Open Access
Add to BibSonomy
Abstract
The influence of Mo interlayers on the microstructure of films and boundaries, and the reflective characteristics of Ru/Be multilayer mirrors (MLM) were studied by X-ray reflectometry and diffractometry, and secondary ion mass spectrometry (SIMS). An increase in the reflection coefficients of MLM at a wavelength of 11.4 nm to record values of R = 72.2% and FWHM to Δλ1/2 = 0.38 nm is shown. The effect of interlayers on the structural and reflective characteristics of MLM is explained by the barrier properties of the Mo layers, which prevent the mutual mixing of the Ru and Be layers, which leads to the formation of beryllides and a decrease in the X-ray optical contrast at the boundaries.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Due to its low density and, as a result, low absorption in almost the entire spectral range of X-rays, beryllium is an extremely promising material for use as a weakly absorbing material in multilayer mirrors in EUV (extreme ultraviolet) [1–6] and X-ray [7–9] ranges. However, it was shown for the first time in [10] that, in the wavelength range above 17.1 nm, Be can also act as a scattering material, simultaneously providing record values of peak reflection coefficients and spectral selectivity. The latest results on peak reflection coefficients of beryllium-containing MLMs can be found in [11].
Largely due to the prospects for using EUV lithography in the region of 11.2 nm, Mo/Be MLMs are the most studied [3–6,12–15]. In particular, the authors of [12] studied in detail the interfaces, as well as the influence of the B4C, C, and Si interlayers on them. Theoretically, Ru/Be MLMs have large peak and integral reflection coefficients (R = 78%, Δλ=0.43 nm). Thus, in the case of a hypothetical 10-mirror scheme for lithography, the efficiency of Ru/Be optics at a wavelength of 11.2 nm exceeds that of Mo/Si optics at a wavelength of 13.5 nm by more than one and a half times [16].
However, the experimental results of Ru/Be MLM turned out to be much more modest. The reflection coefficient was 63.7% at a spectral reflection width at half maximum of 0.34 nm [4]. The authors of this work attribute such a strong discrepancy between the experimental and theoretical reflective characteristics to a high level of roughness of interlayer boundaries in a multilayer system. However, the numerical values of the interfaces are not given in the article. Layer mixing also affects the X-ray optical characteristics. As shown by the results of [4], the Ru/Be system consists of alternating layers of pure Ru and an alloy of Ru with Be instead of a layer of pure Be.
Also, the MoxR1-x/Be MLM was studied [17]. In this system, instead of a layer of ruthenium, a ruthenium-molybdenum alloy was deposited. The maximum reflection coefficient of this MLM is 69.3%, and the spectral width is 0.35 nm. The stoichiometric composition of the Ru-Mo alloy in the multilayer system was not indicated.
The effective method for reducing the length of interfaces in MLM is the deposition of interlayers into the structure. In this wavelength range, B4C and C are usually used as the most transparent materials. However, the purpose of this work is to study Ru/Be MLM with Mo interlayers at the interfaces. This approach is based on the results of recent studies of the microstructure of Mo-on-Be and Be-on-Mo interfaces in Mo/Be MLM, carried out using X-ray reflectometry, electron microscopy of cross sections, X-ray absorption spectroscopy (EXAFS) and photoelectron spectroscopy (XPES) [12–15]. Based on the results of these studies, both the widths of the transition regions and their stoichiometric composition were established: Be-on-Mo∼0.3 nm, composition MoBe12 or MoBe22 and Mo-on-Be∼0.7 nm, composition MoBe2. Calculations show that at such interfaces, the reflection coefficients of Ru/Be MLM with Mo interlayers in the region of 11 nm should be much higher than 70%.
