In this study we examine the effects on lithographic performance due to repetitive cleans on a mask patterned for use in
extreme ultraviolet lithography (EUVL). Exposures were carried out at the SEMATECH Berkeley micro-exposure tool
(MET) on both a cleaned mask and a reference (uncleaned) mask with the same mask architectures. The performance is
measured against the process window for lines and spaces, line edge roughness (LER), and contact size variation
measured using scanning electron microscopy (SEM). Mask properties such as surface roughness were used as metrics
of the cleaning process effects. We also introduce a new method involving the correlation in LER of a single line from
exposure to exposure at the same dose and focus. If mask cleaning were to introduce significant damage to either the
capping layer or the absorber, we might expect an increase in LER correlation from exposure to exposure of the same
feature, as uncorrelated effects due to the resist cause a second order change depending on aerial image contrast loss.
We look at these metrics on the same mask used in previous cleans studies, now for a 50X and 75X cleans.
In this study, the impact of repetitive cleaning of EUV masks on reflectivity, surface roughness and lithographic
performance was evaluated. Two masks were fabricated and patterned with the same layout using commercially
available EUV blanks; one was subjected to 33 cleaning cycles and the other was kept as a reference. Wafers were
patterned using both masks on the SEMATECH Berkeley 0.3 NA micro-field exposure tool (MET), and the data was
used to determine process latitude and line edge roughness at regular intervals between cleaning cycles. Additionally,
mask surface roughness and EUV reflectivity were also measured. After a total of 33 cleaning cycles, minimal
degradation was observed in lithographic performance compared to the reference mask, as well as surface roughness and
reflectivity.
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