The National Institute of Standards and Technology (NIST) has a multifaceted program in AFM dimensional metrology. Two major instruments are being used for traceable measurements. The first is a custom in-house metrology AFM, called the calibrated AFM (C-AFM), and the second is the first generation of commercially available critical dimension AFM (CD-AFM) the Veeco SXM320. Both of these instruments have useful applications in photomask metrology.The NIST C-AFM has displacement metrology for all three axes traceable to the 633 nm wavelength of the Iodine-stabilized He-Ne laser. In the current generation of the system, the approximate levels on the relative standard uncertainty of pitch and step height measurements were 0.1 % for pitches at the micrometer scale and 0.1 % for step heights at the 100 nm scale. Although the ultimate limits have not been fully validated, we expect further improvements in performance in the near future.If the uncertainties of step height measurements can be sufficiently reduced, this could have implications in the calibration of phase offset for alternative aperture phase shifting mask (AAPSM) standards. For example, the uncertainties of some recent attempts at phase calibration using traceable step heights might be reduced by at least a factor of three. The inclusion of phase shift targets is being considered for the next generation of NIST photomask standard reference material (SRM).Providing linewidth reference metrology is an important application of CD-AFM. The most recent release of the NIST photomask standard which is SRM 2059 was supported by CD-AFM reference measurements. The SXM320, since it has the capability of measuring vertical sidewalls, complements the NIST C AFM. Although it does not have intrinsic traceability, it can be calibrated using standards measured on other instruments including the C-AFM. In addition to supporting the photomask SRM, we are using this tool to support the development of electrical resistance metrology for measurement of on-mask test structures. In a preliminary comparison of AFM and electrical measurements, we observed agreement usually less than 5 nm - and consistent within partial uncertainty estimates.
Proceedings Title: Proceedings of SPIE
Conference Dates: September 18, 2007
Conference Location: Monterey, CA
Conference Title: Photomask Technology 2007
Pub Type: Conferences
calibration, CD-AFM, metrology, photomask, SRM, standards, traceability, uncertainty