Photon Factory Activity Report 2017 #35 (2018)

BL-11D/2017G638

Evaluation of second order light of BL-11D, the Photon Factoryaround a wavelength of 13.5 nm using La/C multilayer mirrors
Tadashi HATANO*IMRAM, Tohoku University, 2-2-1 Katahira, Aoba-ku, Sendai 980-8577, Japan

2 Multilayer fabrication
Multilayers of which reflection peaks appear in 170 –200 eV depending on the angle of incidence weredesigned. Material pair of Lanthanum (La) and C wereused. Thickness ratio of La and C layers were 1:1. Figure

1 shows calculated spectral reflectance of a La/C
multilayer with period number of N = 100, period
thickness of D = 3.69 nm and angle of incidence of φ =22°. Interface roughness was assumed to be σ = 1 nm
rms. A sharp peak appears at 183.7 eV. A broad peakaround 120 eV can be attributed to La N4.5 absorption.
When this reflectance is measured using a
monochromatized light with second order impurity
contained, a small but sharp peak will appear at 91.8 eV(13.5 nm). Two La/C multilayers with N = 100, D = 3.69
nm and N = 200, D = 3.37 nm were deposited on siliconwafers by ion beam sputtering.
3 Measuements
0.020
La/C multilayer
N = 100
D = 3.69 nm
σ = 1 nm
φ = 22°0.015
reflectance

1 Introduction
In spectroscopic measurements using diffraction
gratings, higher order lights deteriorate quantitative
accuracy. Output of EUV and soft X-ray beamlines
contains typically 10% second order light in flux. Even inspectrally purer experimental condition with higher order
light absorbing filters, residual impurity has to beanalyzed to discuss the accuracy of measured data. Oneof most frequently used spectral regions in opticalelement beamlines is 13.5 nm wavelength EUV.
Reflectances of mirrors, transmittances of filters,sensitivities of detectors and photo-resist used in study fordeveloping EUV lithography technologies are measured.
Therefore high spectral purity is required especiallyaround a wavelength of 13.5 nm.
BL-11D of the Photon Factory is dedicated tocharacterization of EUV and soft X-ray optical elements.
The monochromator was designed with variable deviation
angle type to suppress higher order lights. Higher orderlights have been detected only in the carbon (C) Kabsorption region where higher order lights are relatively
not so low than other regions [1]. By multilayer
reflectometry, it was found that 560 eV second order lightand 840 eV third order light were contained by 7% and
1%, respectively, when the energy of first order light was
set to 280 eV. Although higher order lights are expected
to be low enough around 13.5 nm EUV region, highsensitivity detection of them are needed for confirmationof high spectral purity.
When reflectance of a multilayer reflecting 180 eV
EUV is measured around 90 eV, a pseudo reflection peak
due to the second order light will be observed, which will
be an evidence of the existence of the second order lightand will give a fraction of them by the ratio of thenominal reflectance at 90 eV to the intrinsic reflectance at
180 eV. The energy of evaluation is tunable by changing
the angle of incidence.

0.010

main peak

0.005
La N4,50.000
80

100

120

140160180200photon energy (eV)Fig. 1: Calculated spectral reflectance of a La/Cmultilayer.
Measurements were performed at BL-11D of thePhoton Factory. Measured spectral reflectances aroundthe peaks are shown in Fig. 2. The monochromator wasmoved to the half energy of the reflection peak andspectral reflectances were measured. Results are shown inFig. 3. Simple oscillation was observed. The oscillation
periods agreed with 1/N times of the peak energies for N= 100 and 200, which confirms that the oscillation is awell-known tail structure of multilayer reflection. Here,pseudo peaks that would appear if 1% second order light
were contained in the incident light were plotted bydotted lines in Fig. 3. They are reduction of measured

Photon Factory Activity Report 2017 #35 (2018)reflectance curves plotted in Fig. 2 by 1/100 and 1/2times, along ordinate and abscissa, respectively. The band
width of pseudo peak are similar to the widths of a peakor a valley (half of the period) of tail oscillation because
multilayer bandwidth approximately agree with 1/N timesof the peak energy. From the experimental results that no
pseudo peak were detected in spectral reflectances around91.8 eV, it was found that the fraction of the second order
light was no higher than 0.1% and a 0.1% accuracy ofEUV element characterization at BL-11D of the Photon
Factory was ensured.
0.03
N = 200
D = 3.37 nm
φ = 5°

reflectance

0.025
0.02
N = 100
D = 3.69 nm
φ = 22°

0.015
0.01
0.005
0175

180185190

195

photon energy (eV)Fig. 2: Measured spectral reflectances of La/C multilayers
around their peaks.
6E-04
N = 200
D = 3.37 nm
φ = 5°

5E-04

reflectance

4E-04
3E-04
2E-04

N = 100
D = 3.69 nm
φ = 22°

1E-04
0E+00859095100photon energy (eV)Fig. 3: Measured spectral reflectances of La/C multilayers
in energy region of half of the peaks.

Acknowledgement
This work was partly supported by CAMS Project
2017, IMRAM, Tohoku University.

References[1] T. Hatano and T. Harada, J. Electron Spectrosc. Relat.
Phenom. 196, 156 (2014).
* hatanotadashi@tohoku.ac.jp