Characterization of Fluence Limiting Defects in _ HafniaKilica Multilayer Coatings Manufactured for the National Ignition Facility

A variety of microscopic techniques are employed to characterize fluence limiting defects in hafniakilica multilayer coatings manufactured for the National Ignition Facility. This work was performed under the auspices of the U.S. DOE by LLNL under contract No. W-7405-Eng-48. Characterization of fluence limiting defects in hafniakilica multilayer coatings manufactured for the National Ignition Facility Z. L. Wu, C. J. Stolz, J. M. Yoshiyama, and A. Salleo University of California Lawrence Liver-more National Laboratory P. 0. Box 808, L-487, Livermore, CA 94550 Name: Phone: Fax: Email: z. L. Wu 5 10 422-8266 510 422-1210 [email protected] C. J. Stolz 5 10 422-3562 510 422-1210 stolzl @llnl.gov J. M. Yosiyama A. Salleo 5 10 422-2925 510 424-3811 510 4221210 510 422-1210 yoshiyamal @llnl.gov salleol @llnl.gov Electron-beam deposition processes are being optimized to improve the laser damage resistance of large aperture (up to 0.34 m2) optical coatings for the National Ignition Facility (NIF). One modification over previous deposition process is the replacement of hafnia as a starting material with hafnium for source ejection reduction.’ This modification has led to a significant reduction (-10 times) in the nodular defect density. Although considerable effort has been spent on studying nodular defects, unfortunately the damage threshold of coatings can also be limited by a multitude of different defect types, i.e. damage sites are also present in nodule-free regions of large aperture optics. In this work, a variety of microscopic techniques are employed to characterize fluence limiting defects. Photothermal microscopy, utilizing the surface thermal lensing technique, is used to map the absorption and thermal characteristics of 3 mm x 3 mm areas of the coatings. High resolution subaperture scans, with a 1 pm step size and 3 pm pump beam diameter, are conducted on the defects to characterize their photothermal properties. Optical and atomic force microscopy is used to visually identify defects and characterize their topography. These defects are then irradiated by a damage testing laser in single shot mode at increasing fluence until damage occurred. The results are analyzed to determine the role of nodular and non-nodular defects in limiting the damage threshold of the multilayer coatings. Some fully characterized nodules are also cross-sectioned by a focused ion beam (FIB) for determination of the defect seed composition and depth for comparison of measured photothermal signals with results of future theoretical modeling of the thermo-mechanical response of the defects. Figure 1 shows the results of a 3 mm x 3 mm area of a coating scanned by the photothermal microscope. The sample in this case is a 1.06 pm Brewster’s angle plate polarizer with an optimized non-quarterwave design. The amplitude map of the photothermal signal reflects the absolute value of the laser-induced surface deformation, which is proportional to the localized optical absorption. The phase map is more related to localized thermal properties as well as the depth of the absorptive defect sites. Figure 2 shows the results of high resolution subaperture photothermal scans of defect la at different modulation frequencies. From the images, semi-quantitative information can be obtained on the absorption and thermal properties of the local defects. For this specific defect, the photothermal images show that the absorption size of the defect is less than 10 pm (image at 19.48 KHz), and that the defect is both an absorptive (amplitude and phase images at 34.2 Hz) and thermal defect (phase image at 19.48 KHz the discontinuity in the phase map suggests a thermal barrier between the defect and the adjacent multilayer coating). This semi-quantitative evaluation is consistent with scanning electron microcopy (SEM) image and FIB cross section of the same defect after photothermal scanning, as shown in Figure 3. Defect la is an absorptive nodular defect surrounded by micro-cracks (Figure 3a) in poor thermal contact with the host coating materials (Figure 3b).