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We evaluated the irradiation damage induced by hard X-ray free-electron lasers to platinum/carbon multilayers intended for use in a focusing reflective mirror. In order to determine the damage threshold, we compared X-ray reflectivities before and after irradiation at the first-order Bragg angle using a focused X-ray free-electron laser with a beam size of approximately 1 μm and a pulse energy ranging from 0.01 to 10 μJ at a photon energy of 10 keV. We confirmed that the damage threshold of the platinum/carbon multilayer with a bilayer period of 3 nm was 0.051 μJ/μm2, which is sufficiently higher than that in practical applications.
© 2015 Optical Society of America
1. Introduction
In the X-ray regime, X-ray free-electron lasers (XFELs) provide unprecedented high brilliance, excellent coherence, and ultrafast pulse duration. In the past few years, the Linac Coherent Light Source (LCLS) [1] and SPring-8 Angstrom Compact free electron LAser (SACLA) [2] have achieved lasing in the hard X-ray regime. The use of focusing optics enables further enhancement of X-ray intensity, which expands the applicable range of XFELs. Several optical devices, such as Fresnel zone plates [3,4], refractive lenses [5], and reflective mirrors [6,7] have been utilized for this purpose. Among these devices, the reflective mirror can achieve the highest focusing efficiency with a long working distance, which provides significant advantages for various applications.
In SACLA, a 50-nm focusing beam with a power density of 1020 W/cm2 has been achieved using total-reflection focusing mirrors [7]. This focused beam was applied to studies of nonlinear X-ray optical phenomena such as two-photon absorption [8], saturable absorption [9], and Cu-atomic inner-shell lasers [10]. In order to achieve smaller spot sizes with higher intensities, the utilization of multilayer mirrors instead of the single-layer metal-coated mirrors used for the 50-nm focusing system may be considered because multilayer mirrors have large numerical apertures with large grazing incidence angles of over 10 mrad.
One of the critical requirements of X-ray optical elements for XFEL utilization is the possession of a sufficient tolerance for intense X-ray irradiation. For this reason, the X-ray damage thresholds of various optical materials at normal and grazing incidence conditions have been extensively investigated. The damage threshold at the normal incidence condition was theoretically estimated using the amount of absorption energy per atom, while that at the grazing incidence condition this threshold was evaluated by taking into consideration the energetic photoelectrons that can remove deposited energies from an interaction region [11]. These models have been experimentally verified [12–21]. However, the X-ray damage of multilayers induced by XFEL exposure has not been investigated in the hard X-ray regime. In order to study this damage, we used a focused XFEL with a photon energy of 10 keV, which provides a sufficiently high intensity.
2. Experiment
An experiment was performed at SACLA using a 1-μm focusing optics [6] with a photon energy of 10 keV, a pulse duration of less than 10 fs [22], and a repetition rate of 10 Hz. A schematic of the experimental system is shown in Fig. 1. The details of the irradiation chamber and the setup for reflectivity measurements are presented in references [17] and [18], respectively. As a sample, we tested a platinum/carbon (Pt/C) multilayer with 40 bilayers, consisting of a multilayer period of 3 nm with a ratio of the thickness of the high-Z layer to the total thickness of the bilayer, γ, of 0.5. This multilayer was deposited on a commercially available silicon (100) wafer using a DC magnetron sputtering system [23]. The sample was placed at the focal point and pre-aligned to the first-order Bragg condition at a reduced X-ray
intensity. The irradiation tests were performed using 100-shot exposure in a fluence range of 0.01 to 10 μJ/μm2 using silicon attenuators with various thicknesses. The fluctuation of the pulse energies in 100 shots was approximately 10%. The damage threshold was determined by comparing the reflectivities, which were measured at the reduced intensity condition, before and after the 100 shots of fluence-controlled irradiation. We also monitored changes in reflectivity during the 100-shot exposure.
3. Results and discussion
The reflectivities before and after the fluence-controlled irradiation are shown in Fig. 2. We found that the reflectivities did not change at fluences below 0.051 μJ/μm2, while these notably decreased as the fluence exceeded this value, which corresponds to the damage threshold of the Pt/C multilayer.
Typical reflectivity changes during the 100-shot exposures at different fluence conditions are shown in Fig. 3(a). We also present images of the sample surfaces after exposure, which were observed with an optical microscope (OM) [Fig. 3(b)]. For a fluence of 0.051 μJ/μm2, we did not observe any damage in either the reflectivity or the OM image. In contrast, when the fluence became considerably greater than the damage threshold, we observed a noticeable scratch in the OM image as well as a rapid decrease in the reflectivity to a value of almost zero. Interestingly, only the first pulse exhibits the original reflectivity because the temporalduration of the X-ray pulses was much faster than the changes in the multilayer structure. For a fluence of 0.11 μJ/μm2 that was slightly larger than the damage threshold, we found that the reflectivity gradually decreased, with little damage to the OM image.
