Systems for and methods of measuring photomask flatness with reduced gravity-induced error

A method of interferometrically measuring flatness of a photomask, comprising:

• recording at first and second measurement angles θ1 and θ2 respective first and second interferograms IF1 and IF2 of a surface of the photomask supported by an adjustable support apparatus, wherein θ2>θ1 and wherein θ1 and θ2 are each within 11.46° of vertical;
• measuring at the first and second measurement angle θ1 and θ2 respective first and second normal forces F1 and F2 of the photomask on the adjustable support apparatus, wherein a force difference is defined as ΔF=F2−F1;
• defining a difference map DM=IF2−IF1 as a difference between the second and first interferograms IF2 and IF1;
• defining either a first scaled difference map DM1=(F1/ΔF)·DM or a second scaled difference map DM2=(F2/ΔF)·DM;
• and
• defining a compensated flatness measurement CFM by either:
• i) subtracting the first difference map from the first interferogram (CFM=IF1−DM1);
• or
• ii) subtracting the second difference map from the second interferogram (CFM=IF2−DM2).