In-vacuum bonding of X-ray optics by means of laser-based soldering

X-ray optics, such as e.g. for ESA's ATHENA (Advanced Telescope for High-ENergy Astrophysics) mission, often require that their aligmnent takes places under vacuum conditions, in particular when high precision and the compensation for vacutun-related distortion states is mandatory. In order to fixate the aligmnent state, standard bonding technologies for x-ray optics, such as clamping or adhesive bonding, only can be carried out at ambient conditions, causing a potential change of the alignment state when transiting from ambient condition to vacuum. In a study issued by ESA, the use of a specific laser-based soldering technology to be carried out under vacuum conditions was successfully conducted. The socalled "Solderjet Bumping" (SJB) technology combines solder alloy preform singulation and handling as well as laser-based reflow and placement at the desired, metallized bonding areas for x-ray optics. Achieving dc-alignment less than a millirad during soldering under <1e-4 mbar vacuum condition is reported for complex and 3D x-ray optics geometries. Also, ca. twice the bonding strength was reached, compared to adhesive bonds carried out at ambient conditions.