Multi Layer Resist
Multi-Layer Resist and Anti-Reflective Coating Stacks are the engineered optical and etch-transfer film stacks used in photolithography to control reflecitivity, improve CD uniformity, and enable pattern transfer from thin imaging layers to thick etch masks — where the combination of bottom anti-reflective coating (BARC), silicon-containing interlayer (SiARC), and photoresist forms a precisely tuned optical system that suppresses standing waves, eliminates reflective notching, and provides the etch selectivity chain necessary for high-fidelity pattern definition.
Why Anti-Reflective Coatings
- Without BARC: Light passes through resist → reflects off substrate → interferes with incoming light.
- Standing waves: Interference creates intensity oscillations in resist → CD variation with thickness.
- Reflective notching: At topography steps → reflected light undercuts resist → pattern distortion.
- BARC absorbs reflected light → no interference → uniform exposure → better CD control.
Stack Options
| Stack | Layers | Use Case |
|---|---|---|
| Single BARC | PR + BARC | Relaxed pitch (>60nm) |
| Bilayer | PR + SiARC + BARC | Mid-pitch (30-60nm) |
| Trilayer | PR + SiARC + SOC | Tight pitch (<30nm) |
| Quad-layer | PR + SiARC + SOC + CVD-C | Most advanced |
SiARC (Silicon Anti-Reflective Coating)
- Material: SiON or SiO₂-rich film, deposited by CVD or spin-on.
- Dual function: Anti-reflective (tuned n and k) + etch-transfer interlayer.
- Optical: n=1.6-1.9, k=0.1-0.5 at 193nm → absorbs reflected light.
- Etch: Contains silicon → resists O₂ plasma → serves as hard mask for SOC etch.
Optical Tuning
Incident light (193nm)
↓
[Photoresist] n=1.7, k≈0
↓
[SiARC] n=1.8, k=0.3 ← absorbs + impedance matches
↓
[SOC/BARC] n=1.5, k=0.5 ← absorbs remaining light
↓
[Substrate] (metallic or oxide)- Goal: Total bottom reflectivity < 1% → minimal standing wave effect.
- Tuning: Adjust n, k, and thickness of each layer → destructive interference for reflected light.
- Different substrates: Metal substrate (high reflectivity) needs different tuning than oxide substrate.
BARC Types
| Type | Deposition | Pros | Cons |
|---|---|---|---|
| Organic BARC | Spin-on | Low cost, good planarization | Develops during resist develop |
| CVD BARC (SiON) | PECVD | Precise thickness, no develop issue | Not planarizing |
| Graded BARC | CVD (variable composition) | Broadband anti-reflection | Complex process |
| Developer-soluble BARC | Spin-on | Removed during develop | Limited to specific resists |
Reflectivity Impact on CD
| Bottom Reflectivity | CD Variation (3σ) | Impact |
|---|---|---|
| 15% (no BARC) | ±8-12nm | Unacceptable |
| 5% (basic BARC) | ±3-5nm | Marginal |
| 1% (optimized stack) | ±1-2nm | Target |
| <0.5% (advanced) | <±1nm | Best achievable |
EUV-Specific Considerations
- EUV (13.5nm): Most materials are highly absorbing → BARC less critical.
- Thin resist (30-40nm): Standing waves less severe due to high absorption.
- Under-layer: Still needed for etch transfer, but optical BARC role reduced.
- New challenge: EUV flare and out-of-band DUV → may need DUV-specific BARC even for EUV.
Multi-layer resist stacks and anti-reflective coatings are the optical engineering foundation that makes high-resolution lithography reproducible — without precise reflectivity control through carefully tuned BARC and SiARC layers, CD variations from substrate reflectivity would make advanced patterning impossible, and without the etch-selectivity chain provided by multi-layer stacks, thin imaging resists could not transfer patterns into the thick films required for subsequent etch processing.