Accuracy and traceability in dimensional measurements

While the importance of the concepts of measurement accuracy and measurement traceability has long been recognized, they have not always been applied in a consistent or rigorous manner. In 1993 the International Organization for Standards (ISO) published two documents which establish consistent definitions of these and other metrology terms and provide an unambiguous way to calculate measurement uncertainty. These documents are widely accepted in the international metrology community. Vendors and buyers of metrology tools and standards will benefit from the improved communication that results from using this standardized metrology vocabulary. Dimensional measurements of importance to microlithography include feature sizes and feature placement on photomasks and wafers, overlay eccentricities, defect and particle sizes on masks and wafers, step heights, and many others. A common element in these measurements is that the object sizes and required measurement uncertainties are often on the order of the wavelength of light or less. This can lead to interesting challenges for certain applications where measurement traceability is desirable. The necessary and sufficient conditions for traceability will be outlined, and some examples will be given.