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Resolution enhancement techniques (RETs) will no doubt play a crucial role in the future of optical lithography. The only question is how much the resolution limit can be extended. While techniques such as attenuated phase-shifting mask (PSM) and selective line biasing are beneficial, the lithography limit can only be pushed with implementation of strong RETs such as alternating phase-shifting mask and off-axis illumination. The characteristic of these strong enhancement techniques is a tremendous improvement in process latitude with restricted applicability. The enabling factor then is to limit pattern configurations to those amenable to strong RETs, for example, using assist features with forbidden pattern pitches such that features with periodicities having small process windows are not allowed (see Fig. 4.8). In addition to mask fabrication capabilities, successful implementation of RETs also requires sophisticated algorithms and computer-aided design tools. These include not only programs for pattern shape manipulation and layout optimization indispensable for alternating phase-shifting mask and optical proximity correction applications, but also mask inspection algorithms. Die-to-die inspection is sensitive to minor variations of phase structures; lack of resemblance between mask patterns and desired images poses challenges for die-to-database inspection. These CAD tools will be the liaisons that draw the two traditionally disparate disciplines of circuit design and manufacture closer.
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