Subnanometer measuring method of highest precision with ten decades measuring range

Background

"Three orders of magnitude create a new science" - this is how particle physicist Murray Gell-Mann, the discoverer of quarks, describes what awaits researchers on their journey into the world of the very smallest. And the world of the very smallest, for nanoscientists, is the realm of molecules and atoms. Their size is a fraction of a nanometer - the atomic lattice distance of car­bon is 0.246 nm. This corresponds to the 1/300 000 of a hair diameter. In the field of nanotech­nologies, it is preparing to produce semiconductor structures with almost atomic dimensions on wafers hundreds of millimeters in diameter. This naturally requires measuring instruments of the highest precision.
The most accurate length measuring systems today include laser interferometers, whereby the highly accurate "immaterial" length scale is formed by the laser wavelength, which makes it pos­sible to measure already the one-millionth part of a hair diameter. Here, the Ilmenau researchers around Prof. Eberhard Manske have made a unique contribution. In just two years of research work, they have succeeded in improving the measurement uncertainty of this length scale, i.e. the wavelength of the laser light used, by an enormous factor of 1000, i.e. by a further three or­ders of magnitude to 10^-12.

Direct combination of meter and second in a precision measuring machine

They achieved this by controlling a special laser using the latest frequency comb technology to the high-precision signal of GPS satellite-based atomic clocks and by directly and thus permanently feeding it back to the unit of the second. Until now, the unit of length was determined by the accu­racy of the wavelength of the lasers used in the precision length measuring systems. According to the state of the art, relative uncertainties of 10^-8 to 10^-9 can be achieved here.  However, the meas­urement uncertainty of the second, embodied by the frequency of atomic clocks, is ten to one hun­dred thousand times lower.

Based on many years of experience in the field of laser frequency control and stabilization, the Ilmenau researchers succeeded in stabilizing a normal He-Ne laser directly at a constant differ­ence frequency to a highly stable crest line of such a frequency comb with uncertainties of about 10^-12. The great advantage is that this enormous accuracy is permanently available due to the cou­pling to the GPS signal. This provides a direct and permanent traceability of the laser wavelength as a linear scale to the time standard, the highly accurate reference frequency of GPS atomic clocks.

Finally, the new quality of laser stability was transferred to a direct three-dimensional coordinate measurement by connecting it to the NPMM-200 nanopositioning and nanomeasuring machine, also developed at the TU Ilmenau, with a measuring range of 200 mm x 200 mm x 25 mm and a resolution of 20 pm (corresponds to one tenth of the atomic lattice spacing). This closes the meas­urement chain for a new subnanometer measurement method spanning ten decades.