Investigation of reticle defect formation at DUV lithography

Defect formation on advanced photomasks used for DUV lithography has introduced new challenges at low k1 processes industry wide. Especially at 193-nm scanner exposure, the mask pattern surface, pellicle film and the enclosed space between the pellicle and pattern surface can create a highly reactive environment. This environment can become susceptible to defect growth during repetitive exposure of a mask on DUV lithography systems due to the flow of high energy through the mask. Due to increased number of fields on the wafer, a reticle used at a 300-mm wafer fab receives roughly double the number of exposures without any cool down period, as compared to the reticles in a 200-mm wafer fab. Therefore, 193-nm lithography processes at a 300-mm wafer fab put lithographers and defect engineers into an area of untested mask behavior. During the scope of this investigation, an attenuated phase shift mask (attPSM) was periodically exposed on a 193-nm scanner and the relationship between the number of exposures (i.e., energy passed through the mask during exposures) versus defect growth was developed. Finally, chemical analysis of these defects was performed in order to understand the mechanism of this “growth”.