Plasma Processing of Thin Chromium Films for Photomasks

Abstract

The preparation of photomasks from thin chromium films on glass substrates is described using both optical and electron beam resists. Plasma processing techniques are used both for pattern definition and for a passivation step prior to either plasma or liquid etching that enables the image to be reversed, i.e., to produce a negative image with a positive resist or vice versa. Plasma etching conditions, using mixtures of carbon tetrachloride and oxygen are described that allow the use of PBS resist which is much less robust than other electron beam or optical resists. Pattern reversal is achieved either by first exposing the patterned chromium to a tungsten hexafluoride plasma after which the resist is stripped and the unpassivated chromium is plasma etched, or the patterned chromium is exposed to an oxygen, argon, or sulfur hexafluoride plasma, the resist is stripped, and the chromium is liquid etched in a mixture of ceric ammonium sulfate and sulfuric acid. In the first case, passivation of the exposed chromium is achieved by the deposition of a thin tungsten layer that is resistant to plasma etching and in the second case by the deposition of material back sputtered from the substrate electrode (e.g., silicon dioxide) that is insoluble in the liquid etchant.