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Transmission-grating spectrometer for highly efficient and high-resolution soft X-ray emission studies
Journal of Electron Spectroscopy and Related Phenomena 188 (2013) 155–160
Contents lists available at ScienceDirect
Journal of Electron Spectroscopy and Related Phenomena journal homepage: www.elsevier.com/locate/elspec
Transmission-grating spectrometer for highly efficient and high-resolution soft X-ray emission studies Hiroyuki Yamane a , Nobuhiro Kosugi a,∗ , Takaki Hatsui b,∗∗ a b
Institute for Molecular Science and The Graduate University for Advanced Studies, Okazaki 444-8585, Japan RIKEN SPring-8 Center, RIKEN, Hyogo 679-5148, Japan
a r t i c l e
i n f o
Article history: Available online 23 June 2012 Keywords: Soft X-ray emission spectroscopy Resonant inelastic soft X-ray scattering Wolter mirror Transmission grating Normal-incident CCD
a b s t r a c t A high-resolution soft X-ray emission spectrometer has been developed for site-specific electronic structure analysis of functional materials in the photon energy range of 50–600 eV on the in-vacuum undulator beamline BL3U of the UVSOR facility. In order to realize the high detection efficiency without sacrificing the energy resolution to the signal intensity, the present X-ray emission spectrometer adopts an optical design, which was originally adopted in the field of astrophysics such as Chandra X-ray observatory. This new-type X-ray emission spectrometer consists of a Wolter type I mirror, a freestanding transmission grating, and a charge coupled device (CCD) detector, allowing the focusing with 20 times larger acceptance angle (1.5 × 10−3 sr) than commercially available compact spectrometers with reflection-type gratings such as VG-SCIENTA XES350. The X-ray emission energy resolution of 15–34 meV has successfully been achieved in the photon energy of 60–100 eV by using a transmission grating with the groove density of 5555 lines/mm. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The high-resolution soft X-ray emission spectroscopy (XES) and resonant inelastic X-ray scattering (RIXS) works based on the photon-in (excitation) photon-out (de-excitation) process have been increasing its importance, in combination with synchrotron radiation, for site-specific precise characterization of valence electronic states and excitation dynamics of gases, liquids, and solids [1]. Furthermore, since the attenuation length of soft X-rays is typically tens or hundreds of nanometers, the photon-based detection in XES/RIXS is an inherent bulk-sensitive method as compared with the electron-based detection, and makes in situ measurements possible under various experimental conditions such as ambient pressure and applied electromagnetic fields. A major problem in high-resolution XES/RIXS experiments is rather low X-ray emission/scattering cross-section, which requires long exposure of incident X-rays from third-generation highly brilliant synchrotron radiation sources, causing the possible radiation damage of the samples. Therefore, soft X-ray emission spectrometers are still under improvement in terms of detection efficiency as well as energy resolution. Since the pioneering development of a high-resolution soft X-ray emission spectrometer was based on the
∗ Corresponding author. Tel.: +81 564 55 7390; fax: +81 564 54 7079. ∗∗ Corresponding author. E-mail addresses: [email protected] (N. Kosugi), [email protected] (T. Hatsui). 0368-2048/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.elspec.2012.06.006
Rowland circle mount and grazing-incident detector configuration [2,3], a similar optical layout has been widely used in the development of soft X-ray emission spectrometers [4–8]. In this case, the horizontal acceptance angle is mainly limited by the detector size; however, in order to achieve higher energy resolution, the Rowland radius (R) or the spectrometer size, should be larger due to the relationship of / ∝ R. It is therefore desired to design a novel optical design focusing the emitted X-rays not only in the vertical dispersion direction but also in the horizontal direction. Recent advance in the charge-coupled device (CCD) makes it a promising soft Xray detector. It is not trivial to combine it with the high-resolution Rowland spectrometer, since the CCD detector has low quantum efficiency in the soft X-ray region at the small grazing-incidence angle. In the present work, we have developed a high-resolution soft Xray emission spectrometer with a compact and easy-to-use design incorporating a basic concept of the X-ray imaging and spectroscopy telescope: a Wolter type I mirror and a freestanding transmission grating. One of the advantages of this spectrometer is its high light gathering capability without sacrificing the energy resolution to the signal intensity and with making the high energy resolution possible in the region hout = 50–300 eV. The prefocusing Wolter type I mirror focuses soft X-rays with a large collection angle. The transmission grating with a groove density higher than 5000 lines/mm has been recently available owing to the advance in lithography. Combination of the Wolter mirror and the transmission grating realizes a normal-incident configuration of
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Fig. 1. Layout of the soft X-ray in-vacuum undulator beamline BL3U at the UVSOR facility. The distances along the beam from the center of the in-vacuum planar undulator are shown in mm-scale. The S1X and M2X slits can be replaced with the other S1 slit so that experiments can be carried out at either the multi-purpose or XES endstation. In the XES setup, the sample is placed at 5–10 mm downstream of the S1X slit.
the CCD detector, mounted on a Rowland torus. The spectrometer is installed on the in-vacuum undulator beamline BL3U at the UVSOR facility to characterize functional materials such as lithium-ion batteries, 4f electron systems, and organic/biological electronics and is working in the energy region of 50–600 eV.
