As lithography is extended to 157 nm, the molecular absorptivity becomes high for most organic polymers. Polymer photochemistry depends on photon absorption, and the higher energy associated with 157 nm light should lead to higher quantum yields of photoproducts. Polymers representative of those commonly employed in 193 or 248 nm resists were selected for this study. A gel permeation chromatography based method was developed to determine quantum yields for chain scission and crosslinking on thin polymers films coated on silicon wafers. This method was applied to determine the ΦS and ΦX of a number of lithographically significant homopolymers and copolymers at both the 157 and 248 nm wavelengths. It was found that polymers containing hydroxystyrene only undergo crosslinking while acrylate and methacrylate polymer only undergo chain scission. The film loss of 157 nm exposed poly-t-butyl acrylate and polymethyl methacrylate was found to be very high and attributed primarily to side chain cleavage of the esters, while no film loss of polyhydroxystyrene was detected. The analysis of outgassing materials showed that ester elimination in poly-t-butyl acrylate was responsible for all outgassed products and that the sum of the quantum yields of all outgassed products exceeded one, implying a reaction mechanism that recycled the initially produced radical. Direct polymer photolysis is significant at 157 nm and must be considered in resist design given the relatively high absorbance of most organic molecules at 157 nm.
This paper outlines the critical issues facing the implementation of 157-nm lithography as a sub-100-nm technology. The status of the present technology for mask materials, pellicles, optical materials, coatings, and resists is presented.
Optical lithography at ultraviolet (UV) wavelengths is the standard process for patterning 90-nm state-of-the-art devices in the semiconductor industry, and extensions to 45 nm and below are currently being explored. With such high resolution, the inherent high throughput of optical lithography will enable the development of a broad range of applications beyond semiconductor electronics. In this article, we will review progress toward nanopatterning with UV light in a variety of materials and geometries.The common thread is the use of short wavelengths, 193 nm or 157 nm, coupled with immersion to further reduce the effective wavelength. Densely spaced, 32-nm (and even smaller) features have been patterned, facilitating the future preparation of large-area, deeply scaled microelectronics, nanophotonics, nanobiology, and molecular-scale self-assembly.
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