Immersion Lithography Production Ramp
 By Frank Burkeen
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Senior Product Marketing
Director at KLA-Tencor
Frank.Burkeen@kla-tencor.com.
The introduction of fluid between the scanner head and the wafer surface in immersion lithography has helped keep Moore's Law on track, but the additional complexity induced by mechanically coupling the scanner head with the wafer surface has produced significant challenges. One area of concern is the behavior of the BARC, photoresist, and top coat films near the wafer's edge. Immersion lithography drags a fluid bubble across the wafer stage at high speed, generating a virtual tsunami at the edge of the wafer. Particles from these film edges and other sources can contaminate the wafer's surface, or can migrate onto the lithography tool's exposure stage.
Figure 1: VisEdge Images showing a Topcoat peel of using 3 different detection methods. Development of processes resistant to this type of defect source is critical for scanner contamination and wafer yield. Historically there have been two technology approaches for edge inspection. The first is a laser-scatter detection system coupled to a digital camera. Optimized for chips, cracks and particles, scatter detection tends to miss low-scatter film defects and residues, severely limiting the value of these tools for IC manufacturing.
The second technology involves imaging the wafer's edge with CCD-based digital microscopy. This technology has significant drawbacks, primarily because its capability to image defects is affected by the curvature of the wafer's edge. At the magnification level necessary to see 2 to 5ºm defects, the depth of field severely limits the “in-focus” field of view, forcing the use of multiple cameras to accommodate complete wafer-edge coverage.
Neither of these established edge inspection technologies provides adequate sensitivity to immersion lithography (iLitho) film-type defects. To promote process development and production ramp of iLitho, an advanced inspection capability that is able to differentiate the various film edges, analyze their defects, and even measure their concentricity and overlap is required. To automate this inspection and provide objective data for development feedback is the ultimate goal.
Figure 2: Automated Defect Classification in Top Near-Edge region. Defects in resist, top coat, and BARC regions were classified using a rule-based classification approach. One new solution that addresses this challenge is the VisEdge™ edge inspection system, which employs four simultaneous imaging techniques to ensure no defect type is missed. This system is capable of scanning all five wafer zones-top near edge, top bevel, apex, bottom bevel, and bottom near edge-to detect and control all edge-related excursions that can impact yield. The tool's sensitivity and design enable a state-of-the-art automatic defect classification capability to simplify the trending of these defects by type, and the resolution of their root cause.
This new inspection technology, sensitive to all defect types, including film-related issues at or near the wafer's edge, supports critical immersion-lithography inspection requirements, enabling the shorter ramps and higher yields needed to be competitive in today's semiconductor market.
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