⇦Chinese Physics Letters, 2019, Vol. 36, No. 12, Article code 120701 Stress, Roughness and Reflectivity Properties of Sputter-Deposited B$_{4}$C Coatings for X-Ray Mirrors * Jia-Li Wu (吴佳莉)1,2, Run-Ze Qi (齐润泽)2,3, Qiu-Shi Huang (黄秋实)2,3, Yu-Fei Feng (冯宇飞)2,3, Zhan-Shan Wang (王占山)2,3**, Zi-Hua Xin (辛子华)1** Affiliations 1Department of Physics, Shanghai University, Shanghai 200444 2MOE Key Laboratory of Advanced Micro-Structured Materials, Shanghai 200092 3Institute of Precision Optical Engineering, School of Physics Science and Engineering, Tongji University, Shanghai 200092 Received 27 May 2019, online 25 November 2019 *Supported by the National Key R&D Program of China under Grant No 2016YFA0401304, the National Natural Science Foundation of China under Grant Nos 61621001, U1732268 and 11875203, and the Shanghai Municipal Science and Technology Major Project under Grant No 2017SHZDZX02.
**Corresponding author. Email: wangzs@tongji.edu.cn; zhxin@staff.shu.edu.cn Citation Text: Wu J L, Qi R Z, Huang Q S, Feng Y F and Wang Z S et al 2019 Chin. Phys. Lett. 36 120701    Abstract Boron carbide (B$_{4}$C) coatings have high reflectivity and are widely used as mirrors for free-electron lasers in the x-ray range. However, B$_{4}$C coatings fabricated by direct-current magnetron sputtering show a strong compressive stress of about $-3$ GPa. By changing the argon gas pressure and nitrogen-argon gas mixing ratio, we are able to reduce the intrinsic stress to less than $-1$ GPa for a 50-nm-thick B$_{4}$C coating. It is found that the stress in a coating deposited at 10 mTorr is $-0.69$ GPa, the rms roughness of the coating surface is 0.53 nm, and the coating reflectivity is 88%, which is lower than those of coatings produced at lower working pressures. When the working gas contains 8% nitrogen and 92% argon, the B$_{4}$C coating shows not only $-1.19$ GPa stress but also a low rms roughness of 0.16 nm, and the measured reflectivity is 93% at the wavelength of 0.154 nm. DOI:10.1088/0256-307X/36/12/120701 PACS:07.85.Fv, 68.55.-a, 81.15.Cd © 2019 Chinese Physics Society Article Text Single-layer mirrors are widely applied in free-election lasers (FELs) and synchrotron light sources.[1–2] For photon transmission systems, their reflective performance influences the beam alignment, focusing and imaging.[3–5] FELs possess ultra-high brightness and full coherence beams, and have produced important discoveries in the fields of physics, materials, and life sciences.[6–8] Due to the stringent specification of FELs, research of x-ray mirrors with excellent reflective performance is a considerable challenge. Boron carbide (B$_{4}$C) coatings are currently used as x-ray mirrors. B$_{4}$C is a promising coating material with high desirable properties such as superior stability, high hardness, and high damage threshold.[9–11] In addition, it has low absorption edge and can provide high reflectivity in the total reflection. These properties make it important for grazing-incidence mirrors in the photon energy range of operation. B$_{4}$C is mostly utilized as a spacer or barrier layer in multilayers used in astronomy, EUV lithography, biological microscopy, and plasma diagnosis.[12–14] B$_{4}$C single layer or multilayer possess large compressive stress, which may cause delamination of the coatings.[10,14] Kulikovsky et al. deposited B$_{4}$C coatings 1.5-µm-thick on substrates that were either grounded or else biased at $-100$ V, and then they varied the annealing temperature.[15] The stress in the B$_{4}$C coatings decreased as the annealing temperature increased. Soufli et al. fabricated 50-nm-thick B$_{4}$C coatings under 10 mTorr Ar pressure for the linac coherent light source (LCLS) soft x-ray mirror offset system (SOMS) mirrors.[7] The intrinsic stress in the coating dropped from the original $-2.5$ GPa to $-1.1$ GPa as a result of optimizing the gas pressure. In the LCLS and the European XFEL, the optimum thickness of the B$_{4}$C coating was determined to be about 50 nm, to ensure good reflectivity and to suppress the higher harmonics of the FEL beam.