Resist Characterization at Soft X-Ray Wavelengths

Abstract

The characterization of resist materials for soft X-ray projection lithography (SXPL) in the 75-350 Å spectral region is an important issue, although it has received little attention to date. With the recent demonstration of diffraction-limited SXPL at 140 Å[1], the need to optimize resists at specific exposure wavelengths for future SXPL commercialization has become acute. Near 300 Å resist absorbance is so large that a single-layer resist strategy does not appear feasible, whereas near 80 Å it does. At intermediate wavelengths, for example where Mo/Si multilayer reflective coatings are efficient, it is not yet possible to determine whether a surface imaging or single-layer method is optimum. To estimate the maximum developed resist depth attainable at a given wavelength, the appropriate mass absorption coefficients may be used to calculate film absorbance. The accuracy of these estimates is questionable, however, especially at longer wavelengths where solid state effects can be pronounced. We have undertaken a study of single-layer resists in the spectral region relevant to SXPL and have characterized the sensitivity, contrast, and absolute absorbance of poly methylmethacrylate (PMMA), polysilane, and diazonapthoquinone/novolak resists. In addition, we have evaluated the lithographic performance of these materials at an exposure wavelength of 140 Å using an SXPL instrument illuminated by a laser plasma source (LPS) of high average power.