Characteristics of Photomask Substrate in Optical Lithography

The characteristic of a photomask strongly impacts lithography performance since it is part of the optical system. In this paper, we investigate characteristics of photomask substrate flatness impact on optical lithography. It is demonstrated that high and mid spatial frequencies of substrate flatness variation are too small enough to affect lithographic performance. However, a low spatial frequency of flatness variation could cause a focal plane deviation. We show that the flatness of exposure area after tilt and curvature correction directly corresponds to an image plane deviation.