2. Design of soft X-ray beamline A soft X-ray in-vacuum undulator beamline of BL3U was designed and constructed for high-resolution soft X-ray spectroscopic studies [9,10], in parallel with the upgrade of the UVSOR storage ring (UVSOR-I to UVSOR-II) in 2002–2003. The BL3U is equipped with a varied-line-spacing plane (VLSP) grating monochromator with a short arm length, in order to satisfy high energy resolution of / = 104 and small width of the X-ray slit opening. Fig. 1 represents the layout of the BL3U. The in-vacuum planar undulator composed of 50 periods of 3.8 cm period length is installed in a straight section, where the electron beam parameters are x = 602 m, x = 49.9 mrad, y = 61.3 m, and y = 40.6 mrad. The cylindrical mirror (M0) vertically focuses the beam on the entrance slit (S0) with the demagnification of 1/7.57. The typical beam size at S0 is the full-width at half maximum (FWHM) of 22 m. Due to the short arm length, the S0 opening width corresponding to the resolving power of / = 104 becomes smaller than the beam size. This mismatch causes the beam loss of 12–63%. Varied-line-spacing parameters are calculated by minimizing the aberrations in the energy range of interest. The analytical solution of the aberrations for an S0-M1-VLSP-S1 optical system derived by Amemiya et al. [11] is used. The obtained parameters give resolving power higher than / = 104 in the photon energy range of 50–600 eV by using three interchangeable gratings with the center groove densities of 240, 600, and 1200 lines/mm. The BL3U has two endstations; a multi-purpose setup and a XES setup as shown in Fig. 1. In the multi-purpose setup, the beam is focused on the exit slit (S1) only vertically and then refocused in the both directions on the sample by a toroidal mirror (M2). In this setup, the beam size at the sample position is typically 30(vert.) × 170(horiz.) m2 . In the XES setup, the beam is horizontally focused on the exit slit by a plane-elliptical mirror (M2X), which is located at the downstream of the VLSP gratings. A sample holder is placed at 23 mm downstream of the exit slit (S1X).
In this setup, the beam on the sample has a Gaussian distribution with FWHM of 60 m horizontally. The vertical beam size is close to the opening width of S1X. Although the beam is diffracted by S1X, the vertical size of the beam can be down to ∼10 m. The smallest beam size is defined as the beam size, where the opening width of S1X matches FWHM of the beam at the sample position, which is estimated within Fraunhofer diffraction. This diffraction effect was evaluated by measuring the 0th order diffraction of our spectrometer since the optical mirror in the present spectrometer has a magnification of 10, which enables us to measure the vertical beam size at the sample position. The M2X mirror and the S1X slit are designed to be easily interchangeable with the S1 slit. Since FY2010, a top-up operation keeping a beam current of 300 mA under the 0.75 GeV full-energy injection has been started and the detection efficiency has been improved. For both the multi-purpose and XES setups, we have confirmed that the photon flux over 1010 –1011 photons/s is achieved under / = 104 in the photon energy range of 50–600 eV. From FY2012, the beam brilliance will be increased due to the upgrade of the storage ring; UVSOR-II with the beam emittance of 27 nm-rad to UVSOR-III with 17 nm-rad.
3. Design of soft X-ray emission spectrometer 3.1. Optical design Soft X-ray emission spectrometers with reflection-type gratings have a serious problem of rather low detection efficiency of emitted soft X-rays. The low efficiency arises from (i) low emission probability of ∼0.1% for light elements, (ii) number of soft X-ray focusing optics in the Rowland circle mount, wherein the focusing direction is limited to the vertical direction only, and (iii) low acceptance angle and low quantum efficiency at the grazing-incident CCD configuration in the soft X-ray region. This problem can be solved by the use of highly brilliant soft X-ray beam of over 1012 photons/s; however, another problem of radiation damage is brought about. The radiation damage is crucial particularly in soft matters such as organic and biological samples. Based on the above background, combination of an omnidirectional optical focusing mirror and a normal-incident CCD configuration, which has recently been adopted in the field of
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Fig. 2. Cross sectional CAD image of the transmission-grating spectrometer. The emitted X-rays are focused omnidirectionaly and diffracted by the Wolter mirror and transmission grating. The diffracted X-rays are detected by the CCD at the normal-incident configuration on the Rowland torus. The distance between the transmission grating and the CCD detector is 1472 mm.
astrophysics [12], is desired for the soft X-ray emission spectrometer. The main advantage of this concept is its high light gathering capability without sacrificing the energy resolution to the signal intensity. The omnidirectional optical focusing and the normalincident CCD configuration can be realized by the use of a Wolter type I mirror and a transmission grating. In order to evaluate the aberrations of this optical design, a ray-tracing code TGSGUI was originally developed by one of the authors (TH). Fig. 2 shows the schematic layout of the XES endstation. Incident soft X-rays, dispersed by the VLSP grating, are horizontally focused by the plane-elliptical mirror (M2X) with the demagnification of 20. The exit slit (S1X) monochromatizes the incident soft X-rays. On the other hand, the vertical beam size is determined by the slit opening of S1X and the diffraction effect. The distance between S1X and the sample position is set to 22.5 mm. In order to focus the emitted soft X-ray efficiently, the Wolter type I mirror is introduced as the pre-focusing mirror with a magnification of 10. The transmission grating is placed at 67 mm downstream at the edge of the Wolter mirror, in the normal-incident geometry. A back-illuminated CCD detector, whose position is changed along the Rowland torus with scanning the photon energy, is located at 1472 mm downstream from the transmission grating. This optical design realizes the large acceptance angle and the low aberration; as a result, we have succeeded in realizing the high detection efficiency without sacrificing the energy resolution. Details of this transmission-grating spectrometer are given below. 3.2. X-ray slits Soft X-ray emission spectrometers generally require small beam size at the sample position, because a smaller opening width of the spectrometer entrance slit is needed to achieve higher energy resolution. Such a beam is usually produced by refocusing optics downstream of the exit slit. In the case of very limited experimental space, the adoption of such refocusing optics is impossible. On the other hand, the monochromator with short arm lengths requires a small exit-slit opening width for obtaining practical resolution; in the present system, high-precision X-ray slits with the opening widths of 5–100 m and 1–30 m are required for the exit slit (S1X)
for the incident soft X-rays and the entrance slit (S2X) for the emitted soft X-rays, respectively. Furthermore, in order to realize the high detection efficiency, a very small distance of 0.3 mm is required between the sample surface and the S2X slit. The high accuracy for each components and the compact size for the overall shape are also essential. The S1X-slit blade was manufactured by the electrolytic inprocess dressing (ELID) method. On the other hand, the S2X-slit blade, wherein the minimum opening width should be 1 m, was manufactured by the high-precision surface grinding within the blade straightness of 0.06–0.08 m. The S1X and S2X slits are opened or closed by the combination of the elastic hinges and the piezo actuator, wherein the displacement of the piezo actuator is enlarged to eight times by the elastic hinges (Fig. 3). The opening width of the S1X and S2X slit is controlled by the feedback system using the capacitance sensor.
