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2 June 2000 Quantifying the effect of pattern density on focus offset
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Abstract
Stepper autofocus systems can involve reflecting a light beam off the top surface of the wafer (i.e. the top of the photoresist). The density of underlying circuitry causes this relative position to vary, and this will then cause focus errors comparable to the step-height of the underlying pattern. The typical depth of focus in sub-micron lithography is of the same order of magnitude as the maximum step-height on the wafer surface due to topography, so these effects have become significant. Therefore, the optimum focus offset may need to be experimentally determined for each product. For a fab which deals with many products (e.g. a foundry) this requires significant amounts of work. This paper investigates the pattern density effect for metal layers and considers the influence of layer thickness, and the planarization properties of the intermetal dielectric on the optimum focus offset. The possible method for calculating the optimum focus offset by taking the average pattern density under the area of the focus beam from a CAD layout is presented. The usefulness of techniques such as Multi-Point Autofocus is also discussed. Finally, the side-benefits of CMP in reducing this effect are quantified.
© (2000) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Shane Geary and James Thompson "Quantifying the effect of pattern density on focus offset", Proc. SPIE 3998, Metrology, Inspection, and Process Control for Microlithography XIV, (2 June 2000); https://doi.org/10.1117/12.386513
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