Characterization of subsurface damage in fused silica using non-destructive optical coherence tomography and a destructive approach by iterative plasma jet etching

In this work, a Vickers indenter was used to systematically produce defined and artificial indentations on the surface of fused silica glass to systematically study subsurface damage (SSD). Various measurement methods, such as optical microscopy, white light interferometry (WLI) and optical coherence tomography (OCT), were used to examine SSD depths and morphologies. Tomographic OCT measurements were performed to non-destructively characterize the defects. SSD depths were validated with a destructive preparation method using iterative plasma jet etching (PJE) and subsequent topography measurements with WLI. A total of eight PJE steps were performed to successively remove material, opening and widening surface and subsurface defects. SSD depths in this destructive characterization approach were deduced by combining the PJE etching depth and the corresponding surface roughness parameter Sv. Additionally, measurement methods were verified twice through OCT measurements performed after different etching steps. The increased surface roughness from PJE reduced OCT imaging artifacts and improved the OCT measurement accuracy. The final SSD depth, determined by adding the OCT-measured SSD values after PJE process to the respective PJE etching depth, was highly reliable. The SSD depth of the Vickers indentation determined by the combined use of OCT and PJE showed excellent agreement with the SSD depth estimated using a commonly applied empirical formula for Vickers indentations, providing additional confirmation of the SSD depth and further demonstrated the robustness of the combined OCT-PJE approach.