3.3. Wolter mirror The soft X-rays emitted from the sample are collected by the Wolter type I mirror with a magnification of 10. As shown in Fig. 4, the Wolter mirror is a tube-shaped composite mirror with hyperboloidal and elliptical inner-surfaces, which are 27 and 33 mm long, respectively. The incident angle of 89◦ at the interconnection of the two mirror surfaces yields an acceptance angle as large as 1.5 × 10−3 sr, which is 20 times larger than the case of commercially available compact X-ray emission spectrometers with reflectiontype gratings such as VG-SCIENTA XES350. The slope error for each mirror estimated by the imaging property was better than 2.0 mrad rms. The Wolter type I mirror was developed and examined by Hamamatsu Photonics K.K. The mirror was manufactured by using the replica method, whose quality was tested by measuring an image of a zone plate. The image was taken by using an X-ray zooming tube. The groove period of 1 m was clearly resolved, which corresponds to a point spread function with FWHM smaller than 10 m. This performance is sufficiently adaptable to the present design of the transmission-grating spectrometer.
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Fig. 3. CAD image of (a) the incident X-ray exit slit (S1X) and (b) the emitted X-ray entrance slit (S2X).
The collected soft X-rays with a focus of 1500 mm downstream of the interconnection of two mirror surfaces are dispersed by the transmission grating. 3.4. Transmission grating In the case of the transmission grating, the groove density of more than 5000 lines/mm (200 nm period) is required for the highresolution soft X-ray emission study. There are two choices for fabricating the transmission grating; one is a freestanding type, wherein the structural preservation is supported by the groove structure itself, and the other is a supported type, wherein the structural preservation is supported by a metal thin film. In the latter case, the lowering of the diffraction efficiency is critical at the photon energy region of below 500 eV due to the X-ray absorption by the supporting metal film. We have thus selected the freestanding transmission grating. Fig. 5 shows the photograph, schematic illustration, and scanning electron microscope (SEM) images of our freestanding transmission grating. These transmission gratings are manufactured using the combination of the electron-beam lithography and the reactive ion etching by NTT Advanced Technology (NTT-AT) Co. Ltd. The groove structure is formed on a SiC membrane, wherein the crosspiece structure attains the structural preservation. Furthermore, in order to obtain the high diffraction efficiency, the gap/period ratio is designed as about 0.5 and the SiC membrane
Fig. 4. Photograph of the Wolter type I mirror.
thickness is designed as about 500 nm [13]. The flatness of the SiC membrane is identically deflected as confirmed by a ZYGO method (not shown). The SEM images show a poor straightness of the groove in the case of the density of 6250 lines/mm (160 nm period) and a rather good straightness in the case of 5555 lines/mm (180 nm period). This large difference between 6250 and 5555 lines/mm originates from the technological limitation in the present manufacturing processes. Note that, in the case of the SiC-based transmission grating, UV/visible stray lights from band-gap materials may be detected in addition to soft X-ray signals. We confirmed that such a problem can be solved by the use of a nickel-based transmission grating, which was manufactured by the electron-beam lithography and an electroprinting method, since the Ni membrane can play as a UV/visible stray-light filter. 3.5. CCD detector The transmission grating has smaller linear dispersion property than the reflection-type grating. The detector can be mounted in the normal-incident configuration in order to obtain higher quantum efficiency. This configuration requires that the detector should have better spatial resolution than 5 m. In the case of hard X-rays, one photon produces a charge cloud in the CCD with 20–300 electrons and the sub-pixel spatial resolution of less than 1 m is achievable for a CCD with the pixel size of 12.0 m × 12.0 m, by using the centroid calculation of split pixel events (Fig. 6) [14]. In the case of soft X-rays, one photon produces a charge cloud with about 10 times fewer electrons than in the case of hard X-rays. This means that the intensity of the nearby pixels (1–10 electrons) must be accurately measured in order to analyze the centroid of the charge cloud. Therefore, we have developed a low noise read-out system compatible with UHV. Our detector consists of a back-illuminated and enhancedprocess CCD chip (e2v CCD 42-40-A33) with the 2048 × 2048 pixel format with the pixel size of 13.5 m × 13.5 m, which is cooled down to ∼160 K with a liquid nitrogen cryostat in order to reduce the dark current. A 16 bit ADC drive system preamplifier (Meisei Co. Ltd.) realizes a low noise level of 6 electrons rms at the 250 kHz/pixel read-out rate. The CCD conversion factor is about 1 e− /ADU. By measuring the center of the electron cloud generated in the depletion layer, the spatial resolution of the CCD was found to be less than 2 m for the photon energy larger than 500 eV. We could not characterize the spacial resolution in the case of below 500 eV precisely due to weak signal intensities of the nearby pixels in comparison with the read-out noise.
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Fig. 5. (a) Scanning electron microscope (SEM) images of the freestanding transmission grating with the groove density of 6250 lines/mm (left) and 5555 lines/mm (right) with a cross sectional view. (b) Photograph of the transmission grating.
