The development of efficient dielectric coatings in the far UV (FUV) is demanded for upcoming space instrumentation, such as LUMOS (LUVOIR Ultraviolet Multi-Object Spectrograph) in LUVOIR mission, among other applications. Multilayers (MLs) based on MgF₂ and LaF₃ have been developed in the last decades for the 157-nm and 193-nm lithography, demonstrating excellent optical properties. Yet, the deposition procedure to obtain coatings with optimal performance has not been fully detailed in the open literature, such as the dependence of ML performance with deposition and post-deposition temperature. This research investigates the effect of the substrate deposition temperature of MgF₂/LaF₃ ML coatings prepared by thermal evaporation on FUV reflectance, stress, roughness, as well as the performance of the coatings and their evolution over time. The relatively higher expansion coefficient of these two fluorides in comparison with fused silica substrates results in a large tensile stress for coatings deposited at high temperature and later cooled down to room temperature. Such stress may result in coating cracking and delamination. A compromise deposition temperature of ∼240°C and 13 bilayers was found for optimal ML reflectance peaked at ∼160 nm before cracks are generated. Above this deposition temperature, stress increased, which resulted in an extension of the cracked area and in a slight roughness increase and FUV reflectance decrease. MLs that were deposited at room temperature and later annealed resulted in a similar reflectance and stress to those of hot-deposited coatings for a given temperature.

中文翻译：

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优化 MgF ₂ /LaF ₃窄带反射 FUV 多层膜的沉积参数

未来的空间仪器需要开发远紫外 (FUV) 中的高效介电涂层，例如 LUVOIR 任务中的 LUMOS（LUVOIR 紫外多目标光谱仪）等应用。在过去的几十年中，基于 MgF ₂和 LaF _{3 的}多层 (ML)已开发用于 157-nm 和 193-nm 光刻，展示了出色的光学性能。然而，获得具有最佳性能的涂层的沉积程序尚未在公开文献中得到充分详细说明，例如 ML 性能与沉积和沉积后温度的相关性。本研究调查了 MgF ₂ /LaF ₃衬底沉积温度的影响通过热蒸发制备的 ML 涂层对 FUV 反射率、应力、粗糙度以及涂层的性能及其随时间的演变进行了研究。与熔融石英基材相比，这两种氟化物的膨胀系数相对较高，导致在高温下沉积并随后冷却至室温的涂层产生较大的拉伸应力。这种应力可能导致涂层开裂和分层。发现约 240°C 的折衷沉积温度和 13 个双层的最佳 ML 反射率在裂纹产生之前在约 160 nm 处达到峰值。高于此沉积温度，应力增加，导致裂纹区域扩大，粗糙度略有增加，FUV 反射率降低。