Exposure characteristics of high-resolution negative resists

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Abstract
Positive and negative resist systems are both essential in the microfabrication of experimental devices. While numerous positive resists have been shown to have high-resolution, negative resists were considered to have poor contrast and resolution. In this paper, the electron-beam exposure characteristics of several negative resist systems including epoxy novolac-type systems [K. J. Stewart, M. Hatzakis, J. M. Shaw, and D. E. Seeger, J. Vac. Sci. Technol. B 7, 1734 (1989); M. Hatzakis, K. Stewart, J. Shaw, and S. Rishton, Microelectron. Eng. 11, 487 (1990)] and two selected vendor resist systems have been investigated. These resist exposure characteristics are related to resist resolution, image profile, edge acuity, and process latitude. The sensitivity of various resists to slight exposures which may be caused by electron scattering in electron-beam lithography or mask diffraction in optical or x-ray lithography varies from resist to resist. The resist system that is least sensitive to these slight exposure effects offers the highest resolution. High-resolution images down to 0.25-μm lines and spaces were delineated only in resist systems with a gamma value higher than 2.5. Resist images of 50 nm dimensions in 0.4-μm-thick resist films have been obtained from the epoxy novolac system with vertical sidewalls and good edge acuity. In addition, results of high-voltage electron-beam exposures showed that residues between small shapes are substantially reduced. Similarly, cleanly resolved features were obtained when a high-contrast mask is used in x-ray synchrotron exposures.

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