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[[Image:Trench doubling.svg|left|thumb| '''Double Expose, Double Etch (trenches):''' Photoresist coating over first pattern; etching adjacent to previous features; Photoresist removal]]
[[File:Pitch splitting.png|thumb|right|150px|'''Double patterning by pitch splitting.''' Double patterning by pitch splitting involves assigning adjacent features to two different masks, indicated by the different colors. It remains the simplest multiple patterning approach practiced today, and adds less cost than EUV.]]
[[File:Stitch Double Patterning.png|right|thumb|Sometimes, it is necessary to "stitch" two separately printed features into a single feature.]]▼
The earliest form of multiple patterning involved simply dividing a pattern into two or three parts, each of which may be processed conventionally, with the entire pattern combined at the end in the final layer. This is sometimes called ''pitch splitting'', since two features separated by one pitch cannot be imaged, so only skipped features can be imaged at once. It is also named more directly as "LELE" (Litho-Etch-Litho-Etch). This approach has been used for the 20 nm and 14 nm nodes. The additional cost of extra exposures was tolerated since only a few critical layers would need them. A more serious concern was the effect of feature-to-feature positioning errors (overlay). Consequently, the self-aligned sidewall imaging approach (described below) has succeeded this approach.
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This is best described by considering a process example. A first exposure of photoresist is transferred to an underlying hardmask layer. After the photoresist is removed following the hardmask pattern transfer, a second layer of photoresist is coated onto the sample and this layer undergoes a second exposure, imaging features in between the features patterned in the hardmask layer. The surface pattern is made up of photoresist features edged between mask features, which can be transferred into the final layer underneath. This allows a doubling of feature density.
▲[[File:Stitch Double Patterning.png|right|thumb|Sometimes, it is necessary to "stitch" two separately printed features into a single feature.]]
Sometimes, it is necessary to "stitch" two separately printed features into a single feature.<ref>S-Min Kim et al., Proc. SPIE 6520, 65200H (2007).</ref><ref>Y. Kohira et al., Proc. SPIE 9053, 90530T (2014).</ref><ref>[https://www.linkedin.com/pulse/application-specific-lithography-sense-amplifier-driver-chen-wnfuc/ Application-Specific Lithography: Sense Amplifier and Sub-Wordline Driver Metal Patterning in DRAM]</ref>
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