Evolution of Fused Silica Subsurface Damage in Chemical Etching and Its Influence on Surface Loss

Mitigating the surface loss induced by subsurface damage is of critical importance in the manufacturing of fused silica devices with ultra-low mechanical loss. Chemical etching technology has been applied as an effective approach in controlling surface loss, while fundamental research on its mechanism is still lacking. In this work, chemical etching treatment on lapped fused silica samples was carried out for different periods of time. The evolution of surface micro-topography and roughness was observed. A set of apparatus for the Q-factor measurement was built, and an evaluation method for surface loss was proposed. The changes in the Q-factor and surface loss of samples during the etching process were demonstrated. The mechanism of surface loss induced by subsurface damage was revealed based on the friction of micro-interfaces. A surface loss model was derived from the definition of surface loss and the perspective of kinetic energy conservation. The results show that the surface micro-topography and roughness of samples depend on the evolution of subsurface cracks in etching. The Q-factor of samples is improved by about 50 times via chemical etching, from an initial value of 2.736 × 105 to a maximum value of 1.338 × 107. The subsurface-damage-induced surface loss of the sample lapped in this experiment is about 3.58 × 10−6, and most of the loss is caused by cracks in the near-surface layer. This work provides profound insights into the mechanism of surface loss caused by subsurface damage and holds guiding importance for the manufacture of fused silica resonators.