2. Experiment
For work with Be, we have a certified laboratory with two magnetron sputtering installations. All safety measures are observed in the laboratory, more details can be found in [11]. Ru/Be MLM were fabricated by DC-magnetron sputtering (IRu = 0.6 A, IBe = 0.8 A, IMo = 0.2 A) in an Ar medium at a gas pressure of ∼0.1 Pa. The MLM were made on smooth (RMS roughness ∼0.2 nm) silicon wafers for microelectronics. The deposition rates were ∼0.2 nm/s for Ru, ∼0.17 nm/s for Be, and ∼0.05 nm/s for Mo. The low rate of Mo deposition is due to the low value of the current on the magnetron in order to increase the controllability of the process during the deposition of ultrathin layers. Structural parameters of MLM: average film thicknesses and thickness dispersion, RMS, and interface profile were determined by joint processing of reflection curves in the region of 11.4 nm and at a wavelength of 0.154 nm using the software packages IMD [18] and Multifitting [19]. In addition to interface parameters, the Multifitting program [19] makes it possible to reconstruct electron density profiles and material distribution profiles. A gradient interface model was used when fitting, discretization step ∼0.1 nm for all samples. Also, to describe the transition layer, a linear combination of several functions was used, namely, the error function (erf) and the linear function (lin), more details can be found in [19].
The angular dependences of the reflection coefficients at a wavelength of 0.154 nm were measured on a PANalytical X’pert-PRO four-crystal high-resolution diffractometer (Netherlands). The spectral and angular dependences of the reflection coefficients in the vicinity of a wavelength of 11.4 nm were carried on a laboratory reflectometer [20,21]. To estimate the absolute accuracy of measurements of the reflection coefficients and the spectral selectivity of the laboratory reflectometer in the vicinity of 11.4 nm, Mo/Be MLM measured on the optical channel of the BESSY-II synchrotron [22,23] was used as a reference. More details about laboratory methods of metrology can be found in [11].
The microstructure of the film materials, as well as possible connections at the interfaces, were studied by wide-angle diffraction on a Bruker D8 Discover diffractometer (Bruker AXS, Germany).
The depth distribution of chemical elements with a resolution of up to 1 nm was carried out by the method of secondary ion mass spectroscopy according to the procedure described in [24,25].
3. Results and discussion
Measurements in the vicinity of the operating wavelength of 11.4 nm were carried out on a laboratory reflectometer. In this regard, we first compared the results of measurements of the reference Mo/Be MLM on a laboratory reflectometer and on BESSY-II (Fig. 1). As can be seen from Fig. 1, the reflection coefficients of both measurements coincided with an accuracy of no worse than ±1%. The resonant wavelength coincided in exactly the same way. This test indicates the accuracy of measurements of the reflection coefficient in the experiment at the level of 1% and the accuracy of determining the FWHM at the level of Δλ1/2 = 0.013 nm.
Fig. 1. Comparison of measurements of the angular and spectral dependencies of the reflection coefficients for an MLM Mo/Be, performed using a synchrotron (solid line) and laboratory (symbols) reflectometers.
3.1 Reflectometry research MLM Ru/Be
A series of Be/Ru samples was prepared without Mo interlayers and with Mo interlayers at the interfaces. The parameters of the samples are shown in Table 1. The periods of all MLMs were close and amounted to about 6 nm, the number of periods N = 80.
Table 1. Experimental samples, their structural parameters, and methods for studying samples
The structural parameters of MLM listed in Table 1 were determined by joint fitting of the reflection curves at wavelengths of 0.154 nm and 11.4 nm. An example of fitting for sample #7 of the Ru/Be MLM is shown in Fig. 2. The solid lines show the fitting results, the dots show the experimental data.
Fig. 2. Fitting of the experimental angular dependences of the reflection coefficients of Be/Ru MLM, sample #7, at wavelengths of 0.154 nm (a) and 11.4 nm (b). The solid lines show the fitting results, the symbols show the experimental data.
