Litho-etch-litho-etch (LELE) is a form of double patterning. LELE is also called pitch splitting. In LELE, two separate lithography and etch steps are performed to define a single layer, thereby doubling the pattern density. Initially, this technique separates the layouts that cannot be printed with a single exposure, forming two lower-density masks. Then, it uses two separate exposure processes. This, in turn, forms two coarser patterns. They are combined and superimposed, which enables a single finer image on the wafer.
LELE imposes new layouts, physical verification and debug requirements on the designer. For example, on the design side, the mask layers are assigned colors, based on spacing requirements. The mask layers are split, or decomposed, from the original drawn layout into two new layers.
One key methodology decision is as basic as whether or not you want the designers to see colors at all—called a “colorless” design flow. The alternative is a two-color flow, in which the designer tapes out two masks, choosing one of several decomposition options. Of course, there are trade-offs with any design flow.
At 20nm, foundries are using several different double patterning design flows. One of the more common flows does not actually require the design team to decompose their layers into two colors. However, there are situations in which the designer may want to know what the color assignments will be. As reasonable as this may sound, seeing the double patterning colors will most likely degrade debug productivity.