Wilhelm Ulrich

Trends in optical design of projection lenses for UV and EUV lithography

The continuing trend towards higher integration density of microelectronic circuits requires steadily decreasing feature sizes. The SIA roadmap defines the technologies needed to meet this challenges. One of the fundamental requirements for lithography with a resolution of 100 nm and below is the development of new high-performance optical designs for projection lenses.

Reflective high-NA projection lenses

Dioptric systems are usually the first choice for the design of an optical system, e.g. a projection lens or a microscope. But in some cases refractive designs suffer from serious drawbacks like chromatic aberration or material problems (cost, quality, absorption, birefringence, etc.). In such cases reflective systems are an attractive alternative. Reflective systems can be subdivided into two classes: on one hand there are systems with central pupil obscuration, e.g. reflective microscopes or telescopes in astronomy, which have a high aperture but only a small field size, on the other hand there are unobscured systems, e.g. reflective relay systems or EUV projection lenses, which have a large field but only small aperture. By the combination of an unobscured and an obscured mirror system one obtains systems with large field and high numerical aperture. We present new designs, which prove this design principle.

Catadioptric projection lenses for immersion lithography

Recently, the development of high NA lenses for immersion lithography turned from dioptric concepts to catadioptric design forms. The introduction of mirrors involves the new challenge to deal with the inevitable obscuration of either field or pupil. We review the strategies used in this regard for microlithography, while focussing on the two most favored ones, folded and inline concepts. Although the vignetting situation is more complicated for inline systems, we report progress in this field of optical design yielding similar system performance for inline and folded designs. Since inline optical systems are much easier to realize, these are the concept of choice.