The profiles of dispersion additions to the real parts of the permittivity (1-ε) reconstructed from these data and depth distributions of materials are shown in Fig. 3(a) and 3(b), respectively. The dependence of the square wave in Fig. 3(a) corresponds to the tabular values of the permittivity, smooth curves is a fitting. In Fig. 3(b), the table value of the density of each of the materials is taken as 1. When fitting the theoretical reflection curves, the formation of chemical bonds between materials was not taken into account. The fitting results showed that the interface widths were: Ru-on-Be σ≈1.1 nm, Be-on-Ru-σ≈0.45 nm. Such a large value of interface widths qualitatively agrees with the results of [4], where a high level of roughness of interlayer boundaries is noted.
Fig. 3. Electron density profile reconstructed from X-ray reflectometry data (a) and material thickness distribution (b) in Ru/Be MLM, sample #7.
As can be seen from the material distribution pattern in the structure, there is no absolutely “pure” layer in the system. The materials of the multilayer system penetrate each other, which is in good agreement with the data of [4], where it is indicated that the Be layer is a solid solution of Ru with Be.
The optimal proportion of Ru in the period γ=h(Ru)/d, where h(Ru) is the layer thickness of Ru, d is the MLM period, corresponding to the maximum reflection coefficient, was chosen for further studies of the influence of Mo interlayers on the structural and reflective characteristics. Figure 4 shows reflectance and FWHM for a series of Ru/Be MLMs, samples #1 - 11.
Fig. 4. Dependences of the reflection coefficient (a) and FWHM (b) Ru/Be MLM at a wavelength of 11.4 nm on the proportion of Ru in the period, samples # 1 - 11.
The maximum reflectivity of the Be/Ru MLM is about R≈67% and falls at γ≈0.46. In this case, the spectral width is Δλ≈0.34 nm. It should be noted that this value of the reflection coefficient is ∼3.3% higher than the reflection coefficient obtained earlier in [4]. This discrepancy in reflectances can be explained by the improvement in the growth technology of MLMs over the past 20 years. In this case, the spectral width is at the same level.
3.2 Reflectometry research MLM Ru/Be/Mo and Ru/Mo/Be
After determining the optimal share of Ru in the period of the structure, which was γ≈0.46, a series of samples with Mo interlayers at different boundaries was prepared. The interlayer thicknesses were 0.2, 0.4, and 0.6 nm. The increase in the thickness of the Mo layer was carried out by reducing the thickness of the Ru layer, while the thickness of the Be layer remained unchanged. Figure 5 shows the dependences of the reflection coefficient (a) and FWHM (b) of the Be/Ru MLM with Mo interlayers at different interfaces, at a wavelength of 11.4 nm, on the thickness of the Mo layer in the period.
Fig. 5. Dependence of the reflection coefficient - a) and FWHM - b) at a wavelength of 11.4 nm on the thickness of the Mo interlayer in the period Be/Mo/Ru MLM, samples # 7 (Ru/Be), 13, 15, 17(Be/Mo/Ru) and 7, 19, 20 and 22 (Ru/Mo/Be)
The addition of the Mo interlayer in the Ru/Be MLM to the Ru-on-Be interface leads to an increase in both the reflectivity and FWHM. In this case, the maximum reflection value R≈69% is achieved at a layer thickness of Mo = 0.2 nm. With a further increase in the thickness of Mo, the value of the reflection coefficient decreases, but, nevertheless, remains higher than the reflection of the Ru/Be MLM. The fitting results showed that the Mo interlayer had little effect on the change in the Ru-on-Be interface. The value of the interlayer changed to the level σ≈0.9-1.0 nm.
Figure 6, a shows the electron density profile reconstructed from X-ray reflectometry data in Be/Mo/Ru MLM, sample # 13. It can be seen from the figure that the main change in the microstructure of layers and interfaces is the increase in optical contrast. Compared to Ru/Be MLMs (Fig. 3), the electronic density of Ru layers increased, while Be, on the contrary, decreased. This indirectly indicates less mixing of materials at the boundary.
