Etching of diamond microstructures for precise measurement in the extremely remote infrared range using a SENTECH ICP-RIE System

The European Space Agency's satellite mission planned for 2027 wants to analyse the earth's heat balance to better understand global warming and the global climate system. For the spectrometer on board the satellite, researchers from Jena needed to develop a new diamond structure that enables precise measurements in the extremely remote infrared range. A spectrometer is used on the satellite that records the heat radiation of the earth in the extremely remote infrared range which is, from approx. 10 to 100 micrometres.

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Figure 1 © Fraunhofer IOF

The key component here is the beam splitter [Figure 1] of the spectrometer. that is transparent across the entire spectral range. Conventional anti-reflective coatings, such as those used in lenses, are not suitable for this application, as they are made of layers of different materials and are not transparent across the entire spectral range and this is where diamonds really do become the project's best friend.

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Figure 2 © Fraunhofer IOF

Researchers from The Fraunhofer Institute for Applied Optics and Precision Mechanics IOF and the Institute of Applied Physics IAP at the Friedrich Schiller University developed and manufactured an innovative diamond microstructure using the SENTECH SI 500-400 ICP-RIE Multiwafer System [Figure 2], for the highly precise measurement of spectra in the extremely remote infrared range using a special technology in which microscopic pyramids are etched into a diamond inspired by the eyes of a moth, which are antireflective across a broad spectrum.

The concept has been used by Fraunhofer IOF for some time however, an etching process needed to be developed that created structures of the necessary shape accurately and reproducibly [Figure 3]. Furthermore, the extreme application requirements for this project due to the material properties of diamond, known to be one of the hardest materials in the world, make it a challenge to structure with the extreme shape fidelity required.

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Figure 3 © Fraunhofer IOF

The application process was developed using the SENTECH SI 500-400 Multiwafer inductively coupled plasma reactive ion etch (ICP-RIE) system for multi wafers and large substrates. The system is equipped with the proprietary Planar Triple Spiral Antenna (PTSA), a unique inductively coupled plasma (ICP) source that enables low-damage etching. ICP-RIE plasma is a preferred method for diamond plasma etching due to its high selectivity, low non-uniformity, and low damage. The low-damage processing is possible due to the narrow ion energy distribution and very low ion energies. Efficient PTSA source coupling of the driving power into the plasma leads to stable plasma at low operation pressure.

This project demonstrates the great potential of the use of diamond as an optical material through nanostructuring in the extreme range and shapes the future use of diamond meta-optics, wafer-scale diamond magnetic field sensors, resonant diamond mirrors, and many more applications.

The FORUM mission marks a significant milestone in the study of climate change and the application of diamond structures in space technology. In total, the Jena research team which consists of employees from the Fraunhofer IOF and the IAP, has worked on the development of the new diamond structure for four years. The development work is carried out in close coordination with the client OHB SE and ESA as a mission provider.

The air-ready jet splitter for use in the FORUM mission was handed over to OHB in December 2023.

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