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IC manufacture has to meet stringent requirements pushing the imaging tools beyond their limits. The key performance attribute of the imaging tool is the quality of the image projected on wafer plane. The image quality is controlled by the wavefront aberrations present in the projection lens pupil. Therefore the quality of the lenses can be represented by either various image quality metrics or by the data on the lens pupil aberration residua. Projection lens quality can be quantified by interferometers capturing the lens pupil residual aberration, leading to estimates of the image quality. These various techniques can be used off-line, testing projection lenses installed on a dedicated test bench, used during or after lens manufacture, or in-situ, testing the lenses installed in the projection tools, often at the IC manufacturing floor. These techniques have inherent tradeoffs in terms their accuracy, portability, ease-of-use and completeness of the aberration and imaging metrics. Such tradeoffs determine which technique is the most appropriate for various applications ranging from lens quality control during imaging tool manufacture, to tool qualification during its installation and setup, to tool monitoring and tuning during the IC manufacture. It is acknowledged within the scanner engineering community that qualification and maintenance of tools used for critical level pattering requires in-situ lens monitoring technique. Such method would also help to select and to fine tune the imaging tools to design-specific requirements of IC critical patterns. A preferred method of aberration monitoring should be highly compatible with routine scanner operation and should be independent of resist process conditions. This paper presents aerial image-based technique to monitor and to diagnose the quality of projection lenses used in scanners. The method involves aerial image sensor, AIS. We start with a discussion of the fundamental principles of operation and the key design issues impacting the accuracy of the technique. We follow with an examples of the AIS aberration test. These tests lead to a discussion of the method's capabilities to quantify the performance of the imaging tools.
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