Calibration of the displacement with nano- and picometer precision
In metrology it is assumed that the accuracy of the gauge should be higher than the required accuracy of measurement at least an order of magnitude. In the field of nano values implementation of this principle is difficult, and in the range of a few picometers is technically impossible. In addition, the use of traditional "static" standards requires the solution of a whole complex of problems associated with maintaining consistency in their sizes, in particular, the prevention of adsorption and dust. Therefore, we chose another way – use of "active" systems, displacement gauges on the basis of materials with the reverse piezoelectric effect. We use synthetic single crystals, such as lithium niobate, which are characterized by a better linearity than the piezoelectric ceramics. The geometrical sizes of such crystals change in proportion to the applied voltage. The magnitude of these changes depending on the applied voltage is determined using a high-precision optical interferometers. Thus, on the basis of lithium niobate crystals without the use of nanotechnology a new type of calibration gauges for nano- and picometer ranges is created that can be used for the calibration of measuring instruments: scanning probe and electron microscopes, as well as for other tasks. Our invention is protected by four patents.
Neither the atmospheric composition nor the composition of the reference surface, nor the magnetic field does not affect the magnitude of the displacement. A few degrees of temperature change also does not have any significant effect, and large variations are excluded in modern laboratories and high-tech production facilities.
How to calibrate the optical interferometer, which controls the amount of deformation of the piezoelectric crystal, and what is the measurement error?
For calibration of the optical interferometer measures of Rosstandart are used. The cesium cells are used to stabilize the frequency of laser radiation in our interferometers, so the measurement accuracy is high enough for use not only in nano-but also in the picometer range. At single measurements the noise level is 50 picometres, but when a series of measurements are performed the signal-to-noise ratio can be improved by an amount defined as the square root of the number of measurements. The measurement time is very short, so in practice it is realistic to perform from 100 to 10 thousand measurements, reducing the noise level in 10-100 times. For example, we experimentally showed the possibility of measurement of picometers displacements. It should be noted that all the used methods of measurements and results processing are standard and widely used in scientific and industrial practice.
Is it possible to integrate calibration system and measuring instrument?
We have developed several types of optical interferometers, including miniature, which can be integrated in atomic force microscopes. This will allow to calibrate the operation of the cantilever drives and improve the accuracy of measurements.
What are the prospects for the use of your developments in metrological practice?
Our displacement gauges were highly appreciated by the All-Russian research Institute of metrological service (VNIIMS), and there are prerequisites to the fact that they will acquire the status of the state standards. Participation in international exhibitions and conferences in Europe, Asia and America showed that interest in our development is great at devices manufacturers, educational institutions, and metrology organizations. In particular, we conducted preliminary tests in German metrological Institute PTB (Physikalisch-Technische Bundesanstalt) and presented joint reports at several conferences. In the future we expect wide implementation of our displacement gauges abroad as a reliable and easy to use nano- and picostandards are required in all countries that are developing nanotechnologies.
Of course, one of the necessary conditions for the implementation of the invention into practice is the education of specialists. We organize training courses, and remote learning with the use of the remote control of the calibration system. An optional lecture for students are held at Lomonosov MSU and MIREA, so I hope that a new generation of professionals will be able to provide high accuracy measurements in nano- and picometrer ranges.