SCALPEL Projection Electron Beam for Sub-Optical Lithography

The performance of integrated circuits has been improving steadily for the past three decades. The linear dimension of the smallest feature on a chip decreases by nu2 every three years giving a two fold increase in density. This progress has been largely driven by progress in optical lithography. There have been improvements in lens design, fabrication, imaging materials, and the use of shorter wavelengths for the exposing radiation. Current generation chips with 350 nm minimum features are printed using 365 nm UV light (I-line) while the next generation will use 248 nm (250 nm features) likely followed by 193 nm deep-UV exposure. Further progress is limited then by opacity of lens and mask materials. It is unlikely that conventional transmission optical printing methods can be used at wavelengths below 193 nm or feature sizes much below 180 nm. Optical lithography runs into other problems at shorter wavelengths as well. Although resolution improves as lambda/NA, the depth of focus decreases as lambda/NA sub 2, greatly reducing process margins. At this time, the lithography choice for the 0.13microns generation is not clear.