Development of 157 Photoresist Materials

The purpose of this document is to provide timely public access to the 157 nm resist development work being done in our laboratories under contract from International SEMATECH. It is our goal and SEMATECH's goal to disseminate this information as rapidly and widely as possible in hope that doing so will accelerate the availability of 157 nm resist materials to the members of International SEMATECH.

Backbone Module 

The backbone subunit is essentially responsible for tethering the various subunits together into an actual polymer. It should carefully be selected to offer minimal impact upon the overall absorbance and mechanical properties of the final polymer. Choosing materials that are capable of imparting these characteristics to a polymer design while offering good transparency at 157 nm is a significant challenge. For example, polyethylene, although it is comprised entirely of methylene units, has been shown to be highly absorbing in the VUV spectral region (1). In order to identify alternative polymer structures that offer reduced absorbance at157 nm, researchers at MIT Lincoln Labs have surveyed the absorbance of various materials at 157 nm. The goal of their research is to determine structure-property relationships between functional groups and their associated absorbance contributions at 157 nm (2). Largely as a result of these material surveys, the details necessary for designing transparent materials for 157 nm photoresists are beginning to emerge.

From this research effort has come at least two very important trends that should be considered in the development of polymers for 157 nm photoresists. First, this research shows that although even the simplest hydrocarbon-based materials absorb significantly at 157 nm, these properties can be reduced through the appropriate use of electron-withdrawing groups such as fluorine or oxygen atoms along the polymer backbone. For example, Teflon AF (trademark of DuPont) which is essentially poly(tetrafluoroethylene), offers the highest transparency of any carbon-based material surveyed to this date. Poly(vinyl alcohol), another analog of polyethylene with electron-withdrawing hydroxyl groups located on alternating carbons along the backbone, has also been shown to be much less absorbing at 157 nm than polyethylene. The second trend regarding the transparency of materials at 157 nm that has emerged from the MIT Lincoln Labs surveys is that silicon-based polymers such as siloxanes and silesquioxanes are the most transparent materials at 157 nm surveyed thus far.

We have chosen to pursue both highly-fluorinated carbon-based materials as well as silicon-based materials for potential backbone structures. The following pages describe current progress in both of these areas.

Siloxane backbone materials

Flourinated hydrocarbon backbone materials

1)Palik, E. D., ed. “Handbook of Optical Constants of Solids” Academic Press, San Diego, CA., 1991, page 212.

2) a) Bloomstein, T. M., Horn, M. W., Rothschild, M., Kunz, R. R., Palmacci, S. T., Goodman, R. B. J. Vac. Sci. Technol. B. 1997, 15(6), 2112. b) Bloomstein, T. M., Rothschild, M., Kunz, R. R., Hardy, D. E., Goodman, R. B., Palmacci, S. T. J. Vac. Sci. Technol. B. 1998, 16(6), 3154. c) Kunz, R. R., Bloomstein, T. M., Hardy, D. E., Goodman, R. B., Downs, D. K., Curtin, J. E. Proc. Soc. Photo-Opt. Instrum. Eng. 1999, 3678, 13.

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