Abstract We have studied the characteristics and limitations of optical projection lithography 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 λ = 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 W min = kλ/NA, we find that with an ARC and careful focusing one can work at k ∼ 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 (λ = 453 nm) inserted.
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