EUV lithography is one of the most promising methods for next-generation lithography below 22 nm half pitch.
However, critical issues such as availability of a clean powerful source, resist resolution and sensitivity, and defect-free
masks have yet to be overcome.
Flare is one of the key issues for EUV lithography critical dimension (CD) control.1 The cause of flare is scattered light
due to the surface roughness of the mirrors in the projection optics. Mirror surface control techniques have reached the
angstrom level and are approaching physical limits.
Therefore, it is important to understand and evaluate "actual" flare and begin developing a mitigation strategy.
In this paper, we report on two evaluations of short-range flare using the ASML Alpha Demo Tool (ADT) in Albany,
NY.2 First, a series of donut shaped patterns of varying size are evaluated in order to determine the impact of flare on the
imaging of a central post surrounded by a clear annulus. A spillover parameter is used to estimate the ADT flare point
spread function. The results, which show roughly a slope of -1 on a log-log plot, are in agreement with those expected
from the power spectral density (PSD) due to mirror surface roughness. Second, an investigation into out-of-band (OoB)
radiation was performed, which leverages OoB reflection from the ADT's reticle masking (REMA) blades. We estimate
that the amount of OoB radiation is on the order of 3 to 4 percent of the EUV light.