New parallel wavelength-dispersive spectrometer based on scanning electron microscope Alexei Erko,1,* Alexander Firsov,1 Renat Gubzhokov,2,Anjuar Bjeoumikhov,2,4 Andreas Günther2, Norbert Langhoff,2 Mario Bretschneider,2 Yvonne Höhn,3 and Reiner Wedell3 1
Institute for Nanometer Optics and Technology Helmholtz-Zentrum Berlin (HZB), Albert-Einstein-Str. 15, 12489 Berlin, Germany 2 Institute for Scientific Instruments GmbH (IFG), Rudower Chaussee 29, 12489 Berlin, Germany 3 Institut für angewandte Photonik e. V. (IAP Rudower Chaussee 29/3, 12489 Berlin, Germany 4 Institute of Computer Science and Problems of Regional Management, Kabardino Balkarian Scientific Center, Russian Academy of Sciences, ul. Armand 37a, Nalchik, 360000 Russia *[email protected]
Abstract: A new wavelength - dispersive X-ray spectrometer for scanning electron microscopy (SEM) has been developed. This spectrometer can cover an energy range from 50 eV to 1120 eV by using an array made of seventeen reflection zone plates. Soft X-ray emission spectra of simple elements of Li, Be, B, C, N, Ti, V, O, Cr, Mn, Fe, Co, Ni, Cu, Zn and Ga were measured. The overall energy resolving power on the order of E/ΔE ~80 to 160 has been demonstrated. Spectrometer with 200 reflection zone plates has been used as a multi-channel analyser in the energy range of 100 – 1000 eV for quasi - continuous spectra measurements. The predicted energy-resolving power on the order of E/ΔE = 50 has been achieved in the entire energy range. © 2014 Optical Society of America OCIS codes: (300.6560) Spectroscopy, x-ray; (180.5810) Scanning microscopy; (340.0340) Xray optics; (050.1950) Diffraction gratings.
References and links 1.
M. Terauchi, M. Koike, K. Fukushima, and A. Kimura, “Development of wavelength-dispersive soft X-ray emission spectrometers for transmission electron microscopes an introduction of valence electron spectroscopy for transmission electron microscopy,” J. Electron Microsc. (Tokyo) 59(4), 251–261 (2010). 2. M. Terauchi, H. Takahachi, N. Handa, T. Murano, M. Koike, T. Kawachi, T. Imazono, M. Koeda, T. Nagano, H. Sasai, Y. Oue, Z. Yonezawa, and S. Kuramotol, “A new WDS spectrometer for Valence Electron Spectroscopy based on Electron Microscopy,” JEOL News 47, 23 (2012). 3. T. Imazono, M. Koike, T. Kawachi, M. Koeda, T. Nagano, H. Sasai, Y. Oue, Z. Yonezawa, S. Kuramoto, and K. Sano, “Diffraction efficiencies of holographic laminar and blazed types gratings for use in a flat-field spectrograph in the 50-200 eV range for transmission electron microscopes,” Proc. SPIE 8139, 81390V (2011). 4. M. Idir, A. Mirone, G. Soullie, Ph. Guerin, F. Ladan, and P. Dhez, “2D focusing with an off-axis elliptical Bragg-Fresnel multilayer lens and application to X-ray imaging,” Opt. Commun. 119(5-6), 633–642 (1995). 5. T. Harada, M. Itou, and T. Kita, “A grazing incidence monochromator with a varied-space plane grating for Synchrotron Radiation,” Proc. SPIE 0503, 114–119 (1984). 6. A. Erko, A. Firsov, and F. Senf, “Novel parallel VUV/X-Ray fluorescence spectrometer,” Spectrochimca Acta B 67, 57–63 (2012). 7. A. Ya. Faenov, S. A. Pikuz, A. I. Erko, B. A. Bryunetkin, V. M. Dyakin, G. V. Ivanenkov, A. R. Mingaleev, T. A. Pikuz, V. M. Romanova, and T. A. Shelkovenko, “High-performance x-ray spectroscopic devices for plasma microsources investigations,” Phys. Scr. 50(4), 333–338 (1994). 8. T. Wilhein, D. Hambach, B. Niemann, M. Berglund, L. Rymell, and H. M. Hertz, “Off-axis reflection zone plate for quantitative soft x-ray source characterization,” Appl. Phys. Lett. 71(2), 190–192 (1997). 9. J. Metje, M. Borgwardt, A. Moguilevski, A. Kothe, N. Engel, M. Wilke, R. Al-Obaidi, D. Tolksdorf, A. Firsov, M. Brzhezinskaya, A. Erko, I. Yu. Kiyan, and E. F. Aziz, “Monochromatization of femtosecond XUV light pulses with the use of reflection zone plates,” Opt. Express 22(9), 10747–11760 (2014). 10. M. Brzhezinskaya, A. Firsov, K. Holldack, T. Kachel, R. Mitzner, N. Pontius, J. S. Schmidt, M. Sperling, C. Stamm, A. Föhlisch, and A. Erko, “A novel monochromator for experiments with ultrashort X-ray pulses,” J. Synchrotron Radiat. 20(4), 522–530 (2013).
