In an off-axis leveling procedure a height map of a mask is generated at a measurement
station. The height map is referenced to a physical reference surface of a mask
support. The physical reference surface may be a surface in which is inset a transmission
image sensor. At the exposure station the height of the physical reference surface
is measured and related to the focal plane of the projection lens. The height map
can then be used to determine the optimum height and/or tilt of the mask support
to position the exposure area on the mask in best focus during exposure.
