Numerical evaluation of protection schemes for EUVL masks in carrier systems against horizontal aerosol flow

A numerical model, based on the commercial code Fluent, was developed to study the effectiveness of protection schemes for extreme ultraviolet lithography (EUVL) photomasks in mask carriers against particle contamination under atmospheric pressure conditions. The model included the effect of gravity, diffusion, drag force, thermophoresis, and electrophoresis on the particles and was validated against experimental data. Due to good agreement, the model could be extended down to a particle size of 50 nm, which could not experimentally be detected. It was found that electrophoresis can offer very effective protection if the particle charge distribution is unipolar. Thermophoresis also showed very promising results, however, only a small fraction of the particles could be intentionally deposited within a particle trap, surrounding the mask. Maintaining the mask facing down mainly protects the mask against large gravity-driven particles, whereas the protection against small particles requires the use of phoretic contamination control.