Nuclear Instruments and Methods in Physics Research A 649 (2011) 163–165

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Nuclear Instruments and Methods inPhysics Research A

0168-90

doi:10.1

n Corr

Hyogo 6

E-m

journal homepage: www.elsevier.com/locate/nima

Performance of focusing mirror device in EUV beamline of SPring-8 CompactSASE Source (SCSS)

Haruhiko Ohashi a,b,n, Yasunori Senba a,b, Mitsuru Nagasono a, Makina Yabashi a,b, Kensuke Tono a,b,Tadashi Togashi a,b, Togo Kudo a,b, Hikaru Kishimoto b, Takanori Miura b, Hiroaki Kimura a,b,Tetsuya Ishikawa a,b

a SPring-8 Joint Project for XFEL, 1-1-1 Koto, Sayo, Hyogo 679-5148, Japanb JASRI/SPring-8, 1-1-1 Koto, Sayo, Hyogo 679-5198, Japan

a r t i c l e i n f o

Available online 17 December 2010

Keywords:

EUV

Ablation

X-ray free electron laser

X-ray mirror

Beamline

Beam size

SPring-8

02/$ - see front matter & 2010 Elsevier B.V. A

016/j.nima.2010.11.174

esponding author at: SPring-8 Joint Project

79-5148, Japan. Tel.: +81 791 58 0831; fax: +

ail address: hohashi@spring8.or.jp (H. Ohashi

a b s t r a c t

A focusing mirror device was designed and installed in an extreme ultraviolet (EUV) beamline of the

SPring-8 Compact SASE Source (SCSS). The horizontal and vertical sizes of the beam at the focal point were

measured to be 22 and 26 mm with a working distance of 0.94 m at a wavelength of 60 nm. A high power

density over 20 TW/cm2 was achieved. Ablation properties of some materials such as silicon, diamond and

tantalum have been studied for determining the focused beam profile with a single shot irradiation.

& 2010 Elsevier B.V. All rights reserved.

1. Introduction

The SPring-8 Compact SASE Source (SCSS) [1] is a 250 MeVlinac-based free electron laser (FEL), constructed as a prototype of acompact X-ray FEL (XFEL) at SPring-8 [2]. After successful achieve-ment of the expected performance, the SCSS has been used toexplore new frontiers of sciences as well as to develop anangstrom XFEL.

Focusing mirror is a key device for developing broad fields ofapplications. In this report, we describe the design and perfor-mance of focusing mirror device installed in the EUV beamline.

2. Focusing mirror device at the EUV beamline of SCSS

A schematic view of the beamline is shown in Fig. 1. The two-dimensional focusing mirrors, which are combined with an ellipticcylindrical mirror (M4e, tangential focusing in horizontal direc-tion) and a cylindrical mirror (M5e, sagittal focusing in verticaldirection), were installed at an E-branch of the beamline. Theparameters of these mirrors are shown in Table 1. For covering avariety of applications with these mirrors, we adopted a longworking distance (0.92 m) between the M5e and the focal point. Aconvergence half-angle of the beam from the final mirror to the

ll rights reserved.

for XFEL, 1-1-1 Koto, Sayo,

81 791 58 0830.

).

focus is estimated to 0.221 and a diffraction limited size is 7 mm at awavelength of 60 nm. As a result of ray tracing the estimated valueof the focused spot is 18 mm.

Both the mirrors give horizontal deflections in order to maintainthe beam height. Since the focal point is designed to be located onthe unfocused beam path, we can easily switch the focus andunfocus conditions only with a horizontal translation of M4b whilefixing the position of the experimental chamber. The differentialpumping system with a 100 mm long, tapered aperture (diametersof 10–15 mm) is attached to the exit flange of the focusing mirrorchamber for separating vacuum environment between the mirrorchamber and the experimental chamber. The aperture can bemoved in vacuum to enable switching between the direct and thefocused beams.

3. Measurement of the focused beam profile

Beam profiles were measured by scanning a nickel pinhole witha diameter of 10 mm. The pinhole was placed in front of a gold plate,which gives photocurrent being proportional to the photon inten-sity as shown in Fig. 2. Since the fluence of the incident beam is veryhigh, the intensity was attenuated to 1% using a tin foil (thickness of500 nm) in order to avoid ablation of the pinhole and the gold plate.

To optimize the focusing condition, a Foucault test was carriedout as the follow procedure; a beam profile in the vicinity of thefocal point was measured while inserting a knife edge that blocks ahalf plane of the incident beam to the mirror. This profile is

H. Ohashi et al. / Nuclear Instruments and Methods in Physics Research A 649 (2011) 163–165164

compared to that measured without the knife edge. A shift of thepeak positions indicates a deviation from the optimal focusingcondition, and a correction angle of the mirror. The process wascarried out until the peak position was maintained.

