Immersion Lithography is the resolution-enhancing technique that places a thin layer of ultra-pure water between the projection lens and the wafer — increasing the numerical aperture (NA) from 0.93 (dry) to 1.35, reducing the minimum printable feature size by ~30%, and enabling patterning of features down to ~38 nm half-pitch at 193 nm wavelength, which was the key technology that extended DUV lithography through the 7nm node.
How Immersion Improves Resolution
- Rayleigh resolution: $CD_{min} = k_1 \times \frac{\lambda}{NA}$
- NA (dry) = n_air × sin(θ) = 1.0 × sin(θ) → max NA ~0.93.
- NA (immersion) = n_water × sin(θ) = 1.44 × sin(θ) → max NA ~1.35.
- Resolution improvement: 0.93 → 1.35 = 31% smaller features.
Immersion Fluid
| Property | Requirement | Why |
|---|---|---|
| Refractive index at 193 nm | 1.44 | Higher NA than air (n=1) |
| Absorption at 193 nm | < 0.05 /cm | Must not absorb exposure light |
| Purity | Semiconductor grade | No particles, dissolved gases |
| Temperature stability | ±0.01°C | n(T) changes → focus error |
| Compatibility | No resist interaction | Must not swell or dissolve resist |
- Only ultra-pure water (UPW) meets all requirements at 193 nm.
- Higher-n fluids (n > 1.6) were researched but never adopted due to absorption and contamination issues.
Scanner Implementation
- Water confined between lens and wafer by immersion hood — meniscus formed by surface tension.
- Wafer moves at high speed (700+ mm/s) under the water puddle — no air bubbles allowed.
- Water flow rate: 200-500 mL/min — continuously refreshed.
- Watermark defects: If water residue remains on resist after exposure → causes pattern defects.
Immersion-Specific Defects
| Defect | Cause | Mitigation |
|---|---|---|
| Watermark | Water droplet residue on resist | Topcoat, fast wafer drying |
| Bubble | Air trapped in water → exposure gap | Degassed water, flow optimization |
| Immersion particle | Particle in water → prints on wafer | Filtration, water quality monitoring |
| Resist leaching | Resist components dissolve into water | Topcoat barrier, resist formulation |
Topcoat
- Thin hydrophobic coating applied over photoresist.
- Prevents resist-water interaction (leaching) and reduces watermark defects.
- Must be transparent at 193 nm and removable during develop step.
- Some advanced resists are topcoat-free — built-in hydrophobic surface.
Immersion in Technology Nodes
- 45-32nm: Single patterning with immersion.
- 22-14nm: Immersion + double patterning (SADP/LELE).
- 10-7nm: Immersion + quadruple patterning (SAQP) — extremely complex.
- 5nm and below: EUV replaced most immersion multi-patterning layers.
- Immersion still used at 3nm/2nm for non-critical layers where EUV is not needed.
Immersion lithography is one of the most impactful innovations in semiconductor history — by simply putting water between the lens and wafer, it extended 193 nm optical lithography across five technology nodes, delaying the need for EUV by over a decade and enabling the chips that power today's smartphones and data centers.
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