ASIC design flow considering lithography-induced effects
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As VLSI technology scales towards 65nm and beyond, both timing and power performance of integrated circuits are increasingly affected by process variations. In practice, people often treat systematic components of the variations, which are generally traceable according to process models, in the same way as random variations in process corner-based methodologies. In particular, lithography-induced process variations are usually estimated by a universal worst-case value without considering their layout environment. Consequently, the process corner models based on such estimation are unnecessarily pessimistic. A new ASIC design methodology that captures lithography-induced polysilicon gate length variations including both the layout dependent systematic components and random components is proposed. This methodology also shows that lookup table methodology is sufficient to handle back end of line lithography process variations in timing analysis. In addition, a new technique of dummy poly insertion is suggested to shield inter-cell optical interferences. This technique together with standard cells characterised using the new methodology will let current design flows comprehend the variations almost without any changes. More importantly, by separating systematic lithography effect from random process variations, this methodology greatly reduces pessimism in timing analysis, thus enabling both aggressive design implementation and easier timing signoff. Experimental results on industrial designs indicate that the new methodology can averagely reduce timing variation window by 11 and power variation window by 55 when compared with a worst-case approach. © The Institution of Engineering and Technology 2008.
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