Sensitivity Requirements in Immersion
Lithography Inspection  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, hotoresist, 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. 
Figure 1: VisEdge Images showing a Topcoat peel off using 3 different detection methods. Development of processes resistant to this type of defect source is critical to prevent scanner or wafer contamination. Particles from these film edges and other sources can contaminate the wafer's surface, or can migrate onto the lithography tool’s exposure stage and lens. Recent work has demonstrated that these particles can be much smaller than 0.5um. Optical inspection of the wafer edge can provide some information on film issues in the immersion lithography resist stack. This inspection is typically done with a CCD camera and a broadband illumination. However, most of the particle defects generated from the film issues are well beyond the detection capabilities of this CCD-based inspection system. Thus, a second, more sensitive inspection is required to determine if the macro-scale film defects are causing micro-scale defects to be generated. To promote process development and production ramp of immersion lithography, 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. What is needed is an edge inspection technology with sensitivity to immersion lithography film conditions (macro) and the potential small particle defects (micro) that will reduce yield. 
Figure 2: <0.5um particles generated from immersion hood interaction with iLitho film stack 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 (macro) and particle (micro) 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. To learn more about wafer edge inspection, go to: www.kla-tencor.com/08edgeUS | 
