Birefringence variation of quartz substrates during mask process

Hyper-NA lithography with polarized light illumination is introduced as the solution of 45nm or 32nm node technology. In that case, consideration of new characteristics of masks and substrates has been required. One of these is birefringence of quartz substrates. It has been said that birefringence of substrates affects printed CD on the wafer and is required to control on the masks or substrates. Current birefringence measurement system requires some millimeters windows if it measures on the masks. So, it is impossible to measure the birefringence on the entire field of the patterned masks. And it is said that birefringence is caused by inside stress of the material. That of quartz substrate may be affected by film stress, thermal effect during bake process or pressure of pellicle mounting. In order to confirm birefringence variation during mask process, we measured the birefringence in between each mask processes. We have prepared ultra low birefringence quartz substrates to check the small volume of birefringence change. Number of measurements for each mask and each process was 11 x 11 = 121 locations which covers 126mm square on the mask. 4mm square window for each measurement locations were opened at the mask process. It was enabled to measure birefringence after mask process as well as before and after pellicle mounting. Substrate's birefringence is typically called as a maximum birefringence value. We analyzed maximum birefringence change and calculated the shift of birefringence for each process to confirm the effect of birefringence change for each processes.