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ArF water immersion lithography is expected to be used down to the 22nm hp node or below. However, such
advancements in technology nodes have led to extremely small process margins. This necessitates more accurate means
of process control. CD uniformity of the photo-resist (PR) image is affected by many sources. In the case of the
exposure tool-CD error on the reticle, as well as exposure dose and focus errors are the key factors. For the PR process,
heterogeneity of the stacked PR film thickness, post exposure bake (PEB) plate temperature, and development have an
impact. Further, the process wafer also has error sources that include under-layer uniformity and wafer flatness.
Fortunately, the majority of these factors is quite stable in a volume production process and can be compensated for by
adjusting exposure dose and focus in the scanner.
A technique to calculate exposure dose and focus correction values simultaneously from the measured PR image feature
was reported previously [1]. In addition, a demonstration of a correction loop using a neural network calculation model
was reported in SPIE 2010 [2], and the corrected CD uniformity was less than 1.5 nm (3-sigma) within the wafer. In
this paper, we will report the latest CD uniformity correction results achieved with the NSR-S620D ArF immersion
scanner using correction values estimated by scatterometry and CD-SEM.
The method of correction using CD-SEM is newly developed. A maximum of nine parameters extracted from the PR
profile are used in this correction. In general, the CD variation of an isolated line pattern caused by focus error is more
sensitive than that of a dense pattern. Thus, we estimate the focus error from the isolated pattern, with the dose error
estimated using both isolated and dense patterns.
The Nikon CDU Master then derives the optimal control parameters for each compensation function in the scanner
using the exposure dose and focus correction data, and the NSR-S620D is able to control higher order dose and focus
distribution. This advanced level of control capabilities enables precise correction of the complicated CD error
distribution that is caused by heterogeneities in the process.
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