#210775 - $15.00 USD (C) 2014 OSA
Received 23 Apr 2014; revised 23 May 2014; accepted 26 May 2014; published 2 Jul 2014 14 July 2014 | Vol. 22, No. 14 | DOI:10.1364/OE.22.016897 | OPTICS EXPRESS 16897
11. O. Aita, K. Tsutsumi, K. Ichikava, M. Kamada, M. Okusawa, H. Nakamura, and T. Watanabe, “Fluorescent emission and scattering spectra of lithium fluoride by using synchrotron radiation,” Phys. Rev. B 23(11), 5676– 5680 (1981).
1. Introduction Scanning electron microscopy (SEM) as well as transmission electron microscopy (TEM) are applied not only for investigation of a surface topology, but can also be used for the definition of the chemical composition of a sample. In this way, an excitation of a small volume of a sample with an electron beam enables to perform an elemental analysis with high spatial resolution. Fluorescence radiation, emitted from the sample under electron beam interaction must be therefore detected and spectroscopic analyzed. For the purpose of spectroscopic analysis, one should use a wavelength-dispersive spectrometer (WDS). But all existing types of WDS have very small acceptance angles and a small number of fluorescence photons must be analyzed in an energy scan mode requiring very long measuring time for the full spectra measurements. In addition, electron beam current in SEM must be maximized, which effects on spatial resolution and in some case a sample modification. With the use of parallel spectrometers with a large aperture the current and the measuring time could be considerably reduced. Therefore, the specialization of electron microscope instrumentation on micro-beams with high beam current for element analysis and low e-beam current for surface imaging are no more necessary. Conventional SEMs can be used for material analysis. In reality, parallel energy-dispersive (EDS) spectrometers based on semiconductor technology have very poor energy resolution in the energy range 50 eV – 1000 eV. Especially in the energy range of 50 eV – 250 eV the advance of WDS is evident. WDS or diffraction spectrometers (DiS) built on the basis of gratings and crystals have much better energy resolution but do not allow parallel spectra registration. The dispersion elements must be mechanically scanned to obtain full spectra. With an increase of the fluorescence line energy the difference becomes smaller. But in real situation exactly at low energies the density of fluorescence lines is maximal and the possibility of wrong spectra interpretation is very high. The maximum of up to 36 lines lies in the energy range between 100 and 2000 eV. Several different concepts for WDS spectrometers were developed to cover the low energy gap in the spectroscopic measurements with SEM and TEM. Terauchi et al [1] reported that the spectrum of the Al-L band in Al metal had successfully been observed with a high energy resolution of 0.2 eV (E/ΔE ~140) using a TEM equipped with a WDS spectrometer. In the paper by H. Takahachi et al. [2] an improved WDS with a specially designed diffraction grating [3] has been developed. This can measure spectra with wider energy range from 50 to 200 eV in which important energy peaks such as Li-K (55 eV), Al-L (70 eV), Si-L (100 eV), Be-K (109 eV) and B-K (185 eV) are included. In this paper we are reporting about a new concept of a WDX for SEM. The present test WDX instrument has been designed to cover an energy range from 50 eV to 1150 eV by using a reflection zone plate array (RZPA) of laminar-type. No additional optics had been used. Since the solid angle of this optics is small, it is possible to incorporate a WDS spectrometer with a scanning electron microscope without any changes in electron optics. Proposed WDS can be used simultaneously with other detectors, for example secondary electron detector, and others. The demerit of a small detection angle is compensated by absence of any other optics (no absorption losses) and a short distance between source and spectrometer grating. 2. The spectrometer concept An off-axis reflection zone plate (RZP) imprinted as a projection of a conventional transmission zone plate on a totally reflecting mirror surface is shown in Fig. 1. The structure, being a laminar grating of variable line spacing in two dimensions, is capable of imaging the source by diffraction onto a certain distance R2 along the optical axis, acting as both a
#210775 - $15.00 USD (C) 2014 OSA
Received 23 Apr 2014; revised 23 May 2014; accepted 26 May 2014; published 2 Jul 2014 14 July 2014 | Vol. 22, No. 14 | DOI:10.1364/OE.22.016897 | OPTICS EXPRESS 16898
dispersive and focusing optical element. However, owing to the high chromaticity of a zone plate, i.e. the dependence of the focal length on wavelength, different energies are focused on different positions along the optical axis.
Fig. 1. Layout of an off-axis reflection zone plate and explanation of the corresponding parameters (see text).
