Cr migration on 193nm binary photomasks

A new type of chrome-on-glass (COG) photomask defect was observed in 2006. Absorber material migrated into vias on dark field masks, partially obscuring the incident 193nm light and thereby causing the imaged photoresist to be underexposed. Through detailed characterization of new and defective photomasks and their histories it was determined that the migration is not caused by any unusual line events or faulty mask handling procedures. Rather, it is an inevitable result of mask use under specific conditions. Four essential elements have been identified: the presence of Cr, 193nm light exposure, charge, and water vapor and their roles elucidated through modeling studies and existing literature. We have reproduced Cr migration in the laboratory, demonstrating that these four elements are necessary and sufficient for this type of defect to occur. The only way to avoid Cr migration is to avoid reactions with water vapor.