[14,16–17] Control of intrinsic stress can also be achieved by means of reactive sputtering with nitrogen, as previously demonstrated in W/B$_{4}$C, Pd/B$_{4}$C, Ru/B$_{4}$C and Co/C multilayers.[18–21] This method not only inhibits the interface diffusion but also improves thermal stability.[22] Some researchers believe that the incorporation of nitrogen has little effect on the optical constants of B$_{4}$C materials in the soft and hard x-ray bands.[18] In addition, high quality of surface morphology is required for good reflective performance.[23] The first hard x-ray free-electron laser in China, called Shanghai High Repetition Rate Hard XFEL and Extreme Light Facility (SHINE), is under construction. The superconducting linac can produce electron beams with 8 GeV energy. After traversing through undulators, the emitted photons cover an energy range of 0.4–25 keV.[24,25] In this Letter, we focus on exploring the optimum deposition conditions, in particular, the sputtering Ar pressure and the N$_{2}$/Ar ratio, to fabricate B$_{4} $C single layers with low stress and low roughness for applications in FELs in the x-ray range. The evolution of stress and roughness of these B$_{4}$C coatings as the deposition conditions changes are discussed. A series of test coupons were prepared, each consisting of a 50-nm-thick B$_{4}$C coating deposited on a D263 glass substrate (each $40\times 15\times 1$ mm), a 5-nm-thick tungsten layer was deposited between the glass and the B$_{4}$C to improve the adhesion. All of the coatings were fabricated by the dc magnetron sputtering technique.[26] The deposition chamber was evacuated to a base pressure of less than $1 \times 10^{-4}$ Pa, and each magnetron cathode was operated in the constant power mode at 100 W and 40 W for the B$_{4}$C and W targets, respectively. Two groups of B$_{4}$C coatings were deposited under different argon pressures with different nitrogen-argon gas mixtures. The first group consisted of B$_{4} $C coatings at variable Ar pressures of 1, 2.5, 5, and 10 mTorr, respectively. The second group included coatings deposited at various N$_{2}$/Ar ratios of 4%, 8% and 15%. For the second group, the working pressure was maintained constant at 1 mTorr during deposition. Grazing incidence x-ray reflectivity (GIXRR) measurements were made in the $2\theta$–$\omega$ mode using an x-ray diffractometer (XRD) comprising a sealed-tube Cu anode and a Ge crystal monochromator tuned to the Cu $K_\alpha$ line (8.04 keV). Fitting of the measured XRR data was used to determine the coating thickness and roughness. Roughness measurements were made using an atomic force microscope (AFM) and an optical profiler covering a wide range of spatial frequencies from $2 \times 10^{-3}$ to $10^{2 }$ µm$^{-1}$. In the high spatial frequency range (HSFR), AFM scans of $1 \times 1$ µm$^{-1}$ and $5 \times 5$ µm$^{-1}$ were performed, and the data obtained were stored in a $256 \times 256$ pixel array. In the middle spatial frequency range (MSFR), optical profiler scans were performed with two objective lens magnifications, 10$\times$ and 50$\times$, at each location, and the data from each scan were stored in a $640 \times 480$ pixel array. The same series of B$_{4}$C coatings without the tungsten adhesive layer, deposited on polished quartz substrates in thickness of 1 mm thick and diameter of 3 mm, were used in the stress measurements. The film stress was measured with a Fizeau interferometer by detecting the changes in the curvature of the quartz substrates before and after the deposition. The stress value was calculated using the Stoney equation.[27] The film stress consists of thermal stress and intrinsic stress. We studied the latter stress, which is a large compressive stress. Although the thermal stress of B$_{4} $C coatings is a tensile stress, its value is negligible relative to the compressive stress and can be ignored.