Proceedings PaperPhase-shifting and other challenges in optical mask technology
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Optical lithography has benefitted from progresses in wavelength reduction, lenses, resist systems, alignment, focussing, table accuracy, and insute metrology. As a result, the minimum feature size of integrated circuits has been reduced through many generations from 2 micrometers to 0.5 micrometers in manufacturing. There are many opportunities in improving the mask to help continue the progress. The image contrast can be restored by reducing stray light with mask anti-reflection coating at the absorber-air interface. Pre-distorting the mask against the distortion of the imaging lens can drastically improve the overlay performance. Adjusting the gray level or the feature size individually according to the pattern proximity environment can create a larger common exposure-defocus window for different feature shapes. Introducing phase shifts in the mask can simultaneously improve resolution and depth of focus, with the potential of a two-generation improvement with any given projection imaging equipment, provided the overlay capability is upgraded accordingly. In addition to describing and comparing these opportunities, the phase-shifting technology is given a special focus on the working principle, the different approaches to phase shifting, their imaging characteristics in terms of exposure-defocus diagrams, a systems view, and the scope of its development.