Mask substrate birefringence requirements for hyper-NA lithography

For the 45nm node lithography, the mask substrate properties will have a significant impact on imaging performance. Analysing the optical stress birefringence in mask blanks reveals a wide variation within the available population of mask substrates. Both magnitude and orientation of birefringence result in polarization changes in an optical system. These effects potentially produce image degradation, leading to intra-field CD variations. Besides the inherent properties of the mask blank, additional stress birefringence can be generated during the patterning processing and on-scanner reticle use. The main causes are: deposition of the absorber stack, patterning, chucking of the mask in an exposure tool and the mounting of a pellicle. In this paper, experimental results will be shown on the stress birefringence introduced by the different process steps. The imaging effects of mask birefringence will be determined through simulations and experimental validation on high-NA lithographic exposure tools. This study makes clear how mask birefringence impacts high- and hyper-NA lithography, and gives an indication of the allowed tolerances.