Proceedings Article | 5 October 2011
Proc. SPIE. 8171, Physical Optics
KEYWORDS: Lithography, Point spread functions, Optical lithography, Lithographic illumination, Modulation, Distortion, Ray tracing, Source mask optimization, Critical dimension metrology, Fiber optic illuminators
Source Mask Optimization (SMO) is one of the most important techniques available for extending ArF immersion
lithography. The combination of freeform source shape and complex mask pattern, determined by SMO, can extend the
practical resolution of a lithography system. However, imaging with a small k1 factor (~0.3 or smaller) is very sensitive
to many imaging parameters, such as illumination source shape error, lens aberration, process property, etc. As a result,
care must be taken to insure that the source solution from SMO can be produced by the real illuminator, which is subject
to its own imaging constraints. One approach is to include an illuminator simulator in the SMO loop so that only
realizable illumination pupils are considered during optimization. Furthermore, the real source shape must be re-adjusted
to realize expected imaging performance as may be seen, for example, in an Optical Proximity Effect (OPE)
curve.
In this paper we present and describe both the illuminator simulator, which can predict the real pupilgram on the
exposure tool quickly, and an illumination pupilgram re-adjustment method that can effectively control the various
illumination parameters to get optimum imaging performance, which is required for the lithography process design.
The adjusting method uses pupilgram modulation functions, which are similar to Zernike polynomials used in wavefront
aberration analysis for lithographic projection lens, to describe the optimal pupilgram adjustment, and the resulting
modulation can then be realized by the illuminator system.