As depicted in Fig. 1, not the full elliptical lens structure is used but only an off-axis section marked by the blue rectangle, as it was first proposed in [4]. Recalling the fact that a zone plate is a hologram of a point source, the radiation from the section is focused along the optical axis too, but at high dispersion due to the high off-center average line density. In addition, the specular reflex (zero order of the grating) can be nicely separated and, most valuable for monochromatization, a slit in a plane perpendicular to the optical axis can be applied for energy selection. The calculation below foots on the fact that each infinitesimal part of the lens has to fulfill the grating equation. The input beam as emitted by the source is dispersed along the optical axis. The same phenomenon that is used in variable line-spacing (VLS) gratings in one dimension was proposed by Harada et al. [5] to provide spectral dispersion of the input radiation. The focal distance F(λ) along the axis depends on the radiation wavelength. The principal of multichannel RZPA spectrometer was first reported in [6]. In comparison with a single RZP used in [7, 8], the new spectrometer can cover an energy range from 50 eV up to 1116 eV simultaneously. Recently, an RZPA was successfully applied also as a monochromator in a high harmonic generation beamline [9]. 3. High-resolution 17-element spectrometer Figure 2a and 2b show a photo of the prototype WDX instrument, attached to a scanning electron microscope Zeiss EVO 40. This spectrometer consists of only one optical element, RZPA, and a detector. Two exchangeable RZPA with 17 elements and 200 elements are examined as dispersive/focusing elements. They have a gold coated surface and a length of 40 mm along the incoming X-ray path irradiated at average grazing incidence angle of 2°. The detector is a 2-dimensional back-illumination type charge-coupled device CCD which has 512 x 2048 pixels; each has a size of 13.5 µm. This small-size pixel detector has an advantage for obtaining high energy resolution for smaller energy-dispersion conditions for higher-energy X-rays. The total distance between source and detector is 250 mm; therefore the total solid angle on the detector is 27.7 mrad (vertical) x 110 mrad (horizontal). The fully in-air alignment system produces no thermal distortion on the mechanical system and provides excellent long-term stability of the measurements. The RZPA is mounted into a special exchangeable holder. The spectrometer optical element can be changed with minimal alignment using a bayonet system. The WDS working range of 54 eV – 1116 eV was chosen because of the lack of high-resolution parallel X-ray detectors in this range. The large energy span makes the fabrication of the diffraction element for the spectrometer, with high energy resolution, rather complicated. We have chosen 17 fluorescence sub-ranges in the full working range. Their energies and the corresponding chemical elements are listed in Table 1.
#210775 - $15.00 USD (C) 2014 OSA
Received 23 Apr 2014; revised 23 May 2014; accepted 26 May 2014; published 2 Jul 2014 14 July 2014 | Vol. 22, No. 14 | DOI:10.1364/OE.22.016897 | OPTICS EXPRESS 16899
Here is listed also the sagittal angular aperture of each RZP. The meridional aperture of each RZP is the same and equal to 21.7 mrad.
Fig. 2. (a) Photograph of a newly constructed WDX instrument attached to a scanning electron microscope of Zeiss-EVO 40; (b) schematic figure of the WDS design
The K and L characteristic fluorescence energies of the elements are shown because other lines have very low intensities and are difficult to register using this optical arrangement. Table 1. Characteristic fluorescence line energies for the RZPs and their sagittal angular apertures Emission Li Be B C Ca N Ti V O Cr Mn Fe Co Ni Cu Zn Ga Line, eV 54 109 183 277 341 392 452 511 525 573 636 704 775 849 933 1022 1116 mrad 14.9 14.8 14.7 9.7 7.9 6.9 5.9 5.3 5.1 4.7 4.2 3.8 3.5 3.2 2.9 2.6 2.4 sagittal
The difference in sagittal angular aperture is following to the technological resolution limit of the RZP production process. In addition, the areas of Li, Be and B RZPs are enlarged to compensate an extremely low fluorescence conversion coefficient for these elements. The optical layout of the presented spectrometer consists of 17 RZP as it is shown in Fig. 3.
Fig. 3. Top view schematics of the parallel diffraction spectrometer designed for the energy range (54–1116 eV). A schematic showing nine RZPs placed on the same substrate. The sagittal CCD size is 27 mm.
Each RZP in Fig. 3 is designed for a spectral dispersion around a particular characteristic fluorescence energy line. Together, the 17 elements cover the full range of 54–1116 eV with an average calculated resolving power of λ/Δλ~87. The grazing incidence angle has been designed to 2°, which corresponds to an optimal combination of the angular acceptance and reflection efficiency at this energy range.
