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Reducing process contributions to CD error range using the Sigma7500 pattern generator and ProcessEqualizerTM

Photomask CD control requirements continue to tighten due to inevitable device scaling, in addition to the increasing mask error enhancement factor (MEEF). Managing the total CD error on production masks requires not only that the error contributions from the pattern generator should be minimized, but also that there is a way to handle errors due to the mask process. This paper addresses process-related contributions such as the CD signatures from mask developing and etching, and how their impact on the total CD error range can be reduced. This is accomplished using the on-line ProcessEquilizerTM function in the Sigma7500 DUV laser pattern generator. Long range and medium range process CD errors are major contributors to the total CD error range. These errors can be classified as being either pattern-independent or pattern-dependent. Pattern-independent errors may occur in the bake, develop and etch process steps. These errors are by definition static from mask to mask, and can therefore be mapped and compensated by local sizing of the mask pattern data. Pattern-dependent errors typically originate from loading effects, for example, in plasma etching. If such errors can be predicted from the pattern density variation across the mask, then they can also be corrected for with local sizing. The on-line ProcessEqualizerTM function performs local sizing, and offers a significant advantage over off-line solutions that have the drawback of requiring flattening and refracturing of the pattern data. Local sizing is performed in parallel with writing in the Sigma7500, and has no effect on throughput. In this paper the ProcessEqualizerTM function is described, including how it is operated in the maskshop. Results are presented demonstrating the performance of the ProcessEqualizerTM for handling global CD error signatures.