Picometer-scale accuracy in pitch metrology by optical diffraction and atomic force microscopy

We measured the pitch of a 144-nm pitch, two-dimensional grid in two different laboratories. Optical Diffraction gave very high accuracy for mean pitch and Atomic Force Microscopy measured individual pitch values, gaining additional information about local pitch variation. The measurements were made traceable to the international meter. Optical diffraction gave mean value 143.928 +/- 0.015 nm (95% confidence limit, per GUM). AFM gave mean value 143.895 +/- 0.079 nm. Individual pitch values had standard deviation 0.55 nm and expanded uncertainty +/- 1.1 nm. Mean values measured by the two methods agreed within 0.033 nm. Because this was less than the uncertainty due to random variation in the AFM results, it suggests that the AFM measuring and analysis procedures have successfully corrected all systematic errors of practical significance in microscopy. We also discuss what precision may be expected from the AFM method when it is applied to measure smaller pitches.