self aligned multiple patterning

**Self-Aligned Multiple Patterning (SAMP)** is the **lithographic pitch-halving technique that achieves feature pitches below the resolution limit of the lithography scanner — using sacrificial spacer deposition on sidewalls of mandrel (core) structures, then removing the mandrel to leave the spacers as the pattern, effectively halving the pitch with each application: SADP (Self-Aligned Double Patterning) halves pitch once, SAQP (Self-Aligned Quadruple Patterning) halves it twice, enabling 10-20 nm pitch patterns using 193 nm immersion lithography at the 10/7/5 nm nodes before EUV adoption**. **SADP (Self-Aligned Double Patterning)** Achieves pitch = ½ of lithographic pitch. 1. **Mandrel Patterning**: Lithography + etch creates mandrel (core) lines at the lithographic pitch (e.g., 80 nm pitch, 40 nm CD). 2. **Spacer Deposition**: Conformal film (SiO₂ or SiN, 10-20 nm) deposited by ALD over the mandrels. The spacer thickness = desired final feature CD. 3. **Spacer Etch (Etchback)**: Anisotropic etch removes spacer from horizontal surfaces (top of mandrel, field), leaving spacer only on vertical sidewalls. 4. **Mandrel Pull (Core Removal)**: Selectively remove mandrel material, leaving free-standing spacer lines on both sides of where the mandrel was. 5. **Result**: Spacer lines at half the original pitch (40 nm pitch from 80 nm lithographic pitch). Feature CD = spacer thickness (set by ALD, not lithography). **SAQP (Self-Aligned Quadruple Patterning)** Achieves pitch = ¼ of lithographic pitch by applying SADP twice: 1. **First SADP**: Creates spacers at ½ pitch. 2. These spacers become mandrels for a second round. 3. **Second SADP**: New spacers formed on the first spacers, then first spacers removed. 4. **Result**: Features at ¼ of the original lithographic pitch (e.g., 20 nm pitch from 80 nm lithographic pitch). SAQP was used extensively at the 7 nm node for metal layers (M1-M3/M4) before EUV was available. **Key Properties** - **Self-Aligned**: Feature placement is determined by the conformal spacer deposition, not by a second lithography alignment. Eliminates overlay error between adjacent features. - **CD = Spacer Thickness**: ALD thickness control (~0.5 Å uniformity) provides much tighter CD uniformity than lithographic CD control. This is a key advantage of spacer-based patterning. - **Pitch Rigidity**: All features are at the same pitch — the technique cannot create arbitrary mixtures of different pitches. Design rule restriction: tracks must be on a regular pitch grid. - **Cut Masks**: After SAMP creates the regular array of lines, separate lithography steps ("cuts") remove unwanted line segments to create the desired circuit pattern. **Process Complexity** SAQP requires ~15 process steps per layer (deposition, etch, strip × multiple rounds) vs. ~3 for a single litho-etch step. At the 7 nm node, M1-M2 using SAQP added significant process cost and cycle time. EUV lithography (single exposure down to ~13 nm half-pitch) replaced SAMP for most critical layers at 5 nm and below, reducing process steps by 5-10× per layer. **SAMP + EUV Complementarity** Even with EUV, SAMP may return at High-NA EUV nodes or for the tightest pitches: - High-NA EUV single exposure: ~8 nm half-pitch (16 nm pitch). - Below 16 nm pitch: SADP + EUV = ~8 nm pitch. - SAMP provides the self-aligned precision while EUV provides the initial higher-resolution mandrel. Self-Aligned Multiple Patterning is **the pitch-multiplication technique that extended 193 nm lithography far beyond its native resolution** — a creative process engineering solution that achieves patterning precision set by atomic layer deposition rather than optical resolution, enabling two additional generations of logic scaling before EUV lithography was ready for volume production.