Etching Diamond Microstructures for Precise Measurements in the Far Infrared Using a SENTECH ICP-RIE System
Supporting the ESA FORUM Mission
The European Space Agency’s FORUM satellite mission, planned for launch in 2027, aims to analyse the Earth’s heat balance. The mission will help scientists better understand global warming and the Earth’s climate system by measuring heat radiation emitted from the planet. To achieve this, the satellite will carry a highly sensitive spectrometer capable of recording thermal radiation in the far infrared spectral range, from approximately 10 to 100 micrometres. Researchers in Jena, Germany, were tasked with developing a novel diamond microstructure that would enable precise measurements across this challenging wavelength range.
Finding the Right Anti-Reflective Solution
A critical component of the spectrometer is the beam splitter. This optical element must remain transparent across the entire far infrared spectrum. Conventional anti-reflective coatings, such as those commonly used on lenses, are not suitable for this application. These coatings typically consist of multiple layers of different materials and do not maintain transparency across such a broad spectral range. To overcome this challenge, researchers from the Fraunhofer Institute for Applied Optics and Precision Engineering IOF and the Institute of Applied Physics (IAP) at Friedrich Schiller University Jena developed an innovative diamond microstructure using a SENTECH ICP-RIE system. The solution is inspired by the structure of a moth’s eye. Microscopic pyramid-shaped features are etched directly into the diamond surface, creating an anti-reflective structure that operates across an exceptionally broad wavelength range. This approach enables highly precise spectral measurements in the far infrared while maintaining excellent optical transparency.
Development of the Diamond Etching Process
Although the underlying concept had already been explored by Fraunhofer IOF, a specialised plasma etching process was required to create the diamond structures with the necessary accuracy, reproducibility and shape fidelity. Developing this process presented a significant challenge due to the unique properties of diamond. As one of the hardest known materials, diamond is extremely difficult to structure while maintaining the precise geometries required for advanced optical applications. The process was developed using a SENTECH ICP-RIE system equipped with the proprietary Planar Triple Spiral Antenna (PTSA). The PTSA source is a unique inductively coupled plasma (ICP) technology designed to enable low-damage plasma processing.
ICP-RIE is a preferred method for diamond plasma etching because it offers high selectivity, low non-uniformity and minimal surface damage. The low-damage characteristics are achieved through a narrow ion energy distribution and very low ion energies. In addition, the efficient coupling of RF power into the plasma provided by the PTSA source enables stable plasma operation at low process pressures.These capabilities allow the precise and reproducible fabrication of complex diamond microstructures required for demanding optical applications.
Looking to the Future
This project demonstrates the significant potential of nanostructured diamond as an advanced optical material for far infrared applications. The successful development of the diamond beam splitter not only supports the FORUM mission but also opens new opportunities for future technologies. Potential applications include diamond meta-optics, wafer-scale diamond magnetic field sensors, resonant diamond mirrors and many other advanced photonic devices. The FORUM mission represents an important milestone in both climate research and the application of diamond microstructures in space technology. Researchers from Fraunhofer IOF and the IAP spent four years developing the new diamond structure, working in close collaboration with OHB SE and the European Space Agency.
In December 2023, the flight-ready beam splitter for the FORUM mission was successfully delivered to OHB, marking a major achievement in the development of advanced diamond-based optical components for space applications.