Fig. 6. Electron density profile reconstructed from X-ray reflectometry data in Be/Mo/Ru MLM, sample # 20 (a) and Ru/Mo/Be MLM, sample # 20 (b)
The addition of a Mo interlayer in Ru/Be MLM to the Be-on-Ru interface also leads to an increase in both reflectivity and FWHM. In this case, the maximum value of the reflection coefficient R∼69% is achieved at a layer thickness of Mo = 0.4 nm. With a further increase in the thickness of the Mo layer, the value of the reflection coefficient decreases, but remains higher than the reflection of the Ru/Be MLM. The spectral width also decreases with increasing thickness of the Mo layer, but also remains larger than in the Ru/Be MLM. The fitting results showed that the interfaces did not change drastically, however, as in the previous case, the optical contrast at the boundaries increased (Fig. 6, b).
3.3 Reflectometry research MLM Mo/Ru/Mo/Be
After finding the optimal thicknesses of Mo interlayers at different interfaces, Ru/Be MLMs were fabricated with Mo interlayers at both interfaces simultaneously, samples # 7, 24, 25, 27, and 28. Figure 7 shows the corresponding dependences.
Fig. 7. Dependences of the reflection coefficient - a) and FWHM - b) at a wavelength of 11.4 nm on the thickness of the Mo interlayer in the period in Mo/Be/Mo/Ru MLM, samples # 7, 24, 25, 27, and 28
The addition of Mo interlayers to both interfaces simultaneously led to a significant improvement in the X-ray optical characteristics. The maximum value of the reflection coefficient was R = 72.2% at the thickness of the interlayers Mo: Ru-on-Be - 0.2 nm, and Be-on-Ru - 0.4 nm. These thicknesses correspond to the best thicknesses for Ru/Be MLM with a Mo interlayer at one boundary. It should also be noted that for all studied thicknesses of the Mo interlayers, the reflection coefficient is higher than 70%, and the FWHM is wider than that of Ru/Be MLM. The obtained reflection coefficients are also higher than in MoxRu1-x/Be MLM [14].
Figure 8 shows the electron density profiles (Fig. 8 a) and material distribution (Fig. 8 b) reconstructed from X-ray reflection data at wavelengths of 0.154 nm and 11.4 nm in Mo/Be/Mo/Ru MLM, sample # 27.
Fig. 8. Electron density profile reconstructed from X-ray reflectometry data (a) and material thickness distribution (b) in Mo/Be/Mo/Ru MLM, sample #27.
If we compare similar curves for Ru/Be MLM (Fig. 3), we can see that the values of the transition layers have not changed much: the Be-on-Ru transition layer remained unchanged σ(Be-on-Ru) ≈0.45 nm, while the Ru-on-Be transition layer decreased to the level σ(Ru-on-Be) ≈0.8 nm. These values are somewhat higher than in Mo/Be MLM [12], but lower than those of Ru/Be MLM. An additional effect leading to an increase in the reflection coefficient of Mo/Be/Mo/Ru MLMs is a decrease in the mixing of Ru and Be layers. A “pure” Be region appears on the electron density profile. The observed slight decrease in the density of Ru is due to the replacement (decrease) of its part by Mo interlayers. Thus, it follows from the reflectometric data that the Mo interlayers play the role of antidiffusion barriers between the Ru and Be layers.
3.4 X-ray phase analysis and SIMS
To confirm this conclusion, diffraction measurements were carried out. We hoped to discover new phases, for example, beryllides and alloys. Figure 9 and Table 2 show the measurement results of X-ray phase analysis from the MLM, samples #2, 12, 15, 16, 18, 20, 21, 23, 26, 27 and 28. Vertical lines indicate the positions of various phases of Mo, Ru and their alloys, as well as Si in the region of diffraction peaks recorded in the experiment.