#210775 - $15.00 USD (C) 2014 OSA
Received 23 Apr 2014; revised 23 May 2014; accepted 26 May 2014; published 2 Jul 2014 14 July 2014 | Vol. 22, No. 14 | DOI:10.1364/OE.22.016897 | OPTICS EXPRESS 16900
The local grating densities (in lines/mm) in each RZP were calculated using the procedure published in [10]. Large variation in local grating density required different depths of profile of a lamellar grating. To optimize this parameter, the full structure was divided into three areas with 45 nm (54-183 eV), 17 nm (277 – 573 eV) and 8 nm (636-1116 eV) depth of profile correspondently. They were written by e-beam and then etched separately on the same substrate. The Line density for reflection zone plates, designed for different energies starting from 50 eV to 1000 eV, are in the range of 10 lines/mm to 900 lines/mm. In the following experiments, the accelerating voltage of the SEM was 5000 eV; the beam size on specimen area was on the order of 100 nm and the e-beam current was 25 nA. The spectrometer was tested using different samples containing pure metallic elements as well as chemical combinations of different elements: Be, C, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, LiF, FeO, NiCr, CuNi etc. Only one element (Ca) was not detected due to effective self-absorption of the Ca-L fluorescence line in a carbon contamination layer on the surface of the sample. Li-K fluorescence line in LiF was measured in accordance with [11]. The results of the measurements are shown in Table 2. The count rates of the signals depend on the sample composition, the RZP efficiency, and the conversion coefficient of electron beam to fluorescence radiation. The overall efficiency of the spectrometer (events / nA / second) is also presented in the table. Table 2. Acquisition time, total number of events, overall efficiency and bandwidth at different energies Emission Time, s events x 103 events/s/nA FWHM (eV)
Li Be B C N Ti V O Cr Mn Fe Co 250 1200 270 277 230 300 700 300 150 140 250 80 1.4
170
38
425 431 236 527 117 295
0.2 1.0
5.7 3.1
5.7 148 75 3.9 5.1 6.8
30 32 15 6.2 11.0 8.4
79 5.3
Ni 75
Cu 25
Zn 75
Ga 90
59
44
351 455 176 540 1048
17 7.0
71 176 243 282 288 466 7.1 7.8 8.5 6.9 9.1 11.0
The results of the bandwidth “Full width half maxima” (FWHM) are shown in Table 2. They depend not only on the spectrometer resolution, but also on the natural line shape for each element. Figure 4 shows the examples of the measured fluorescence at the WDX energy lines of B K and highest WDS energy of Ga L. The spectral resolution measurements were done in dispersion direction. The energy dispersion in the focal plane as easily derived from equation:
ΔE E 2 d sin β = Δh R2' hc
(1)
here Δh is the distance in dispersion direction, and R´2 is the corresponding RZPA-focus distance. The values are based on the geometrical parameters and Eq. (2), taking into account the CCD pixel size of 13.5 µm.
Fig. 4. Measured spectra of Be K and Ga L fluorescence lines.
#210775 - $15.00 USD (C) 2014 OSA
Received 23 Apr 2014; revised 23 May 2014; accepted 26 May 2014; published 2 Jul 2014 14 July 2014 | Vol. 22, No. 14 | DOI:10.1364/OE.22.016897 | OPTICS EXPRESS 16901
4. Quasi-continuous spectrometer design
The same mechanical design of the spectrometer was used in combination with the “multichannel” dispersion optical element. The optical layout of the spectrometer is also similar with the previously described one, as it is shown in Fig. 3, but in this case 200 RZPs were fabricated to cover energy range from 100 to 1000 eV. An energy step for such a multichannel spectrometer is optimized for a fixed energy spectral resolving power. The following equations were applied for the energy step and energy resolving power calculations: 1 N E ΔE (2) 2 and = 2 − 1 E1 E Where N is the total number of channels, En is the energy of nth channel; E1 and E2 are the minimum and maximum energy of the spectrometer, and λ/Δλ is the designed spectral resolving power. The spectrometer consists of 200 channels and has a resolving power in horizontal (Ε/ΔΕ = 44) and vertical planes (Ε/ΔΕ = 100). The corresponding spectral interval between the RZPs is Ε/ΔΕ = 87. The principle of the multichannel spectrometer can be used in the whole entire energy range for all fluorescence lines in between 100 to 1000 eV. Taking into account geometrical confederations, the distance between focal spots on the detector corresponds to 9.26 pixel/channel, the energy calibration can be defined by the equation. Figure 5 shows an example of spectra measured in multichannel mode: Ni L and Cu L. n
E N En = E1 2 E1
Fig. 5. The spectra of Ni L and Cu L lines measured with the multichannel spectrometer. (100 s acquisition time).
In contrary to previous spectra, the sagittal resolution is dependent on the energy interval between the RZPs. No specific elements are defined and pre-designed. In vertical direction the resolving power is still the same as for a 17 RZP spectrometer. 5. Conclusions
A newly designed soft X-ray spectrometer attached to a conventional scanning electron microscope has a broad detection range from 50 eV to 1116 eV by using a total external reflection zone plate array. This spectrometer enables us to detect Li K emission from an identified LiF specimen area by scanning electron microscopy. Thus, it is successfully shown that this WDS instrument provides new possibilities for chemical analysis, as well as methods for chemical analysis for a wide variety of new functional materials and basic research of compounds. Acknowledgments
The authors acknowledge support by ZIM-SOLO financed by BMWi. This work was also financially supported by the BMBF project No. 05K12CB4. We are grateful to A. Hafner for his support in the experimental data evaluation.
