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

Abstract

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 XUV 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 µJ to 40 µJ on the sample surface. The irradiation was carried out at five fluence levels between 0.1 J/cm 2 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) µm 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. A Raman micro-probe 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 et al., Proc. SPIE 5917, 91 (2005)] on an a-C sample irradiated by a large number of femtosecond pulses of XUV high-order harmonics (HHs).

Ablation and transmission of thin solid targets irradiated by intense extreme ultraviolet laser radiation

“…Such lasers can enable fundamental studies and be utilized for applications such as nanomachining 14,15 and the development of nanoprobes. [16][17][18] We present the results of experiments undertaken with an argon capillary discharge laser, producing nanosecond pulses at a wavelength of 46.9 nm, focused onto thin parylene-N targets. This type of plastic contains equal proportions of hydrogen and carbon, which simplifies radiation-hydrodynamic simulations.…”

Multiple free electron laser pulse illumination of a carbon coated silicon substrate

“…A simultaneously non-thermal desorption of the carbon coating and an extrusion of the Si substrate could be supposed processes. This conclusion is reasonable due to the findings of [15] where a ablation caused by multiple laser shots below the single shot damage threshold could be observed and [13] with the findings of extrusions for a-C under illumination with fluences below 0.2 J/cm 2 at a wavelength of 32.5 nm at FLASH. Also the radiation in the present experiment with a wavelength of 21 nm has a transmission through the carbon coating to the silicon substrate of 50 % under an angel of α=80° [4].…”

“…In later experiments the damage threshold of a 40 nm thick a-C coated mirror was determined to 43.7 ± 13 mJ/cm 2 for a FLASH wavelength of 7 nm (177 eV) [14]. In further experiments to analyze the behavior of mirrors under multiple laser shots an a-C coated Si substrate was irradiated with 46.9 nm laser pulses [15]. As light source a Ne-like Ar 8+ XUV laser with a pulse duration of 1.7 ns was applied.…”

Soft X-ray laser ablation for nano-scale chemical mapping microanalysis

“…Irradiation of amorphous carbon with multiple XUV laser pulses at a uence below the single-shot damage threshold still showed an erosion of the material. 93 This is an unexpected behavior because above the single-shot threshold the authors rst observed expansion, then ablation and then a-C lm delamination from the silicon substrate. Henceforth, the character of the process changed dramatically in the vicinity of the single-shot damage threshold, i.e., erosion turns into expansion.…”