FinFET 2010s Pattern Fin with Sidewall Image Transfer Double Patterning

# FinFET (2010s): Step 4 — Pattern the Fin With Sidewall-Image-Transfer Double Patterning

## 1. Why the Scanner Alone Cannot Print the Fin Pitch

Step 3 fixed the fin's governing dimensions; actually placing dense arrays of 8-10nm-wide fins at the roughly 40-60nm pitches early FinFET nodes required went beyond what contemporary 193nm immersion scanners could resolve directly as a single exposure. Rather than wait for a shorter-wavelength scanner, the industry adopted a patterning technique that multiplies the pitch a given scanner can achieve without changing the scanner itself.

## 2. Sidewall-Image-Transfer, Step by Step

A sacrificial mandrel line is first patterned by the scanner at a pitch twice as coarse as the final fin pitch. A conformal film is then deposited over the mandrel and etched back, leaving only sidewall spacers standing on either side of each mandrel line. The mandrel itself is removed, leaving twice as many fin-defining lines, at half the original pitch, as the scanner's own resolution ever printed — a spacer deposition and an etch-back step, not the lithography, is what actually sets the fin pitch.

## 3. Self-Aligned Double and Quadruple Patterning

Running the sidewall-spacer process once (self-aligned double patterning, SADP) doubles the mandrel's pitch; running it twice in sequence (self-aligned quadruple patterning, SAQP) quadruples it, letting later FinFET generations reach sub-30nm fin pitches from scanners that could never print that pitch unassisted. Each additional spacer cycle buys pitch reduction at the cost of tighter overlay and line-edge-roughness budgets, since errors from every prior cycle accumulate into the final fin pattern.

## 4. What This Step Does and Does Not Cover

This step covers how the fin pitch is defined; it does not yet address how the fin's own height and width are made uniform once patterned and etched, which Step 5 takes up as a distinct variability concern.

The Spacer, Not the Scanner, Sets the Final Fin Pitch mandrel pitch from lithography versus the finished fin pitch after spacer-based multiplication mandrel pitch as printed directly by the scanner (coarse, well within lithographic resolution) finished fin pitch after one or two self-aligned spacer cycles (SADP / SAQP) each spacer cycle halves the pitch the scanner itself never had to resolve process generation advances left to right across this chart the scanner prints the mandrel; the spacer deposition and etch-back set the fin's real pitch SADP doubles pitch once; SAQP repeats the cycle to quadruple it for the tightest fin pitches
Sidewall-Image-Transfer, in Three Moves how a coarse mandrel becomes a dense fin array without a finer scanner MANDREL (scanner) sacrificial line patterned at twice the final fin pitch — lithography's actual contribution SPACER DEPOSIT + ETCH conformal film etched back leaves sidewall spacers on both sides of each mandrel MANDREL REMOVED spacers alone remain as the fin-defining lines, at half the original printed pitch SAQP repeats this sequence once more for sub-30nm fin pitches at later FinFET nodes

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