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We evaluated the ablation thresholds of optical materials by using hard X-ray free electron laser. A 1-µm-focused beam with 10-keV of photon energy from SPring-8 Angstrom Compact free electron LAser (SACLA) was irradiated onto silicon and SiO2 substrates, as well as the platinum and rhodium thin films on these substrates, which are widely used for optical materials such as X-ray mirrors. We designed and installed a dedicated experimental chamber for the irradiation experiments. For the silicon substrate irradiated at a high fluence, we observed strong mechanical cracking at the surface and a deep ablation hole with a straight side wall. We confirmed that the ablation thresholds of uncoated silicon and SiO2 substrates agree with the melting doses of these materials, while those of the substrates under the metal coating layer are significantly reduced. The ablation thresholds obtained here are useful criteria in designing optics for hard X-ray free electron lasers.
©2013 Optical Society of America
1. Introduction
X-ray free-electron lasers (XFELs) [1], such as the Linac Coherent Light Source [2] and SPring-8 Angstrom Compact free electron LAser (SACLA) [3], have started to provide intense, coherent, and ultrafast pulses in the hard X-ray region, which promote the development of new approaches in various fields, such as atomic physics [4–6] and structural biology [7,8].
Although XFEL light provides great capabilities, the intense beam could induce damage to optical elements, which would lead to degradation of the beam quality. The irradiation tolerance of optical elements is evaluated by comparing the absorption dose with the melting threshold. The melting threshold has been considered as a reasonable guide in designing optical components [9–11]. Damage by FEL irradiation has been investigated in the extreme ultraviolet (EUV) and the soft X-ray regions [12–16]. David et al. have reported on the ablation phenomenon of gold at a photon energy of 8 keV [17]. In this paper, we report on our systematic study of the damage thresholds for various optical materials by using a hard X-ray free electron laser (FEL).
We used a focused XFEL beam at a photon energy of 10 keV, which has a sufficient power density to study ablation phenomena. We designed and installed a dedicated experimental chamber for the precise alignment of the position and incident angle of the samples. We used uncoated silicon and SiO2 substrates, as well as the metal (platinum and rhodium) coating on these substrates, which are widely used for X-ray optics as samples. We performed irradiation studies on a single shot and for a normal incidence condition.
2. Experiment
The experiments were carried out at beamline 3 (BL3) of the SACLA [3]. During the experiments, SACLA was operated at a mean pulse energy of 130 µJ, a pulse duration of 20 fs [18], and a pulse repetition rate of 10 Hz. The X-ray photon energy was chosen to be 10 keV. The unwanted contamination of higher-order harmonics and gamma-rays were suppressed using a double-mirror system in the optics hutch. The XFEL light was focused down to a diameter of 1 µm (FWHM) using the mirror system [19] that consists of two carbon-coated elliptical mirrors in a Kirkpatrick–Baez configuration. The focusing mirror system was located 115 m downstream from the exit of the final undulator.
An irradiation chamber was designed and installed at the focal point of the focusing mirror system, as shown in Fig. 1(a). The samples were mounted on high precision stages at a motion range of 50 mm in the vertical and horizontal directions perpendicular to the optical axis, as well as at 15 mm along the optical axis, as shown in Figs. 1(b) and 1(c). A rotation stage was used for adjusting the incident angle, the motion range of which is from −10 to + 100 degrees. The stage specifications are summarized in Table 1. The surface of the samples is monitored by an optical microscope at an angle of 30 degrees for the normal incidence condition, and at an angle of 90 degrees for the grazing incidence condition, as shown in Fig. 1(a). A knife-edge scanning method was used for measuring the profile of the focused beam. The excellent pointing stability of the XFEL light from SACLA made it possible to accurately evaluate the beam profile and the irradiation area.