4. Results and discussion At BL3U, we found that the correctable energy range of the transmission-grating spectrometer is 57–600 eV in the case of the 5555 lines/mm grating. After the development of the transmissiongrating spectrometer, XES/RIXS studies at the C K-edge region have been published for biological molecules [15], organic semiconductors [16,17], and graphene-based materials [18]. The energy resolving power (h/Eout ) of 1000–2000 has been achieved for C K␣ XES/RIXS spectra. In the present work, we demonstrate much better performance in our transmission-grating spectrometer below the photon energy range of 100 eV by using the transmission grating with 5555 lines/mm groove density. Note that, due to small signal intensities of the nearby pixels in comparison with the read-out noise at low energy region as mentioned
in Section 3.5, the centroid method is not necessarily applied in the present measurement. 4.1. Evaluation in 100 eV energy region Fig. 7(a) shows a Si L␣ resonant X-ray emission spectrum (elastic peak region) for a Si(1 1 1) single crystal, measured at around the first Si L-edge resonant excitation (hin = 100.0 eV) [19] with FWHM of 13 meV, together with a raw CCD image and its line profile. The spectrum was taken within 90 min with incident photon flux of 1011 photons/s. The vertically dispersed image clearly shows a sharp elastic peak with a two-bumped fluorescence structure, where the lower direction corresponds to the higher X-ray emission energy (hout ). In the fluorescence feature, coherent and incoherent emission structures were clearly observed, showing the
Fig. 6. Schematic of the centroiding algorithm of the CCD detector. The centroid of a charge cloud produced by one soft X-ray photon is determined from the intensity of nearby pixels.
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5. Conclusion The soft X-ray emission spectrometer, based on the focusing optics with the Wolter type I mirror and the normal-incident CCD configuration, has been successfully developed by the combination with the freestanding transmission grating, making the high detection efficiency possible without sacrificing the energy resolution to the signal intensity. The resonant X-ray emission spectra of Si(1 1 1) and LiF(0 0 1) single crystals have shown the high energy resolution of 15–34 meV at the photon energy of 60–100 eV, resulting in the resolving power h/Eout of 4000–3000. The energy resolution is determined by the groove density and accuracy of the transmission grating and the spatial resolution of the CCD detector, and will be improved by more advanced technological progress in the near future. The present results clearly indicate that our transmissiongrating spectrometer is capable of carrying out high-resolution soft X-ray emission studies even under the condition of rather low photon fluxes (<1011 photons/s) and rather low radiation damages. Acknowledgements We would like to thank K. Matsushita of Nagoya University and H. Yoshida, M. Suzui, and T. Horigome of Institute for Molecular Science for their mechanical development of the spectrometer. The authors are grateful for the development of the Wolter mirror to A. Ohba of Hamamatsu Photonics K.K. and of the SiC transmission grating to A. Ozawa of NTT-AT Co. Ltd. This work was partially supported by PRESTO, Japan Science and Technology Agency, and Grant-in-Aid for Scientific Research (B) (no. 20350014) from Japan Society for the Promotion of Science. Fig. 7. Resonant X-ray emission spectra of (a) Si(1 1 1) and (b) LiF(0 0 1), measured at hin = 100.0 and 61.0 eV, with the Gaussian fitting result in the energy resolution (Eout ) of 34 meV (h/Eout = 100/0.034–3000) and 15 meV (h/Eout = 61/0.015–4000), respectively. The raw CCD image and its profile for Si(1 1 1) and LiF(0 0 1) are also shown. The accumulation time (texp ) for the Si-L and Li-K emission spectra is 90 and 60 min, respectively.
same hin dependence as reported in Ref. [19]. From the line profile analysis of the elastic peak, we have succeeded in getting a small FWHM of 34 meV, corresponding to the resolving power of 3000 at h = 100 eV. 4.2. Evaluation in 60 eV energy region Fig. 7(b) shows a Li K␣ resonant X-ray emission spectrum (elastic peak region) for a LiF(0 0 1) single crystal, measured at around the first Li K-edge resonant excitation (hin of 61.0 eV) [20] with FWHM of 10 meV, together with the raw CCD image and its line profile. The spectrum was taken within 60 min with incident photon flux of 1011 photons/s. As shown in the CCD image, the 5555 lines/mm groove density is impossible to detect Li K␣ fluorescence features due to the energy window problem and the higher groove density is necessary. From the line profile analysis of the elastic peak, we have succeeded in getting a rather small FWHM of 15 meV, corresponding to the resolving power of 4000 at h = 61 eV. The elastic peak shows an asymmetric line shape with a broad shoulder structure at the lower hout side. The energy distribution of the shoulder structure (RIXS feature) is ∼100 meV and is linearly shifted to the higher hout side with increasing hin (not shown). Since bulk LiF crystals show no first-order Raman scattering, a possible origin of the observed RIXS structure is a phonon-induced second-order Raman scattering around the wave number of 400–800 cm−1 (i.e., 50–99 meV) [21].
References [1] F. de Groot, A. Kotani, Core Level Spectroscopy of Solids, CRC, New York, 2008. [2] T.A. Callcott, K.L. Tsang, C.H. Zhang, D.L. Ederer, E.T. Arakawa, Rev. Sci. Instrum. 57 (1986) 2680. [3] J. Nordgren, G. Bray, S. Cramm, R. Nyholm, J.E. Rubensson, N. Wassdahl, Rev. Sci. Instrum. 60 (1989) 1690. [4] S. Shin, A. Agui, M. Fujisawa, Y. Tezuka, T. Ishii, Rev. Sci. Instrum. 66 (1995) 1584. [5] C.F. Hague, J.H. Underwood, A. Avila, R. Delaunay, H. Ringuenet, M. Marsi, M. Sacchi, Rev. Sci. Instrum. 76 (2005) 023110. [6] T. Tokushima, Y. Harada, H. Ohashi, Y. Senba, S. Shin, Rev. Sci. Instrum. 77 (2006) 063107. [7] G. Ghiringhelli, A. Piazzalunga, C. Dallera, G. Trezzi, L. Braicovich, T. Schmitt, V.N. Strocov, R. Betemps, L. Patthey, X. Wang, M. Grioni, Rev. Sci. Instrum. 77 (2006) 113108. [8] O. Fuchs, L. Weinhardt, M. Blum, M. Weigand, E. Umbach, M. Bär, C. Heske, J. Denlinger, Y.-D. Chuang, W. McKinney, Z. Hussain, E. Gullikson, M. Jones, P. Batson, B. Nelles, R. Follath, Rev. Sci. Instrum. 80 (2009) 063103. [9] T. Hatsui, E. Shigemasa, N. Kosugi, AIP Conf. Proc. 705 (2004) 921. [10] T. Hatsui, H. Setoyama, E. Shigemasa, N. Kosugi, J. Electron Spectrosc. Relat. Phenom. 144–147 (2005) 1059. [11] K. Amemiya, Y. Kitajima, T. Ohta, K. Ito, J. Synchrotron. Radiat. 