Advanced processes for photomask damage-free cleaning and photoresist removal

Photon induced haze resulting from sulfur residues that remain after cleaning and photoresist stripping is a key challenge for 193 nm photomasks. In previously reported work, sulfur-free processes for cleaning and photoresist removal on mask blanks were shown. Additional characterization and development of the cleaning and strip/clean processes are presented here. For cleaning the particle adder stability, ammonia chemistry residue levels, and chrome oxide anti-reflection coating (ARC) layer integrity were characterized. It was found that process modification was needed to provide acceptable post-clean ammonia levels and reflectivity change per clean. A strip/clean process with acceptable window for complete resist removal without ARC layer damage was found to be challenging and dependent on the mask photoresist/ARC stack. Dry strip, wet strip, and combined dry/wet stripping approaches (all followed by wet clean) were investigated. Oxidizing dry strip chemistry, while easily removing the bulk photoresist layer, gave unacceptable ARC attack. For FEP photoresist an all-wet process was demonstrated, and for iP and NEB resists, promising results were achieved with less oxidizing dry strip chemistry.