Critical processing factors in the lithography process include overlaying the pattern properly to previous layers and
properly exposing the pattern to achieve the desired line width. Proper overlay can only be attained in the lithography
process while the desired line width accuracy is achieved by both lithography and etch processes. Since CD is
substantially influenced by etch processing, therefore, it is possible to say that overlay is one of the most important
processing elements in the lithography process. To achieve the desired overlay accuracy, it is desirable to expose critical
layers with the same exposure tool that exposed the previous or target layer. This need to dedicate a particular exposure
tool, however, complicates the lot dispatching schedule and, even worse, decreases exposure tool utilization. In order to
allow any exposure tool available to print the arriving lot, M&M (Mix and Match) overlay control becomes necessary.
By reducing overlay errors in M&M control, lot dispatching scheduling will become more flexible and exposure tool
utilization will improve.
Since each exposure tool has a unique registration signature, high order errors appear when overlaying multiple layers
exposed with different tools. Even with the same exposure tool, if a different illumination is used, a similar error will be
seen. A correction scheme to make the signature differences has to be implemented, however manually characterizing
each tool's signature per illumination condition is extremely tedious, and is subject human errors. The challenge is to
design a system to perform the corrections automatically.
In the previous paper(1), we have outlined concepts of the system scheme. The system has subsequently been developed
and tested using exposure tools. In this paper test results are shown using automated distortion correction. By analyzing
the results, suggestions for further improvements and further developments are shown.