Basic studies of overlay performance on immersion lithography tool

Immersion lithography with ArF light and Ultra Pure Water (UPW) is the most promising technology for semiconductor manufacturing with 65 nm hp design and below. Since Nikon completed the first full-field immersion scanner, the Engineering Evaluation Tool (EET, NA=0.85) at the end of 2004, Toshiba and Nikon have investigated overlay accuracy with the EET which uses the local fill nozzle. EET successfully demonstrated immersion tools are comparable in single machine overlay accuracy to dry tools, and immersion-dry matching has the same level overlay matching accuracy as dry-dry matching. EET also made it clear that overlay accuracy is independent of scanning speed, and both solvent-soluble topcoats, as well as developer-soluble topcoats can be used without degradation of overlay accuracy. We investigated the impact of the thermal environment on overlay accuracy also, assuming that a key technology of overlay with immersion tools must achieve thermal stabilities similar to dry tools. It was found that the temperature of supply water and loading wafer are stable enough to keep the overlay accuracy good. As for evaporation heat, water droplets on the backside of the wafer lead to overlay degradation. We have decided to equip the wafer holder of S609B, the first immersion production model, with an advanced watertight structure.