Physical mechanisms for anisotropic plasma etching of cesium iodide

The physical mechanisms for the interaction between a reactive plasma and a cesium iodide surface are investigated. Under conditions of ion bombardment and elevated substrate temperature, CsI is found to sputter etch slowly (15nm∕min). If atomic fluorine, fluorocarbon radicals, of SFx radicals are present in the discharge, however, CsI is reactively etched at substantially higher rates (up to 200nm∕min). The roles of plasma radicals and energetic ion bombardment are investigated by first exposing the surface to plasma radicals and then bombarding the surface with argon ions. The optical emission from Cs and I atoms is found to correlate with the etch rate of CsI and is used as an in situ monitor of radical-enhanced etching. Small surface exposures to CFx, SFx, and F radicals are shown to enhance the etch rate of CsI. If the exposure of the CsI surface is increased, however, these same radical species act as etch inhibitors. A simple model for reactive etching of CsI is proposed, and this model is shown to compare reasonably well with experimental etch rates.