Mega-sonic effect on sub-resolution assit features during photomask cleaning

As the shrinkage of feature size and the increase of pattern density, specification requirements for photomask cleaning is getting more and more challenging. Mega-sonic energy is widely applied in photomask cleaning, while it may lead to the damage of Sub-Resolution Assist Features (SRAF) with small size. In this work, different magnitude of mega-sonic energy has been evaluated and its effects on SRAF are studied. High end phase shift mask (PSM) cleaning could be accomplished under the condition that the power of mega-sonic is suitable.