Micro-Assembly Technologies at Fraunhofer IOF

The goal of the ExoMars mission of the European Space Agency (ESA) is to examine the surface of Mars to search for signs of past and present life on the planet and characterize geochemical structures. The Exomars mission will be the first Martian research that comprises the combining capabilities of driving with a rover across the planet, drilling up to 2 m below the surface, and having a stationary science surface platform. For the purpose of analyzing the Martian samples by Raman Spectra measurements from the rover, a doubled 532 nm diode-pumped solid-state laser (DPSSL) was built at the Fraunhofer IOF in cooperation with the Spanish laser manufacturer Monocrom.

The main challenges of this work were to assemble a small DPSSL device (< 50 g), assuring optical performance in terms of wavelength and power stability, but also proving its capability to work in harsh environmental conditions. Moreover, the laser had to guarantee ESA Planetary protection standards and ultra-high cleanliness; outgassing total mass loss (TML) < 0.1 % and collected volatile condensable materials (CVCM) < 0.01 %.

Ordinary assembly techniques such as adhesive based or clamping methods were not able to fulfill the above-mentioned stringent aerospace imposed requirements. Thus, the laser-based soldering Solderjet Bumping technique was used to assemble the miniaturized green laser. Laser-based soldering processes are especially well suited to the joining of optical components made of fragile and/or brittle materials such as glasses, ceramics, and optical crystals due to a localized and minimized input of thermal energy. Solderjet bumping technique uses spherical solder preforms that are molten by a precise laser pulse and jetted to the desired position through a discrete nitrogen laser pulse.

Fraunhofer IOF, in cooperation with Monocrom, handled the required Flight Module lasers assembled by the Solderjet bumping technique to the Spanish National Institute of Aerospace Technology (INTA), where they were mounted to the Raman Spectrometer Instrument (RLS) which will finally be delivered to ESA. The launch of the space mission is scheduled for 2020, when the ExoMars rover will be going into space with a Russian Proton rocket from Baikonur.

Authors: Pol Ribes-Pleguezuelo, Marcel Hornaff, Steffen Gramens, Erik Beckert