[28] The surface quality of the coating is affected by the surface quality of the substrate. Therefore, it is necessary to ensure the lowest possible substrate roughness. The surface morphology of the uncoated substrate and the B$_{4}$C coating were characterized by optical profiler and by AFM. The power spectral density (PSD) was calculated to elucidate the roughness evolution versus spatial frequency.[10] The rms roughness $\sigma_{\rm r} $ can be obtained by $$\begin{align} \sigma_{\rm r}^{2} =\int_{f_{1} }^{f_{2} } {S(f_{x} )df_{x} },~~ \tag {1} \end{align} $$ where $S(f_{x} )$ is the surface one-dimensional PSD, $f_{1}$ and $f_{2}$ define the spatial frequency range of interest. For the HSFR, $f_{1} = 1$ µm$^{-1}$ and $f_{2} = 1 \times 10^{2 }$ µm$^{-1}$, for the MSFR, $f_{1} = 2 \times 10^{-3 }$ µm$^{-1}$ and $f_{2} = 1$ µm$^{-1}$. The measured values were computed from the PSD curves according to Eq. (1) and were averaged between discrete locations on the surface. The PSD curve of the glass substrate is shown in Fig. 1 (solid line). In addition, we computed $S(f_{x})$ and fitted a power-law model in the form[29] $$\begin{align} S(f_{x} )=\frac{K_{n} }{f_{x}^{n} },~~ \tag {2} \end{align} $$ where $K_{n}$ denotes spectral strengths and $n$ denotes spectral indices. The measured data and curve from the fitting model, with $K_{n} = 4.36 \times 10^{-9 }$ µm$^{3-n}$ and $n = 1.14$, are shown in Fig. 1 (dashed line), which demonstrates that the glass substrate possesses the well-known fractal behavior that can reduce its impact on film growth.[30]
cpl-36-12-120701-fig1.png
Fig. 1. PSD curve of a flat glass substrate. The solid line is the measured date, and the dashed line is from the fitting.
The measured roughness data covering the MSFR and HSFR are given in Table 1. From the test results of the optical profiler, it can be seen that the 1 mTorr B$_{4}$C coating improved the morphology of the glass substrate. Overall, the B$_{4}$C roughness increases with increasing working pressure, and it increases significantly in the AFM-measured range. In particular, in the scan of $1 \times 1$ µm$^{2}$, the rms roughness of the B$_{4}$C coating increases from 0.16 nm to 0.53 nm when the Ar pressure changes from 1 mTorr to 10 mTorr. The AFM $1 \times 1$ µm$^{2}$ images (Figs. 2(a)–2(e)) show that the B$_{4}$C coatings deposited at 5 mTorr and 10 mTorr exhibit large voids and grains. With the increase of working pressure, the probabilities of collision/scattering between argon ions and the sputtering particles are increased.[31] Consequently, the energy of particles falling on the substrate is reduced. These porous columnar structures can relax the layer structure, which leads to an increase in roughness. It appears that the working pressure has a considerable influence on the layer structure.
Table 1. Values of rms roughness of the uncoated substrate and the B$_{4}$C coatings at different Ar pressures.
Sample Magnification 10$\times$ (nm) Magnification 50$\times$ (nm) Scan 5 $\times 5$ µm$^{2}$ (nm) Scan $1 \times 1$ µm$^{2}$ (nm)
Glass substrate 0.32 0.30 0.17 0.16
1 mTorr Ar 0.23 0.21 0.18 0.17
2.5 mTorr Ar 0.26 0.28 0.19 0.18
5 mTorr Ar 0.36 0.28 0.23 0.26
10 mTorr Ar 0.35 0.29 0.45 0.53
To understand the surface morphology of the B$_{4}$C coatings, PSD curves are shown in Fig. 3. It can be seen that varying the Ar pressure during deposition of the B$_{4}$C coatings mainly affects the HSFR for spatial frequencies greater than 1 µm$^{-1}$. In the MSFR, the B$_{4}$C coatings nearly replicate the topography of the glass substrate.
cpl-36-12-120701-fig2.png
Fig. 2. (a)–(e) Results for substrate and the coating morphologies. HSFR values were measured with an AFM in a scan of $1 \times 1$ µm$^{2}$.
cpl-36-12-120701-fig3.png
Fig. 3. One-dimensional PSD function of B$_{4}$C coatings at different Ar pressures.
