Method of selecting photomask blank substrates

A method of selecting photomask blank substrates for use in a process where one or more layer of film, including at least a masking film or a phase shift film, is deposited on a top surface of a photomask blank substrate to form a photomask blank, the deposited film is patterned to form a photomask, and the photomask is mounted in an exposure tool, the method comprising the steps of:

• simulating a change caused by a film stress of the masking film or the phase shift film to be formed on the substrate in shape by the top surface of the substrate from prior to film deposition thereon to when the photomask is mounted in the exposure tool;
• determining the shape of the substrate top surface prior to said change that will impart to the top surface a flat shape when the photomask is mounted in the exposure tool;
• and
• selecting, as an acceptable substrate, a substrate having this top surface shape prior to film deposition.