A NEW METHOD FOR ANALYZING LANGMUIR PROBE DATA AND THE DETERMINATION OF ION DENSITIES AND ETCH YIELDS IN AN ETCHING PLASMA

The numerical results of Laframboise for the operation of a cylindrical Langmuir probe are parameterized for arbitrary ratios of probe radius rpto Debye length XD. It is shown that the ion current to the probe can be represented by an expression proportional to a(— X)bwhere a and b are constants depending only on rp/λDand × is a dimensionless probe potential. This allows for ready evaluation of ion densities from probe data even in high-density enhanced discharges. Examples are given for the ion density and electron temperature as a function of position relative to the cathode in a reactive ion etch reactor and in a magnetron ion etch reactor. The determination of the ion flux to the cathode is shown not to require knowledge of the exact location of the sheath edge, but it is important to take into account the variation of the ion density in the presheath. By combining etch rate data with data on the ion flux to the substrate, one can obtain etch yields per impinging ion. It is demonstrated that etch yields are much more useful than etch rates in giving information on plasma etch mechanisms. © 1990, American Vacuum Society. All rights reserved.