Radiation damage to amorphous carbon thin films irradiated by multiple 46.9 nm laser shots below the single-shot damage threshold

• Published in: Journal of applied physics Vol. 105; no. 9; pp. 093117 - 093117-6
• Main Authors: Juha, L., Hájková, V., Chalupský, J., Vorlíček, V., Ritucci, A., Reale, A., Zuppella, P., Störmer, M.
• Format: Journal Article
• Language: English
• Published: United States American Institute of Physics 01.05.2009
• Subjects: AMORPHOUS STATE; ANTIREFLECTION COATINGS; ATOMIC FORCE MICROSCOPY; BLOOD VESSELS; BODY; CAPILLARIES; CARBON; CARDIOVASCULAR SYSTEM; COATINGS; CONFIGURATION; ELECTROMAGNETIC RADIATION; ELEMENTS; EXTREME ULTRAVIOLET RADIATION; LASERS; LAYERS; MATERIALS SCIENCE; MICROSCOPY; NONMETALS; OPTICAL MICROSCOPY; OPTICAL PROPERTIES; ORGANS; PHYSICAL PROPERTIES; PULSES; RADIATION EFFECTS; RADIATIONS; RAMAN SPECTRA; REFLECTIVITY; SEMIMETALS; SILICON; SPECTRA; SPHERICAL CONFIGURATION; SPUTTERING; SURFACE PROPERTIES; THIN FILMS; ULTRAVIOLET RADIATION
• ISSN: 0021-8979; 1089-7550
• DOI: 10.1063/1.3117515

High-surface-quality amorphous carbon (a-C) optical coatings with a thickness of 45 nm, deposited by magnetron sputtering on a silicon substrate, were irradiated by the focused beam of capillary-discharge Ne-like Ar extreme ultraviolet laser ( CDL = capillary -discharge laser; XUV = extreme ultraviolet, i.e., wavelengths below 100 nm). The laser wavelength and pulse duration were 46.9 nm and 1.7 ns, respectively. The laser beam was focused onto the sample surface by a spherical Sc/Si multilayer mirror with a total reflectivity of about 30%. The laser pulse energy was varied from 0.4 to 40   μ J on the sample surface. The irradiation was carried out at five fluence levels between 0.1 and 10   J / cm 2 , accumulating five different series of shots, i.e., 1, 5, 10, 20, and 40. The damage to the a-C thin layer was investigated by atomic force microscopy (AFM) and Nomarski differential interference contrast (DIC) optical microscopy. The dependence of the single-shot-damaged area on pulse energy makes it possible to determine a beam spot diameter in the focus. Its value was found to be equal to 23.3 ± 3.0   μ m using AFM data, assuming the beam to have a Gaussian profile. Such a plot can also be used for a determination of single-shot damage threshold in a-C. A single-shot threshold value of 1.1   J / cm 2 was found. Investigating the consequences of the multiple-shot exposure, it has been found that an accumulation of 10, 20, and 40 shots at a fluence of 0.5   J / cm 2 , i.e., below the single-shot damage threshold, causes irreversible changes of thin a-C layers, which can be registered by both the AFM and the DIC microscopy. In the center of the damaged area, AFM shows a-C removal to a maximum depth of 0.3, 1.2, and 1.5 nm for 10-, 20- and 40-shot exposure, respectively. Raman microprobe analysis does not indicate any change in the structure of the remaining a-C material. The erosive behavior reported here contrasts with the material expansion observed earlier [ L. Juha , Proc. SPIE 5917 , 91 ( 2005 ) ] on an a-C sample irradiated by a large number of femtosecond pulses of XUV high-order harmonics.