3 (1996) 282. [12] http://cxc.harvard.edu/, and references therein. [13] http://henke.lbl.gov/optical constants/. [14] J. Hiraga, H. Tsunemi, E. Miyata, Jpn. J. Appl. Phys. 40 (2001) 1493. [15] W. Hua, H. Yamane, B. Gao, J. Jiang, S. Li, H.S. Kato, M. Kawai, T. Hatsui, Y. Luo, N. Kosugi, H. Ågren, J. Phys. Chem. B 114 (2010) 7016. [16] H. Yamane, T. Hatsui, K. Iketaki, T. Kaji, M. Hiramoto, N. Kosugi, J. Chem. Phys. 135 (2011) 034704. [17] H. Yamane, N. Kosugi, unpublished. [18] J.W. Chiou, S.C. Ray, S.I. Peng, C.H. Chuang, B.Y. Wang, H.M. Tsai, C.W. Pao, Y.C. Shao, Y.F. Wang, S.C. Chen, W.F. Pong, Y.C. Yeh, C.W. Chen, M.-H. Tsai, P. Papakonstantinou, H. Yamane, N. Kosugi, T. Reigier, L. Liu, T.K. Sham, J. Phys. Chem. C, submitted for publication. [19] S. Eisebitt, J. Lüning, J.-E. Rubensson, A. Settels, P.H. Dederichs, W. Eberhardt, S.N. Patitsas, T. Tiedje, J. Electron Spectrosc. Relat. Phenom. 93 (1998) 245. [20] R. Haensel, C. Kunz, B. Sonntag, Phys. Rev. Lett. 20 (1968) 262. [21] T.P. Sharma, R.D. Kirby, S.S. Jaswal, Phys. Rev. B 9 (1974) 1971.
Contents lists available at ScienceDirect
Journal of Electron Spectroscopy and Related Phenomena journal homepage: www.elsevier.com/locate/elspec
Transmission-grating spectrometer for highly efficient and high-resolution soft X-ray emission studies Hiroyuki Yamane a , Nobuhiro Kosugi a,∗ , Takaki Hatsui b,∗∗ a b
Institute for Molecular Science and The Graduate University for Advanced Studies, Okazaki 444-8585, Japan RIKEN SPring-8 Center, RIKEN, Hyogo 679-5148, Japan
a r t i c l e
i n f o
Article history: Available online 23 June 2012 Keywords: Soft X-ray emission spectroscopy Resonant inelastic soft X-ray scattering Wolter mirror Transmission grating Normal-incident CCD
a b s t r a c t A high-resolution soft X-ray emission spectrometer has been developed for site-specific electronic structure analysis of functional materials in the photon energy range of 50–600 eV on the in-vacuum undulator beamline BL3U of the UVSOR facility. In order to realize the high detection efficiency without sacrificing the energy resolution to the signal intensity, the present X-ray emission spectrometer adopts an optical design, which was originally adopted in the field of astrophysics such as Chandra X-ray observatory. This new-type X-ray emission spectrometer consists of a Wolter type I mirror, a freestanding transmission grating, and a charge coupled device (CCD) detector, allowing the focusing with 20 times larger acceptance angle (1.5 × 10−3 sr) than commercially available compact spectrometers with reflection-type gratings such as VG-SCIENTA XES350. The X-ray emission energy resolution of 15–34 meV has successfully been achieved in the photon energy of 60–100 eV by using a transmission grating with the groove density of 5555 lines/mm. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The high-resolution soft X-ray emission spectroscopy (XES) and resonant inelastic X-ray scattering (RIXS) works based on the photon-in (excitation) photon-out (de-excitation) process have been increasing its importance, in combination with synchrotron radiation, for site-specific precise characterization of valence electronic states and excitation dynamics of gases, liquids, and solids [1]. Furthermore, since the attenuation length of soft X-rays is typically tens or hundreds of nanometers, the photon-based detection in XES/RIXS is an inherent bulk-sensitive method as compared with the electron-based detection, and makes in situ measurements possible under various experimental conditions such as ambient pressure and applied electromagnetic fields. A major problem in high-resolution XES/RIXS experiments is rather low X-ray emission/scattering cross-section, which requires long exposure of incident X-rays from third-generation highly brilliant synchrotron radiation sources, causing the possible radiation damage of the samples. Therefore, soft X-ray emission spectrometers are still under improvement in terms of detection efficiency as well as energy resolution. Since the pioneering development of a high-resolution soft X-ray emission spectrometer was based on the
∗ Corresponding author. Tel.: +81 564 55 7390; fax: +81 564 54 7079. ∗∗ Corresponding author. E-mail addresses: [email protected] (N. Kosugi), [email protected] (T. Hatsui). 0368-2048/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.elspec.2012.06.006
Rowland circle mount and grazing-incident detector configuration [2,3], a similar optical layout has been widely used in the development of soft X-ray emission spectrometers [4–8]. In this case, the horizontal acceptance angle is mainly limited by the detector size; however, in order to achieve higher energy resolution, the Rowland radius (R) or the spectrometer size, should be larger due to the relationship of / ∝ R. It is therefore desired to design a novel optical design focusing the emitted X-rays not only in the vertical dispersion direction but also in the horizontal direction. Recent advance in the charge-coupled device (CCD) makes it a promising soft Xray detector. It is not trivial to combine it with the high-resolution Rowland spectrometer, since the CCD detector has low quantum efficiency in the soft X-ray region at the small grazing-incidence angle. In the present work, we have developed a high-resolution soft Xray emission spectrometer with a compact and easy-to-use design incorporating a basic concept of the X-ray imaging and spectroscopy telescope: a Wolter type I mirror and a freestanding transmission grating. One of the advantages of this spectrometer is its high light gathering capability without sacrificing the energy resolution to the signal intensity and with making the high energy resolution possible in the region hout = 50–300 eV. The prefocusing Wolter type I mirror focuses soft X-rays with a large collection angle. The transmission grating with a groove density higher than 5000 lines/mm has been recently available owing to the advance in lithography. Combination of the Wolter mirror and the transmission grating realizes a normal-incident configuration of
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Fig. 1. Layout of the soft X-ray in-vacuum undulator beamline BL3U at the UVSOR facility. The distances along the beam from the center of the in-vacuum planar undulator are shown in mm-scale. The S1X and M2X slits can be replaced with the other S1 slit so that experiments can be carried out at either the multi-purpose or XES endstation. In the XES setup, the sample is placed at 5–10 mm downstream of the S1X slit.
