Sub-nm precise measurement of adhesive gaps by imaging interferometry

Adhesives are often used to bond optical components such as mirrors or lenses in high-precision applications. Adhesives absorb moisture in air or release it in vacuum, which is accompanied by expansion or shrinkage, respectively. In this paper, it is proposed to use absolute length measuring imaging interferometry as a method for adhesive gaps with sub-nm accuracy. The method is based on the idea that an adhesive gap is measured as part of an overall length that includes the adhesive gap and the length of a test specimen. For this purpose, a test specimen of known length is bonded to an end plate. A major advantage of the method is that the thickness of the gap can be measured over any period. Test specimens are made of ULE, with very good parallelism of the adhesive gaps when glued to the end plate, which is ensured by an autocollimation setup. Measurements on samples with adhesive gap are carried out in PTB’s precision interferometer and show shrinkage in vacuum over several years and temporary swelling when water vapor of approximately 10 hPa is injected to vacuum, which, in air, corresponds to a relative humidity of approximately 42% at 20°C. Measurement results for the common adhesive 3M Scotch-Weld DP-460EG, gap thickness approximately 100 µm, are shown as an example. To prove that the measured length changes are only due to the behavior of the adhesive gap, a sample without an adhesive bond was used. Measurements on this sample reveal that the length changes without adhesive are within approximately 0.1 nm under comparable conditions. On the adhesive sample, on the other hand, drying in a vacuum and water vapor treatment lead to shrinkage and swelling, respectively. These long-term processes can be described very well by simple but empirical fitting functions. The measurement uncertainty budget results in typical values of approximately 0.3 nm for changes of the adhesive gap thickness. This is orders of magnitude more accurate than was previously possible using traditional methods on adhesive samples.