Advanced photomasks for low-k1 lithography, are prone to various defects sources: contamination, geometry,
transmission, phase, etc. These defects exhibit a complex relation between the signal from an imaging detector and its
print related impact, with important consequences for the performance of the detection scheme under nuisance-ubiquity
We studied numerically several imaging schemes, with respect to their defect detection signal and its relation to the
associated CD effect. We show that for actinic aerial imaging detection the signal is tightly correlated and linearly scaled
with the induced CD variation regardless of defect source and location. Conversely, the correlation of non-actinic and/or
non-aerial (high-resolution based) detection signal with printing effect is poor. Whereas the linear behavior
characterizing aerial imaging is independent of the distribution of defect attributes, the statistics of non-aerial defect
signal is shown to be highly sensitive to defect distribution. Such non-aerial detection schemes would generally have to
compromise detection sensitivity in order to maintain a constant nuisance false alarm rate. Aerial imaging is therefore the
optimal discriminator between printing and non-printing defects.
The tight linear correlation between defect signal and CD effect in aerial inspection systems, allows for an optimized and
effective mask inspection, suitable for all mask types and technologies. Specifically, we show here that such a tool
allows a straightforward migration from 65nm node to 45nm and 32nm with double patterning, by tuning the detection
threshold without being flooded by nuisance induced false alarms.