Characterization of alignment mark to obtain reliable alignment performance in advanced lithography
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The continued downscaling of semiconductor fabrication has imposed increasingly tighter overlay tolerances. Such tight tolerances will require very high performance in alignment. Hence, the objective of this research to establish characterization process for alignment evaluation and to determine the robust alignment strategy for via 1 and metal 1 masking layers. This research covers four aspects, namely to find robust alignment mark for Metal 1 and Vial layer, alignment performance comparison between via mark and metal mark, alignment mark feature size effect on alignment signal, and to evaluate the new metal alignment mark performance. In order to achieve these objectives, a fractional factorial experiment with 4 parameters variation (tungsten thickness, over polish time, aluminum thickness, and final oxide thickness) and one duplicate was developed. Fifteen alignment Mark types were evaluated. Based from the characterization experiment, B2 mark with highest capability score (4979) and weighted Cpk score (75.16) is the 11I0St robust alignment mark for Metal 1 layer. A3 is the most robust alignment mark for Via 1 layer. A3 mark gives the highest total score in weighted average capability analysis (2173.52) and Cpk analysis (2800). Based on this work, contact mark is more sensitive to process variation as it pattern formation involved 6 processing steps compared to 3 steps for metal mark. For via //lark, big mark size (more than 2.6 f.1m) gives bad alignment signal quality compared to the smaller feature size. Regardless of mark size, alignment signal generates by metal mark gives comparable results. Two types of new metal alignment mark designs (B8 and B9) were evaluated in this experiment. The results were compared with standard metal mark (B6) and standard via mark (B4). B8 gives the best overall alignment and overlay performance since it gives the highest total in weighted average analysis (/40.96(alignment) and 53.43 (overlay)). The research findings becomes a baseline for C18 technology alignment process and already implemented in our production line.
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