Dimensional metrology of NPL photomask standards

This paper describes measuring machines, tools and calibration techniques developed at the National Physical Laboratory (NPL) for the calibration of optical metrology standards. Such standards include one dimensional line pitch and linewidth scales, two dimensional photomasks plates (X-Y), circular optical fibres and reference stage graticules used for the calibration of image analysers. Specific details are given on the design of a fully automatic length measuring machine (400 mm machine) having an absolute accuracy of 1 part in 10⁷ and modifications to a commercial photomask comparator are presented. Individual results from these measuring machines are plotted and their mathematical combination which has resulted in a state of the art capability for the calibration of seven inch two-dimensional photomask standards (uncertainty of 8 parts in 10⁷) is discussed