The complexity of integrated circuits processing has made CD control a challenging work,
especially in a high volume manufacturing environment such as the one where this study was
done. Many factors contribute to linewidth variability, namely previous process variation,
consistency of equipment performance, and human intervention.
To effectively reduce CD variations in such a high volume manufacturing Fab, a two
phase study was carried out. In phase I, several sources of variation were evaluated through
Variance Components Analysis. This analysis gave a better understanding of where variation
seen in some critical layers was coming from as well as providing information needed to
prioritize efforts in variance reduction. Variables selected for inclusion in this study were
etchers, steppers, day to day litho process fluctuations, and lot to lot poly substrates.
Findings from phase I showed that of those variables included in the study, stepper to
stepper variation was the major contributor to the CD variation.
In order to reduce this effect of stepper variation a two step plan was formulated. The
first step was to place a customized exposure correction into the software for each stepper,
thus bringing the stepper means together. A second step was initially considered but not
implemented as of this writing for logistical reasons. The objective of this second step was
to reduce the variance by grouping the least sensitive steppers (lowest variance) to process
the most sensitive masking layers.