(164g) Chemical Composition, Structure Morphology, Contaminant Cleaning and Laser-Induced–Damage Threshold in Coarse Fused-Silica Gratings

Previous work at the Laboratory for Laser Energetics had established that organic contaminants that accumulate on the surface of the multilayer-dielectric (MLD) gratings during the fabrication steps contribute to lowering the laser-induced–damage thresholds (LIDT’s), and lead to premature grating failure when compared to unprocessed MLD substrates. The geometrical morphology (e.g., surface roughness, uniformity) of the grating structures can also be affected by changes in the manufacturing conditions or cleaning steps, leading to the changes in the mechanical stability or optical performance of the grating devices. To determine which contaminants lower LIDT values and to propose contaminant-specific cleaning steps, we have investigated how grating manufacturing steps and cleaning conditions affect the amount of interfacial organic impurities and the geometrical morphology of the grating features. Because different grating sections (pillar side wall, pillar top, and sole) are exposed to different chemical environments during the manufacturing, we conducted these studies using coarse, grating-like structures (continuous ~5-mm X 1-µm lines) to separately analyze different grating sections using surface-specific spectroscopic and microscopic techniques. The goals of this study were to (1) investigate which steps in the grating fabrication have the highest impact on the damage-threshold values; (2) determine the composition, thickness, and spatial distribution (e.g., top, bottom, or pillar side wall) of the contaminant material that decreases the damage threshold; and (3) investigate the effect of the cleaning steps on the contaminant concentration and grating feature morphology. We found that the organic materials left after the photolithographic processing and fluorinated coatings developed during the reactive ion etching can both lead to contamination of the grating interfaces. We suggested a modified cleaning protocol that targets specific removal of these materials. We also discovered that the geometrical morphologies of the pillar side wall and the grating sole are affected by the etching conditions and the cleaning steps. The laser-induced–damage performance of the grating sections was measured to determine correlations between the amount of surface impurities and LIDT values. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DENA0003856, the University of Rochester, and the New York State Energy Research and Development Authority.