Table 2. Result of processing X-ray phase analysis of MLM
X-ray phase analysis data show - only one peak is observed. The position of the peak in Ru/Be MLM (#2) is close to the position of the reflection of the Ru (002) (2θ=41,985°) phase with hexagonal close packing (hcp). This is due to the characteristic texture of hcp metals, which is characteristic of the prepared of multilayer by DC-magnetron sputtering. Also, the peak shifts towards smaller angles when a Mo interlayer is added to Ru/Be MLM. The greater the displacement, the greater the thickness of the Mo interlayer and the smaller the thickness of the Ru layer. The position of the peak at the maximum Mo thickness in MLM is close to the position of the (002) reflection for the phase of the Mo0.3Ru0.7 (2θ=41,335°) solid solution based on the hcp ruthenium structure. In this case, the intensity of the peak increases with increasing Mo thickness and, consequently, with decreasing Ru layer thickness. Also, FWHM decreases, which indicates an increase in the size of the coherent scattering region (CSR), i.e. “crystallite size”. It is suggested that this dependence is formed either due to additional texturing of the Ru layers or due to an increase in the effective thickness of the “coherently scattering” Ru layer when a Mo interlayer is added to the structure. The rocking curves, samples #2, 21 and 28 were measured to determine the texture scattering angle. The width of the peak of the rocking curve (texture angle) does not change with the change in the thickness of the Ru and Mo layers in the structure and is 11-12° for the measured samples. Therefore, the change in peak intensity is not associated with additional texturing of the layer due to the addition of Mo to the structure. We attribute such a change in the peak amplitude to an increase in the effective thickness of the Ru layer in the structure. The effective thickness of Ru increases, apparently due to the better “separation” of the layers of the multilayer system, as a result of which the reflectivity of the MLM improves. The confirmation of this assumption using SIMS measurements is presented below.
The data of X-ray phase analysis did not give an exhaustive answer to the mechanism of improvement of the optical characteristics of Ru/Be MLM when Mo interlayers are added to the interfaces. SIMS studies have been carried out to answer this question. Samples #2 (Ru/Be), 12 (Be/Mo/Ru), 20 (Mo/Be/Ru), 27 and 28 (Mo/Be/Mo/Ru) were tested under the same measurement parameters. Figure 10 shows SIMS data Ru/Be MLM, sample #2.
Figure 10 shows the amplitude values of the SIMS measurement of the sample under study. The normalized values of element concentrations are not given due to the influence of matrix effects on the amplitude and the absence of reference samples of different concentrations to take into account this influence. More details about matrix effects and the need for reference samples can be found in [26]. In this regard, all SIMS measurement results will be presented with the amplitude values of the MLM materials, and not normalized.
Figure 11 and 12 show the results of SIMS measurements of Ru/Be MLM samples with Mo interlayers at different interfaces (Fig. 11) and at both interfaces simultaneously (Fig. 12). Sample numbers # 12 and 20 Fig. 11 and # 27 and 28 Fig. 12.
Fig. 11. SIMS data Ru/Be MLM with Mo interlayers at different interfaces, samples #12 (Be/Mo/Ru) (a) and #20 (Mo/Be/Ru) (b)
Fig. 12. SIMS data Ru/Be MLM with Mo interlayers at both interfaces simultaneously, samples # 27 (a) and # 28 (b)
When a Mo interlayer is added to any interface in Ru/Be MLM, the amplitude of both Be and Ru increases. However, the maximum increase in amplitude is achieved by adding Mo interlayers to both interfaces simultaneously. Figure 13 shows the comparative SIMS results of Ru/Be MLM (#2) and Ru/Be MLM with Mo interlayers at both interfaces simultaneously (#27), which has the maximum value of reflectivity.
The addition of Mo interlayers to both interfaces simultaneously increased a contrast, a ration max to min intensity on the SIMS curves, for Be by a factor of ∼2.2 and for Ru by a factor of ∼5.4 from the initial values. An increase in the contrast indicates less mixing of the MLM materials with each other, which confirms all earlier assumptions: as a result of less mixing of materials, the reflective characteristics of the MLM improve, as well as an increase in the effective thickness of Ru, which gives a reflection in the X-ray phase analysis patterns. An even greater increase in the contrast, and hence less mixing, is observed in the sample with the largest thicknesses of the Mo interlayers at both interfaces (#28), however, it has lower reflective characteristics due to the lower X-ray optical benefit of such a system - at an operating wavelength of 11.4 nm Ru is more preferable as a scattering material than Mo.
