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

•Increase of critical dimension of phase shift mask after exposure to ArF excimer laser.•Critical dimension increase resulted from oxidation of MoSiON layer.•Plasma exposure or furnace annealing suppressed increase of critical dimension.•Slight oxidation of top surface of MoSiON is related to the suppression. 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=193nm), 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 5nm 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.