New displacement-angle differential interferometer: Meeting the future of the semiconductor industry

Highly sensitive measurements often do not take place under ideal laboratory conditions, but are exposed to disturbing environmental influences in the industrial environment, which quickly falsify the results. For applications like these, the German manufacturer SIOS Messtechnik GmbH therefore develops high-precision differential interferometers that have ultra-stable thermal and physical properties: Fluctuating parameters such as temperature, air pressure and humidity do not affect the very high measurement accuracy, even over longer periods of time. The new and hitherto unique displacement-angle differential interferometer combines the advantages of the differential interferometer with those of the proven three-beam interferometer and makes it possible for the first time to record several degrees of freedom with both long-term stability and synchronously.

Tolerances in the nano to picometer range

"High-tech display technology, in which a screen has several million pixels, exemplifies the current challenges of industrial metrology and the semiconductor industry," knows Dr. Denis Dontsov, Managing Director of SIOS Messtechnik. "The requirement for positioning accuracy in so-called stitching, i.e. lining up the individual structures in the manufacturing process, is around 45 nm per meter. This order of magnitude is comparable to the perfect lining up of cherry pits on the total area of Thuringia (a little more than 16,200 km2, editor's note)." Semiconductor manufacturing faces the same issue: The more compact and powerful chips become, the higher the demands on the precision of photolithography as a central manufacturing step. But refractive index measurements in the optics industry and expansion measurements of materials also require increasingly accurate results and only allow tolerances in the nanometer or even picometer range.

At the same time, the complexity of the individual measurement tasks increases, such as position controls of x-y tables, detection of thermal material expansions, investigations of the creep and drift behavior of objects, refractive index measurements, and high-precision length and angle measurements. The problem here is that all these measurements take place in production environments that cannot provide optimal laboratory conditions. The technology used must therefore be able to compensate for fluctuating environmental influences such as temperature, air pressure and humidity without falsifying its results, in order to also enable stable repeatability of multiple measurements. To this end, SIOS uses high-precision differential interferometers that achieve 25 times the stability of comparable measurement systems.

Requirements for x-y positioning mastered

However, the increasing demand for suitable solutions, especially for x-y positioning, further challenged the know-how of the measurement technology specialists at SIOS. Long-term stable measurements of several degrees of freedom were to be made possible simultaneously, even over greater distances. To realize this, they combined their high-precision SP 5000 TR three-beam interferometer, which is designed for simultaneous displacement and angle measurements, with the SP 5000 DI differential interferometer to benefit from its long-term stability. With the SP 5000 TR-DI displacement-angle differential interferometer, SIOS has thus developed the world's only system that uses two times three laser beams to measure displacement and angle highly synchronously and - thanks to compensation for environmental factors - with ultra-stability at the same time. "The new SP 5000 TR-DI is now able to detect not only the tiniest movements but also the smallest tilts over larger areas without the results being subject to thermal and physical environmental influences," explains Dr. Dontsov.

The ultra-stable and at the same time fast displacement-angle difference interferometers also have a reference beam for each of the three measuring beams. "The total of six laser beams are guided out of the sensor head in parallel and strike a flexible and a static reflector. In this way, a large part of the distance between the interferometer and the measurement location can be optically compensated. The actual measurement concentrates on the difference in length between the measurement and reference beams, so that environmental influences that could affect the result can only act on this small measurement range. Thanks to the highly symmetrical design of the sensor head, it is therefore also possible to measure a greater distance between the sensor and the measured object without having to accept falsification of the result. In addition, all differential interferometers are equipped with long-term stable sensors that have a temperature sensitivity of <20 nm/K.

Precisely fitting differential interferometers for a wide range of measurement tasks

For more complex, multi-axis measurement setups, several differential interferometers can simply be combined and operated with just one control unit. Thanks to integrated optical adjustment aids, the correct alignment of the measuring arrangement is quick and uncomplicated, even over several meters and axes. Thanks to the modular design principle, in which the different components and versions are compatible with each other, SIOS can generally react flexibly to individual measurement tasks and adapt optics, hardware or interfaces to specific applications within a short time. "It does not always remain with individual solutions: Like the new SP 5000 TR-DI displacement-angle differential interferometer, a large part of our product range has been developed on the basis of individual customer requests," sums up Dr. Dontsov. n