4. Conclusion
In this work, X-ray reflectometry, diffraction, and SIMS are used to study the effects of Mo interlayers on the X-ray optical and structural characteristics of Ru/Be MLM. A reflection coefficient was obtained at the level R = 67% at FWHM Δλ1/2 = 0.335 nm with the optimal share of Ru in the period γ=0.46, corresponding to the maximum reflection coefficient Ru/Be MLM in the vicinity of a wavelength of 11.4 nm. The main factors leading to a decrease in the reflectance compared to the theoretical limit, about 78%, are interfaces: σ(Be-on-Ru)≈0.45 nm and σ(Ru-on-Be)≈1.1 nm, as well as mixing layer materials. The Ru and Be layers contain traces of an alternative material.
The addition of Mo interlayers to both one and the other interface leads to an increase in the reflection coefficient to R∼69% and FWHM to Δλ1/2 = 0.36-0.37 nm. In this case, a slight decrease in the length of the interface σ(Ru-on-Be)≈0.9-1.0 nm is observed. However, on the distributions of electron density and chemical elements over depth, an increase in the contrast of occurrence of Ru and Be is clearly observed, which indicates a decrease in the mixing of the materials of the layers - the Mo interlayers act as barriers from the mutual diffusion of materials. At the same time, the optimal thicknesses of the buffer layers when applied to different boundaries differ. The optimal thicknesses of Mo buffer layers are: 0.2 nm at the Ru-on-Be interface and 0.4 nm at the Be-on-Ru interface.
When Mo interlayers are added to both interfaces simultaneously, the positive effect is even more enhanced - the reflection coefficient and FWHM in the vicinity of a wavelength of 11.4 nm increase to R = 72.2% and Δλ1/2 = 0.38 nm, respectively. The interface lengths change insignificantly: the Ru on Be interface improves to σ(Ru-on-Be)≈0.8 nm, while the Be on Ru interface remains unchanged σ(Ru-on-Be)≈0.45 nm. Some decrease in the average density of Ru layers is observed, which is associated with the replacement of a part of dense Ru by less dense Mo. However, in this case, the Be density drops to the tabular value, which indicates the complete absence of Ru in the Be layer, which significantly reduces the absorption in the structure under resonant reflection conditions and, as a result, leads to an increase in the reflection coefficient.
From a methodological point of view, a good agreement between the X-ray reflectometry and SIMS data should be noted. Despite the rather low depth spatial resolution of SIMS, on the order of 1 nm, this method is nevertheless quite effective in determining the chemical composition of layers and interfaces in MLMs. The observed change in the contrast of different materials with respect to depth under certain ionic effects on the composition of the MLM provides a qualitative understanding of the diffusion processes in the MLM.
Funding
Center of Exellence «Center of Photonics» funded by The Ministry of Science and Higher Education of the Russian Federation (contract № 075-15-2022-316).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
References
1. O. Renner, M. Kopecky, E. Krousky, F. Schafers, B. R. Muller, and N. I. Chkhalo, “Properties of laser-sputtered Ti/Be multilayers,” Rev. Sci. Instrum. 63(1), 1478–1481 (1992). [CrossRef]