Figure 4 shows the stress of the B$_{4}$C coatings fabricated at different Ar pressures. It is demonstrated that with the increasing working pressure, the changes in stress and roughness trend in opposite directions as functions of argon pressures. Coatings deposited at sufficiently low pressures were in a state of high compression. As the working pressure increases from 1 to 10 mTorr, the compressive stress of the B$_{4}$C coating gradually decreases from $-3.06$ GPa to $-0.69$ GPa. This is similar to the previously reported results of Soufli et al.[7] This inverse correlation between film stress and surface roughness can be described by structure-zone models with the low temperature ($T/T_{\rm m} < 0.3$).[32] The low-pressure sputtered films are characterized by a zone T-type microstructure comprising over-dense columnar structure with large compressive stresses, as the pressure increases, the films possess loose columnar structure with lower stresses, or even tensile stresses. We term this a zone-1-type microstructure comprising of porous material with large surface roughness.
cpl-36-12-120701-fig4.png
Fig. 4. Stress of the B$_{4}$C coatings fabricated at different Ar pressures (solid squares). The rms roughness of the same samples were measured with an AFM in a scan of $1 \times 1$ µm$^{2}$ (hollow triangles).
The change of stress can also be explained by the atomic peening effect of energetic particles on the growing film. Lower gas pressures result in bombardment of the film surface by highly energetic particles, resulting in the growth of compact films with compressive stress. Meanwhile, higher gas pressures cause less atomic peening, which results in less compact films containing a larger number and size of atomic-scale voids. These voids allow the compressive stress to relax towards the tensile direction.[32] Under pure argon conditions, boron mainly exists in the elemental form, but a small amount exists in the form of the compounds B$_{4}$C and ${\rm B}_{2} $O$_{3}$.[20] Interstitial atoms like Ar could exist in the voids between the columns. As the working pressure was increased to 10 mTorr, the pure B$_{4}$C deposition rate decreased from 2.91 nm/min to 2.14 nm/min due to the reduced mean free path and the enhanced scattering collisions of the depositing atoms. The x-ray reflectivity (at the Cu $K_\alpha$ line) of a B$_{4}$C coating deposited at 1 mTorr is about 95% at incidence angles of about 0.22 deg, and the film density is about 2.39 g/cm$^{3}$. For the B$_{4}$C coatings deposited at 2.5 mTorr or 5 mTorr, the reflectivity is about 92%. However, for the 10 mTorr B$_{4}$C coating, the reflectivity deceases to 88%. As shown in Table 2, the roughness of the B$_{4}$C coatings increased slightly as the working nitrogen gas ratio increased from 4% to 15% in the HSFR and MSFR. At an N$_{2}$ ratio of 4%, the B$_{4}$C coating shows the smallest roughness. Compared with the uncoated substrate, there is no obvious change in the surface morphology of B$_{4}$C coatings that were fabricated at different N$_{2}$ ratios, even under a 15% N$_{2}$ ratio. These results are consistent with those shown in the corresponding PSD curves in Fig. 5.
Table 2. Values of rms roughness of B$_{4}$C coatings at different nitrogen/argon gas mixture ratios.
