Reevaluation of the accuracy of NIST photmask linewidth standards

Every artifact measurement standard has some uncertainty associated with its calibration, and the NIST Photomask Linewidth Standards are no exception. The methods of estimating measurement uncertainty, however, and the interpretation of its meaning, have been subjects of discussion for many years. The International organization for Standardization (ISO) has recently published a Guide to the Expression of Uncertainty in Measurement, which outlines an operational procedure for estimating and combining the effects of the various contributing factors. A new NIST policy on measurement uncertainty has led to a change in the method of determining the uncertainty of NIST Photomask Linewidth Standards to comply with ISO recommendations. Consequently, their calibration uncertainty has changed. The new method of evaluating systematic components leads to increased systematic uncertainty, and the new method of combining uncertainty components leads to reduced calibration uncertainty. This paper describes the old and new methods and compares their results.