Fig. 3 (a) Change in the reflectivity as a function of the number of pulses at each irradiation fluence. (b)–(d) Optical microscopy images of the sample surface after 100 pulses of irradiation.
In order to investigate detailed mechanisms behind the changes observed at the last condition, we performed a θ–2θ scan before and after the irradiation conditions with a reduced intensity, which indicated a shift of the Bragg angle from 22.05 mrad to 21.98 mrad, together with a decrease in the reflectivity [Fig. 4(a)]. This shift corresponds to an expansion of the multilayer period of approximately 0.3% as an average over the irradiated area. We also measured cross-sectional transmission electron microscope images at a central part of the footprint before and after irradiation [Figs. 4(b) and 4(c)], which indicated that the multilayer expanded by approximately 10%. This means the degree of the expansion widely distributes in the footprint. This expansion may originate from intermixing at the interface and the introduction of vacancies into the Pt layer, although further investigation is required in order to clearly understand this phenomenon.
Fig. 4 (a) The Bragg angle shift measurement by a θ-2θ scan in the vicinity of the first-order Bragg angle. The measured pre-irradiation and post-irradiation Bragg angles were 22.05 mrad and 21.98 mrad, respectively. (b, c) Cross-sectional bright-field transmission electron microscopy images of the irradiated and non-irradiated areas, respectively. Dark and bright layers correspond to the platinum and carbon layers, respectively.
The damage threshold fluence Fth, can be converted to the damage threshold dose Dth for a single atom using the following expression [11,24,25]
where R and θ are the reflectivity and grazing incidence angle, respectively. In this estimation, R and θ are 0.76 and 21.6 mrad, respectively, which were theoretically obtainable from the design parameters of the multilayer. Here, QPt is the approximate quantity of Pt atoms, given bywhere NA, ρ, and A are Avogadro’s constant, the density of the Pt and C layers, and the atomic weight of the Pt and C layers, respectively. In this estimation, the quantity of C atoms is included as a converted quantity using a ratio of the general electron penetration depth, which is approximately inversely proportional to the density [26]. Moreover, d is the energy deposition depth, given by d = √(dx2 + de2), where dx and de are the X-ray penetration depth and the electron collision range [27], respectively. In this case, de was assumed to be 70 nm, which is twice the de value of a Pt single layer (35 nm [28]), because electrons penetrate the C layers more easily than the Pt layers. On the other hand, dx was estimated using an X-ray standing wave (XSW) field intensity of 1/e. The XSW field intensity at each interface of the multilayer can be obtained using a recursive methodology [29,30], and the inside of the jth layer can be interpolated to be an interference between the transmitted and reflected plane waves, given by [30,31]where z is depth from the top of the jth layer, and Ejt and Ejr are the transmitted and reflected field amplitudes at the top of the jth layer, respectively. Moreover, k'jz and k”jz are the real and imaginary parts of the component kjz, respectively, which is defined as the z-component of the wave vector in the material of the jth layer, and δ is the phase difference between Ejr and Ejt. In this calculation, we assume that the sample was surrounded by vacuum and that the parameters of the materials were the same as those for the bulk materials [32]. The calculated XSW field intensity is shown in Fig. 5. From Fig. 5(b), dx is 22.5 nm at the Bragg angle. According to Eq. (1), the Dth value is calculated to be 0.58 eV/atom, which agrees reasonably well with the threshold dose of a Pt single layer (0.52 eV/atom [17]).Fig. 5 Calculated X-ray standing-wave field-intensity distribution in the multilayer structure as a function of (a) the grazing incidence angle and the depth of the multilayer medium and (b) the line profile at the first-order Bragg angle.
The typical beam size of SACLA at a photon energy of 10 keV is 200 μm in diameter (FWHM), and the pulse energy reaches 400 μJ, which implies that the fluence should reach 0.01 μJ/μm2. This fluence is sufficiently lower than the damage threshold of the Pt/C multilayer. Thus, we confirmed the feasibility of multilayered Pt/C as an optical component of XFELs.
4. Summary
We measured the damage threshold of a Pt/C multilayer using a 1-μm focused hard XFEL beam with a photon energy of 10 keV. We confirmed that the damage threshold of a Pt/C multilayer with a bilayer period of 3 nm is 0.051 μJ/μm2, and that the multilayer can thus be used in XFEL focusing optics. Our calculated value of the threshold atomic dose in the multilayers was similar to that for the bulk material. This threshold result should be a useful criterion for designing multilayer optics in sub-10-nm XFEL focusing and related fields.
Acknowledgments
This research was mainly supported by a Grant-in-Aid for Scientific Research (S) no. 23226004 and a Grant-in Aid for JSPS Fellows no. 25129 from the Ministry of Education, Sports, Culture, Science and Technology, Japan. The XFEL experiments were performed at the BL3 of SACLA with the approval of the Japan Synchrotron Radiation Research Institute (Proposal No. 2012B8052 and 2013A8063). We are grateful to SACLA beamline staff for their help during the beam time.
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