Fig. 3 shows typical results of the profiles measured at a wavelengthof 60 nm. Horizontal and vertical sizes were estimated to be 22 and26 mm, respectively, by deconvoluting the slit aperture.

φ 10um

Gas filter

M4e

Au Thin filter

M5e

18VSignal EUV-FEL

TriggerOscilloscope

Fig. 2. Measurement of focused beam profile by scanning a pinhole with a diameter

of 10 mm placed in front of a gold plate.

4. Ablation test under the high fluence condition

To observe the focused beam shape by a single shot, we studiedablation of several materials such as silicon, diamond, silicondioxide, sapphire and PMMA (poly-methyl-methacrylate). Clearcraters were formed on these materials with single shot, high-fluence irradiation. Fig. 4(a) and (b) shows a typical image and thedepth profile of ablated silicon after a single shot of 10.5 mJ; theseresults were obtained by performing measurements using ascanning probe microscope (SPM). The diameter of the outer rimof the crater was approximately 26 mm. The value corresponds withthe beam size measured by scanning the pinhole.

From the SPM measurement, we can estimate the ablated volumefor each substrate. The dependence on the pulse energy of EUVradiation for the silicon substrate is shown in Fig. 5. The radiationwavelength was 60 nm with an energy bandwidth of �1% [1]. Thepulse energy, which was measured with an ion chamber calibrated byusing a cryogenic radiometer [3], was controlled by Ar gas attenuator.

5.58 m

Focusing mirrors

Focal

Diagnosticsbranch

Switching mirrorsChopperFocal

point

G

pp

Gas monitor,

attenuator

Filter, Sn Switching mirrors

Shielding wall (2.8m)

,(400nm)

Focal point

12.16 m3.837 m

1.2

m

0.8

m

Fig. 1. (a) Top and (b) side views of the EUV beamline at the SPring-8 Compact SASE

Source (SCSS).

Table 1Parameters of the mirrors (M1a, M2b, M3d, M3f, M4e and M5e) at the EUV beamline. T

Name M1a M2b

Purpose Horizontally

deflecting mirror

Horizontally

deflecting mirror

Deflection Horizontal Horizontal

Surface shape Flat Flat

Coating Au (SiC) Au (SiC)

Substrate SiO2 SiO2

Mirror size (mm) L 150, W 30, T 30 L 150, W 30, T 30

Deflected angle (deg) 170 170

Distance between source

and mirror (m)

5.577 7.88

Distance between mirror

and focus (m)

� �

The ablation threshold is estimated to 1.5 mJ. The value is equivalent tobe 0.2 J/cm2, which agrees with the damage threshold of 0.25 J/cm2

measured at wavelength of 32.5 nm [4].

5. Conclusion

A focused beam less than 30 mm in diameter is available at theEUV beamline of the SCSS. The power density of the focused beamexceeded 20 TW/cm2, assuming a pulse duration of 100 fs. Thebeam size was measured by scanning the 10 mm-diameter pinholeplaced in front of the gold plate under the incident intensity

he source point is at the exit of the 2nd ID.

M3d, M3f M4e M5e

Horizontally

deflecting mirror

Horizontally

focusing mirror

Vertically focusing mirror

Horizontal Horizontal Horizontal

Flat Elliptic cylinder Cylinder (r¼167.2 mm)

Au (SiC) SiC SiC

SiO2 SiO2 SiO2

L 150, W 30, T 30 L 140, W 30, T 30 L 120, W 30, T 30

170, �170 171 170

20.04 23.877 23.989

� 1.112 1

Vertical position (µm)

Horizontal position (µm)

Fig. 3. Typical beam profiles measured by scanning a pinhole with a diameter of

10 mm: (a) vertical and (b) horizontal profiles.

0.4

0.6

0.8

-0.2

0.0

0.2

Dep

th (µ

m)

-20 -10 0 10 20-0.6

-0.4

Position (µm)

Fig. 4. (a) Typical SPM image and (b) depth profile of Si ablated by a single shot from the focused EUV laser at a wavelength of 60 nm.

50

40

30

20

10

Abl

ated

vol

ume

(µm

3 )

00

Incident power (µJ)2 4 6 8 10 12 14

Fig. 5. Dependence of ablated Si volume on the incident power. The incident power

to the focusing mirrors was varied by using an Ar gas attenuator.

H. Ohashi et al. / Nuclear Instruments and Methods in Physics Research A 649 (2011) 163–165 165

attenuated to 1%. The threshold of silicon ablation with a singleshot irradiation is about 0.2 J/cm2. The beam size was approxi-mately estimated using the ablation profile.

Acknowledgements

This work was supported by the SCSS Test Accelerator OperationGroup.

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