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In lithographic scanner, many different physical factors could impact to image quality and CD uniformity. In optical
systems, the pupil filling quality (source shape), wavefront error and stray light can decrease the intensity contrast and
shrink the process window. In mechanical domain, the vibration and scanning synchronization error have the similar
effect to imaging process.
Imaging in scanner is a dynamic exposure process and in this process, aerial image should keep the same relative
position to the wafer. It requests the lithographic tool must have a very stable mechanical frame and very good motion
control performance. In addition, the wafer stage, reticle stage's coordinate and projection lens' grid should be matched
exactly, include the scanning direction and velocity ratio. The tool's alignment system can calibrate the statistic
coordinate for overlay, but it cannot calibrate the dynamic coordinate in scanning direction very well because projection
lens' grid has a small asymmetric signiture. This systematic error should be calibrated for CDU improvement.
An imaging model considering the motion blurring is represented in this paper and based on this model, the dynamic
coordinate's error could be analyzed. Furthermore, exposure method can be used to calibrate the dynamic coordinate and
improve the CD uniformity.
Exposure latitude will be used to check and calibrate the lithographic tool's dynamic coordinate. We designed a special
calibration process to obtain the best dynamic coordinate setting for scanner. In this process, some tool's coordinate
parameters (scanning skew and scale) have been changed for every field to obtain the multi-dimensions' exposure
information. Exposure window can be represented from this result, and in this exposure window, the best dynamic
coordinate setting could be found. After the dynamic coordinate calibrated, the CDU is improved.
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