Ultrasound characterization of contact bonded bulk infrared optics

The optical community frequently relies on optical adhesives to join multi-component optical assemblies. However, options for bonding infrared materials are limited, since most optical adhesives are organic which means poor index matching to high index materials, and the introduction of their own IR absorption bands. Contact bonding is a potential solution, as it provides adhesion through intermolecular forces, capillary forces, and covalent bonds between bonding surfaces. However, to be successful, a reliable non-destructive characterization technique must be designed to evaluate bond quality. Many optical applications require low temperature processing to avoid degradation of any optical coatings. At room temperature, trapped water in the interface is a dominant contributor to the strength of contact bonded assemblies. Therefore, evaluating the interface is a crucial portion of determining bond quality. This report will analyze the potential of acoustic microscopy as a non-destructive technique to probe trapped water in the bonded interface. Several experiments were conducted to ascertain the sensitivity of acoustic microscopy to trapped water: (i) FTIR spectroscopy was used to provide a benchmark, (ii) bulk silicon samples were bonded in ambient atmosphere and aged over several weeks, (iii) and various annealing procedures were also performed to demonstrate water content evolution. A spring model of the bonded interface is used to quantify the amount of water in these studies through interfacial stiffness. Through these experiments, acoustic microscopy is shown to be an effective non-destructive technique to observe changes in interfacial water and may be used alongside other methods to screen high-value optical assemblies.