Optical Projection Lithography Using Lenses with Numerical Apertures Greater Than Unity.

摘要

The characteristics and limitations of optical projection lithography are studied using high numerical-aperture (NA) optical microscopy lenses. With oil-immersion, NA's as large as 1.4 were realized, and linewidths as fine as 140 nm were achieved in 70 nm-thick commercial photoresist films using illumination of wavelength lambda = 453 nm. Previous researchers have also obtained deep-submicron linewidths using high NA lenses. The new elements in our work are: (1) use of a photoresist-compatible oil to achieve NA = 1.4; (2) use of modern antireflection coatings (ARC) and tri-level processing to circumvent the problem of substrate back-reflection; (3) use of NA > 1 optical-projection to make masks for x-ray lithography, and the x-ray replication of such masks. Our motivation in pursuing this work was to explore the limits of high-NA optical projection, and to develop a simple, quick-turn-around method of making sub-quarter-micron-linewidth x ray masks. Using the formula for minimum feature width Wmin = k lambda/NA, we find that with an ARC and careful focusing one can work at k about 0.43. Focusing and alignment are done by projecting the reticle image onto the resist with yellow light, which does not expose the resist. For exposure, the yellow filter is removed and a narrow-band blue filter (lambda = 453 nm) inserted. Reprints. (jhd)