Improve the LIDT of high-reflection coatings by planarizing nodular defects
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摘要: 探究了节瘤缺陷平坦化技术中平坦化层(刻蚀层)厚度和种子源尺寸之间的刻蚀规律，同时解释了平坦化技术提高节瘤缺陷的损伤阈值的机制。在双离子束溅射系统中，使用SiO2微球模拟真实的种子源置于基板上，镀制1064 nm HfO2/SiO2高反膜，制备人工节瘤缺陷。对类似于实际种子源的SiO2微球一系列不同刻蚀程度的实验得出了节瘤缺陷平坦化技术的刻蚀规律：只要平坦化层(刻蚀层)的厚度稍大于节瘤缺陷的种子源粒径，就可以将种子源完全平坦化。使用时域有限差分法(FDTD)模拟不同平坦化程度的节瘤缺陷内电场增强的结果与节瘤缺陷的损伤形貌进行对比实验，将损伤形貌和损伤阈值与电场强度分布之间建立联系，表明平坦化技术可以改变节瘤缺陷原有的几何结构，有效抑制节瘤缺陷的电场增强效应。最后，通过对未经平坦化和经过平坦化处理后的节瘤缺陷进行损伤阈值测试，对比结果直接验证了节瘤缺陷平坦化技术可以实现对节瘤缺陷的调控，大幅度提高了节瘤缺陷的损伤阈值。Abstract: Nodular defects planarization was investigated to improve the laser-induced damage threshold (LIDT) of high-reflection coatings. The monodisperse SiO2 microspheres were deposited on the substrate surface by spin coating process. In the dual ion beam sputtering system, the artificial nodules were grown from these engineered seeds in 1064 nm HfO2/SiO2 multilayer coatings. After a series of coating and etching steps, the SiO2 microspheres were smoothed by a single SiO2 planarization layer. The relation between the thickness of the planarization layer and the size of the microspheres has been investigated. When the planarization layer (etching layer) thickness was slightly larger than the diameter of the seeds, the seeds could be completely planarized to obtain smooth thin films. Furthermore, the three-dimensional finite-difference time-domain code (FDTD) was used to simulate the electric-field intensity distributions in the artificial nodular defects. The comparison between the electric-field intensity distributions and the nodular morphologies of the non-planarized nodular defects and partially planarized nodular defects indicates that the nodular defects planarization has changed the geometry of nodular defects and effectively suppressed the electric field enhancement in nodular defects. Finally, nodular defects with different thickness planarization layers were tested by raster scan damage tests. For the nodular defects with an adequate planarization layer, the laser damage threshold test results show that the ejection fluences has been greatly raised, which has verified that nodular defect planarization could improve the damage resistance of thin films.
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图 3 三组不同平坦化程度1064 nm高反膜的透射光谱图
Figure 3. Transmission spectra of 1064 nm high-reflection coatings with different thickness planarization layers
图 5 2 μm SiO2微球形成的人工节瘤缺陷的剖面图
Figure 5. Cross-sectional SEM image of artificial nodular defects with 2 μm SiO2 microspheres
图 6 经过1.25 μm SiO2层平坦化后节瘤缺陷的剖面图
Figure 6. Cross-sectional SEM images of nodular defects with 1.25 μm-thick SiO2 Planarization layer
图 7 经过2.5 μm SiO2层平坦化后节瘤缺陷的剖面图
Figure 7. Cross-sectional SEM images of nodular defects with 2.5 μm-thick SiO2 planarization layer
图 8 2 μm种子源形成的不同平坦化程度的节瘤缺陷的剖面图
Figure 8. Cross-sectional SEM images of nodular defects that grow from 2 μm-diameter seeds with different planarization layer thickness
图 9 2 μm种子源未经平坦化和部分平坦化后形成的节瘤缺陷的剖面图、节瘤缺陷内的电场强度分布模拟图以及激光辐照后的损伤形貌图
Figure 9. Cross-sectional SEM images, simulated electric field intensity(EFI) distributions and damage morphologies of nodular defects that grow from 2 μm-diameter seeds
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