Surface modification of a MoSiON phase shift mask to reduce critical dimension variation after exposure to a 193-nm ArF excimer laser

Introduction of a MoSi-based phase shift mask (PSM) improves photolithography resolution by causing light to shift phase by 180° thus canceling the overlap. However, when MoSiON PSM was exposed to an ArF excimer laser (λ = 193 nm), a significant increase in patterned critical dimension (CD) was observed. It was confirmed that the CD increase resulted from oxidation progression into the MoSiON layer. In this study, N₂O or NH₃ plasma treatment and thermal annealing in NH₃ effectively suppressed CD variation after ArF laser exposure. While the compositional ratio of Si, N, O, and Mo elements in the MoSiON layer was not changed, an increase in oxygen content only in the top 5 nm was observed. Therefore, it is concluded that slight oxidation of the top surface of MoSiON PSM by introducing either N₂O or NH₃ plasma treatment or thermal annealing in NH₃ suppresses an increase in the patterned CD of MoSiON PSM after exposure to a 193-nm ArF excimer laser.