#210775 - $15.00 USD (C) 2014 OSA
Received 23 Apr 2014; revised 23 May 2014; accepted 26 May 2014; published 2 Jul 2014 14 July 2014 | Vol. 22, No. 14 | DOI:10.1364/OE.22.016897 | OPTICS EXPRESS 16902
Institute for Nanometer Optics and Technology Helmholtz-Zentrum Berlin (HZB), Albert-Einstein-Str. 15, 12489 Berlin, Germany 2 Institute for Scientific Instruments GmbH (IFG), Rudower Chaussee 29, 12489 Berlin, Germany 3 Institut für angewandte Photonik e. V. (IAP Rudower Chaussee 29/3, 12489 Berlin, Germany 4 Institute of Computer Science and Problems of Regional Management, Kabardino Balkarian Scientific Center, Russian Academy of Sciences, ul. Armand 37a, Nalchik, 360000 Russia *[email protected]
Abstract: A new wavelength - dispersive X-ray spectrometer for scanning electron microscopy (SEM) has been developed. This spectrometer can cover an energy range from 50 eV to 1120 eV by using an array made of seventeen reflection zone plates. Soft X-ray emission spectra of simple elements of Li, Be, B, C, N, Ti, V, O, Cr, Mn, Fe, Co, Ni, Cu, Zn and Ga were measured. The overall energy resolving power on the order of E/ΔE ~80 to 160 has been demonstrated. Spectrometer with 200 reflection zone plates has been used as a multi-channel analyser in the energy range of 100 – 1000 eV for quasi - continuous spectra measurements. The predicted energy-resolving power on the order of E/ΔE = 50 has been achieved in the entire energy range. © 2014 Optical Society of America OCIS codes: (300.6560) Spectroscopy, x-ray; (180.5810) Scanning microscopy; (340.0340) Xray optics; (050.1950) Diffraction gratings.
References and links 1.
M. Terauchi, M. Koike, K. Fukushima, and A. Kimura, “Development of wavelength-dispersive soft X-ray emission spectrometers for transmission electron microscopes an introduction of valence electron spectroscopy for transmission electron microscopy,” J. Electron Microsc. (Tokyo) 59(4), 251–261 (2010). 2. M. Terauchi, H. Takahachi, N. Handa, T. Murano, M. Koike, T. Kawachi, T. Imazono, M. Koeda, T. Nagano, H. Sasai, Y. Oue, Z. Yonezawa, and S. Kuramotol, “A new WDS spectrometer for Valence Electron Spectroscopy based on Electron Microscopy,” JEOL News 47, 23 (2012). 3. T. Imazono, M. Koike, T. Kawachi, M. Koeda, T. Nagano, H. Sasai, Y. Oue, Z. Yonezawa, S. Kuramoto, and K. Sano, “Diffraction efficiencies of holographic laminar and blazed types gratings for use in a flat-field spectrograph in the 50-200 eV range for transmission electron microscopes,” Proc. SPIE 8139, 81390V (2011). 4. M. Idir, A. Mirone, G. Soullie, Ph. Guerin, F. Ladan, and P. Dhez, “2D focusing with an off-axis elliptical Bragg-Fresnel multilayer lens and application to X-ray imaging,” Opt. Commun. 119(5-6), 633–642 (1995). 5. T. Harada, M. Itou, and T. Kita, “A grazing incidence monochromator with a varied-space plane grating for Synchrotron Radiation,” Proc. SPIE 0503, 114–119 (1984). 6. A. Erko, A. Firsov, and F. Senf, “Novel parallel VUV/X-Ray fluorescence spectrometer,” Spectrochimca Acta B 67, 57–63 (2012). 7. A. Ya. Faenov, S. A. Pikuz, A. I. Erko, B. A. Bryunetkin, V. M. Dyakin, G. V. Ivanenkov, A. R. Mingaleev, T. A. Pikuz, V. M. Romanova, and T. A. Shelkovenko, “High-performance x-ray spectroscopic devices for plasma microsources investigations,” Phys. Scr. 50(4), 333–338 (1994). 8. T. Wilhein, D. Hambach, B. Niemann, M. Berglund, L. Rymell, and H. M. Hertz, “Off-axis reflection zone plate for quantitative soft x-ray source characterization,” Appl. Phys. Lett. 71(2), 190–192 (1997). 9. J. Metje, M. Borgwardt, A. Moguilevski, A. Kothe, N. Engel, M. Wilke, R. Al-Obaidi, D. Tolksdorf, A. Firsov, M. Brzhezinskaya, A. Erko, I. Yu. Kiyan, and E. F. Aziz, “Monochromatization of femtosecond XUV light pulses with the use of reflection zone plates,” Opt. Express 22(9), 10747–11760 (2014). 10. M. Brzhezinskaya, A. Firsov, K. Holldack, T. Kachel, R. Mitzner, N. Pontius, J. S. Schmidt, M. Sperling, C. Stamm, A. Föhlisch, and A. Erko, “A novel monochromator for experiments with ultrashort X-ray pulses,” J. Synchrotron Radiat. 20(4), 522–530 (2013).
