The National Institute of Standards and Technology (NIST) interferometer for measuring graduated length scales has been in use since 1965. It was developed in response to the redefinition of the meter in 1960 from the prototype platinum-iridium bar to the wavelength of light. The history of the interferometer is recalled, and its design and operation described. A continuous program of modernization by making physical modifications, measurement procedure changes and computational revisions is described, and the effects of these changes are evaluated. Results of a long-term measurement assurance program, the primary control on the measurement process, are presented, and improvements in measurement uncertainty are documented.

We at NIST are building a metrology instrument called the Molecular Measuring Machine (M3) with the goal of performing two-dimensional point-to-point measurements with one nanometer accuracy over a 50 mm by 50 mm area. The instrument combines a scanning tunneling microscope (STM) to probe the surface and a Michelson interferometer system to measure the probe movement, both with sub-nanometer resolution. The instrument also features millidegree temperature control at 20 °C, an ultra-high vacuum environment with a base pressure below 1 O Pa, and seismic and acoustic vibration isolation.High-accuracy pitch measurements have been performed on one-dimensional gratings. In one experiment, the M3 STM probe imaged an array of laser-focused, atomically deposited chromium lines over an entire 5 jtm by 1 mm area. Analysis of the data yielded an average line spacing of 212.69 nm with a 5 pm standard uncertainty. The uncertainty estimate is derived from an analysis of the sources of uncertainty for a 1 mm point-to-point measurement, including the effects of alignment, Abbe offset, motion cross-coupling, and temperature variations. In another measurement, the STM probe continuously tracked a holographically-produced grating surface for 1 0 mm, counting out 49,996 lines and measuring an average line spacing of 200.01 1 run with a 5 pm standard uncertainty.We are continuing the development of M3, making modifications that will lower the measurement uncertainty toward the one nanometer goal.

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