1 January 1988 Studies Of Ultrathin Polymer Films For Lithographic Applications
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The advantages of minimizing polymer resist thickness to the macromolecular dimensions are manifold. In optical lithography, ultrathin resists can improve exposure and focus latitude and alleviate the problem of absorption in conventional resists for deep ultra-violet (UV) exposure. In electron beam lithography it is desirable to reduce the resist thickness to minimize forward electron scattering in the resist, and to allow penetration when using the scanning tunneling microscope (STM) as a very low voltage exposure source. Moreover, we can better understand many phenomena such as the nature of the pinholes and etching resistance, which are still puzzles for the thicker films. Here we describe the preparation and characterization of resists of ultrathin polymer films (10 nm to 200 nm) formed by both the Langmuir-Blodgett (LB) and spin-cast techniques. The characterization techniques include fluorescence spectroscopy, electron beam exposure and development, and etch resistance. The fluorescence spectroscopy results of spin cast polystyrene films suggest a dramatic increase in the ordering of the molecular structure as the film thickness is reduced below 0.2 μm; this increase is most marked for materials of large molecular weights. This suggested that the lithographic properties and the defect (pinhole) density of such films might be significantly different from the results expected by extrapolation from thicker (>200 nm) films. Ultrathin poly(methylmethacrylate) (PMMA) films (thinner than 20 nm) prepared by LB and spin-cast techniques and novolac resists prepared by the spin-cast technique were explored as electron beam resists, using a Perkin Elmer MEBES I pattern generation system for exposure. The results have demonstrated the resolution and etch resistance capabilities of such films for patterning 0.1 μm features in 50 nm of chromium film. The most surprising result has been that the pinhole density in these films has been far lower than previously expected levels.
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S. W. J. Kuan, S. W. J. Kuan, C. C. Fu, C. C. Fu, R. F. W. Pease, R. F. W. Pease, C. W. Frank, C. W. Frank, "Studies Of Ultrathin Polymer Films For Lithographic Applications", Proc. SPIE 0920, Advances in Resist Technology and Processing V, (1 January 1988); doi: 10.1117/12.968342; https://doi.org/10.1117/12.968342

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