Turning an immersion litho “defect” into a double patterning “feature”

Polarization effects may allow double patterning by frequency doubling with a multilayer resist stack

by Bob Haavind, Editorial Director, Solid State Technology

Immersion lithography introduced a new polarization problem, because of contrast differences for the TE and TM modes of the exposure light. But a paper presented by Bruce Smith, et. al., of the Rochester Institute of Technology at the recent SPIE Advanced Lithography conference suggested the intriguing possibility of turning this “defect” into a “feature” by using the effect to do double patterning with frequency doubling imaging. The technique might provide a single-exposure alternative to double exposure/double patterning schemes at future nodes.

While polarized light works fine with dry lithography, immersion litho has favored transverse electric (TE) polarized illumination for high-NA patterning, because the image modulation from the transverse magnetic (TM) component falls off rapidly at large angles. In fact, the RIT group finds, by using large oblique angles for imaging while controlling the resist/substrate reflectivity, TM polarization can be employed. Smith showed that by putting a highly reflective film under the resist and using TM illumination, the lines and spaces of an alternating line-space pattern were reversed compared to TE polarized light.

It should be possible, the RIT researchers suggested, to use a multilayer resist stack (doing TE and TM imaging at different large angles) to perform a double exposure in one resist stack, rather than going to the double exposure/double patterning schemes now being devised for sub-45nm lithography. They devised a method for frequency doubling in a single exposure at large angles using the reflective component at the resist/substrate interface for their experiments. Maximum contrast was achieved at 45° angles. Using conventional polyacrylate 193nm resist, the RIT group was able to achieve 20nm half-pitch pattern resolution with a 1.20NA, water immersion exposure tool, and they also showed results for 22nm half pitch at 1.05NA. A resist stack was used with light areas in one layer and dark areas in another layer. There were 30-40nm thick regions in the resist, and the exposure was not too sensitive to thickness variations.

Smith explained that while the technique appears to work well for lines in two directions, it did not work so well for elbows. — B.H.

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