Photon Factory Activity Report 2018 #36 (2019)

BL-11D/2018G012

Diffraction Efficiency Measurements of Laminar-Type Diffraction Gratings
Overcoated with Lanthanum Series Layers for Boron-K Emission Spectroscopy
Tadashi HATANO 1*, Masato KOIKE 2, Masami TERAUCHI 1, Alexander S. PIROZHKOV 2,Nobukazu HAYASHI 3, Hiroyuki SASAI 3, and Tetsuya NAGANO 3
1

Institute of Multidisciplinary Research for Advanced Materials, Tohoku University
2-1-1 Katahira, Aoba-ku, Sendai, Miyagi 980-8577, Japan2Kansai Photon Science Institute, Quantum Beam Sciences Directorate,
National Institutes for Quantum and Radiological Science and Technology
8-1-7 Umemidai, Kizugawa, Kyoto 619-0215, Japan
3Device Department, Shimadzu Corporation
1 Nishinokyo-Kuwabaracho, Nakagyo-ku, Kyoto, Kyoto 604-8511, Japan

Experimental studies have been performed to evaluate the enhancement of diffraction efficiency of diffraction gratings byovercoating with lanthanum series layers on conventional metal coated laminar-type gratings around B-K emission (6.76nm, 184 eV) spectrum. In the previous study we observed this effect with a laminar-type Ni-coated diffraction grating
overcoated with diamond-like carbon (DLC), TiO2, and CeO2 layers which have high reflectivity and small extinction
coefficient. In this study, LaF3 and La/C layers were overcoated on Ni-coated gratings, and diffraction efficiency wasmeasured by use of the reflectometer at the PF-BL11D beamline. As the results, the enhancements of spectral flux at 6.76
nm are 3.62 and 4.41 times at the incident angles of 85.005° and 85.050° for LaF3 and La/C overcoated gratings, respectively,compared with that of a conventional Ni coated grating used at an incidence angle of 87.070°.
1 Introduction
There has been common practice to use precious metals,
Au and Pt typically, as the coating materials of the soft Xray diffraction gratings for general use. Furthermore, thegratings are usually illuminated at a grazing incidence
condition beyond the critical angle to attain highdiffraction efficiency taking an advantage of totalreflection condition [1]. However, usually the larger
incidence angle invites the higher diffraction efficiency
and less incoming flux. For the sake of tradeoff relation ofbetween diffraction efficiency and incoming flux it isessentially difficult to increase the spectral flux which isthe product of incoming flux and diffraction efficiency, andthe parameter directly affecting to sensitivity of the applied
grating spectrometer. Also, multilayer coating is aneffective method to enhance the diffraction efficiency at aninterested spectral range even sacrificing the spectral
bandwidth [2]. Therefore, we have initiated feasibility
studies to explore the methods to enhance spectral flux
essentially not merely increase diffraction efficiency at a
traditionally predefined incidence angle as before. In theprevious report we showed simulation and experimental
results which demonstrate that enhancement of spectral
flux around B-K emission spectrum (6.76 nm, 186 eV) byovercoating popular materials of TiO2 and CeO2 layers onconventional Ni coated laminar-type gratings. As theresults, the enhancements of spectral flux at 6.76 nm were
2.2 and 3.4 times for the cases of TiO2 and CeO2
overcoatings, respectively [3-5].
The purpose of this report is to show simulation andexperimental results which demonstrate that similar

enhancement of diffraction efficiency can be obtained byuse of lanthanum series layers which is known as apromising material for multilayer mirrors with enhanced
spectral selectivity for the next generation extreme
ultraviolet lithography in the 6 nm range [6,7].
2 Experiment
Table 1 shows the list of common, past and newly
investigated coating materials, and their theoretical density,ρ , complex refractive index defined by n = (1– δ) + iβ (β:extinction coefficient), and /C, i.e., a grazing incidence
angle (θ) of 5.5°, divided by critical angle measured fromthe surface, C , at = 6.76 nm.
Table 1: Density, optical constant, and grazing incidence
angle normalized with critical angle. Here is density, and are components of complex refractive index, and Cis critical angle measured from the reflecting surface at =
6.76 nm.  = 5.5° is grazing angle of incidence.
NiAu
Pt
CDLC
TiO2
CeO2
La
LaF3
La2O3

 (g/cm3)
8.900
19.32021.450
2.2 0
3.1 0
4.260
7.300
5.950
5.900
6.510


0.025200
0.014819
0.014598
0.008900
0.012550
0.015919
0.0221300.016448
0.019676
0.019656
0.018933
0.008417
0.014647
0.000757
0.001066
0.004000
0.003725
0.001420
0.003093
0.002218

/C
0.428
0.558
0.562
0.720
0.606
0.537
0.456
0.529
0.484
0.484

Photon Factory Activity Report 2018 #36 (2019)

