Immersion lithography

By using a liquid with a higher refractive index than air, immersion lithography allows for smaller features to be created on the wafer.

[1] Immersion lithography replaces the usual air gap between the final lens and the wafer surface with a liquid medium that has a refractive index greater than one.

Current immersion lithography tools use highly purified water for this liquid, achieving feature sizes below 45 nanometers.

The lenses in the highest resolution "dry" photolithography scanners focus light in a cone whose boundary is nearly parallel to the wafer surface.

However,[clarification needed] the depth of focus, or tolerance in wafer topography flatness, is improved compared to the corresponding "dry" tool at the same resolution.

[4] The idea for immersion lithography was patented in 1984 by Takanashi et al.[5] It was also proposed by Taiwanese engineer Burn J. Lin and realized in the 1980s.

[7] Defect concerns, e.g., water left behind (watermarks) and loss of resist-water adhesion (air gap or bubbles), have led to considerations of using a topcoat layer directly on top of the photoresist.

In immersion lithography, light travels down through a system of lenses and then a pool of water before reaching the photoresist on top of the wafer.
Throughput of immersion lithography tools vs. dose. The throughput vs. dose is compared to for different pulse powers at the same slit width.
Double patterning by pitch splitting. Double patterning by pitch splitting involves assigning adjacent features to different masks, indicated by different colors.
Triple patterning by pitch splitting. Triple patterning by pitch splitting involves assigning adjacent features to 3 different masks, using three colors.