Defects on an extreme ultraviolet (EUV) mask blank strongly depend on the defects on the mask blank substrate. Any imperfection on the substrate surface in the form of a particle, pit, and scratch will appear on the EUV mask blank. In this article, we study the effect of the cleaning process on the creation of defects on the EUV substrate and mask blank. Added particles could be removed by improving the cleaning tool and the cleaning process. Pits are generally created when many large defects, particularly glass-like materials, are present on the surface and the substrate is exposed to a high energy cleaning step. Comparison of different high energy steps in a typical cleaning process suggests that the megasonic step most likely creates pits. Current cleaning processes developed in the Mask Blank Development Center (MBDC) have been optimized so that no added pits or particles are observed after using them.
Low thermal expansion material (LTEM) substrates were cleaned with recipes developed to clean blank quartz substrates. These recipes were capable of cleaning the LTEM without damaging the LTEM substrate. No effect of etching doped metals in LTEM was observed in these experiments. However, LTEM substrates currently require multiple cleaning cycles to obtain the same removal or cleaning efficiencies as quartz substrates. In addition, no change in the surface roughness or degradation of the backside choromium layer was observed.
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