In conventional photolithography a photoresist is exposed to light through a mask. The photoresist is modified by the exposure in such a way that either the exposed or unexposed portions of the resist can be removed during subsequent development. Any photolithographic process has limitations, whereby there is a critical dimension below which features are too fine to be resolved. That resolution limit is a critical barrier in reducing the scale of integrated circuit devices.
Self-aligned double patterning is a technique for forming features having a finer pitch than would be possible by the direct application of a photolithographic process. Self-aligned double patterning involves forming a mandrel having line-shaped features. A spacer formation process is then used to form spacers on the sides of the mandrel features. The mandrel is then stripped leaving the spacers defining two sets of lines. A first set of lines corresponds to the mandrel. A second set of lines is formed between each adjacent pair of the mandrel's line-shaped features, the second set of lines being form between adjacent spacers.