2. N. I. Chkhalo, M. V. Fedorchenko, N. V. Kovalenko, E. P. Kruglyakov, A. I. Volokhov, V. A. Chernov, and S. V. Mytnichenko, “Status of X-ray mirror optics at the Siberian SR Centre,” Nucl. Instrum. Methods Phys. Res., Sect. A 359(1-2), 121–126 (1995). [CrossRef]
3. C. Montcalm, S. Bajt, P. B. Mirkarimi, E. A. Spiller, F. J. Weber, and J. A. Folta, “Multilayer reflective coatings for extreme-ultraviolet lithography,” Proc. SPIE 3331, 42–51 (1998). [CrossRef]
4. S. Bajt, R. D. Behymer, P. B. Mirkarimi, C. Montcalm, and M. A. Wall, “Experimental investigation of beryllium-based multilayer coatings for extreme ultraviolet lithography,” Proc. SPIE 3767, 259–270 (1999). [CrossRef]
5. P. B. Mirkarimi, “Stress, reflectance, and temporal stability of sputter-deposited Mo/Si and Mo/Be multilayer films for extreme ultraviolet lithography,” Opt. Eng. 38(7), 1246–1259 (1999). [CrossRef]
6. M. Singh and J. J. M. Braat, “Design of multilayer extreme-ultraviolet mirrors for enhanced reflectivity,” Appl. Opt. 39(13), 2189–2197 (2000). [CrossRef]
7. Y. Utsumi, H. Kyuragi, and T. Urisu, “Spatial period division with synchrotron radiation bandwidth control by W/Be multilayer mirror,” J. Vac. Sci. Technol., B: Microelectron. Process. Phenom. 8(3), 436–438 (1990). [CrossRef]
8. J. I. Takahashi, T. Urisu, and H. Maezawa, “Soft x-ray W/Be multilayer and its application to a diffraction grating Yuichi, Utsumi,” Rev. Sci. Instrum. 60(7), 2024–2026 (1989). [CrossRef]
9. A. A. Akhsakhalyan, Y. A. Vainer, S. A. Garakhin, K. A. Elina, P. S. Zavertkin, S. Y. Zuev, D. V. Ivlyushkin, A. N. Nechay, A. D. Nikolenko, D. E. Pariev, R. S. Pleshkov, V. N. Polkovnikov, N. N. Salashchenko, M. V. Svechnikov, and N. I. Chkhalo, “Set of Multilayer X-Ray Mirrors for a Double-Mirror Monochromator Operating in the Wavelength Range of 0.41–15.5 nm,” J. Surf. Invest.: X-Ray, Synchrotron Neutron Tech. 13(1), 1–7 (2019). [CrossRef]
10. N. I. Chkhalo, D. E. Pariev, V. N. Polkovnikov, N. N. Salashchenko, R. A. Shaposhnikov, I. L. Stroulea, M. V. Svechnikov, Y. A. Vainer, and S. Y. Zuev, “Be/Al-based multilayer mirrors with improved reflection and spectral selectivity for solar astronomy above 17 nm wavelength,” Thin Solid Films 631, 106–111 (2017). [CrossRef]
11. V. N. Polkovnikov, N. N. Salashchenko, M. V. Svechnikov, and N. I. Chkhalo, “Beryllium-based multilayer X-ray optics,” Phys.-Usp. 63(1), 83–95 (2020). [CrossRef]
12. M. V. Svechnikov, N. I. Chkhalo, S. A. Gusev, A. N. Nechay, D. E. Pariev, A. E. Pestov, V. N. Polkovnikov, D. A. Tatarskiy, N. N. Salashchenko, F. Schafers, M. G. Sertsu, A. Sokolov, Y. A. Vainer, and M. V. Zorina, “Influence of barrier interlayers on the performance of Mo/Be multilayer mirrors for next-generation EUV lithography,” Opt. Express 26(26), 33718–33731 (2018). [CrossRef]
13. S. A. Kasatikov, E. O. Filatova, S. S. Sakhonenkov, A. U. Gaisin, V. N. Polkovnikov, and R. M. Smertin, “Study of Interfaces of Mo/Be Multilayer Mirrors Using X-ray Photoelectron Spectroscopy,” J. Phys. Chem. C 123(42), 25747–25755 (2019). [CrossRef]
