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Immersion defect performance and particle control method for 45nm mass production

Water-based immersion technology has overcome various challenges and is starting to be used for the 45nm-node mass production. However, even though immersion technology is being used in memory device production, significant improvement in defect performance is needed before the technology can be used for logic devices. Canon has developed an immersion exposure system, the FPA-7000AS7, with numerical aperture of 1.35. In the AS7 immersion tool, there is little influence of vibration and evaporative cooling. The AS7 has an in-situ cleaning system in order to remove particles carried into the exposure tool. We evaluated the contamination of the projection lens and immersion nozzle due to photoacid generator (PAG) leaching from resist to water. We evaluated the cleaning effects of various cleaning processes and found the suitable processes for cleaning the projection lens and immersion nozzle from the view that it does not adversely affect the exposure tool: damage-free and easy drainage treatment. In addition, we evaluated the influence of particles on the wafer stage, since there is a major concern that particles entering the water may increase the defects. The number of particles adhering on the wafer during an exposure sequence can be reduced with the wafer stage cleaning. Periodical cleaning keeps the wafer stage clean, thus preventing the increase of exposure defects caused by particles. We performed a defect evaluation with the AS7. The average defect density was 0.042/cm2 in the continuous exposure process of 25 wafers with a developer-soluble topcoat. Circle defects and bubble defects were not observed.