Double patterning is a technique developed for photolithography to typically double the feature density. Double patterning may be used as early as 65 nm node and will be the predominant lithography technique for 32 nm node. It can in principle be extended to 22 nm node as well. Beyond 22 nm node will require more than double patterning; perhaps multiple patterning would become appropriate.
There are several types of double patterning. The three most common types are: double exposure, spacer mask, and intermediate pattern accumulation.
Double Exposure
Double exposure is a sequence of two separate exposures of the same photoresist layer using two different photomasks. This technique is commonly used for patterns in the same layer which look very different or have incompatible densities or pitches. Unfortunately, the sum of the exposures cannot improve the minimum resolution limit. This technique allows manufacturability of minimum pitch features in a layout that may contain a variety of features. The 65 nm node saw the introduction of alternating phase-shift masks (see photomask) in manufacturing. This technology is typically a double exposure approach.
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
A "brute force" approach, intermediate pattern accumulation involves a sequence of (at least) two separate exposures and etchings of independent patterns into the same layer. For each exposure, a different photoresist coating is required. When the sequence is completed, the pattern is a composite of the previously etched subpatterns. By interleaving the subpatterns, the pattern density can theoretically be increased indefinitely, the half-pitch being inversely proportional to the number of subpatterns used. This composite pattern can then be transferred down into the final layer.
Concerns
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.
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.