Understanding contributors to critical dimension uniformity (CDU) is becoming ever more important. We propose a systematic CDU breakdown into contributions that can be attributed to local CDU, intra-field CDU, field-to-field CDU, intra-lot CDU and lot-to-lot CD variations using smart metric definitions. We checked the CDU breakdown on synthetic data as well as on real DRAM production data. Comparing our CDU breakdown to results from nested ANOVA made with statistical software packages, we could verify that the results were very close, while our calculation method was orders of magnitude faster.
The demands on process windows for focus and dose optimization in optical lithography are increasing due to tighter specs caused by shrinking of design patterns. Additionally, as the number of scanners, products and critical layers increase, the amount of processed focus exposure matrix (FEM) wafers also increases. However, standard analysis methods based on Bossung plots and manual analysis are still common. These methods are mostly script based and require manual input. We present a novel automatable method of FEM analysis with regards to process window and working point optimization geared towards the high-volume manufacturing (HVM) fab environment. Our multi-dimensional approach can optimize a number of critical dimension (CD) measurements simultaneously, along with the inclusion of quality parameter like line edge roughness or local uniformity. Fine tuning specs, limits, and minimal desired focus or dose windows is now possible, resulting in an even more optimized process window, tailored to the unique properties and requirements of the product at hand. All these possibilities have been evaluated with various state of the art memory chips, including devices with 3Xnm dense line/space patterns and 10Xnm dense contacts. In all cases the optimization resulted in larger combined process windows and _ne-tuned working points.
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