Accurate and traceable calibration of two-dimensional gratings

Two-dimensional (2D) gratings are widely used for calibrating the xy-plane of nearly all kinds of microscopes. The mean pitch, orthogonality and local pitch uniformity of the 2D gratings have to be calibrated prior to usage. In this paper, a method of accurate calibration of 2D gratings using a metrological large-range scanning force microscope is presented. A new measurement strategy is proposed, where the 2D gratings are measured in two narrow rectangular areas for determining all desired measurands and a small square area for viewing the grids in detail. The proposed strategy greatly shortens the measurement time, reduces the drift and eases the data processing procedure. Different data evaluation methods are introduced. Several 2D gratings with mean pitches from 100 nm to 10 mu m m have been calibrated, the results agree well with the values determined by an optical diffractometer. The expanded uncertainty (k = 2) of the mean pitch was approximately 15 pm for a 2D grating with a nominal mean pitch of 1000 nm, i.e. a relative uncertainty of 1.5 x 10(-5).