gradient coil configuration, but the uniformity of plasma worsens with the rising source power or pressure due to the strong localization in the vertical coil geometry. Moreover, when the discharge is sustained in a reactor with a stepped dielectric window at r = 0.135 m, the best plasma uniformity is obtained at 400 W and 20 mTorr because the ionization rate is enhanced at the outermost coil, and the dielectric window at r = 0.135 m blocks the diffusion of plasma towards the axis. In addition, higher source power and lower gas pressure produce more uniform plasma for the designs with a stepped window near the symmetry axis. When the dielectric window is stepped at r = 0.135 m, the non-uniformity of plasma initially decreases and then increases with the increase in source power or gas pressure. When the dielectric window is stepped at the radial edge of the chamber, the plasma uniformity is improved by increasing the source power and gas pressure due to the enhanced ionization at the larger radius caused by the severe localization.