Sample Magnification 10$\times$ (nm) Magnification 50$\times$ (nm) Scan $5 \times 5$ µm$^{2}$ (nm) Scan $1 \times 1$ µm$^{2}$ (nm)
96%Ar$+$4%N$_{2}$ 0.28 0.22 0.18 0.15
92%Ar$+$8%N$_{2}$ 0.32 0.22 0.18 0.16
85%Ar$+$15%N$_{2}$ 0.33 0.23 0.18 0.17
cpl-36-12-120701-fig5.png
Fig. 5. One-dimensional PSD function of B$_{4}$C coatings obtained at different nitrogen/argon gas ratios.
cpl-36-12-120701-fig6.png
Fig. 6. Stress of the B$_{4}$C coatings fabricated at different nitrogen/argon gas ratios (solid squares). The rms roughness of the same samples were measured with an AFM in a scan of $1 \times 1$ µm$^{2}$ (hollow triangles).
The stress measurement results for the nitrided samples are shown in Fig. 6. There was a sharp and significant reduction in stress in the nitrided B$_{4}$C coatings compared to the pure B$_{4}$C coatings, which demonstrates that nitrogen addition can dramatically reduce the stress of the coatings. The B$_{4}$C coating fabricated with pure Ar exhibits a large compressive stress of $-3.09$ GPa. After adding 4% or 8% N$_{2}$ to the working gas, the stress was significantly reduced to $-1.34$ GPa or $-1.19$ GPa, respectively. With a higher N$_{2}$ ratio of 15%, the stress further decreased down to $-0.84$ GPa. For these nitrided B$_{4}$C single layers, the trend of the stress reduction is similar to the earlier reported results of Windt et al.[18] To clarify the influence of nitrogen on the film stress, we analyzed the elemental valence and atomic fraction of coatings with x-ray photoelectron spectroscopy (XPS). In Fig. 7(a), the B $1s$ central peak locates at about 188.6 eV, corresponding to B–C bonds.[13,33] The peaks at the binding energy of 189.4 eV and 191 eV corresponding to elemental state of boron and oxides, respectively.[20,34,35]
cpl-36-12-120701-fig7.png
Fig. 7. (a) B $1s$ spectrum of the B$_{4}$C coating deposited with pure Ar. (b) B $1s$ and (c) N $1s$ spectra of the nitrided coating deposited with N$_{2}$ ratio of 15%. (d) Atomic fraction of B, C and N for the B$_{4}$C coatings deposited with N$_{2}$ ratio of 15%.
Figures 7(b) and 7(c) show the B $1s$ and N $1s$ spectra for the B$_{4}$C coatings with N$_{2}$ ratio of 15%. Compared with the B $1s$ spectrum of the B$_{4}$C coating deposited with pure Ar, the B $1s$ peak of the nitrided B$_{4}$C coating shifts to higher binding energy. In Ref. [32], the peak centered at 190.7 eV was fitted with B–N bonds. Small peaks at 189.6 eV and 188.6 eV were attributed to elemental boron and B$_{4}$C, respectively. The ones at higher binding energy are identified at more than 192 eV, corresponding to BC$_{2}$O, BCO$_{2}$ and ${\rm B}_{2}$O$_{3}$.[36] The atomic fraction of boron in the chemical states of nitrides is 79%, while in the chemical states of carbides it is dramatically decreased down to 7%. The N $1s$ central peak located at about 397.9 eV corresponds to N–B bonds.[20,33] A small peak at energy of 399.4 eV was fitted to N–C bonds.[20,36] Figure 7(d) shows the atomic concentration of the boron, carbon and nitrogen in the nitrided B$_{4}$C coatings. The ratio of B/C is close to 4:1 for B$_{4} $C coating under pure Ar pressure. However, as the proportion of nitrogen increased, this ratio decreased dramatically to $\sim$3.3 at N$_{2}$ ratio of 15%. Meanwhile, the increasing concentration of nitrogen atoms was apparently observed. In the composition analysis of nitrided B$_{4}$C coatings, we found that the B–C bonds decreased significantly, accompanied by formation