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System cleanliness is a major issue facing the lithographic community as the prospects of integrating EUV lithography
into integrated circuit manufacturing progress. Mask cleanliness, especially of particles in the sub-micron
range, remains an issue for the implementation of EUV lithography since traditional mask cleaning processes
are limited in their ability to remove nanometer scale contaminants. The result is lower wafer throughput
due to errors in pattern transfer to the wafer from the particulate defects on the mask. Additionally, carbon contamination and growth on the collector optics due to energetic photon interactions degrade the mirror and shortens its functional life. Plasma cleaning of surfaces has been used for a variety of applications in the past,
and now is being extended to cleaning surfaces for EUV, specifically the mask and collector optics, through a process developed in the Center for Plasma-Material Interactions (CPMI) called Plasma Assisted Cleaning by Metastable Atom Neutralization (PACMAN). This process uses energetic neutral atoms (metastables) in addition
to a high-density plasma (Te ≈ 3 eV and ne ≈ 1017 m-3) to remove particles. The PACMAN process is
a completely dry process and is carried out in a vacuum which makes it compatible with other EUV related
processing steps. Experiments carried out on cleaning polystyrene latex (PSL) nanoparticles (30 nm to 500 nm)
on silicon wafers, chrome coated mask blanks, and EUV mask blanks result in 100 % particle removal with a
helium plasma and helium metastables. Removal rates greater than 20 nm/min have been achieved for PSL
material. Similar removal rates have been achieved for the PACMAN cleaning of carbon from silicon wafers
(simulating collector optic material) with 100% removal with helium plasma and helium metastables. The
PACMAN cleaning technique has not caused any damage to the substrate type being cleaned either through
roughening or surface sputtering. Current results of cleaning various particle types from surfaces through the
PACMAN process are presented.
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