Fig. 1: Calculated reflectivity of Ni, □, Au, ■, Pt ◇, C,●, DLC, ○, TiO2, ▽, CeO2, ▼, La, ▲, La2O3, ◆, LaF3,△, at = 6.76 nm and glancing angle of incidence=5.5°. The abscissa indicates the value of θ normalized toθC. Reflectance curves for 0.0, 0.05, 0.1, 0.2, 0.4, 0.8,and 1.0 are also shown.
Figure 1 shows the graph of the calculated reflectivity ofNi, □, Au, ■, Pt ◇, C, ●, DLC, ○, TiO2, ▽, CeO2,▼,La, ▲, La2O3, ◆, LaF3, △, at = 6.76 nm and glancing
angle of incidence= 5.5°plotted against respective
values of C. The reflectivity curves of β/δ =0, 0.05, 0.10,0.20, 0.40, 0.80, and 1.00 are also indicated. This figure
indicates that lanthanum series materials, i.e., La, La2O3,and LaF3 would exhibit high diffraction efficiency
competitive with the materials investigated previously [3].
Lanthanum exhibits the highest reflectivity, but in air thedeposited surface is easily oxidized to smash. Therefore, aprotective cap layer of a stable material such as carbon isneeded. La2O3 is one of the materials widely used for
optical coating having a high melting temperature(2,315 °C). Therefore, the deposition phenomenon occurs
for the substrate heated up to a high temperature range over
300°C, and it is difficult to apply to the devices which are
undurable to heat load like replica gratings. LaF3 is also themost commonly used material for UV optical coatings, butdefers from La2O3, and can be deposited by various
methods including thermal evaporation method for thesake of its moderate melting temperature (1,493 °C).
Finally, we chose three types of coatings for theexperiment, i. e., Ni single layer for a reference (Type A),
Ni/LaF3 coating (Type B), and Ni/La/C coating (Type C).
Figure 2 shows the schematic diagrams of the cross sectionof the three types of gratings. Table 2 summarizes theparameters of the fabricated gratings. In this table, thesubscript i of the parameter means a, b, or c.
Fabrications of the laminar-type gratings and Ni coatingwere performed at Shimadzu Corp., Kyoto, Japan, andresidual coatings were conducted at Institute ofMultidisciplinary Research for Advanced Materials,
Tohoku University. The measurements of absolutediffraction efficiencies of the gratings were carried out at
the BL11D beamline at the Photon Factory, KEK, Tsukuba,Japan.

Fig. 2: Schematic diagram of laminar-type gratings coatedwith single, double, and triple layers, denoted as Types,A, B, and C, respectively.
Table 2: Parameter list of the fabricated gratings. The
subscript i of parameters means a, b, or c in Fig. 2.

coatingi(lines/mm)
li (nm)li /ihi (nm)d1i (nm)
d2i (nm)
d3i (nm)Type A
Ni

Type B
Ni/LaF3Type C
Ni/La/C

1200.00

1200.00

1200.00

341.67
0.41
14.00
30.00
——

316.67
0.38
16.50
30.00
31.20
—

357.14
0.40
15.00
60.00
25.40
4.00

3 Results and Discussion,Figure 3 shows the relation of the incidence angle withcalculated and measured diffraction efficiencies of the firstorder light (m = + 1) at λ = 6.76 nm for the gratings coatedwith Ni (30.0), Type A, Ni/LaF3, Type B, and Ni/La/C,
Type C. The dashed and full curves show calculated andPhoton Factory Activity Report 2018 #36 (2019)

measured ones. The diffraction efficiencies of the gratingof Types A, B, and C were 0.139, 0.223, and 0.254 at thedefault incidence angle of 87.070°, respectively. The
maximum diffraction efficiencies were 0.173, 0.296, and0.363 at the incidence angles of 84.420°, 85.005°, and85.050° for the gratings of Types A, B, and C, respectively.
For the case of the width of the diffraction grating inview of the incident light direction is proportional to theamount of incident flux ( cos, the gains of theincidence flux are 1.90, 1.70, and 1.69 times, and finallythe enhancements of diffracted spectral flux are 2.33, 3.62
and 4.41 times compared to those of the Ni coated gratingused at the incidence angle of 87.070°, respectively.

Fig. 4: Calculated and measured diffraction efficiencies
for the gratings of Types A, B, and C, respectively, vs.
wavelength at incidence angles of 87.070° for the gratingof Type A and 84.200° for those of Types B and C. For
details see text.

Fig. 3: Calculated and measured diffraction efficiencies
for the gratings of Types A, B, and C, coated with Ni,Ni/LaO3, and Ni/La/C layers, respectively, vs. incidence
angle at  = 6.76 nm. For details see text.
Figure 4 shows wavelength dependence of thediffraction efficiencies of the gratings of Types A, B, andC for the incidence angles of 87.070°, 84.200° and 84.200°,respectively. The dashed and full curves indicate calculated
and measured ones. The deviations of the real peak
positions and values of diffraction efficiencies appeared
for Types B and C gratings from the calculated results are
considered to be due to the discrepancies of real opticalconstants and thickness of the coating layers from thoseassumed vales for the calculations in addition to the effectof surface roughness, and which imply the possibilities offurther improvements. Regarding to oxidization oflanthanum it is necessary to observe whether the protective
layer of carbon works effectively as well as well as theaging over time. We will discuss this subject elsewhere.

Acknowledgements
The measurements of absolute diffraction efficiencies wereperformed as a part of the PF synchrotron radiation joint
experiment of “Development of low incidence angle highdiffraction efficiency diffraction grating in soft x-rayregion,” (2018G012, April 2018-March 2020). A.S.P
acknowledges the support of Strategic Grant (CreativeResearch) by President, QST (No.20, April 2018-March2020).
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[4] T. Imazono et al., AIP Conf. Proc. 1741, 040043 (2016),(4 pages); doi: 10.1063/1.4952915.
[5] M. Koike et al., Photon Factory Activity Report 2017,#35 (2018) (3 pages).
[6] Y. Platonov et al., Pros. SPIE 8076 (2011), doi:
10.1117/12.889519.
[7] N. Chkhalo et al., Appl. Phys. Let., 102 (2013), doi:
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*hatanotadashi@tohoku.ac.jp