Patent ID: 8304146

Claim:
A method for manufacturing a photomask comprising the steps of: providing a photomask blank comprising a transparent substrate, a phase shift film thereon comprising a silicon base material at least on its side contiguous to a light-shielding film, and a light-shielding film disposed on the phase shift film and including a layer of silicon base material and an optional outer layer of chromium base material, said phase shift film and said light-shielding film being a combination of a phase shift film of single or multilayer structure including an outermost layer of a silicon base material containing oxygen and/or nitrogen and a transition metal and a light-shielding film of single or multilayer structure including a layer of a silicon base material containing oxygen and/or nitrogen and a transition metal, or a combination of a phase shift film of single or multilayer structure including an outermost layer of a silicon base material containing nitrogen or nitrogen and oxygen and optionally a transition metal and a light-shielding film of single or multilayer structure including a layer of a silicon base material containing nitrogen or nitrogen and oxygen and optionally a transition metal, the silicon base material layer of said phase shift film being disposed contiguous to the silicon base material layer of said light-shielding film, and selectively etching away the silicon base material layer of the light-shielding film disposed on the silicon base material layer of said phase shift film in said photomask blank while retaining the silicon base material layer of said phase shift film, wherein the silicon base material layer of said phase shift film has a total content C1 (mol %) of nitrogen and oxygen, and the silicon base material layer of said light-shielding film has a total content C2 (mol %) of nitrogen and oxygen, the difference between these total contents (C1−C2) is at least 5, and the etching step comprises the step of etching said silicon base material layer of said light-shielding film by chlorine dry etching using oxygen-containing chlorine gas in a molar ratio of oxygen to chlorine between 0.0001 and 1.