The current photomask linewidth Standard Reference Material (SRM) supplied by the National Institute of Standards
and Technology (NIST), SRM 2059, is the fifth generation of such standards for mask metrology. An in house optical
microscope tool developed at NIST, called the NIST ultra-violet (UV) microscope, was used in transmission mode to
calibrate the SRM 2059 photomasks. Due to the limitations of available optical models for determining the edge
response in the UV microscope, the tool was used in a comparator mode.
One of the masks was selected as a master standard - and the features on this mask were calibrated using traceable
critical dimension atomic force microscope (CD-AFM) dimensional metrology. The optical measurements were then
used to determine the relative offsets between the widths on the master standard and individual masks for sale to
customers. At the time of these measurements, however, the uncertainties in the CD-AFM reference metrology on the
master standard were larger than can now be achieved because the NIST single crystal critical dimension reference
material (SCCDRM) project had not been completed.
Using our CD-AFM at NIST, we have performed new measurements on the SRM 2059 master standard. The new AFM
results are in agreement with the prior measurements and have expanded uncertainties approximately one fourth of
those of the earlier results for sub-micrometer features. When the optical comparator data for customers masks are
reanalyzed using these new AFM results, we expect to reduce the combined reported uncertainties for the linewidths on
the actual SRMs by at least 40 % for the nominal 0.25 μm features.