the CCD detector, mounted on a Rowland torus. The spectrometer is installed on the in-vacuum undulator beamline BL3U at the UVSOR facility to characterize functional materials such as lithium-ion batteries, 4f electron systems, and organic/biological electronics and is working in the energy region of 50–600 eV.
2. Design of soft X-ray beamline A soft X-ray in-vacuum undulator beamline of BL3U was designed and constructed for high-resolution soft X-ray spectroscopic studies [9,10], in parallel with the upgrade of the UVSOR storage ring (UVSOR-I to UVSOR-II) in 2002–2003. The BL3U is equipped with a varied-line-spacing plane (VLSP) grating monochromator with a short arm length, in order to satisfy high energy resolution of / = 104 and small width of the X-ray slit opening. Fig. 1 represents the layout of the BL3U. The in-vacuum planar undulator composed of 50 periods of 3.8 cm period length is installed in a straight section, where the electron beam parameters are x = 602 m, x = 49.9 mrad, y = 61.3 m, and y = 40.6 mrad. The cylindrical mirror (M0) vertically focuses the beam on the entrance slit (S0) with the demagnification of 1/7.57. The typical beam size at S0 is the full-width at half maximum (FWHM) of 22 m. Due to the short arm length, the S0 opening width corresponding to the resolving power of / = 104 becomes smaller than the beam size. This mismatch causes the beam loss of 12–63%. Varied-line-spacing parameters are calculated by minimizing the aberrations in the energy range of interest. The analytical solution of the aberrations for an S0-M1-VLSP-S1 optical system derived by Amemiya et al. [11] is used. The obtained parameters give resolving power higher than / = 104 in the photon energy range of 50–600 eV by using three interchangeable gratings with the center groove densities of 240, 600, and 1200 lines/mm. The BL3U has two endstations; a multi-purpose setup and a XES setup as shown in Fig. 1. In the multi-purpose setup, the beam is focused on the exit slit (S1) only vertically and then refocused in the both directions on the sample by a toroidal mirror (M2). In this setup, the beam size at the sample position is typically 30(vert.) × 170(horiz.) m2 . In the XES setup, the beam is horizontally focused on the exit slit by a plane-elliptical mirror (M2X), which is located at the downstream of the VLSP gratings. A sample holder is placed at 23 mm downstream of the exit slit (S1X).
In this setup, the beam on the sample has a Gaussian distribution with FWHM of 60 m horizontally. The vertical beam size is close to the opening width of S1X. Although the beam is diffracted by S1X, the vertical size of the beam can be down to ∼10 m. The smallest beam size is defined as the beam size, where the opening width of S1X matches FWHM of the beam at the sample position, which is estimated within Fraunhofer diffraction. This diffraction effect was evaluated by measuring the 0th order diffraction of our spectrometer since the optical mirror in the present spectrometer has a magnification of 10, which enables us to measure the vertical beam size at the sample position. The M2X mirror and the S1X slit are designed to be easily interchangeable with the S1 slit. Since FY2010, a top-up operation keeping a beam current of 300 mA under the 0.75 GeV full-energy injection has been started and the detection efficiency has been improved. For both the multi-purpose and XES setups, we have confirmed that the photon flux over 1010 –1011 photons/s is achieved under / = 104 in the photon energy range of 50–600 eV. From FY2012, the beam brilliance will be increased due to the upgrade of the storage ring; UVSOR-II with the beam emittance of 27 nm-rad to UVSOR-III with 17 nm-rad.
3. Design of soft X-ray emission spectrometer 3.1. Optical design Soft X-ray emission spectrometers with reflection-type gratings have a serious problem of rather low detection efficiency of emitted soft X-rays. The low efficiency arises from (i) low emission probability of ∼0.1% for light elements, (ii) number of soft X-ray focusing optics in the Rowland circle mount, wherein the focusing direction is limited to the vertical direction only, and (iii) low acceptance angle and low quantum efficiency at the grazing-incident CCD configuration in the soft X-ray region. This problem can be solved by the use of highly brilliant soft X-ray beam of over 1012 photons/s; however, another problem of radiation damage is brought about. The radiation damage is crucial particularly in soft matters such as organic and biological samples. Based on the above background, combination of an omnidirectional optical focusing mirror and a normal-incident CCD configuration, which has recently been adopted in the field of
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Fig. 2. Cross sectional CAD image of the transmission-grating spectrometer. The emitted X-rays are focused omnidirectionaly and diffracted by the Wolter mirror and transmission grating. The diffracted X-rays are detected by the CCD at the normal-incident configuration on the Rowland torus. The distance between the transmission grating and the CCD detector is 1472 mm.
