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Limitation of removal particles in positive photoresist

The pore size of membrane filter used in the filtering of photoresist for ULSI manufacturing is now shifting from 200 nm to 100 nm, but the photoresist end-users are asking for a much finer filter pore size. By knowing the size of its polymer it is possible to know the limit of the pore size required to filter the photoresist. From the results of ultra filtration and measurement of light scattering of the photoresist, the limitation of the filter pore size for the photoresist filtration and the detectable particle size in the photoresist were determined. In the ultra filtration of OFPR-800 (positive photoresist of Tokyo Ohka Kogyo Co., Ltd.) molecules larger than 13.5 nm in the original solution have been removed using a 13.5 nm filter. The maximum particle size of OFPR-800 novolak resin molecules measured by dynamic light scattering spectrophotometer was 32 nm and the average particle size was 5.4 nm. Furthermore, particles were observed larger than 32 nm. Because a positive photoresist is a highly concentrated polymer solution of 20 - 30 wt% it has been determined that those particles larger than 32 nm are large polymer groups similar to macro-molecules resulting from the intertwisting of novolak resin. It has been determined that the limit of the filter pore size that can filter a positive photoresist is 50 nm and the detectable particle size is 100 nm.