The photo-lithographic process of very large scale integrated (VLSI) microelectronics fabrication is based upon the presentation, placement, photoreduction and fine movement of pattern-masks to generate submicrometre features on planar microcircuit 'wafer' substrates. For this, two major issues of accuracy arise: the two-dimensional positioning of features on the photomasks and the precise (stepper) motions which are essential if transfer of patterns from photomasks to wafers are to meet current overlay accuracy requirements of <100nm. (Boworth et al 1995) This has considerable implications for the metrology of artefacts and equipment and, as part of UK (NPL Teddington), FRG (PTB Braunschweig) and European Community (BCR Brussels) standards program for the development of a two dimensional high precision photomask standard, an appropriate areal metrology facility was required.
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