#210775 - $15.00 USD (C) 2014 OSA
Received 23 Apr 2014; revised 23 May 2014; accepted 26 May 2014; published 2 Jul 2014 14 July 2014 | Vol. 22, No. 14 | DOI:10.1364/OE.22.016897 | OPTICS EXPRESS 16897
11. O. Aita, K. Tsutsumi, K. Ichikava, M. Kamada, M. Okusawa, H. Nakamura, and T. Watanabe, “Fluorescent emission and scattering spectra of lithium fluoride by using synchrotron radiation,” Phys. Rev. B 23(11), 5676– 5680 (1981).
1. Introduction Scanning electron microscopy (SEM) as well as transmission electron microscopy (TEM) are applied not only for investigation of a surface topology, but can also be used for the definition of the chemical composition of a sample. In this way, an excitation of a small volume of a sample with an electron beam enables to perform an elemental analysis with high spatial resolution. Fluorescence radiation, emitted from the sample under electron beam interaction must be therefore detected and spectroscopic analyzed. For the purpose of spectroscopic analysis, one should use a wavelength-dispersive spectrometer (WDS). But all existing types of WDS have very small acceptance angles and a small number of fluorescence photons must be analyzed in an energy scan mode requiring very long measuring time for the full spectra measurements. In addition, electron beam current in SEM must be maximized, which effects on spatial resolution and in some case a sample modification. With the use of parallel spectrometers with a large aperture the current and the measuring time could be considerably reduced. Therefore, the specialization of electron microscope instrumentation on micro-beams with high beam current for element analysis and low e-beam current for surface imaging are no more necessary. Conventional SEMs can be used for material analysis. In reality, parallel energy-dispersive (EDS) spectrometers based on semiconductor technology have very poor energy resolution in the energy range 50 eV – 1000 eV. Especially in the energy range of 50 eV – 250 eV the advance of WDS is evident. WDS or diffraction spectrometers (DiS) built on the basis of gratings and crystals have much better energy resolution but do not allow parallel spectra registration. The dispersion elements must be mechanically scanned to obtain full spectra. With an increase of the fluorescence line energy the difference becomes smaller. But in real situation exactly at low energies the density of fluorescence lines is maximal and the possibility of wrong spectra interpretation is very high. The maximum of up to 36 lines lies in the energy range between 100 and 2000 eV. Several different concepts for WDS spectrometers were developed to cover the low energy gap in the spectroscopic measurements with SEM and TEM. Terauchi et al [1] reported that the spectrum of the Al-L band in Al metal had successfully been observed with a high energy resolution of 0.2 eV (E/ΔE ~140) using a TEM equipped with a WDS spectrometer. In the paper by H. Takahachi et al. [2] an improved WDS with a specially designed diffraction grating [3] has been developed. This can measure spectra with wider energy range from 50 to 200 eV in which important energy peaks such as Li-K (55 eV), Al-L (70 eV), Si-L (100 eV), Be-K (109 eV) and B-K (185 eV) are included. In this paper we are reporting about a new concept of a WDX for SEM. The present test WDX instrument has been designed to cover an energy range from 50 eV to 1150 eV by using a reflection zone plate array (RZPA) of laminar-type. No additional optics had been used. Since the solid angle of this optics is small, it is possible to incorporate a WDS spectrometer with a scanning electron microscope without any changes in electron optics. Proposed WDS can be used simultaneously with other detectors, for example secondary electron detector, and others. The demerit of a small detection angle is compensated by absence of any other optics (no absorption losses) and a short distance between source and spectrometer grating. 2. The spectrometer concept An off-axis reflection zone plate (RZP) imprinted as a projection of a conventional transmission zone plate on a totally reflecting mirror surface is shown in Fig. 1. The structure, being a laminar grating of variable line spacing in two dimensions, is capable of imaging the source by diffraction onto a certain distance R2 along the optical axis, acting as both a
#210775 - $15.00 USD (C) 2014 OSA
Received 23 Apr 2014; revised 23 May 2014; accepted 26 May 2014; published 2 Jul 2014 14 July 2014 | Vol. 22, No. 14 | DOI:10.1364/OE.22.016897 | OPTICS EXPRESS 16898
dispersive and focusing optical element. However, owing to the high chromaticity of a zone plate, i.e. the dependence of the focal length on wavelength, different energies are focused on different positions along the optical axis.
