subtractive metal etch
**Subtractive Metal Patterning for Interconnects** is the **alternative to the damascene process that patterns metal lines by first depositing a blanket metal film, then etching away unwanted metal through a patterned hard mask** — reversing the traditional damascene flow (etch trench in dielectric → fill with metal → CMP) and offering advantages in metal grain structure, void-free lines, and compatibility with barrier-free metals like ruthenium and molybdenum at the tightest pitches where damascene fill becomes unreliable.
**Damascene vs. Subtractive**
```
Damascene (current): Subtractive (emerging):
1. Deposit dielectric 1. Deposit blanket metal
2. Etch trenches 2. Pattern hard mask
3. Deposit barrier + seed 3. Etch metal
4. Electroplate Cu fill 4. Deposit dielectric fill
5. CMP planarize 5. CMP planarize
```
**Why Subtractive Is Returning**
- History: Aluminum interconnects (pre-2000) used subtractive etch.
- Cu replaced Al: Cu is very difficult to etch → damascene became standard.
- Now: At sub-20nm pitch, damascene has severe fill challenges:
- Narrow trenches (10-12nm) → difficult to fill without voids.
- Barrier consumes too much of trench width.
- Grain structure is poor in narrow trenches → high resistivity.
- Subtractive: Blanket film → excellent grain structure → etch to define lines.
**Subtractive Metal Candidates**
| Metal | Etchable? | Etch Chemistry | Advantage |
|-------|----------|---------------|----------|
| Ru (ruthenium) | Yes | O₂-based plasma | No barrier needed |
| Mo (molybdenum) | Yes | Cl₂/O₂ plasma | Low resistivity, no barrier |
| W (tungsten) | Yes | SF₆/Cl₂ plasma | Established etch process |
| Cu (copper) | Not well | No volatile Cu halides | Still needs damascene |
| Co (cobalt) | Marginal | Cl₂-based | Possible but challenging |
**Subtractive Ru Process Flow**
1. Deposit 15-30nm blanket Ru by ALD or CVD → large grain, low resistivity.
2. Deposit hard mask (SiO₂ or TiN) → pattern with EUV lithography.
3. Plasma etch Ru: O₂ + Cl₂ plasma → RuO₄ (volatile) + RuCl₃ etch products.
4. Strip hard mask.
5. Deposit conformal dielectric (SiO₂ or low-k) between Ru lines.
6. CMP to planarize dielectric.
**Advantages of Subtractive Approach**
| Property | Damascene Cu | Subtractive Ru/Mo |
|----------|-------------|-------------------|
| Metal grain size | Small (confined growth) | Large (blanket film) |
| Void/seam risk | High at narrow pitch | None (solid film) |
| Barrier needed | Yes (TaN, 2-3nm) | No (Ru/Mo don't diffuse) |
| Effective metal width | Width - 2×barrier | Full width |
| Resistivity at 10nm width | ~10 µΩ·cm (Cu + barrier) | ~10-13 µΩ·cm (barrier-free Ru) |
| CMP challenge | Metal CMP (Cu dishing) | Dielectric CMP (easier) |
**Challenges**
| Challenge | Issue | Status |
|-----------|-------|--------|
| Metal etch damage | Plasma can damage sidewalls → roughness | Optimizing etch chemistry |
| Dielectric gap fill | Must fill high-AR gaps between metal lines | Flowable CVD, SOD |
| Line edge roughness | Etch transfers LER from mask → resist | Hard mask optimization |
| Metal redeposition | Etch byproducts redeposit on sidewalls | In-situ clean + chemistry tuning |
| Industry inertia | 20+ years of damascene infrastructure | Gradual adoption |
**Semi-Damascene (Hybrid)**
- Vias: Damascene (etch hole in dielectric → fill with metal).
- Lines: Subtractive (pattern metal by etch).
- Combines advantages: Damascene vias (proven) + subtractive lines (better grain, no barrier).
- Intel's approach for future nodes: Semi-damascene with Ru or Mo lines.
Subtractive metal patterning is **the return of an old paradigm enabled by new metals** — what was abandoned when aluminum gave way to copper is now being revived for ruthenium and molybdenum, where the ability to etch these metals and their freedom from barrier requirements make subtractive patterning the most promising path to achieving viable metal resistance at sub-20nm interconnect pitches where damascene copper has reached its fundamental scaling limits.