Alexander Sou-Kang Ko

Advanced mask cleaning techniques for sub-100-nm technology nodes

Sub-pellicle defects and haze increase due to photon reaction with cleaning chemistry residues are especially problematic on photomasks for 193 nm and shorter exposure wavelengths. In addition to mask cleaning, these chemistries are also used for photoresist stripping from photomasks. In this paper sulfuric acid free processes are shown to be effective for mask cleaning and photoresist removal. Bulk removal of photoresist was accomplished with both oxygen based dry plasma stripping and with wet oxidizing chemistry. Surface preparation prior to the main cleaning step was necessary to render Cr surface hydrophilic and enable targeted cleaning performance. This was accomplished with an O3/DI pre-treatment step. Full mask megasonics improved particle removal efficiency of moderately to heavily contaminated masks.

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.