1 February 1992 Surface morphology of As-deposited and laser-damaged dielectric mirror coatings studied in-situ by atomic force microscopy
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Abstract
Atomic force microscopy was used to determine in situ the nanometer-scale morphological changes that occur on dielectric optical coatings as a result of laser-induced damage. The optical film studied was a highly reflective dielectric multilayer mirror (HR) consisting of many alternating HfO2 and SiO2 layers of quarter-wave thickness at 1.06 micrometers . The top layer was a (lambda) /2 SiO2 overcoat. Laser beam specifications were: 1.06 micrometers wavelength, 8 ns pulselength, and 110 micrometers to 300 micrometers beam diameter. The laser fluence was determined by beam profiling and total energy measurements. The maximum scan-range of the AFM was 80 micrometers . A survey of the as-deposited surface shows mostly hillocks of approximately 200 nm width and 10 nm height. Comparison of this hillock structure to that of a single layer of SiO2 and a surface layer of HfO2 was made. Irregularities (i.e., defects) on the surface of the HR consisted of micrometer-scale domes, and occasional craters, of micrometer planar dimension and depth extending over many layers. Three types of coating defects were identified which could be related to the classic nodule-type coating defect. Nodule defects were found to be easily ejected from the coating surface by laser illumination, leaving craters from which further damage would propagate. After laser damage with fluences above 30 J/cm2 the hillocks coalesced into structures with heights and widths 5 - 10 times that of the as-deposited film. A pattern of concentric surface distortions (ripples or cracks) appeared at higher fluences, in some cases following the full circumference of the beam.
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Mark R. Kozlowski, Mark R. Kozlowski, Michael C. Staggs, Michael C. Staggs, Mehdi Balooch, Mehdi Balooch, Robert J. Tench, Robert J. Tench, Wigbert J. Siekhaus, Wigbert J. Siekhaus, } "Surface morphology of As-deposited and laser-damaged dielectric mirror coatings studied in-situ by atomic force microscopy", Proc. SPIE 1556, Scanning Microscopy Instrumentation, (1 February 1992); doi: 10.1117/12.134889; https://doi.org/10.1117/12.134889
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