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Chapter 6:
Immersion Defects and Defect-Reduction Strategies
Defectivity is the most important issue yet to be resolved in order to move 193i lithography into high-volume manufacturing. Typically, unoptimized 193i processes have 4–20% more defects than dry 193-nm processes have. Defects can be introduced by the scanner, the track, the materials, or the process. In general, sources of defects that are present in dry 193-nm lithographic processes are also present in immersion 193i lithography. For example, coating defects introduced during BARC and resist coating processes are commonly observed in both dry and immersion 193-nm lithography. Optimized 193-nm dry processes can serve as starting points for 193i processes. Unfortunately, however, immersion processes introduce additional sources of defects. During exposure, the immersion water may contain particles or insoluble compounds that can be deposited onto the wafer, forming defects. Bubbles in the water can scatter the exposure light, distorting the aerial images and forming bubble defects in the resist pattern. Topcoats can also introduce additional material-related defects, including particles from the topcoat itself or microbridges caused by intermixing problems between resist and topcoat layers. Defects can also be introduced by the wafer edge. Resist or topcoat films at the edge of the wafer can peel off to generate particles. These particles may be brought to the wafer center by the movement of the immersion exposure head. Watermark defects are generated by water leakage from the meniscus and are unique to the 193i process. This chapter focuses on these immersion-related defects. The formation mechanism of each defect type is analyzed with the help of simulation and experimental results. A series of defect reduction methods are proposed based on these defect mechanisms. 6.1 The Basics of Defect Detection After the lithographic process, the desired patterns must be printed in all locations, faithfully reproducing the mask pattern. Any deviations from the desired patterns are called defects. Examples of defects include missing pattern segments, pattern scum, and particles.
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