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Measuring spheres with nanometer accuracy

Interferometric measuring device for silicon spheres and other spherical objects manufactured with high accuracy

PTBnews 3.2022
01.12.2022
Especially interesting for

precision manufacturing

device for in-process measurement

metrology institutes

Within the scope of the new definition of the kilogram – the SI unit of mass –, a procedure for manufacturing nearly perfectly round silicon spheres was developed at PTB. The form errors of these spheres only range from approx. 10 nm to 100 nm, and the spheres must be polished for several weeks. During this period, their form and roughness must be checked on a daily basis. A novel measuring device was developed specifically for this purpose. It can determine the form error of a sphere within a few minutes.

The sphere is located on a rotating stage and is measured at two opposing points by means of laser light.

Interferometry is based on an incident laser beam overlapping with the reflected laser beam. In this way, lengths on the nanometer scale can be measured precisely. Such measurements may, however, be influenced by disturbing displacements of the sphere during its rotation. The sphere is therefore measured simultaneously at two opposite points, the nearly perfectly polished surface of the sphere serving as a reflector. The distance signals of the two collinear beam paths thus obtained are detected while the sphere is being rotated and are recorded as the sum of the two signals. Without further geometric error correction, this sum represents the change in the diameter of the sphere along the corresponding equator. The advantage of this measurement principle is that disturbing displacements of the sphere while it is being rotated only have very little influence on the measurement result. If the sphere is shifted along the beam path, the sum signal does not change. Therefore, the rotational axis does not require exceptional precision.

Moreover, other features of the measuring device ensure that it has a low sensitivity to environmental influences such as temperature or vibrations. The entire setup is robust and easy to operate. It does not require any mechanical probes that might scratch the surface. A special transport cylinder is used to protect the sphere in the laboratory. This transport cylinder also allows the sphere to be placed into the measuring device without any collisions and it serves as a thermal protection system during the measurement, too.

With the experiences gathered while using an experimental model, SIOS Messtechnik GmbH, in cooperation with PTB, has optimized the entire setup. The repeatability of the measurements is a few nanometers. The measuring device has been in operation for many months and has provided reliable results every day. These results are necessary for manufacturing the spheres. Adapting this device to spherical transparent objects of a similar diameter would be conceivable – such a device is already available as a concept.

Contact

Rudolf Meeß
Department 5.5 Scientific Instrumentation Department
Phone: +49 531 592-5560
Opens local program for sending emailrudolf.meess(at)ptb.de

Patent

Rudolf Meeß, DE 10 2017 106 184 A1 2018.09.27, Method for measuring the form error of a sphere and sphere measuring fixture for this purpose

Scientific publication

R. Meeß, D. Dontsov, E. Langlotz: Interferometric device for the in-process measurement of diameter variation for the manufacturing of ultraprecise spheres. Measurement Science and Technology 32, 074004 (2021)