Although EUVL is widely considered to be used from the node of hp32nm, there are serious problems. One of them is the defect problem at the mask blank. The defect concentration increases rapidly as its size becomes smaller, but the current defect level of around 0.1/cm2(>100nm) is orders of greater than the required level of 0.005/cm2(>32nm). Also the present defect detection limit of around 50nm is much greater than the required defect size of 23nm for hp32nm and 16nm for hp22nm. Therefore 8X mask, having double-larger patterns, is helpful because of its double-larger size for the required minimum defect. Moreover the double-larger patterns have much fewer killer particles, which is also helpful for the no-pellicle mask. However changing the mask magnification to 8X has been reported to decrease the exposure-tool throughput to around 40% of that of 4X. Since this decreased throughput was estimated for KrF/ArF, and not for EUVL, throughputs of an 8X EUVL scanner are calculated. The calculated results have cleared that an 8X scanner can give around 64% of the 4X throughput, and that a 9” 8X scanner and a double-long 8X scanner can give 89-98% of the 4X throughput at the resist sensitivity of 5mJ/cm2. This is due to a double-higher scan speed obtained by 8X. Another advantage by changing to 8X is the smaller line edge shift by the shadowing effect to 1/2-1/3 because of the higher magnification and the possibility of decreasing the illumination incident angle. Similarly the mask surface flatness requirement can be loosened by 2-3 times.
Kiwamu Takehisa, Kiwamu Takehisa,
"Mask magnification of 8X will be also helpful for EUVL", Proc. SPIE 5853, Photomask and Next-Generation Lithography Mask Technology XII, (28 June 2005); doi: 10.1117/12.617278; https://doi.org/10.1117/12.617278