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In the absence of EUV pellicles, EUV masks need to be cleaned frequently. Even after the implementation of the pellicles, EUV masks need to be cleaned multiple times. When pellicle studs need to be removed, aggressive cleaning recipes are required to remove glue from the surface mask. Therefore, surfaces in contact with chemicals during mask cleaning should be stable against aggressive, acidic and basic chemicals. In addition to, chemical durability, EUV masks are repeatedly exposed to EUV and UV radiation. In particular, with increasing the EUV source power, it is expected that EUV mask surface be exposed to EUV light with energy densities > 5 W/cm2 as well as out-of-band radiation in UV region. Such high energy radiation can oxidize Ru cap layer or promote Ruthenium silicide formation under the capping layer. Such mechanism can result in Ruthenium peel –off by multiple cleaning of EUV masks. When acidic chemistries are used to remove particles from pattern EUV masks, absorber layer can be etched and as a result mask CD will change by multiple cleanings. During the chucking process in an EUV scanner, there might be dents form in the backside conductive layer (e.g. CrN) which results in thinning backside conductive layer in the certain areas. Meantime, more aggressive megasonic cleaning is required to remove micron size particles from the backside of the EUV masks. Combination of multiple chucking, dent formation and aggressive cleaning may result in damage in the backside film. Finally, in the EUV masks with OPC sub-resolution-assist-feature (SRAF) can be easily damaged by megasonic during cleaning.
This paper will discuss cleaning durability challenges for sub 10 nm half pitch nodes when high power EUV sources expect to be used. In particular, we present our latest results of multiple cleaning of Applied Materials blanks with conventional cleaning chemistries. The change in the EUV reflectivity of Ru cap multilayers by 100x cleaning will be presented. Impact of cleaning on conventional absorber (TaN) and newly developed thin absorber films in Applied Materials will be discussed. The impact of the cleaning processes on the substrate and CrN backside conductive film will be presented. Multiple surface characterization techniques will be used for study of cleaning impact on different films.
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