Impact of DUV exposure on reticle repairs

摘要

The reticle manufacturing process induces various defects on the mask that need to be repaired. Missing absorber or clear defects are often repaired by depositing a carbon-based material (depo) using a Focused Ion Beam (FIB) tool. Few cases of such depo repairs on defects in between nested contacts on attenuated phase shift masks were found to fail upon use in high volume wafer manufacturing factories. With the goal of first reproducing the problem in the mask shop, a controlled set of depo repairs were performed on a test reticle and sequentially exposed on a DUV flood exposure system, emulating stepper exposure. The repair AIMS™ printability and AFM height profiles were measured before and after each exposure step. With incremental exposures, AIMS™ results showed the repaired contacts gradually printing larger in size and AFM results showed the tail of the depo repair (also referred to as depo overspray or halo) correspondingly receding with exposure. This suggests that the tail of the depo presumably contributes to the correct print CD of the repaired contact, and its gradual recession with exposure was likely causing the contacts to print larger, ultimately even bridging with the neighboring nested contact in some cases. This mechanism was confirmed by checking similar repairs on several production masks already being used in the wafer factories, at different stages of exposure. Subsequently, a novel post-repair process was developed which achieves rapid overspray removal thereby avoiding any further change in these repairs and associated wafer yield impact upon prolonged use on scanners.