Fig. 1 (a) Photograph of experimental chamber. OM: Optical microscope for observation of the sample surface. (b) Photograph of area around sample holder inside chamber. (c) Schematic drawing of sample stage configuration. The sample holder is mounted on the XYZ translation stages. These stages are placed on the rotation stage.
Table 1. Stage specifications
The pulse energy was controlled by silicon attenuators of various thicknesses inserted in front of the focusing mirrors. The shot-to-shot fluctuations of the pulse energy were monitored by using a scattering-based beam intensity monitor [20], which was calibrated by a cryogenic radiometer [21]. Measured pulse energy accuracy was within the range of ± 3.5% around the X-ray energy used in this experiment. Single shot irradiations at a normal incidence condition at a pulse energy ranging from 0.001 to 100 µJ were used in this experiment. The sample was moved with constant speed during the exposure. The number of shots was controlled by using a pulse selector [22]. The ablation thresholds of the samples were evaluated by measuring the diameters of the imprinted ablation profiles using scanning probe microscopy (SPM) and scanning electron microscopy (SEM).
3. Results and discussion
Figure 2 shows one of the typical imprints of silicon irradiated at high fluence without any attenuators. The optical microscope image for the surface is shown in Fig. 2(a). The irradiated fluence inside the crater was 57 µJ/µm2, which was several tens of orders of magnitude higher than the melting dose of silicon. Spallation and cracks were observed around an area of 40 µm on the surface. The cross sectional SEM image prepared using focused ion beam sampling is shown in Fig. 2(b). For the cross sectional SEM image, the crater depth and the diameter were measured to be 40 and 4 µm, respectively. A large volume of melted and/or evaporated silicon was ejected from the inside, and a straight side wall was formed. Therefore, a deep ablation phenomenon was observed. Although the attenuation length of silicon is 134 µm for 10 keV of photon energy, the 40-µm crater depth is small. The reason for this may be as follows. Ablated silicon in the region deeper than the bottom of the crater cannot be ejected outside, so solidification occurs and it is probably in the amorphous state. It is difficult to observe from the SEM images because they are only sensitive to the surface morphology.
Fig. 2 (a) Optical microscope image of irradiated silicon viewed from surface at fluence of 57 µJ/µm2. (b) Cross sectional SEM image of (a) prepared by focused ion beam sampling.
We evaluated the ablation thresholds of uncoated silicon and SiO2 substrate by varying the intensity using Liu’s technique [23]. The imprint areas were plotted as a function of the fluence, as shown in Fig. 3(a). The obtained threshold fluence Fth was 0.78 ± 0.04 µJ/µm2 (4.5 ± 0.7 µJ/µm2) in silicon (SiO2), which was converted to the dose D for a single atom [9–11], as follow; the dose D is given by , where μ, A, σ, ρ, and NA are the absorption coefficient, the average atomic weight, the RMS beam size, the average density, and the Avogadro’s constant, respectively. Converted dose was 0.73 ± 0.04 eV/atom (1.7 ± 0.3 eV/atom). This value reasonably agrees with the calculated melting dose of 0.88 eV/atom (1.1 eV/atom). The melting dose was calculated from the thermodynamic properties, which took into consideration the temperature dependent heat capacity and the latent heat of melting [24]. Note that we did not include effects of electron transport in this calculation.
Fig. 3 Imprint areas plotted as function of fluence. (a) Uncoated Si and SiO2 substrates, (b) Pt coating layer and SiO2 substrate under this layer.
