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throughput of double patterning |
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==Spacer mask==
A spacer is a film layer grown on the sidewall of a pre-patterned feature. A spacer is formed by deposition of the film on the previous pattern, followed by etching to remove all the film material on the horizontal surfaces, leaving only the material on the sidewalls. By removing the original patterned feature, only the spacer is left. However, since there are two spacers for every line, the line density has now doubled. The spacer technique is applicable for defining narrow gates, for example.
==Intermediate pattern accumulation==
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With double patterning, the two main immediate concerns are reduced throughput and greater sensitivity to overlay error. The two concerns are an apparent tradeoff. Generally, lithography tool manufacturers (notably ASML, Canon and Nikon) have always maintained a trend of improvement in both areas, but double patterning elevates the priority.
It is often claimed that throughput is reduced by a factor of two by double patterning, but this is actually incorrect. Since a layer is split into two layers, the wafer effectively sees one extra layer of processing added to a sequence already containing many layers. Since double patterning is only applied to at most a few critical layers, the overall throughput of a wafer is affected by a small amount.
A third concern is increased cost due to higher consumption of materials and tool time, as well as new hardware and processes associated with double patterning.
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