The National Institute of Standards and Technology is developing a simple one-dimension certified pitch standard (or scale) covering the range 1 um to 10 mm, intended for the calibration of microscope magnification and dimensional metrology instrument scales. While NIST has long provided length scale calibrations, this traceable artifact is a new product. Called SRM 2800, Optical Microscope Magnification Standard, it consists of symmetrical nested linear pitch patterns in decade ranges. The patterns are printed in etched chrome on a quartz microscope slide, using photomask techniques to assure high quality. The arrays of chrome parallel lines are printed on a clear background to facilitate use in transmission mode optical microscopes as well as in reflection mode. This pitch standard is also useful in atomic force microscopes, and in scanning electron microscopes and scanning tunneling microscopes when coated with a conducting film (although there are other standards from NIST which are more suitable for SEMs). It can be used to calibrate the scales of micromachining tools. The positions of the centers of the lines relative to the origin in the center of the pattern will be certified. The calibration will be performed optically, using the NIST UV Microscope. This is a transmission mode scanning optical microscope which is also used to calibrate photomask linewidth standards. A subset of a production lot will be calibrated, with the results applied to the remainder of the lot taking into account the sampling statistics. Expected calibration uncertainty is better than 50 nm for lot-calibrated units, and 5 to 20 nm for individually calibrated units. Calibrations will be traceable to the meter through the NIST Line Scale Interferometer. The linewidths are not calibrated. While this standard facilitates accurate magnification and pitch measurements, care must be taken when measuring the size (left edge to right edge, or linewidth) of an object. There are fundamental differences between linewidth and line spacing measurements. The appropriate definition for edge becomes an important issue, and proximity effects and edge effects are important in optical measurements when the required measurement uncertainty is less than the wavelength of the light used.
Proceedings of American Society for Precision Engineers