Lesson 3 showed resolution has a floor set by wavelength, NA and a process factor that itself has a floor. Lesson 4 showed how OPC and phase-shift masks push against that floor for a single exposure. This lesson covers the two further levers the industry reached for when a single exposure, even a heavily RET'd one, still could not resolve a layer's target pitch: splitting the pattern across more than one exposure, and dropping wavelength outright with EUV.
Reading time: about 9 minutes.
5.1 Why split a pattern across two exposures
If a target pitch is finer than a single 193 nm immersion exposure can resolve even with RET, one option is to split the features into two interleaved sets, print each set with its own mask at a pitch the exposure tool can handle (each individual mask is at double the final pitch), and etch them into the substrate together so the combined result is the fine final pitch. This is multiple patterning, and it trades mask count and process steps for resolution the optics alone cannot deliver.
5.2 LELE: litho-etch-litho-etch
The most direct scheme is LELE (litho-etch-litho-etch): expose and etch the first set of features with mask A, then expose and etch the second, interleaved set with mask B, into the same film. Each individual exposure is at the coarser, resolvable pitch; the two etches together leave the fine final pitch in the film. LELE's main cost is overlay: mask A's features and mask B's features must land registered to each other to a small fraction of the final pitch, or the finished lines come out unevenly spaced. Lesson 6 covers overlay measurement and control directly, and LELE is the layer type that makes overlay budget tightest.
5.3 Self-aligned double and quadruple patterning
Self-aligned double patterning (SADP) sidesteps the overlay problem by not using a second mask to define the second set of features at all. Instead, a single, coarser-pitch mandrel pattern is etched first; a conformal spacer film is then deposited over the mandrels and etched back (an anisotropic spacer etch, covered in lesson 7) so that only sidewall spacers remain, one on each side of every mandrel. Removing the original mandrel leaves only the spacers, now at half the mandrel pitch, and twice as many lines as the original mask had. Because both spacers of a pair are defined by etching around the same mandrel edge, not by aligning a second independent exposure, SADP has no litho-to-litho overlay error between the two interleaved sets; the price is a much more involved film stack and etch sequence, and a specific geometric relationship: SADP naturally produces evenly spaced lines, so it is best suited to regular, grating-like layers, not arbitrary layouts. Self-aligned quadruple patterning (SAQP) repeats the spacer-and-mandrel-removal trick a second time, quartering the original mask's pitch.
| Aspect | LELE | SADP |
|---|---|---|
| Number of exposures | Two independent masks | One mandrel mask, no second exposure |
| Litho-to-litho overlay error | Present, tightest budget on the layer | None, both lines come from one mandrel |
| Layout requirement | Every feature must be colored to mask A or B | Regular, grating-like pattern only |
| Extra step needed | Coloring / decomposition | Cut mask, to turn the grating into real wiring |
5.4 What this means for physical design
For a LELE-type layer, the physical design and mask-data-preparation flows must assign every feature on the layer to mask A or mask B, a step called layout decomposition or coloring. Two features that are too close together to both be assigned the same color, yet too close to be split onto different masks without violating a minimum spacing rule between adjacent-mask features, create a coloring conflict: a layout pattern that multiple patterning simply cannot manufacture as drawn. Design rule checking for a multiple-patterned layer includes coloring-conflict detection for exactly this reason, and router and placer constraints on that layer (track assignment, spacing) are often shaped directly by what colors cleanly, not only by electrical rules. SADP and SAQP largely sidestep coloring, because they do not use two independent masks, but in exchange they constrain the layer to regular, grating-like patterns; cut masks (a separate exposure that selectively removes segments of the otherwise-regular SADP/SAQP grating to create the actual, non-regular wiring pattern a design needs) are then layered on top to turn the regular grating into a usable interconnect pattern.
5.5 EUV: shrinking wavelength instead
EUV lithography uses 13.5 nm light, generated by a laser-produced tin plasma, and an all-reflective optical system (no material transmits 13.5 nm light usefully, so EUV scanners use multilayer mirrors, not lenses). Lesson 3's worked example already showed why this matters: a wavelength more than an order of magnitude smaller than 193 nm resolves a much finer CD without paying the depth-of-focus penalty that pushing 193 nm NA higher would. EUV's practical benefit for multiple patterning is direct: layers that needed LELE or SADP at 193 nm immersion can often be patterned in a single EUV exposure, eliminating that layer's coloring problem and overlay budget entirely. High-NA EUV (scanners with NA above roughly 0.5, roughly double early-generation EUV's NA) pushes this further, resolving finer pitches still in a single exposure, though at a smaller field size per exposure (a consequence of the anamorphic optics high-NA EUV uses), which is itself a new constraint flowing back into how large a reticle field a design must fit within.
5.6 Interview questions
What problem does multiple patterning solve, in one sentence? It resolves a final pitch finer than a single exposure can achieve, by splitting the pattern across two (or more) exposures or process steps that are each individually within the optics' resolvable pitch.
Why does LELE need layout coloring, but SADP does not? LELE uses two independent litho exposures, so every feature must be assigned to one mask or the other (colored); SADP defines both interleaved features by etching a spacer around one mandrel, with no second independent exposure, so there is nothing to color.
What is a coloring conflict? A pair of features too close together to be assigned the same mask color, yet too close to be split onto different masks without violating the minimum same-color or different-color spacing rule, making the pattern unmanufacturable as drawn on that layer.
Why does SADP typically need a separate cut mask? SADP's spacer process naturally produces a regular, evenly spaced grating; a cut mask selectively removes segments of that grating to create the actual, non-regular wiring pattern the design needs.
Why does EUV reduce or eliminate the need for multiple patterning on some layers? Its much smaller wavelength resolves a finer pitch in a single exposure than 193 nm immersion can, so a layer that needed LELE or SADP coloring and overlay management at 193 nm can sometimes be patterned directly, with no coloring conflict and no litho-to-litho overlay at all.
Key takeaways
- Multiple patterning resolves a pitch finer than a single exposure can, by splitting the pattern across exposures or process steps that are each individually within the optics' resolvable pitch.
- LELE uses two independent exposures and needs the layout colored to avoid coloring conflicts; overlay between the two masks is its tightest budget.
- SADP/SAQP build interleaved lines from one mandrel using a spacer etch, with no litho-to-litho overlay, but produce a regular grating that needs a cut mask for a real wiring pattern.
- EUV's much smaller wavelength lets some layers pattern in a single exposure, removing both the coloring and overlay burden multiple patterning imposes.
- High-NA EUV pushes resolution further but at a smaller field size, a new constraint on how large a reticle field a design must fit within.