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EUV lithography is considered the most promising lithography solution for the 16 nm node and beyond. As EUV
light is strongly absorbed by all known materials, reflective optics are used instead of conventional transmittance optics
applied to ArF and KrF lithography. The EUV mask must also need be reflective. It typically consists of a Ta-based
absorber layer, Ru capping layer, Si/Mo multilayer on a low thermal expansion material (LTEM) substrate with a
backside Cr-based metal coating. Because of the strong absorbance of the EUV light, a pellicle is not practical. Therefore,
EUV masks must be cleaned more frequently to maintain the necessary cleanliness. This poses numerous unique
challenges in cleaning processes. For example, the EUV mask integrity, including critical dimension (CD), EUV
reflectivity, and absorber thickness must be kept intact during multiple cleanings throughout the mask's lifetime.
Requirements of defect size for the cleaning, furthermore, are becoming tighter as semiconductor circuit design rules get
smaller. According to the International Technology Roadmap For Semiconductors (ITRS), the smallest defect size that
must be removed is 23 nm for the 18 nm NAND Flash node in 2013. In addition to defects on the frontside, defects
on a backside also need to be minimized since they might lead overlay error due to local distortions of EUV masks on an
electrostatic chuck.
This paper focuses on evaluations of cleaning performances using the Lasertec M1350 and M7360 blank
inspection system, which has a 71 nm and 43 nm sensitivity. The 43nm is the current best sensitivity while keeping a
>90% defect capture rate. First, the cleaning performance using the standard process has been investigated. We found a
mitigation of adders was a key challenge for the EUV mask cleaning. The primary source of the adders was also
identified as pits. Secondly, the megasonic cleaning process has been optimized to mitigate the adders. We could
successfully reduce the adders by 30%. Thirdly, to confirm the entire cleaning process, a backside cleaning process
combined with frontside cleaning was investigated, demonstrating that the backsides of the EUV mask blanks could be
cleaned without additional impact on frontside defectivity.
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