Immersion lithography offers an attractive alternative to 157-nm and EUV
lithography. Due to the tremendous obstacles associated with these exotic
technologies, immersion lithography has become the last technique for the
incremental extension of dioptric/catadioptric optical lithography, if not the last
technique for photon lithography per se.
The basic idea of immersion lithography is to fill up the air space between the
last element of the imaging lens and the surface of the photoresist on the wafer,
as shown in Fig. 7.1. With a higher-index medium, light of higher spatial
frequencies can be coupled to the resist, improving the resolution. Liquid
coupling also minimizes the reflection loss at the two affected interfaces. The
classical interpretation of the improvement of resolution is that NA â
n Â· sinÎ¸ is
increased with the change of refractive index n in the coupling medium. But, how
does one visualize it?