Particle protection capability of extreme ultraviolet lithography mask carriers based on the gap effect and filter effect

In extreme ultraviolet lithography, particle-free mask handling is a critical issue because the use of pellicles is impractical. We measured the long-term change in the number of particle adders on a mask blank during transfer processes using a reticle SMIF pod (RSP) and a dual pod, which consists of an outer pod and an inner pod that holds the mask. In the RSP, the number of particle adders during the transfer test of a load port in air to an electrostatic chuck chamber in vacuum decreased from 0.053/cycle to 0.032/cycle because of a clean-up during the pumping down and purging operations. However, the number of particle adders during vacuum transfer did not change with long-term use. Moreover, we found that particles were added by mask blank sliding on a robot hand during vacuum transfer. In contrast, for the dual pod, no accident was observed during the 2000-cycle transfer test, and the number of particle adders was 0.004/cycle. We confirmed that the filter effect and gap effect for protecting the mask from particles were effective. We concluded that the dual pod was a reliable mask carrier for vacuum transfer.