Spin on Carbon
Spin-On Carbon (SOC) and Trilayer Resist Stacks are the organic planarizing films and multi-layer patterning stacks used in advanced lithography to achieve the etch selectivity, pattern transfer fidelity, and topography planarization that single-layer photoresist cannot provide — where the trilayer stack (SOC + SiON/SiO₂ + photoresist) enables high-aspect-ratio pattern transfer into thick underlying films by distributing the imaging and etch-mask functions across separate optimized layers.
Why Trilayer Stacks
- Single-layer resist: Must simultaneously image pattern AND serve as etch mask.
- At advanced nodes: Resist is thin (30-50nm for EUV) → insufficient etch budget to transfer pattern.
- Trilayer: Thin resist images pattern → transfer to SiON hardmask → thick SOC serves as etch mask.
- Each layer optimized for its function → better overall performance.
Trilayer Stack Structure
Pattern Transfer Sequence
1. Expose and develop: Pattern in photoresist (lithography).
2. Transfer to SiON: Fluorine-based etch (CF₄/CHF₃) → removes SiON where resist is open.
3. Transfer to SOC: Oxygen-based etch (O₂/CO₂) → removes SOC where SiON is open.
4. Transfer to target: Use thick SOC as etch mask → etch target film.
5. Strip SOC: O₂ plasma ashes remaining SOC.
Etch Selectivity Chain
| Step | Etch Chemistry | Selectivity |
|---|---|---|
| Resist → SiON | CF₄/CHF₃ | Resist:SiON ~2:1 |
| SiON → SOC | O₂/CO₂ plasma | SiON:SOC ~10:1 |
| SOC → Target | Target etch chemistry | SOC:Target ~3-5:1 |
- Each layer is selected for high selectivity to the layer below.
- Total amplification: 30nm resist → patterns 200nm SOC → etches 500nm+ target film.
Spin-On Carbon Properties
| Property | Requirement | Typical Value |
|---|---|---|
| Carbon content | High (for O₂ etch mask) | >80% |
| Planarization | Flat surface over topography | >95% |
| Thermal stability | Survive SiON deposition temperature | >400°C |
| Optical properties (n, k) | Tuned for BARC function | n=1.5-1.8, k=0.1-0.5 at 193nm |
| Adhesion | Good to substrate and SiON | No delamination |
| Strippability | Clean removal after etch | O₂ plasma, full removal |
Planarization Function
- Topography from underlying layers: Metal lines, contacts → uneven surface.
- Spin-on: Liquid fills valleys, planarizes → flat surface for lithography.
- Without planarization: Focus variation across field → CD non-uniformity.
- SOC inherently planarizes due to fluid spin-coating → no CMP needed.
SOC vs. CVD Carbon
| Property | Spin-On Carbon | CVD Amorphous Carbon |
|---|---|---|
| Deposition | Spin coat | PECVD |
| Thickness uniformity | Depends on pattern | Excellent |
| Planarization | Good (fluid) | None (conformal) |
| Carbon content | 80-90% | >95% |
| Etch selectivity | Good | Excellent |
| Throughput | High | Lower |
| Use case | General patterning | Critical etch mask |
Spin-on carbon and trilayer resist stacks are the patterning architecture that bridges the gap between thin imaging resist and thick etch masks — by decomposing the conflicting requirements of lithographic imaging (thin film) and etch resistance (thick film) into separate optimized layers connected by high-selectivity etch transfers, trilayer stacks enable the pattern transfer fidelity required at every advanced CMOS node from 14nm through to the latest EUV-based technologies.