Fig. 1. Layout of an off-axis reflection zone plate and explanation of the corresponding parameters (see text).
As depicted in Fig. 1, not the full elliptical lens structure is used but only an off-axis section marked by the blue rectangle, as it was first proposed in [4]. Recalling the fact that a zone plate is a hologram of a point source, the radiation from the section is focused along the optical axis too, but at high dispersion due to the high off-center average line density. In addition, the specular reflex (zero order of the grating) can be nicely separated and, most valuable for monochromatization, a slit in a plane perpendicular to the optical axis can be applied for energy selection. The calculation below foots on the fact that each infinitesimal part of the lens has to fulfill the grating equation. The input beam as emitted by the source is dispersed along the optical axis. The same phenomenon that is used in variable line-spacing (VLS) gratings in one dimension was proposed by Harada et al. [5] to provide spectral dispersion of the input radiation. The focal distance F(λ) along the axis depends on the radiation wavelength. The principal of multichannel RZPA spectrometer was first reported in [6]. In comparison with a single RZP used in [7, 8], the new spectrometer can cover an energy range from 50 eV up to 1116 eV simultaneously. Recently, an RZPA was successfully applied also as a monochromator in a high harmonic generation beamline [9]. 3. High-resolution 17-element spectrometer Figure 2a and 2b show a photo of the prototype WDX instrument, attached to a scanning electron microscope Zeiss EVO 40. This spectrometer consists of only one optical element, RZPA, and a detector. Two exchangeable RZPA with 17 elements and 200 elements are examined as dispersive/focusing elements. They have a gold coated surface and a length of 40 mm along the incoming X-ray path irradiated at average grazing incidence angle of 2°. The detector is a 2-dimensional back-illumination type charge-coupled device CCD which has 512 x 2048 pixels; each has a size of 13.5 µm. This small-size pixel detector has an advantage for obtaining high energy resolution for smaller energy-dispersion conditions for higher-energy X-rays. The total distance between source and detector is 250 mm; therefore the total solid angle on the detector is 27.7 mrad (vertical) x 110 mrad (horizontal). The fully in-air alignment system produces no thermal distortion on the mechanical system and provides excellent long-term stability of the measurements. The RZPA is mounted into a special exchangeable holder. The spectrometer optical element can be changed with minimal alignment using a bayonet system. The WDS working range of 54 eV – 1116 eV was chosen because of the lack of high-resolution parallel X-ray detectors in this range. The large energy span makes the fabrication of the diffraction element for the spectrometer, with high energy resolution, rather complicated. We have chosen 17 fluorescence sub-ranges in the full working range. Their energies and the corresponding chemical elements are listed in Table 1.
#210775 - $15.00 USD (C) 2014 OSA
Received 23 Apr 2014; revised 23 May 2014; accepted 26 May 2014; published 2 Jul 2014 14 July 2014 | Vol. 22, No. 14 | DOI:10.1364/OE.22.016897 | OPTICS EXPRESS 16899
Here is listed also the sagittal angular aperture of each RZP. The meridional aperture of each RZP is the same and equal to 21.7 mrad.
Fig. 2. (a) Photograph of a newly constructed WDX instrument attached to a scanning electron microscope of Zeiss-EVO 40; (b) schematic figure of the WDS design
The K and L characteristic fluorescence energies of the elements are shown because other lines have very low intensities and are difficult to register using this optical arrangement. Table 1. Characteristic fluorescence line energies for the RZPs and their sagittal angular apertures Emission Li Be B C Ca N Ti V O Cr Mn Fe Co Ni Cu Zn Ga Line, eV 54 109 183 277 341 392 452 511 525 573 636 704 775 849 933 1022 1116 mrad 14.9 14.8 14.7 9.7 7.9 6.9 5.9 5.3 5.1 4.7 4.2 3.8 3.5 3.2 2.9 2.6 2.4 sagittal
The difference in sagittal angular aperture is following to the technological resolution limit of the RZP production process. In addition, the areas of Li, Be and B RZPs are enlarged to compensate an extremely low fluorescence conversion coefficient for these elements. The optical layout of the presented spectrometer consists of 17 RZP as it is shown in Fig. 3.
Fig. 3. Top view schematics of the parallel diffraction spectrometer designed for the energy range (54–1116 eV). A schematic showing nine RZPs placed on the same substrate. The sagittal CCD size is 27 mm.
Each RZP in Fig. 3 is designed for a spectral dispersion around a particular characteristic fluorescence energy line. Together, the 17 elements cover the full range of 54–1116 eV with an average calculated resolving power of λ/Δλ~87. The grazing incidence angle has been designed to 2°, which corresponds to an optimal combination of the angular acceptance and reflection efficiency at this energy range.