of B–N bonds. The bonding mode of boron atoms was mainly B–N bonds, but no longer B–C bonds. Thus, it can be concluded that most nitrogen atoms react with boron atoms to form boron nitride (BN) compounds, decreasing the content of B$_{4}$C. This change trend of B$_{4}$C content is similar to that of stress. In the nitrided B$_{4}$C coating, the chemical environment and the bonding structure of boron atoms have changed. Different from the change of working pressure, nitrogen filling not only reduces the stress but also smooths the surface of the film. In addition, a large increase in B$_{4}$C deposition rate with the nitrogen ratio was observed, and the deposition rate of B$_{4}$C coating at an N$_{2}$ ratio of 15% was 4.20 nm/min, which is advantage for the given low deposition rate of pure B$_{4}$C. For the B$_{4}$C coatings deposited at 4% N$_{2}$ or 8% N$_{2}$, the reflectivity is about 93%. Upon adding more nitrogen to the sputtering gas, the reflectivity decreases to 87%. Owing to the essentially relaxed stress and lower roughness, preparation of the B$_{4}$C coating with a ratio of 8% N$_{2}$ can be of interest for practical applications. In conclusion, to preserve high reflectivity and a perfect x-ray wavefront, we have investigated different deposition conditions including the working pressure and the composition of the working gas to achieve both low stress and low roughness of B$_{4}$C coatings. A gas mixture with 8% N$_{2}$ and 92% Ar is found to be the appropriate condition for generating B$_{4}$C coatings with $\leq$0.32 nm roughness and $-1.19$ GPa stress. This will be of practical interest if further studies can show good thermal stability. Nevertheless, when working under the ultra-intense FEL beam, the stress value is too high to withstand the long-time multi-pulse exposure. Thus, using an optimized working pressure or gas mixture is not sufficient to reduce the intrinsic stress of B$_{4}$C films. Further consideration of other techniques, such as annealing, will be required to obtain a B$_{4}$C coated mirror with high reflectivity and lower stress than those achieved presently. References Predicting the coherent X-ray wavefront focal properties at the Linac Coherent Light Source (LCLS) X-ray free electron laserX-ray Spectroscopy at Free Electron LasersWavelength dependence of the damage threshold of inorganic materials under extreme-ultraviolet free-electron-laser irradiationStructural modifications in Pd/B 4 C multilayers for X-ray optical applicationsThe influence of residual gas on boron carbide thin films prepared by magnetron sputteringEffect of air annealing on mechanical properties and structure of amorphous B4C filmsCoatings for FEL optics: preparation and characterization of B 4 C and PtPreparation and characterization of B 4 C coatings for advanced research light sourcesNitridated Pd/B_4C multilayer mirrors for soft X-ray region: internal structure and aging effectsThermal stability of nitride thin filmsDesign, modeling, and optimization of precision bent refocus optics—LCLS AMO KB mirror assemblySystematic design and three-dimensional simulation of X-ray FEL oscillator for Shanghai Coherent Light FacilityThe high level application architecture of the control system for SHINEOptimization of Gas Sensing Performance of Nanocrystalline SnO 2 Thin Films Synthesized by Magnetron SputteringThe Tension of Metallic Films Deposited by ElectrolysisSPIE ProceedingsSPIE ProceedingsFractal surface finishStress Changes and Stability of Sputter-Deposited Mo/B 4 C Multilayer Films for Extreme Ultraviolet MirrorsThe microstructure of sputter‐deposited coatingsStructure and depth profile composition analysis of Cr/(B4C)/V/(B4C) multilayer for water window applicationOptical constants of magnetron-sputtered boron carbide thin films from photoabsorption data in the range 30 to 770 eVOptical and XPS studies of BCN thin films by co-sputtering of B4C and BN targets
