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In a previous publication, we introduced Blossom, a multi-layer overlay mark (Ausschnitt, et al. 2006, [1]).
Through further testing carried out since that publication, Blossom has been shown to meet the requirements
on current design rules (Ausschnitt, et al. 2007, [2]), while giving some unique benefits. However, as future
design rules shrink, efforts must be made now to ensure the extensibility of the Blossom technology.
Previous work has shown that the precision component of Total Measurement Uncertainty (TMU) can be
reduced by using extra redundancy in the target design, to achieve performance beyond that of a conventional
box-in-box measurement. However, improvements that single contributor to TMU would not be sufficient for
future design rules; therefore we have also to consider the Tool Induced Shift (TIS) variability and tool to
tool matching contributions to TMU.
In this paper, we introduce a calibration artifact, based on the Blossom technology. The calibration artifact is
both compact, and produced by standard lithography process, so it can be placed in a production scribe line if
required, reducing the need for special sets of calibration wafers compared to other possible calibration
methodologies. Calibration is currently with respect to the exposure tool / process / mask, which is arguably
more pertinent to good yield, and less expensive, than calibration to an external standard; externally
calibrated artifacts would be straightforward to manufacture if needed.
By using this artifact, we can map out remaining optical distortions within an overlay tool, to a precision
significantly better than the operational tool precision, in a way that directly relates to overlay performance.
The effect of process-induced mark uncertainties on calibration can be reduced by performing measurements
on a large number of targets; by taking multiple measurements of each target we can also use the artifact to
evaluate the current levels of process induced mark uncertainty. The former result leads to an improvement
method for TIS and matching capability. We describe the artifact and its usage, and present results from a
group of operational overlay tools.
We show how the use of this information also provides further insight into the layout optimizations discussed
previously (Binns et al. 2006 [3]). It provides the current limits of measurement precision and mark fidelity
with respect to target redundancy, enabling us to use a predictive cost-benefit term in the optimization.
Finally, examining the bulk behaviour of a fleet of overlay tools, allows us to examine how future mark
layouts can also contribute to minimizing TMU rather than just precision.
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