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Photo-induced defects (or haze defects) on 193nm optic masks (haze defects) have been a serious problem not only to reticle engineers
working for mask manufacturing and handling but also to photo-lithography engineers. The most widely accepted explanation of the
root causes of haze defects is the cleaning chemical residues remaining on the mask surface and unavoidable outgassed molecules that
outgas from pellicle materials when exposed to 193nm radiation. These have been significant challenges for reticle cleaning engineers
who need to use cleaning chemicals whose residues do not lead to progressive defect formation on the mask and to find improved
materials to minimize pellicle outgassing.
It is assumed that contamination generation on EUV masks would have a higher probability than on optic masks, primarily since EUV
masks are not protected by a pellicle and amorphous carbon films can accumulate during exposure to EUV light. While there is
potential to mitigate the generation of carbon contamination by improving the exposure tool environment and removing carbon films
using in-situ atomic hydrogen cleaning, it is not yet clear whether the reaction of mask cleaning chemicals to EUV radiation will lead
to creation of progressive defects on EUV mask surfaces.
With the work to being done it has been observed that carbon contamination on EUV masks dominates any effects of solvent
chemicals under normal environmental or exposure conditions (from atmospheric pressure up to a vacuum level of 10-6 Torr) during
EUV exposure. However, it is still unknown whether residual cleaning chemicals will provide a nucleus for progressive defect
formation during exposure. This lack of understanding needs to be addressed by the industry as EUV masks are expected to undergo
more frequent cleaning cycles.
In this work, we will report on an investigation of the molecular behavior of cleaning chemicals on EUV mask surfaces during EUV
exposure. Movement (e.g., migration or aggregation) of cleaning chemical molecules near EUV exposure spots on the top surface and
beneath the mask will be examined under high vacuum (~10-8 Torr). We will also investigate whether EUV exposure can trigger the
evaporation of cleaning chemical residues from the EUV mask surface, possibly contaminating the exposure environment. Better
understanding of the influences of the mask cleaning chemicals during exposure, coupled with knowledge about mask tolerance and
patterning performance affected by the cleaning chemicals, should enable the proper selection of mask cleaning processes and
chemicals to meet EUV requirements.
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