We developed a statistical method that can be applied to overlay metrology tools to improve performance and time-to-results (TTR) of multi-cycle optimization based on the brute force method. First, we evaluated full response surfaces for each combination of the discrete equipment settings and calculated desirability scores using a normalization function. Second, we combined gradient optimization techniques and response surface methodologies to find the important local maxima (center of the islands in quadratic contour) and stationary response points. Once all the stationary response points have been identified, users can choose to rank the solutions by quality or can choose to use analysis of variance (ANOVA) methods to determine which main effects and/or interactions are of interest. Two separate layers were evaluated and compared to the process of reference (POR) brute force method of optimization. Results showed that the best residuals values from recipes optimized using 1-cycle SPOC-based automatic recipe optimization (ARO) and ARO based on the 2- cycle Brute-Force strategy were comparable to known residuals values from the POR recipes. Moreover, SPOC-based ARO was performed with a TTR of under 2 hours, while a 2-cycle Brute-Force ARO typically took 6~ 20 hours depending on specific configurations. The vast reduction in optimization time is primarily attributed to the elimination of multi-cycle refinement, whose data collection dominated the previously observed TTR. In conclusion, we demonstrated the ability to reduce time to solution by a factor of 3 while maintaining or improving on overlay residuals compared to existing brute force methodologies.
Reduction in on product overlay (OPO) is a key component for high-end, high yield integrated circuit manufacturing. Due to the continually shrinking dimensions of the IC device elements it has become near-impossible to measure overlay on the device itself, driving the need to perform overlay measurements on dedicated overlay targets. In order to enable accurate measurement on grid (target) in terms of OPO matching, the overlay mark must be as similar as possible to the device in order to mimic the process impact on the device. Imaging-based optical overlay (IBO) provides the best accuracy and robustness for overlay metrology measurements for many process layers. To further optimize IBO performance, a new robust AIM (rAIM™) IBO target design was developed, using the Moiré effect. rAIM is implemented using significantly smaller pitches compared with the standard AIM® target, hence providing a more device-like target design. This new target design has the potential to improve target accuracy and robustness, to improve measurability, and to meet overlay basic performance requirements, such as total measurement uncertainty (TMU).
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