Excimer Lasers, High-NA Optics Push Lithography

Michael D. Wheeler

BALTIMORE -- Developments in excimer lasers and high numerical aperture optics technology are pushing photonics into the forefront of semiconductor microlithography, where laser-created feature sizes have already dipped below 0.25 µm.
David Shaver of Massachusetts Institute of Technology's Lincoln Laboratory in Cambridge, Mass., told the CLEO '97 plenary audience that development of a generation of systems based on 193-nm ArF excimer lasers is under way. If they prove successful, feature sizes as small as 0.1 µm using phase-shift masking could become the industry standard.
To further reduce feature sizes, researchers must develop high numerical aperture optical elements and systems. High-end microlithography systems use optics with a numerical aperture of 0.6, but most systems still use optics with 0.5.

New materials
Researchers are also considering new optical materials for 193-nm systems and those based on lasers with even shorter wavelengths. These optical materials must not only exhibit initial transparency to ultraviolet light, but also be resistant to the changes that long-term exposure can create. For years, fused silica has been the choice of optical shops. An amorphous material, it is sensitive to short pulses. However, fused silica has its shortcomings. The Japanese are exploring an alternative material, CaF₂, and may be planning a system using all-CaF₂ optics, Shaver said.
Even if a suitable optical material is found, photonics and lithography system manufacturers must still produce a reliable, high-repetition laser, large-field steppers and new photoresists. "Having most of the pieces isn't really good enough," Shaver said.
These shortcomings have not stopped Shaver or others in the field from looking beyond 193-nm lithography. Extending the technology to use 157-nm lasers could produce feature sizes as small as 0.07 µm. That marks a 23 percent reduction in feature sizes from the 193-nm systems, enabling roughly 50 percent more features on a chip.
There is speculation about reducing the wavelength still further, to 126 nm, into the realm where even CaF₂ optics are reflective. Shaver has suggested that Al/MgF₂ coatings could solve that problem. Light sources in this region remain problematic for commercial application because they exhibit poor beam quality and are difficult to use, he said.