August, 2022: Ready for the future of the semiconductor industry: New displacement-angle differential interferometer captures multiple degrees of freedom simultaneously and with long-term stability

Interferometry specialists develop tailored solutions for individual measurement tasks

Tiny microchips and enormous 8K displays could not be more different in their dimensions – and yet they have one thing in common: their structures are becoming ever finer and are placing increasing demands on manufacturing technology. Tolerances in the picometer range have to be dealt with, which repeatedly pushes the measurement methods used to the limits of what is technically feasible. This is because the highly sensitive measurements do not take place under ideal laboratory conditions, but are exposed to disruptive environmental influences in the industrial environment, which quickly falsify the results. For applications like these, SIOS Meßtechnik GmbH develops high-precision differential interferometers with ultra-stable thermal and physical properties: Fluctuating parameters such as temperature, air pressure and humidity do not affect the very high measuring accuracy even over longer periods of time. The new and so far unique displacement-angle differential interferometer combines the advantages of the differential interferometer with those of the well-established triple-beam interferometer and makes it possible to record several degrees of freedom both synchronously and with long-term stability.

“The current challenges of industrial metrology and the semiconductor industry are exemplified by high-tech display technology, in which a screen has several million pixels,” knows Dr. Denis Dontsov, Managing Director of SIOS Meßtechnik GmbH. “The requirement for positioning accuracy in so-called stitching, i.e. the lining up of individual structures in the manufacturing process, is about 45 nm per meter. This order of magnitude is comparable to the perfect lining up of cherry pits on the total area of Thuringia.” Semiconductor manufacturing has to face the same problem: The more compact and powerful chips become, the higher the demands on the precision of photolithography as a central manufacturing step. But also refractive index measurements in the optics industry and expansion measurements of materials 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 offer optimum laboratory conditions. In order to also enable stable repeatability of multiple measurements, the technology used must therefore be able to compensate for fluctuating environmental influences such as temperature, air pressure and humidity without falsifying its results. To do so, SIOS uses high-precision differential interferometers that achieve 25 times the stability of comparable measurement systems.

Displacement-angle differential interferometer masters increasing demands on x-y-positioning

The increasing demand for suitable solutions, especially for x-y-positioning, however, 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 over longer distances. To achieve this, they combined their high-precision SP 5000 TR triple-beam interferometer, which is designed for simultaneous displacement and angle measurements, with the SP 5000 DI differential interferometer in order 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 tree laser beams to measure displacement and angle highly synchronously and – thanks to the compensation of 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 differential interferometers also have a reference beam for each of the three measuring beams. The total of six laser beams are directed out of the sensor head in parallel and hit a flexible as well as a static reflector. In this way, most 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 measuring and reference beams, so that environmental influences that could affect the result can only have an effect on this small measuring 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 risking any distortion 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 only 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 react flexibly to individual measurement tasks and adapt optics, hardware or interfaces to specific applications within a short time. “In doing so, we do not always stick to one-off solutions: Like the new displacement-angle differential interferometer SP 5000 TR-DI, a large part of our product range has been developed on the basis of individual customer requests,” summarizes Dr. Dontsov.

Further information online at https://www.sios-precision.com/en/