What is the resolution limit for immersion lithography?

Resolution Limit for Immersion Lithography

Immersion lithography is an advanced technique used in semiconductor manufacturing to create patterns on silicon wafers with high precision. This method enhances the resolution beyond the conventional limits of photolithography by introducing a liquid with a high refractive index between the optics and the wafer.

Resolution Formula

The resolution of any lithographic process is given by the Rayleigh criterion, which is expressed as:

R = k1 × λ / NA

where R is the resolution, k1 is a process-dependent coefficient, λ is the wavelength of the light used, and NA is the numerical aperture of the lens.

Immersion Lithography Advantage

In immersion lithography, a fluid—typically purified water—is introduced to the space between the final lens and the wafer. Water has a higher refractive index (about 1.44 at 193 nm) than air, which effectively reduces the wavelength of the light in the medium and increases the numerical aperture of the lens system. This yields a smaller R, thus improving the resolution.

Limiting Factors

The resolution limit is affected by not only the optical system but also by the photoresist material's ability to accurately replicate small features. As features sizes approach several nanometers, other limiting factors come into play, such as line edge roughness (LER) and photon shot noise. Moreover, the immersion fluid's ability to maintain purity and resist contamination is crucial, affecting long-term resolution capabilities.

Current Resolving Capability

With immersion lithography currently operating at 193 nm (ArF laser), and considering high-resolution photoresists and immersion fluids, the resolution limit typically reaches well below 40 nm. Leading-edge systems work with k1 values close to 0.3, allowing for the production of devices with feature sizes down to around 10 nm when using advanced techniques like multiple-patterning.

Future Prospects

While immersion lithography has successfully pushed the boundaries of resolution, ongoing research aims to reduce k1 further or develop new high-index immersion fluids and photoresists. Efforts are also focusing on the advent of extreme ultraviolet (EUV) lithography, which uses significantly shorter wavelengths to achieve even finer resolutions that are difficult to reach with current immersion lithography technologies.