Optical Lithography
Resolution limits, wavelength progression, and immersion lithography
The Wavelength Progression
The Wavelength Progression
Lithography has continuously moved to shorter wavelengths to print smaller features:
| Generation | Wavelength | Light Source | Min Feature |
|---|---|---|---|
| g-line | 436 nm | Mercury lamp | ~350 nm |
| i-line | 365 nm | Mercury lamp | ~250 nm |
| KrF | 248 nm | Excimer laser | ~130 nm |
| ArF | 193 nm | Excimer laser | ~65 nm |
| ArF immersion | 193 nm | Excimer laser + water | ~38 nm |
| EUV | 13.5 nm | Tin plasma | ~8 nm |
Immersion Lithography
Immersion Lithography
A breakthrough innovation: placing ultra-pure water between the final lens and the wafer. Since water has a refractive index of 1.44, this increases the effective NA from ~0.93 to 1.35, improving resolution by ~45%.
ArF immersion lithography (ArFi) has been the workhorse technology for nodes from 45nm to 7nm (with multi-patterning). Key challenges:
- Maintaining a stable water puddle on a fast-moving wafer (500 mm/s scan speed)
- Preventing bubbles and contamination in the water
- Controlling defects from water droplets left behind
Key Concept: Multi-Patterning
When features became smaller than ArFi's single-exposure resolution limit (~38nm pitch), the industry developed multi-patterning — exposing multiple times with shifted patterns to achieve tighter pitches. SADP (Self-Aligned Double Patterning) effectively doubles pattern density.
Knowledge Check
Knowledge Check
1 / 2What is the wavelength used in ArF immersion lithography?