On-line monitoring of the refraction index of air for ultra-precise length measurements in the nano-world

In the production of semiconductor microchips, nano-components like MEMS, in the field of optical lithography, an even more precise length measuring devices are necessary for a reliable fabrication. In a lot of cases we can find modern high- resolution laser interferometers. They use a wavelength of the laser beam like a fundamental unit of length. If the interferometers measure distances in atmospheric conditions, the absolute value of the laser wavelength fluctuates in linear form with a refraction index of air. It causes a distortion of the measured values, so that, the repeatability of controlled dimensions can be degraded. To elimination of this influence the on-line monitoring of the value of the refraction index of air is very necessary. On basis of knowledge of the index, the wavelength of the laser beam used in the distance measurement can be corrected to true value. Because the technique of direct measurement of the refraction index are not solved sufficiently now, we put together a new method that uses a special design of an optical resonator working like a laser refractometer. The possible changes of the refraction index of air have the influence to the optical length of the resonator. Principle of monitoring of the length changes of the resonator, down-conversion of the optical frequency changes of a tunable laser into the radio-frequency domain, and experimental verification of the method with any others are key parts of the presented work.