The National Institute of Standards and Technology (NIST) has had a robust program in photomask dimensional metrology since the late 70s when the late Diana Nyyssonen and coworkers developed the first chrome-on-glass (COG) Standard Reference Material (SRM) for linewidth metrology. There have since been four more generations of the NIST photomask SRM the most recent being SRM 2059. An in house tool developed at NIST, called the NIST ultra-violet (UV) microscope, is 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 atomic force microscope dimensional metrology. Since 2001 NIST has cooperated with SEMATECH to implement a traceable critical dimension atomic force microscope (CD-AFM) reference measurement system (RMS) using the most current generation CD-AFM tool at SEMATECH. The initial measurements on the SRM 2059 master were made in 2004 as part of this collaboration. At the time of these measurements, however, the uncertainties in the CD-AFM metrology were larger than can be achieved because NIST single crystal critical dimension reference material (SCCDRM) project had not been completed. Using our CD-AFM at NIST, we are performing new measurements on the SRM 2059 master standard. Recent 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 are reanalyzed using these new AFM results, we expect to reduce the combined reported uncertainties by approximately 40 %.
Proceedings Title: SPIE Proceedings Vol. 7122
Conference Dates: October 5-10, 2008
Conference Location: Monterey, CA
Conference Title: SPIE/BACUS Photomask Technology Conference
Pub Type: Conferences
CD-AFM, metrology, CD, linewidth, photomask, standards, calibration, traceability, SRM