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<sup>−2</sup> 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 SiO<sub>2</sub> layers are wedged with bond thicknesses varying from 30 nm to several micrometres. We suggest a possible explanation for this observation.