I found Senior Editor Paul Nesdore’s editorial titled “DUV Lithography Pushes Contamination Issues” (December 2002) simultaneously interesting and somewhat confusing. If I may, I’d like to offer a few observations.
First, I believe most would agree that the adverse effect of organic contamination on both DUV exposure tools and processes has always been an issue, regardless of wavelength. In fact, it was concern over organic contamination (n-methyl2-pyrolidone) that led IBM to the first use of gas-phase filtration to protect optics, wafer surfaces, and early adaptations of their Apex DUV photoresist (circa 1991). Presently, the world’s leading suppliers of DUV exposure tools are modifying their make-up air filtration requirements for 248 nm tools to include the removal of organics and acids to minimize optical degradation. Unfortunately, some industry “experts” still appear unwilling to acknowledge what most have known for years: that organic contamination of DUV optics has always been a very real concern.
Second, it is curious that a manufacturer of gas-phase contamination measurement and filter products is lamenting the fact that (some?) real-time monitors lack the sensitivity to accurately measure contaminants which are proven to degrade optical elements. The suggestion that the quantity of filters installed on each tool should then be doubled through “stacking” in a serial flow configuration may not be the best, especially for the cost-conscious fab. Truthfully, any filter can be stacked in serial flow to reduce the frequency of filter changes or to reduce output contaminant concentrations. But the lack of any tangible benefit from doing so, particularly when compared with the cost of money invested in these extra filters over time, seems quite problematic. The better approach may be to utilize the most effective contamination measurement technology, the most efficient and effective filter technology, and that filter product which provides the lowest overall cost of ownership. As you can see, air flow, be it parallel or serial, is really nothing more than hyperbole in this context.
Third, while I know you were being facetious, I am fairly certain that most of your readers would disagree with the closing statement proclaiming contamination control at 193 nm process as “solved.” To wit: most operational 193 nm exposure tools are currently being used for process development and R&D work. That fact alone suggests that contamination concerns at 193 nm process are far from solved; many haven’t even been identified. Just as we have seen with 248 nm process, the strategies and technologies used to minimize contamination effects on 193 nm tools and processes are bound to change as the technology evolves.