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High-precision length measurements

In precision manufacturing, ever higher demands are made on the accuracy of the length measuring systems used. Recently, the nanometer comparator – the standard measuring device of PTB for length graduations and for incremental length measuring systems based on them – has contributed considerably to essential progress in precision length measurements.

Measurement results of the three vacuum interference comparators on the incremental length measuring system LIP 382 used as transfer standard (background: Transfer standard on nanometer comparator).

In various areas of industrial production, the processing machines used must position extremely exactly – up to accuracies of a few nanometers. Examples of this are the exposure machines (wafer steppers) used for the lithographic manufacture of integrated circuits, or the diamond turning machines used for ultra-precise surface machining.

To comply with the required positioning accuracies and linearities, the measurement deviations of the length measuring systems used in the manufacturing process must be determined with very small uncertainty. For the assessment of such length measuring systems, the usually applied interferometric measurement methods with light propagation in air are not precise enough. In the case of the nanometer comparator developed in cooperation with the Dr. Johannes Heidenhain GmbH, the interferometric length measurement is, therefore, realized completely under vacuum conditions. Similar length comparators operated under vacuum conditions for longer evaluation lengths are at present not available at any other metrology institute. Two companies, however, which manufacture, among other things, incremental length measuring systems (the Heidenhain company itself as well as the Mitutoyo Corporation in Japan) also have vacuum length comparators at their disposal.

In a recently completed comparison of the three vacuum length comparators on an incremental length measuring system used as transfer standard, an agreement of the results of the length measurement of 5.5 nm was reached over a path of 280 mm in total. This corresponds to a relative agreement of better than 2 ·  10–8. This agreement is by at least a factor of 5 better than the agreement obtained for the preceding length measuring comparisons, where interferometry was operated in air.

This result documents the current efficiency of the vacuum length comparators that participated in the comparison. The requirements for the precision of industrial length and coordinate measurements, which will in future further increase, for example in the case of the imminent implementation of double exposure methods in 193 nm lithography, suggest a goal-oriented further development of the nanometer comparator. The corresponding work has already been started.

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