Strength testing and SEM imaging of hydroxide-catalysis bonds between silicon

Silicon is under consideration as a substrate material for the mirror masses and quasi-monolithic suspension stages of 'third generation' gravitational wave detectors. Identifying a jointing technique to attach the silicon suspension elements to the optics with repeatable high strength and low mechanical loss is critical. Hydroxide-catalysis bonding is the method of choice for current quasi-monolithic silica suspensions. Here we present measurements of the shear strength of hydroxide-catalysis bonds between silicon samples. Strengths of approximately 3.9 N mm−2 are found, comparable to strengths found for silica to silica bonds. Scanning electron microscope imaging shows that the bonds between two silicon parts with thermally grown SiO2 layers are wedged with bond thicknesses varying from 30 nm to several micrometres. We suggest a possible explanation for this observation.