Abstract
In general, depth-of-focus (DOF) decreases as the square of the feature size. As the resolution of optical lithography has improved, with the potential to go below 0.25 /tm, the decrease in usable DOF has been significant. As such, there has been increasing effort put towards ways of improving DOF in manufacturing. This paper will examine several proposed techniques for improving DOF including the use of geometry dependent mask bias, variable numerical aperture, multiple focal-plane exposures (the FLEX method), frequency plane spatial filtering, and annular illumination sources. As will be shown, each method offers the potential for improved DOF, but only for certain cases. None of the methods provides a general solution to the problem of shrinking focal depth, but rather they may simply slow the inevitable progression.
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