FOCAL PLANE DETERMINATION FOR SUBHALF MICRON OPTICAL STEPPERS

In this paper the determination of optimal focus using a unconventiona l method as a function of common lithographic variables such as exposu re wavelength, resist thickness, resist contrast, and numerical apertu re (NA) will be studied. The determination of the focal plane in resis t with now widely used high NA lenses have shown to be problematic due to the asymmetric nature of the depth of focus range (1). In sub-half micron lithography, the usable depth of focus has shrunk to about 1 m icron because of the relatively high NA lens used. For this reason, ha ving the focus set at the center of the usable depth of focus is very critical. An updated application of a previously reported focus determ ination technique will be studied as a function of stepper and resist variables (2).