Halftone phase shift masks with a single-layer SiNx absorption film whose stoichiometric ratio x was controlled were developed at 365 and 248 nm, respectively. Because this stoichiometric ratio x was smaller than 1.33 (Si3N4), the SiNx halftone films were unstable, especially during 248 nm exposure, causing a transmittance error. In this article, we show how to get stable SiN, film for 248 nm exposure.