Low-stress soldered Lenses for High-Perfomance Objectives

Lens assemblies in metal mounts that are highly precise and stable in the long-term are key components for high-performance objectives, e.g. in earth observation instrumentation. Low-stress mounting is important for minimized deformation of optical surfaces, low stress-induced birefringence and, thus, outstanding imaging properties. The accurate centering of the lens within the mount reduces the effort required during system integration using the stack-mounting approach. Adjustment turning allows the optical axis of the lens to be aligned with the mount’s axis of symmetry with accuracy better than one micron. This process is completely independent from the joining, allowing both tasks to be optimized separately.

Laser-based soldering enables an inorganic, material bonded joining. The bonding agent exhibits beneficial properties in comparison to organic adhesives. Stability at short wavelengths, and use under vacuum conditions and at elevated temperatures with resulting high thermo-mechanical loads are of interest for particular application cases, e.g. aero-space. Our laser-based Solderjet Bumping allows flux-free processing and localized joining with minimized thermal influence.

We have established an FEA-based optimization of solder and mount geometries, as well as the design of adapted flexure hinges for two types of lens assembly: transmissive optics with a diameter of up to 74 mm made from fused silica and LAK9G15 (a radiation resistant glass), and mounts with matched thermal expansion made of Invar and TiAl6V4, respectively. Following thermal cycling and vibrational testing, a change of surface deformation within the clear aperture below 100 nm was achieved. The study showed an average increase of stress-induced optical path difference of less than 3.5 nm.

Our proposed technology chain of inorganic, laser-based joining, and adjustment turning addresses new and different application fields with demanding environmental conditions (aero-space) and highly stable optical systems for high laser powers (materials processing) and short wavelengths (UV lithography).


Authors: Marcel Hornaff, Thomas Burkhardt, Andreas Kamm