Dry etching of Cr2O3/Cr stacked film during resist ashing by oxygen plasma

Stacked films of chromium oxide and chromium (Cr2O3/Cr) have been commonly used as a photomask for the lithographic process of integrated circuit fabrication. It has been found, however, that the Cr2O3/Cr films were etched during the oxygen plasma ashing process, which is applied for the resist removal after the Cr2O3/Cr patterning. In order to solve this problem, we investigated the mechanisms of the Cr2O3/Cr film etching during the ashing process. As a result, it was found that the Cr2O3/Cr film is oxidized during the ashing process to generate CrOx (2 less than or equal to x less than or equal to 3) in which Cr has a higher valence than Cr2O3, and that the CrOx (2 less than or equal to x less than or equal to 3) evaporates. It was confirmed by means of a plasma shielding experiment that not only oxygen plasma but also oxygen radicals oxidize the Cr2O3/Cr. It has been found that keeping the photomask temperature below 200degreesC during the ashing process solves the Cr2O3/Cr etching problem.