Photon Factory Activity Report 2016 #34 (2017)

BL-11D/2011G661, 2013G702

Diffraction order sorting of grating monochromator using a multilayer mirror
Tadashi Hatano1,* and Testuo Harada2
1MRAM, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan
2LASTI, University of Hyogo, 3-1-1 Koto, Kamigori-cho, Ako-gun 678-1205, Japan

1 Introduction
Soft X-rays monochromatized by a diffraction grating
are series of diffraction orders being “polychromatic.”
Spectral impurity due to higher order diffraction is one ofsignificant factors causing a measurement error inexperiments at a grating monochromator beamline ofsynchrotron facilities. BL-11D of the Photon Factory is
dedicated for characterization of soft X-ray elements [1],where the spectral purity is more important for quantitive
measurements. To suppress the high harmonic energy
impurities, varied-deviation-angle type was adopted inmonochromator design [2]. Still remaining impurities
have to be evaluated for data correction.
Suppose the monochromator is set at an energy E andits output is a series of harmonic energy E, 2E, 3E and 4E
whose power spectrum are I1, I2, I3 and I4, where “power”
means the signal height of the detector given by theproduct of the source power, beamline throughput anddetector sensitivity. Measured beam power is I1+I2+I3+I4.
The goal of this work is to evaluate each impurity power
In (n = 2, 3, 4) and to obtain I1 as a deduction from thewhole power. Since In (n = 2, 3, 4) is much smaller than I1
in general, the purity factor I1/(I1+I2+I3+I4) will be
relatively insensitive to error in evaluation of I2, I3 and I4.
A multilayer mirror set at an adequate angle of incidence
is proposed as a tool for picking up one out of series ofharmonic energies.
The monochromator at BL-11D covers 230–1200 eVand 60–290 eV with gratings G1 and G3, respectively [3].
The carbon window below C K-edge is one of usefulenergy regions for soft X-ray microscope and activities
on characterization of carbon window multilayer mirrors
are performed at this beamline [4, 5]. Since the carbon
window lies between energy regions where G1 and G3
were optimized, spectral impurity problem might beserious in this region. Therefore the fundamental energy
E = 282 eV were chosen in this work.
2 Principle
When the monochromator is set at an energy E, eachhigh harmonic component is reflected by a multilayer
with a periodic thickness D at an angle of incidence ofapproximately φn ~ arccos(λn/2D) according to the Bragg

condition with refraction neglected, where λn is thewavelength corresponding the photon energy nE.
In multilayer reflectance measurement at the angle φnaround the fundamental energy E, the nominal value ofthe peak reflectance will be RnIn/(I1+I2+I3+I4), where Rn is
the intrinsic peak reflectance for energy nE and angle φn.
If Rn is known, In can be obtained. Such procedure issimilar to the higher order light analysis using anabsorption edge measurement. In the present method,analysis on all n for a particular E is possible by changing
the angle of incidence of the multilayer, and then thepower series I2, I3, I4 can be obtained.
3 Experiment
The multilayer for reflection measurement is a 300period Cr/C deposited on a Si wafer by ion beam
sputtering deposition. Sputtering targets were switched sothat the thickness ratio should be dCr : dC = 3 : 7. The peakenergy in the spectral reflectance measured at a 5° angleof incidence was 282 eV, in advance, which determines
the period thickness dCr+dC as 2.16 nm.
Grating G1 was used in R2, R3 and R4 measurements
above 500 eV. The spectral resolution estimated from theslit widths of the monochromator was 1000. Grating G3
was used in measurements around E = 282 eV with thespectral resolution of 2000. The converging angle ontothe sample surface was 0.1°. The plane of incidence wasvertical and the polarization state was s-polarized. The
detector was AXUV100Ti2 photodiode (IRD Inc.).
4 Results
Reflectance peak angle at 2E = 564 eV was found at φ2= 59.4°. Measured spectral reflectance at φ2 = 59.4° isshown by a blue line in Fig. 1. The energy scale is shownon the upper axis. The peak reflectance was R2 = 2.49%.
A small structure due to Cr L2,3 absorption was observed
around 590 eV. It is not a side band of multilayer
reflection. At the same angle of incidence, spectralreflectance was measured around E = 282 eV. The resultis shown by a red line in Fig. 1. The small peak pointedby arrows is a reflection signal of the energy 2Ecomponent, which gives R2I2/(I1+I2+I3+I4) = 0.18%. Thus
the second order impurity factor was obtained as I2/(I1+I2+I3+I4) = 0.072, which is an ordinary level.

Photon Factory Activity Report 2016 #34 (2017)

signal of the energy 4E component could not be detected,
which means I4 should be negligible. Small peaks
observed around 290 eV originates in C 1s-π* and C 1sσ* transitions at a grazing incidence geometry.

Fig. 1: Measured spectral reflectance of a Cr/C multilayer atangle of incidence of 59.4°.

Reflectance peak angle at 3E = 846 eV was found at φ3= 70.0°. Measured spectral reflectance at φ3 = 70.0° isshown by a green line in Fig. 2. The energy scale isshown on the upper axis. The peak reflectance was R3 =3.92%. At the same angle of incidence, spectralreflectance was measured around E = 282 eV again. Theresult is shown by a red line in Fig. 2. The very small
peak pointed by arrows is a reflection signal of the energy
3E component, which gives R3I3/(I1+I2+I3+I4) = 0.05%.
Thus the third order impurity factor was obtained as I3/(I1+I2+I3+I4) = 0.013.

Fig. 3: Measured spectral reflectance of a Cr/C multilayer atangle of incidence of 75.3°.

Reflectance measurement at the same angle ofincidence was also tried around 2E = 564 eV, but noreflection signal was detected. It means themonochromator is second-order-free when thefundamental energy is shifted to 564 eV. Therefore thevalue of R2 measured above should be reliable.
As a result the second and third order impurity factors
were determined to be 0.072 and 0.013, respectively,
which give the purity factor I1/(I1+I2+I3+I4) = 0.915.
5 Conclusions
A simple method for quantitive analysis of the higher
order impurities in a grating monochromator output wasproposed. A multilayer is used as a bandpass filter fororder sorting with the angle of incidence changed. We
have characterized the BL-11D, PF at 282 eV using a 300
period Cr/C multilayer of a 2.16 nm period thickness. The
power spectrum was determined to be I1 : I2 : I3 : I4 =0.915 : 0.072 : 0.013 : 0.

Fig. 2: Measured spectral reflectance of a Cr/C multilayer atangle of incidence of 70.0°.

Reflectance peak angle at 4E = 1128 eV was found at
φ4 = 75.3°. Measured spectral reflectance at φ4 = 75.3° isshown by a purple line in Fig. 3. The energy scale isshown on the upper axis. The peak reflectance was R4 =
6.71%. At the same angle of incidence, spectralreflectance was measured around E = 282 eV again. The
result is shown by a red line in Fig. 3. The reflection

References[1] T. Hatano and S. Aihara, J. Phys. Conf. Series 425,152018 (2013).
[2] PF Act. Rep. 2000 A #18, 69 (2001).
[3] T. Hatano, PF Act. Rep. 2014 B #32, 364 (2015).
[4] T. Ejima et al., Applied Optics 53, 6846 (2014).
[5] T. Hatano and T. Harada, J. Electron Spectrosco.
Relat. Phenom. 196, 156 (2014).
* tadashi.hatano.e6@tohoku.ac.jp