| Working Group 5.21 |
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Length Graduation
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The Nanometer Comparator
The measurement of length is one of the most common tasks in research and industry. In order to satisfy the highest requirements, which occur for example at the determination of dimensions of semiconductor structures, length measurements have to be performed with uncertainties of only a few nanometer.
The Nanometer Comparator was developed in collaboration of PTB, Dr. Johannes Heidenhain GmbH and Werth Meßtechnik GmbH to provide a highly accurate traceability to the units of length and to allow fundamental investigations of the detection of geometrical structures. Designated measurement objects, which may have a measurement range up to 610 mm, are photo masks, linear encoders, line scales, and interferometers. It is intended to achieve a measurement uncertainty of 5 nm over the full range.
Figure 1: A picture of the Nanometer Comparator
Figure 1 shows a picture of the Nanometer Comparator. The instrument is located in an air-conditioned measurement room (temperature stability of 0.1oC). In such rooms the achievable calibration uncertainty is normally determined by the correction of the wavelength in air ( refractive index of air). The operation of the whole instrument in vacuum does not lead to the lowest possible uncertainty in traceable length measurements because the length measurement systems currently used operate only very rarely in vacuum and because the compressibility of the scales that have to be calibrated is not known sufficiently accuratly. Thus, the currently lowest possible measurement uncertainty will only be achieved if the interferometer is located in vacuum. Therefore a metal bellow was introduced between the beam splitter of the interferometer and the movable measurement reflector.
A schematics of the instrument is shown in fig. 2. The comparator is based on a granite bed. The displacement of the scales is provided by a carriage with air bearings which is driven by a linear motor. The interferometer head that includes the beam splitter as well is located in a vacuum chamber which is mounted on the granite. Starting from this chamber the metal bellow goes to a tracking carriage. A second metal bellow leads from here to the measurement carriage so that the interferometer beam is completely located in vacuum. In order to avoid the introduction of the bellow forces to the measurement carriage the distance of both carriages is kept constant by a controller.
Different systems for the structure localization can be connected to an universal mounting plate of a massive bridge over the measurement carriage. At the time of implementation of the Nanometer Comparator incremental detectors and two photoelectric microscopes are available for this purpose. It is planned to investigate further structure localization methods at a later time. Each air bearing is equipped with a piezo translator which is used by a controller to reduce the intrinsic guidance deviations even further.
Figure 2 : Principle of the Nanometer Comparator
Literature
J. Flügge, H. Dangschat, A. Spies, J. Tschirnich, H. Pieles : Concept of a interferometric length comparator with measurement uncertainties in the nanometer scale, Tagungsband 1. Euspen Conference, Bremen Mai 1999
J. Flügge, G. Dai : Design of the temperature measurement and control system at the PTB nanometer comparator, Tagungsband 1. Euspen Topical Conference, Copenhagen, Mai 2000
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Page created: 2004-05-06, last update: 2006-05-04, D. Schulz |
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