Production of the 2nd generation bond & fuse immersed gratings for the ESA/Copernicus Sentinel-5 instrument.

For atmospheric science from space, the Netherlands has a strong track-record in the development and use of the grating spectrometers SCIAMACHY (2002), OMI (2004), TROPOMI (2017). Scientists and policy makers request ever-increasing detailed information about the abundance of greenhouse gases and their sources and sinks. This calls for instruments with high spectral and spatial (ground) resolution especially at the short-wave infrared (SWIR) wavelengths which is relevant for space-based monitoring of greenhouse gases with high sensitivity down to the Earth’s surface. In order to combine a high resolution and a limited instrument size, SRON and TNO introduced immersed gratings (IGs) for the SWIR spectroscopic channel of the TROPOMI push broom spectrometer (see smart technology boxed text). Such smart diffraction gratings allow in the SWIR spectral range for a volume reduction of more than an order of magnitude compared to classical reflection grating spectrographs.

More recently, a second-generation production process was developed at SRON to enable larger and more performing gratings also allowing for accurate tuning of the desired blaze angle and groove depth (see industrialization boxed text). The second-generation gratings will be employed in the SWIR 1 and SWIR3 channels of the UVNS spectrometer of ESA’s Sentinel-5 instrument on board of EUMETSAT’s METOP-SG series of satellites currently being deveoped. Further use of the IG technology for future GHG monitoring, for example for the ESA/EC projected dedicated CO2 mission Sentinel-7, is under study. SRON will also deliver an IG for the METIS (Mid-infrared E-ELT Imager and Spectrograph) instrument for the European Extremely Large Telescope that will, among other things, be used to characterize exo-planet atmospheres.

Smart technology The use of silicon immersed gratings offers advantages for both space- and ground-based spectrographs. As diffraction takes place inside the high-index medium, the optical path difference and angular dispersion are boosted proportionally, thereby allowing a smaller grating area and a smaller spectrometer size. SRON and our partner TNO were the first to develop (and patented) a production process for immersed gratings for space.

Industrialization The first-generation space gratings are lithographically made in a customized flow with monolithic 50 mm thick “wafers” involving specialized equipment. Prisms are carved out of the monoliths after grating patterning. This led to the worldwide-first space qualified immersed grating.

In the second-generation flow, the lithography is done on industry-standard silicon wafers and on standard equipment in the Philips innovation services labs in Eindhoven (NL). Prisms are shaped and polished independently. Then, in a bonding step, the prism and grating wafer are combined. Processing of standard wafers on standard machines has several advantages: Firstly, it is much faster than working on the monoliths. Therefore, improvement cycles can be faster and batch sizes can be larger. This results in a lean manufacturing flow characterized by: reduced cost, improved performance, and greater flexibility. Secondly, the bonding technology has the advantage that the prism and the grooved surface are produced in parallel; this reduces the risk of the program. Thirdly, the wafer based technology can be easily scaled up to larger grating sizes. We are currently compatible with 150 mm wafer technology. Coupling of the prism and the wafer is done by direct bonding, a well-known technique in the semiconductor industry.