astrophysics [12], is desired for the soft X-ray emission spectrometer. The main advantage of this concept is its high light gathering capability without sacrificing the energy resolution to the signal intensity. The omnidirectional optical focusing and the normalincident CCD configuration can be realized by the use of a Wolter type I mirror and a transmission grating. In order to evaluate the aberrations of this optical design, a ray-tracing code TGSGUI was originally developed by one of the authors (TH). Fig. 2 shows the schematic layout of the XES endstation. Incident soft X-rays, dispersed by the VLSP grating, are horizontally focused by the plane-elliptical mirror (M2X) with the demagnification of 20. The exit slit (S1X) monochromatizes the incident soft X-rays. On the other hand, the vertical beam size is determined by the slit opening of S1X and the diffraction effect. The distance between S1X and the sample position is set to 22.5 mm. In order to focus the emitted soft X-ray efficiently, the Wolter type I mirror is introduced as the pre-focusing mirror with a magnification of 10. The transmission grating is placed at 67 mm downstream at the edge of the Wolter mirror, in the normal-incident geometry. A back-illuminated CCD detector, whose position is changed along the Rowland torus with scanning the photon energy, is located at 1472 mm downstream from the transmission grating. This optical design realizes the large acceptance angle and the low aberration; as a result, we have succeeded in realizing the high detection efficiency without sacrificing the energy resolution. Details of this transmission-grating spectrometer are given below. 3.2. X-ray slits Soft X-ray emission spectrometers generally require small beam size at the sample position, because a smaller opening width of the spectrometer entrance slit is needed to achieve higher energy resolution. Such a beam is usually produced by refocusing optics downstream of the exit slit. In the case of very limited experimental space, the adoption of such refocusing optics is impossible. On the other hand, the monochromator with short arm lengths requires a small exit-slit opening width for obtaining practical resolution; in the present system, high-precision X-ray slits with the opening widths of 5–100 m and 1–30 m are required for the exit slit (S1X)
for the incident soft X-rays and the entrance slit (S2X) for the emitted soft X-rays, respectively. Furthermore, in order to realize the high detection efficiency, a very small distance of 0.3 mm is required between the sample surface and the S2X slit. The high accuracy for each components and the compact size for the overall shape are also essential. The S1X-slit blade was manufactured by the electrolytic inprocess dressing (ELID) method. On the other hand, the S2X-slit blade, wherein the minimum opening width should be 1 m, was manufactured by the high-precision surface grinding within the blade straightness of 0.06–0.08 m. The S1X and S2X slits are opened or closed by the combination of the elastic hinges and the piezo actuator, wherein the displacement of the piezo actuator is enlarged to eight times by the elastic hinges (Fig. 3). The opening width of the S1X and S2X slit is controlled by the feedback system using the capacitance sensor.
3.3. Wolter mirror The soft X-rays emitted from the sample are collected by the Wolter type I mirror with a magnification of 10. As shown in Fig. 4, the Wolter mirror is a tube-shaped composite mirror with hyperboloidal and elliptical inner-surfaces, which are 27 and 33 mm long, respectively. The incident angle of 89◦ at the interconnection of the two mirror surfaces yields an acceptance angle as large as 1.5 × 10−3 sr, which is 20 times larger than the case of commercially available compact X-ray emission spectrometers with reflectiontype gratings such as VG-SCIENTA XES350. The slope error for each mirror estimated by the imaging property was better than 2.0 mrad rms. The Wolter type I mirror was developed and examined by Hamamatsu Photonics K.K. The mirror was manufactured by using the replica method, whose quality was tested by measuring an image of a zone plate. The image was taken by using an X-ray zooming tube. The groove period of 1 m was clearly resolved, which corresponds to a point spread function with FWHM smaller than 10 m. This performance is sufficiently adaptable to the present design of the transmission-grating spectrometer.
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Fig. 3. CAD image of (a) the incident X-ray exit slit (S1X) and (b) the emitted X-ray entrance slit (S2X).
The collected soft X-rays with a focus of 1500 mm downstream of the interconnection of two mirror surfaces are dispersed by the transmission grating. 3.4. Transmission grating In the case of the transmission grating, the groove density of more than 5000 lines/mm (200 nm period) is required for the highresolution soft X-ray emission study. There are two choices for fabricating the transmission grating; one is a freestanding type, wherein the structural preservation is supported by the groove structure itself, and the other is a supported type, wherein the structural preservation is supported by a metal thin film. In the latter case, the lowering of the diffraction efficiency is critical at the photon energy region of below 500 eV due to the X-ray absorption by the supporting metal film. We have thus selected the freestanding transmission grating. Fig. 5 shows the photograph, schematic illustration, and scanning electron microscope (SEM) images of our freestanding transmission grating. These transmission gratings are manufactured using the combination of the electron-beam lithography and the reactive ion etching by NTT Advanced Technology (NTT-AT) Co. Ltd. The groove structure is formed on a SiC membrane, wherein the crosspiece structure attains the structural preservation. Furthermore, in order to obtain the high diffraction efficiency, the gap/period ratio is designed as about 0.5 and the SiC membrane
Fig. 4. Photograph of the Wolter type I mirror.
thickness is designed as about 500 nm [13]. The flatness of the SiC membrane is identically deflected as confirmed by a ZYGO method (not shown). The SEM images show a poor straightness of the groove in the case of the density of 6250 lines/mm (160 nm period) and a rather good straightness in the case of 5555 lines/mm (180 nm period). This large difference between 6250 and 5555 lines/mm originates from the technological limitation in the present manufacturing processes. Note that, in the case of the SiC-based transmission grating, UV/visible stray lights from band-gap materials may be detected in addition to soft X-ray signals. We confirmed that such a problem can be solved by the use of a nickel-based transmission grating, which was manufactured by the electron-beam lithography and an electroprinting method, since the Ni membrane can play as a UV/visible stray-light filter. 3.5. CCD detector The transmission grating has smaller linear dispersion property than the reflection-type grating. The detector can be mounted in the normal-incident configuration in order to obtain higher quantum efficiency. This configuration requires that the detector should have better spatial resolution than 5 m. In the case of hard X-rays, one photon produces a charge cloud in the CCD with 20–300 electrons and the sub-pixel spatial resolution of less than 1 m is achievable for a CCD with the pixel size of 12.0 m × 12.0 m, by using the centroid calculation of split pixel events (Fig. 6) [14]. In the case of soft X-rays, one photon produces a charge cloud with about 10 times fewer electrons than in the case of hard X-rays. This means that the intensity of the nearby pixels (1–10 electrons) must be accurately measured in order to analyze the centroid of the charge cloud. Therefore, we have developed a low noise read-out system compatible with UHV. Our detector consists of a back-illuminated and enhancedprocess CCD chip (e2v CCD 42-40-A33) with the 2048 × 2048 pixel format with the pixel size of 13.5 m × 13.5 m, which is cooled down to ∼160 K with a liquid nitrogen cryostat in order to reduce the dark current. A 16 bit ADC drive system preamplifier (Meisei Co. Ltd.) realizes a low noise level of 6 electrons rms at the 250 kHz/pixel read-out rate. The CCD conversion factor is about 1 e− /ADU. By measuring the center of the electron cloud generated in the depletion layer, the spatial resolution of the CCD was found to be less than 2 m for the photon energy larger than 500 eV. We could not characterize the spacial resolution in the case of below 500 eV precisely due to weak signal intensities of the nearby pixels in comparison with the read-out noise.
