Watermark defect formation and removal for immersion lithography

In immersion lithography, water drop residue has been identified as the source of watermark defects. Many methods have been studied to reduce water drops outside of the immersion area. However, from a physical point of view, the wafer surface is very hard to keep dry after immersion exposure. The water drop residues easily cause watermark defects that ranges from micrometer-size circular defects to sub-micron scum defects. In this paper we describe a few new methods to understand watermark formation. We also describe our studies on various water drop sizes and their impact on CD and defects. Our results show that major watermark defects occur as a result of the presence of relatively small water drops. A few methods have also been studied to reduce the impact of watermarks in order to remove watermark-induced pattern defects on wafers. These methods include post-immersion exposure treatment as well as novel material and process improvement methods. By applying these methods, we are able to remove watermark defects without an additional resist protection layer while still maintaining good resist lithographic performance.