We used a 200-nm-thick platinum layer coated on silicon and SiO2 substrates as the metal coating samples. The inserted adhesive layer was 5-nm-thick chromium. The imprint areas of the platinum layer and the SiO2 substrate under the coating layer were plotted in Fig. 3(b) as a function of the fluence. The ablation threshold of the platinum was evaluated to be 0.023 ± 0.004 µJ/µm2 (0.52 ± 0.09 eV/atom). This value reasonably agrees with the calculated melting dose of 0.78 eV/atom. However, the ablation threshold of the SiO2 substrate under the coating layer was evaluated to be 0.11 ± 0.03 µJ/µm2 (0.04 ± 0.01 eV/atom), while that of the uncoated SiO2 substrate was 4.5 ± 0.7 µJ/µm2 (1.7 ± 0.3 eV/atom) as obtained above. This value was 40 times lower than that of the uncoated substrate. Figures 4(a)–4(c) show the SEM images of the imprints formed on the platinum layer coated SiO2 substrate for a detailed observation of the surface morphology. Figures 4(d)–4(f) show cross sectional profiles of these craters. Shallow craters appeared in the substrate region for the 0.3 µJ/µm2 and 2.6 µJ/µm2 fluences. Notably, these fluences are lower than the threshold fluence for the uncoated SiO2 substrate (4.5 µJ/µm2). Similar results were observed for the silicon substrate and other coating materials such as rhodium. In the case of silicon substrate under platinum coating layer, the ablation threshold was evaluated to be 0.065 ± 0.008µJ/µm2 (0.060 ± 0.007 eV/atom). This value was 10 times lower than that of the uncoated substrate. Furthermore, we used a 75-nm-thick rhodium layer coated on silicon and SiO2 substrates as the other metal coating samples. The inserted adhesive layer was 10-nm-thick chromium. We confirmed that the substrate behavior was the same with platinum coatings.
Fig. 4 (a–c) Imprint SEM images of platinum coated SiO2 at fluences of 0.3, 2.6, and 8.6 µJ/µm2. Observed SEM images were viewed under an angle of 30°. The imprint diameters of the platinum layer were 2.1, 4.0, and 4.9 µm, respectively. (d–f) Cross sectional profiles of these craters measured by SPM. In the case of the crater irradiated by a fluence of 8.6 µJ/µm2, the SPM probe cannot reach the bottom of the crater. The dashed line indicated the interface between the Pt layer and the SiO2 substrate.
The measured threshold fluences and calculated melting doses were summarized in Table 2. The measured threshold values for the uncoated substrate and metal coating layer agreed with the calculated melting dose. However, the thresholds of the substrates under the coating layer were significantly lower than that for the uncoated substrates; the ratio was 1/10 for silicon and 1/40 for SiO2.
Table 2. Measured threshold fluences, corresponding doses, and calculated melting doses.
As seen in Fig. 3(b), differential coefficient of the measured imprint area of SiO2 underneath coating as a function of fluence is changed clearly. The inflection point is nearly threshold fluence of uncoated substrate. For higher fluence than the threshold, large size craters were formed, as shown in Fig. 4(c), by direct interaction with intense X-rays to the substrate. Note that X-ray transmissions through the coating are as high as 95.4% for 200-nm-thick platinum layer and 99% for 75-nm-thick rhodium layer for 10 keV X-rays. On the other hand, for lower fluence, shallow craters were formed as shown in Fig. 4(a) and 4(c). The damage of the substrate underneath coating could originate from collisions of energetic particles (electrons, ions, and neutrals) that are generated with intense X-rays in the coating region.
4. Summary
We have measured the ablation thresholds of optical materials that are widely use as X-ray mirrors. A focusing hard X-ray FEL beam at a beam size of 1 µm was used. We found that the measured ablation thresholds of uncoated silicon and a SiO2 substrate, as well as a metal (platinum and rhodium) thin film are comparable to the melting dose, while the substrates under the metal coating layer showed that they are easily damaged. These results should be useful criteria for designing X-ray optics.
Acknowledgments
The authors would like to sincerely thank Takanori Miura for his support in measuring the samples, and Hikaru Kishimoto and the SACLA engineering team for their help during the beam time. This work was performed at the BL3 of SACLA with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal No. 2012A8056). This research was partially supported by a Grant-in-Aid for Scientific Research (S) (23226004) from the Ministry of Education, Sports, Culture, Science and Technology, Japan (MEXT).
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