Immersion Lithography
Immersion (193i) lithography extends 193 nm ArF deep-ultraviolet lithography by filling the gap between the final projection lens and the wafer with ultra-pure water. Because water's refractive index (n = 1.44) is far above that of air, the numerical aperture climbs past the dry limit of 1.0 \u2014 production 193i scanners reach NA 1.35 \u2014 so the Rayleigh resolution floor drops and mature DUV stays in volume manufacturing from the 45 nm node through multi-patterned 10 nm- and 7 nm-class layers.
where CD is the critical dimension, $k_1$ the process-dependent Rayleigh coefficient, $\lambda$ the exposure wavelength, and NA the numerical aperture: water shrinks the effective wavelength from 193 nm to about 134 nm, which is the entire resolution gain of 193i.
Immersion lithography is the 193 nm ArF patterning technique that puts ultra-pure water between the final projection lens and the wafer, raising the effective numerical aperture and extending deep-ultraviolet lithography far beyond its original dry-optics limit.
The trick is simple but demanding. Water has a higher refractive index than air, so the lens can collect a wider cone of light and print smaller features at the same 193 nm wavelength. Production immersion scanners reached numerical aperture around 1.35, turning ArF into the workhorse for 45 nm, 28 nm, and many multipatterned layers at more advanced nodes.
The scanner had to become a fluid-control machine. The water film must stay clean, bubble-free, temperature-stable, and confined under a rapidly moving lens and wafer stage. Any particle, bubble, or thermal disturbance can become a printable defect or an overlay error, so immersion lithography depends on fluid handling, stage control, resist compatibility, and metrology as much as on optics.
| Challenge | Immersion answer | Manufacturing tradeoff |
|---|---|---|
| Need smaller features at 193 nm | Raise numerical aperture with water | Tighter focus budget |
| Reflection and standing waves | Tune resist and anti-reflective coatings | More stack integration |
| Pitches below single-exposure limit | Use LELE, SADP, or SAQP | More masks and overlay risk |
| EUV transition cost | Keep ArF for support layers | Larger process menu |
Immersion still matters in an EUV fab. EUV prints the hardest layers, but most layers in an advanced process still use DUV or immersion because it is faster, cheaper, and mature. The leading-edge fab is not EUV instead of immersion; it is EUV plus a large installed base of immersion scanners used where they are economically better.