[1]Störmer M, Horstmann C, Siewert F et al 2010 AIP Conf. Proc. (Melbourne Australia 27 September–2 October 2009) 1234 756
[2]Jacobi S, Steeg B, Wiesmann J et al 2002 Proc. SPIE (Seattle WA USA 7–11 July 2002) 4782 113
[3] Barty A, Soufli R, McCarville T et al 2009 Opt. Express 17 15508
[4]Flechsig U, Bahrdt J, Follath R and Reiche S 2016 AIP Conf. Proc. (New York, USA 6–10 July 2015) 1741 040040
[5]Störmer M, Siewert F and Gaudin J 2011 Proc. SPIE (Prague Czech Republic 18–21 April 2011) 8078 80780G
[6]Soufli R, Fernández-Perea M, Hau-Riege S P et al 2011 Proc. SPIE (Prague Czech Republic 18–21 April 2011) 8077 807702
[7]Soufli R, Pivovaroff M J, Baker S L et al 2008 Proc. SPIE (San Diego California USA 10–14 August 2008) 7077 707716
[8] Milne C J, Schietinger T, Aiba M et al 2017 Appl. Sci. 7 720
[9] Hau-Riege S P, London R A, Bionta R M et al 2009 Appl. Phys. Lett. 95 111104
[10]Pivovaroff M J, Bionta R M, Mccarville T J, Soufli R and Stefan P M 2007 Proc. SPIE (San Diego California USA 26–30 August 2007) 6705 67050O
[11]Barthelmess M and Bajt S 2011 Proc. SPIE (Prague Czech Republic 18–21 April 2011) 8077 807710
[12] Morawe Ch, Supruangnet R and Ch J 2015 Thin Solid Films 588 1
[13] Jiang H, Zhu J, Huang Q et al 2011 Appl. Surf. Sci. 257 9946
[14]Soufli R, Baker S L, Robinson J C et al 2009 Proc. SPIE (Prague Czech Republic 20–23 April 2009) 7361 73610U
[15] Kulikovsky V, Vorlicek V, Ctvrtlik R, Bohac P and Lapsanska H 2011 Surf. Coat. Technol. 205 4052
[16] Störmer M, Siewert F, Horstmann C, Buchheim J and Gwalt G 2018 J. Synchrotron Rad. 25 116
[17] Störmer M, Siewert F and Sinn H 2016 J. Synchrotron Rad. 23 50
[18]Windt D L 2007 Proc. SPIE (San Diego California USA 26–30 August 2007) 6688 66880R
[19] Wang Y, Huang Q, Yi Q et al 2017 Opt. Express 25 7749
[20]Huang Q, Liu Y, Yang Y et al 2018 Opt. Express 26 21803
[21]Bellotti J A and Windt D L 2009 Proc. SPIE (San Diego California USA 2–6 August 2009) 7437 743715
[22] Hultman L 2000 Vacuum 57 1
[23] Soufli R, Fernández-Perea M, Baker S L et al 2012 Appl. Opt. 51 2118
[24] Li K and Deng H 2018 Nucl. Inst. Methods Phys. Res. Sect. A 895 40
[25] Lv H, Leng Y, Yan Y and Wang H 2018 Nucl. Inst. Methods Phys. Res. Sect. A 908 167
[26] Panahi N et al 2016 Chin. Phys. Lett. 33 066802
[27] Stoney G G 1909 Proc. R. Soc. London Ser. A 82 172
[28] Morawe C, Ch J and Friedrich K 2010 Proc. SPIE (San Diego California USA 1–5 August 2010) 7802 78020B
[29] Church E L, Takacs P Z and Leonard T A 1989 Proc. SPIE (San Diego, USA 7–11 August 1989) 1165 136
[30] Church E L 1988 Appl. Opt. 27 1518
[31] Niibe M, Nii H and Sugie Y 2002 Jpn. J. Appl. Phys. 41 3069
[32] Thornton J A 1986 J. Vac. Sci. & Technol. A 4 3059
[33]Moulder J F, Stickle W F, Sobol P E and Bomben K D 1995 Handbook of X-Ray Photoelectron Spectroscopy (USA: Physical Electronics Inc) pp 215–230
[34] Li P, Huang Q, Jiang L et al 2016 Vacuum 128 85
[35] Soufli R, Aquila A L, Salmassi F, Fernández-Perea M and Gullikson E M 2008 Appl. Opt. 47 4633
[36] Prakash A and Sundaram K B 2017 Appl. Surf. Sci. 396 484