Overlay measurement of metal interconnect layers that exhibit contrast variation has historically been problematic. The appearance of random grain structure surrounding the overlay target obscures the edge contrast, leading to significant noise in measurement and residual error of fit. Large residuals reduce confidence in modeled scanner correctable terms, increasing the likelihood of false scanner corrections which can lead to poor process control. We have developed a new measurement methodology that minimizes the effect of grain-induced noise for overlay metrology. Measurements which utilize the new parameters have shown a 30% reduction in mean-square (MS) residual error and a 39% reduction in variance. In addition, predicted wafer maximum overlay error has decreased by 14%, with a 16% improvement in wafer-to-wafer variance. This type of performance improvement is expected to have a significant impact on rework reduction. The primary source of visible grain structure in the aluminum interconnect layer has been attributed to the metal hardmask deposition process. By moving to a lower temperature hardmask deposition, the grain has been significantly reduced. This has resulted in a 15% further reduction of MS residual error.
A novel overlay registration target design was developed for the purposes of characterizing pre- and post- etch wafer-induced shift (offset) on metal layers. The new target enables the measurement of metal overlay offset during the same measurement run. The combination target was studied on production wafers and compared with the traditional two step pre- and post- etch characterization methodology. Wafer induced shift data measured by both methods showed good agreement in magnitude and direction across the wafer. The new target design was incorporated into an experiment to determine the effects of chemical-mechanical polishing, metal deposition thickness, and their interaction on the offset. Within the target process windows for both CMP and deposition, no significant dependencies of offset were observed. Analysis of this data indicated strong wafer level rotation and scale signatures, confirming offset arises from CMP and metal deposition processes. For the metal process conditions of the experiment, the offset is stable and reliable with respect to fluctuations in overpolish and deposition thickness.
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