Design and fabrication of customized illumination patterns for low k1 lithography: a diffractive approach

As CDs continue to shrink, lithographers are moving more towards using off-axis illumination schemes to increase their CD budget. There have been several papers over the last few years describing various custom illumination profiles designed for application specific optimization. These include various annular and quadrupole illumination schemes including weak quadrupole, CQUEST, and Quasai(TM). Traditionally, pupil filtering is used to realize these complex illumination modes but this approach tends to introduce significant light loss. Therefore, compromises are made to lithographic performance to minimize the effect on wafer throughput. Diffractive optics, if incorporated into the design of the illumination system, can be used to create arbitrary illumination profiles without the associated light loss, thus maintaining throughput while optimizing system performance. We report on the design and fabrication of such devices for use with KrF, ArF, and potentially F-2 scanners. Extension to I-line steppers is also possible.