For fifty years, Moore’s Law has relied on advances in lithography to propel the semiconductor industry forward. Optical lithography has achieved a 250x improvement in resolution over that time, all while keeping the cost to print a square centimetre of silicon about the same. But current lithography tools have been stuck at a minimum resolution of 40 nm for the last eight years, necessitating expensive multiple patterning to break this limit. As a result, patterning costs are rising faster than they ever have, putting the continuation of Moore’s Law in doubt.
Extreme ultraviolet lithography was supposed to fix this. Its lower wavelength enables sub-20-nm resolution, beyond the capabilities of double patterning with 193-nm immersion lithography. But resolution is not enough to save Moore’s Law – it must be cost-effective as well, while providing the same or higher manufacturing yields. And this is where EUV continues to fall short.
There are three technical roadblocks to a successful introduction of EUV lithography into manufacturing, and each of these plays into its economic potential. The first is mask defectivity. A defect on a wafer can kill one chip, but a defect on the mask can kill every chip. So every mask must be made and kept defect free. We don’t know how to do this yet for EUV masks. Mask blanks, the starting material for making masks, have too many defects. We’re not good at finding every defect that might exist on a mask, and not all of the defects we find can be repaired. And once a mask is made, we don’t yet have a working pellicle (a transparent wrapper that keeps defects away from the mask while in use) to ensure the mask stays defect free.
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