Haze risk reduced mask manufacturing process

Since introduction of 193nm exposure wavelength, the haze formation becomes a serious challenge especially at mask used for big number of exposures. Several compounds present in air as low concentration contaminants are leading to haze building. Well understood is the sulphate based haze formation, however, still causing significant losses and demanding for re-cleaning of the mask during mask life time. There are plenty of publications taking different approaches to reduction of the final sulphate concentration on the mask and reduction of the use of sulphuric acid during the mask manufacturing. Beside traditional process as hot water extraction, UV exposure, baking, IR exposure at vacuum, Ammonia solution treatment more exotic method were published as surface treatment preventing migration of the sulphate ions on mask surface. The number of exposures till haze crystals growth prevents further use of the mask is not solely dependent on the sulphate concentration on the freshly manufactured mask. Additional factors as storage and use conditions are significantly influencing the period till re-clean of the mask is needed. In this work we try to assess above mentioned approaches and provide rough estimate of their limits.