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Length measurements with nanometre-accuracy

Writing ever smaller structures on computer chips also requires measurement devices of increasingly improved accuracy. In cooperation with two German companies in the fields of machine tool control and coordinate measuring devices PTB has developed a length comparator. The new device significantly reduces the hitherto achievable measurement uncertainties for measurement lengths of 2 mm to 610 mm.

The new nano-comparator

Until now world-wide the spacing of structures (as SI-traceable length measurements) can only be measured with an uncertainty of roughly 20 nm. Yet today highly precise position controlling and ever more-densely built integrated circuits already require length calibrations with lower measurement uncertainties. The new nano-comparator allows calibrating one-dimensional structure distances on two-dimensional specimen such as photomasks of the semiconductor industry. Measurement systems to calibrate highly accurate processing and measurement devices can be characterized as well. Objects with a width up to 450 mm can be used.

Due to the high requirements on accuracy the comparator was constructed and installed with great expenditure and effort. For instance, the sliding carriage to move the specimen is fitted on air cushion bearings and equipped with piezo-activators to compensate the guide deviations of the bearings detected by the measurement system. The displacement is measured with a special in-vacuum interferometer set-up, in which the interferometer translation is encapsulated in a metal bellows. This practically eliminates the influence of the refractive index of air on the measurement result – which otherwise would dominate the measurement uncertainties. The interferometer itself consists of three reference beams in an arrangement for differentiation measurements with reflectors on the rigid frame of the comparator which compensate the mechanically and thermally induced shifts between the microscope bridge and the beam-splitter of the interferometer.

First comparative measurements with other length comparators from PTB and external sources using 200 mm line scale standards demonstrate agreements better than 30 nm which correspond roughly to the measurement uncertainty of these comparators. In the near future calibration services with measurement uncertainties below 10 nm are to be offered with the new nano-comparator. Further optimization of the comparator will aim to reduce the measurement uncertainty to below 5 nm. For instance, due to the length of the measurement circles measurement uncertainties below 1 nm appear to be unrealistic even for small measurement lengths. Therefore PTB is preparing a further measurement facility for measurement lengths below 2 mm. First investigations with this facility are presented in the next contribution.

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