Improved immersion scanning speed using superhydrophobic surfaces

Abstract

Failure of the receding meniscus during immersion lithography is one of the well known problems. A thin liquid film left behind on the wafer during scanning may generate imaging defects. Liquid loss occurs at the receding meniscus when the smooth substrate is accelerated beyond a critical velocity of approximately 1 m/s. Nanotexturing the surface with average roughness values even below 10 nm results in critical velocity larger than 2.5 m/s, the upper limit of our apparatus. This unexpected increase in critical velocity is observed for both sticky rough hydrophobic and slippery superhydrophobic surfaces. We attribute this large increase in critical velocity to the increased static receding contact angle, air extraction and in increased slip length for such nanotextured surfaces. We have also presented the experimental proof of the hypothesis which shows that the slip length and static receding contact angle as a significant parameters for the enhanced performance of sticky surface. Further the dynamic contact line behavior on surface with regions of varying wetting behaviour was studied. The preliminary result shows that the water droplet retains its meniscus shape as soon as it transits from hydrophobic to superhydrophobic region. The secondary thin streak of entrained water on the hydrophobic region is formed which can be controlled with higher extraction.