14. R. M. Smertin, V. N. Polkovnikov, N. N. Salashchenko, N. I. Chkhalo, P. A. Yunin, and A. L. Trigub, “The microstructure of transition boundaries in multilayer Mo/Be systems,” Tech. Phys. 65(11), 1800–1808 (2020). [CrossRef]
15. N. Kumar, V. A. Volodin, R. M. Smertin, P. A. Yunin, V. N. Polkovnoikov, K. Panda, A. N. Nechay, and N. I. Chkhalo, “Raman scattering study of nanoscale Mo/Si and Mo/Be periodic multilayer structures,” J. Vac. Sci. Technol., A 38(6), 063408 (2020). [CrossRef]
16. N. I. Chkhalo and N. N. Salashchenko, “Next generation nanolithography based on Ru/Be and Rh/Sr multilayer optics,” AIP Adv. 3(8), 082130 (2013). [CrossRef]
17. S. Bajt, “Molybdenum–ruthenium/beryllium multilayer coatings,” J. Vac. Sci. Technol., A 18(2), 557–559 (2000). [CrossRef]
18. D. L. Windt, “IMD: Software for modeling the optical properties of multilayer films,” Comput. Phys. 12(4), 360–370 (1998). [CrossRef]
19. M. Svechnikov, “Multifitting: software for the reflectometric reconstruction of multilayer nanofilms,” J. Appl. Crystallogr. 53(1), 244–252 (2020). [CrossRef]
20. S. A. Garakhin, I. G. Zabrodin, S. Y. Zuev, I. A. Kas’kov, A. Y. Lopatin, A. N. Nechai, V. N. Polkovnikov, N. N. Salashchenko, N. N. Tsybin, and N. I. Chkhalo, “Laboratory reflectometer for studying optical elements in the wavelength range 5–50 nm: description and results of tests,” Quantum Electron. 47(4), 385–392 (2017). [CrossRef]
21. A. D. Akhsakhalyan, E. B. Kluenkov, A. Y. Lopatin, V. I. Luchin, A. N. Nechay, A. E. Pestov, V. N. Polkovnikov, N. N. Salashchenko, M. V. Svechnikov, M. N. Toropov, N. N. Tsybin, N. I. Chkhalo, and A. V. Shcherbakov, “Current Status and Development Prospects for Multilayer X-ray Optics at the Institute for Physics of Microstructures, Russian Academy of Sciences,” J. Surf. Invest.: X-Ray, Synchrotron Neutron Tech. 11(1), 1–19 (2017). [CrossRef]
22. F. Schäfers, P. Bischoff, F. Eggenstein, A. Erko, A. Gaupp, S. Künstner, M. Mast, J.-S. Schmidt, F. Senf, F. Siewert, A. Sokolov, and T. Zeschke, “The at-wavelength metrology facility for UV- and XUV-reflection and diffraction optics at BESSY-II,” J. Synchrotron Radiat. 23(1), 67–77 (2016). [CrossRef]
23. A. Sokolov, P. Bischoff, F. Eggenstein, A. Erko, A. Gaupp, S. Künstner, M. Mast, J.-S. Schmidt, F. Senf, F. Siewert, T. Zeschke, and F. Schäfers, “At-wavelength metrology facility for soft X-ray reflection optics,” Rev. Sci. Instrum. 87(5), 052005 (2016). [CrossRef]
24. B. Ber, P. Bábor, P. N. Brunkov, P. Chapon, M. N. Drozdov, R. Duda, D. Kazantsev, V. N. Polkovnikov, P. Yunin, and A. Tolstogouzov, “Sputter depth profiling of Mo/B4C/Si and Mo/Si multilayer nanostructures: A round-robin characterization by different techniques,” Thin Solid Films 540, 96–105 (2013). [CrossRef]
25. M. N. Drozdov, Y. N. Drozdov, N. I. Chkhalo, V. N. Polkovnikov, P. A. Yunin, M. V. Chirkin, G. P. Gololobov, D. V. Suvorov, D. J. Fu, V. Pelenovich, and A. Tolstogouzov, “Time-of-flight secondary ion mass spectrometry study on Be/Al-based multilayer interferential structures,” Thin Solid Films 661, 65–70 (2018). [CrossRef]
26. A. Benninghoven, F. G. Rudenauer, and H. W. Werner, “Secondary ion mass spectrometry: basic concepts, instrumental aspects, applications and trends,” Surf. Interface Anal. 10(8), 435 (1987). [CrossRef]
