Phase topography-based characterization of thermal effects on materials and joining techniques.

There are growing demands to characterize the stability of assemblies of optical components for ultrahigh-precision instruments. In this paper we demonstrate how absolute length measurements by interferometry can be applied to measure the thermal and dimensional stability of connections. In order to enable investigation of common joining techniques, including wringing, screwing, and gluing, as well as specialized, inorganic joining techniques such as silicatic bonding, thin-film soldering, and solderjet bumping, representative connections were fabricated. By using gage blocks or prismatic bodies as joining parts, parallelism and flatness were provided which are needed for precision interferometric length measurements. The stability of connection elements used in ultrahigh-precision instruments was investigated longitudinally and laterally to the connection interface, and also mutual tilting of the parts was detected by analysis of the phase topographies. The measurements have an accuracy level of about 1 nm, and the traditional wringing method was also considered as a reference joining technique. The long-term behavior was studied within a period of about 1 year under constant temperature. Further, the thermal dilatation and the reaction of connections to thermal stress were measured. Results show that screwed connections do not exhibit a significant drift of length or orientation. They also did not show response to temperature variations of ±10°C. This is different for adhesive connections, where dimensional changes of up to 100 nm were observed. The specimens produced by using thin-film soldering as well as silicatic bonding revealed stability of length better than 5 nm per year and angular stability within ±0.1   arcsec. Furthermore, these specimens were shown to be insensitive to a temporary temperature variation in a range from 10°C to 40°C. This situation is slightly different for the sample connections produced by solderjet bumping, which show a positive length change of ∼25  nm and a tilt of ∼1  arcsec. These observations can be explained by creeping of the relatively large solder bumps that bridge a gap of about 100 μm between the connected mirror plates. The thermal expansion of the connections shows a strong correlation with the "layer thickness."