The depth of focus in sub-half micron lithography is about plus or minus 5 micrometers , several factors, such as field image curvature, circuit topography, wafer flatness error and auto-focus errors, reduce the usable focus margin. To minimize these focus margin decreasing factors, multi-point focus detection and/or field chip leveling control should be required before every exposure. Even the cases, global deviations of wafer and chuck flatness errors are being corrected with the good field leveling system, the focus sensing accuracy must stay in the plus or minus 0.075 micrometers region for all the exposure field. In the conventional optical auto- focus systems, small spot sizes probe beam was used, and the focus signal can be affected strongly by local variations in reflectance, scattering on the wafer surface and wafer flatness. Whereas probe beam scan type auto-focus system, newly developed in this work, has a large focus measurement area, and it averages out the errors which decrease the usable depth of focus. In this work, a new optical auto-focus system for sub-half micron lithography tools will be presented and its characteristics and applications will be discussed. It is composed of a probe beam scan mirror and a position sensing detector, and is insensitive to the structures on the wafer surface. Also, the theoretical analysis of focus measurement error, per the probe beam width and wafer pattern topology will be discussed.