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Fig. 5. (a) Scanning electron microscope (SEM) images of the freestanding transmission grating with the groove density of 6250 lines/mm (left) and 5555 lines/mm (right) with a cross sectional view. (b) Photograph of the transmission grating.
4. Results and discussion At BL3U, we found that the correctable energy range of the transmission-grating spectrometer is 57–600 eV in the case of the 5555 lines/mm grating. After the development of the transmissiongrating spectrometer, XES/RIXS studies at the C K-edge region have been published for biological molecules [15], organic semiconductors [16,17], and graphene-based materials [18]. The energy resolving power (h/Eout ) of 1000–2000 has been achieved for C K␣ XES/RIXS spectra. In the present work, we demonstrate much better performance in our transmission-grating spectrometer below the photon energy range of 100 eV by using the transmission grating with 5555 lines/mm groove density. Note that, due to small signal intensities of the nearby pixels in comparison with the read-out noise at low energy region as mentioned
in Section 3.5, the centroid method is not necessarily applied in the present measurement. 4.1. Evaluation in 100 eV energy region Fig. 7(a) shows a Si L␣ resonant X-ray emission spectrum (elastic peak region) for a Si(1 1 1) single crystal, measured at around the first Si L-edge resonant excitation (hin = 100.0 eV) [19] with FWHM of 13 meV, together with a raw CCD image and its line profile. The spectrum was taken within 90 min with incident photon flux of 1011 photons/s. The vertically dispersed image clearly shows a sharp elastic peak with a two-bumped fluorescence structure, where the lower direction corresponds to the higher X-ray emission energy (hout ). In the fluorescence feature, coherent and incoherent emission structures were clearly observed, showing the
Fig. 6. Schematic of the centroiding algorithm of the CCD detector. The centroid of a charge cloud produced by one soft X-ray photon is determined from the intensity of nearby pixels.
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5. Conclusion The soft X-ray emission spectrometer, based on the focusing optics with the Wolter type I mirror and the normal-incident CCD configuration, has been successfully developed by the combination with the freestanding transmission grating, making the high detection efficiency possible without sacrificing the energy resolution to the signal intensity. The resonant X-ray emission spectra of Si(1 1 1) and LiF(0 0 1) single crystals have shown the high energy resolution of 15–34 meV at the photon energy of 60–100 eV, resulting in the resolving power h/Eout of 4000–3000. The energy resolution is determined by the groove density and accuracy of the transmission grating and the spatial resolution of the CCD detector, and will be improved by more advanced technological progress in the near future. The present results clearly indicate that our transmissiongrating spectrometer is capable of carrying out high-resolution soft X-ray emission studies even under the condition of rather low photon fluxes (<1011 photons/s) and rather low radiation damages. Acknowledgements We would like to thank K. Matsushita of Nagoya University and H. Yoshida, M. Suzui, and T. Horigome of Institute for Molecular Science for their mechanical development of the spectrometer. The authors are grateful for the development of the Wolter mirror to A. Ohba of Hamamatsu Photonics K.K. and of the SiC transmission grating to A. Ozawa of NTT-AT Co. Ltd. This work was partially supported by PRESTO, Japan Science and Technology Agency, and Grant-in-Aid for Scientific Research (B) (no. 20350014) from Japan Society for the Promotion of Science. Fig. 7. Resonant X-ray emission spectra of (a) Si(1 1 1) and (b) LiF(0 0 1), measured at hin = 100.0 and 61.0 eV, with the Gaussian fitting result in the energy resolution (Eout ) of 34 meV (h/Eout = 100/0.034–3000) and 15 meV (h/Eout = 61/0.015–4000), respectively. The raw CCD image and its profile for Si(1 1 1) and LiF(0 0 1) are also shown. The accumulation time (texp ) for the Si-L and Li-K emission spectra is 90 and 60 min, respectively.
same hin dependence as reported in Ref. [19]. From the line profile analysis of the elastic peak, we have succeeded in getting a small FWHM of 34 meV, corresponding to the resolving power of 3000 at h = 100 eV. 4.2. Evaluation in 60 eV energy region Fig. 7(b) shows a Li K␣ resonant X-ray emission spectrum (elastic peak region) for a LiF(0 0 1) single crystal, measured at around the first Li K-edge resonant excitation (hin of 61.0 eV) [20] with FWHM of 10 meV, together with the raw CCD image and its line profile. The spectrum was taken within 60 min with incident photon flux of 1011 photons/s. As shown in the CCD image, the 5555 lines/mm groove density is impossible to detect Li K␣ fluorescence features due to the energy window problem and the higher groove density is necessary. From the line profile analysis of the elastic peak, we have succeeded in getting a rather small FWHM of 15 meV, corresponding to the resolving power of 4000 at h = 61 eV. The elastic peak shows an asymmetric line shape with a broad shoulder structure at the lower hout side. The energy distribution of the shoulder structure (RIXS feature) is ∼100 meV and is linearly shifted to the higher hout side with increasing hin (not shown). Since bulk LiF crystals show no first-order Raman scattering, a possible origin of the observed RIXS structure is a phonon-induced second-order Raman scattering around the wave number of 400–800 cm−1 (i.e., 50–99 meV) [21].
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