#210775 - $15.00 USD (C) 2014 OSA
Received 23 Apr 2014; revised 23 May 2014; accepted 26 May 2014; published 2 Jul 2014 14 July 2014 | Vol. 22, No. 14 | DOI:10.1364/OE.22.016897 | OPTICS EXPRESS 16900
The local grating densities (in lines/mm) in each RZP were calculated using the procedure published in [10]. Large variation in local grating density required different depths of profile of a lamellar grating. To optimize this parameter, the full structure was divided into three areas with 45 nm (54-183 eV), 17 nm (277 – 573 eV) and 8 nm (636-1116 eV) depth of profile correspondently. They were written by e-beam and then etched separately on the same substrate. The Line density for reflection zone plates, designed for different energies starting from 50 eV to 1000 eV, are in the range of 10 lines/mm to 900 lines/mm. In the following experiments, the accelerating voltage of the SEM was 5000 eV; the beam size on specimen area was on the order of 100 nm and the e-beam current was 25 nA. The spectrometer was tested using different samples containing pure metallic elements as well as chemical combinations of different elements: Be, C, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, LiF, FeO, NiCr, CuNi etc. Only one element (Ca) was not detected due to effective self-absorption of the Ca-L fluorescence line in a carbon contamination layer on the surface of the sample. Li-K fluorescence line in LiF was measured in accordance with [11]. The results of the measurements are shown in Table 2. The count rates of the signals depend on the sample composition, the RZP efficiency, and the conversion coefficient of electron beam to fluorescence radiation. The overall efficiency of the spectrometer (events / nA / second) is also presented in the table. Table 2. Acquisition time, total number of events, overall efficiency and bandwidth at different energies Emission Time, s events x 103 events/s/nA FWHM (eV)
Li Be B C N Ti V O Cr Mn Fe Co 250 1200 270 277 230 300 700 300 150 140 250 80 1.4
170
38
425 431 236 527 117 295
0.2 1.0
5.7 3.1
5.7 148 75 3.9 5.1 6.8
30 32 15 6.2 11.0 8.4
79 5.3
Ni 75
Cu 25
Zn 75
Ga 90
59
44
351 455 176 540 1048
17 7.0
71 176 243 282 288 466 7.1 7.8 8.5 6.9 9.1 11.0
The results of the bandwidth “Full width half maxima” (FWHM) are shown in Table 2. They depend not only on the spectrometer resolution, but also on the natural line shape for each element. Figure 4 shows the examples of the measured fluorescence at the WDX energy lines of B K and highest WDS energy of Ga L. The spectral resolution measurements were done in dispersion direction. The energy dispersion in the focal plane as easily derived from equation:
ΔE E 2 d sin β = Δh R2' hc
(1)
here Δh is the distance in dispersion direction, and R´2 is the corresponding RZPA-focus distance. The values are based on the geometrical parameters and Eq. (2), taking into account the CCD pixel size of 13.5 µm.
Fig. 4. Measured spectra of Be K and Ga L fluorescence lines.
#210775 - $15.00 USD (C) 2014 OSA
Received 23 Apr 2014; revised 23 May 2014; accepted 26 May 2014; published 2 Jul 2014 14 July 2014 | Vol. 22, No. 14 | DOI:10.1364/OE.22.016897 | OPTICS EXPRESS 16901
4. Quasi-continuous spectrometer design
The same mechanical design of the spectrometer was used in combination with the “multichannel” dispersion optical element. The optical layout of the spectrometer is also similar with the previously described one, as it is shown in Fig. 3, but in this case 200 RZPs were fabricated to cover energy range from 100 to 1000 eV. An energy step for such a multichannel spectrometer is optimized for a fixed energy spectral resolving power. The following equations were applied for the energy step and energy resolving power calculations: 1 N E ΔE (2) 2 and = 2 − 1 E1 E Where N is the total number of channels, En is the energy of nth channel; E1 and E2 are the minimum and maximum energy of the spectrometer, and λ/Δλ is the designed spectral resolving power. The spectrometer consists of 200 channels and has a resolving power in horizontal (Ε/ΔΕ = 44) and vertical planes (Ε/ΔΕ = 100). The corresponding spectral interval between the RZPs is Ε/ΔΕ = 87. The principle of the multichannel spectrometer can be used in the whole entire energy range for all fluorescence lines in between 100 to 1000 eV. Taking into account geometrical confederations, the distance between focal spots on the detector corresponds to 9.26 pixel/channel, the energy calibration can be defined by the equation. Figure 5 shows an example of spectra measured in multichannel mode: Ni L and Cu L. n
E N En = E1 2 E1
Fig. 5. The spectra of Ni L and Cu L lines measured with the multichannel spectrometer. (100 s acquisition time).
In contrary to previous spectra, the sagittal resolution is dependent on the energy interval between the RZPs. No specific elements are defined and pre-designed. In vertical direction the resolving power is still the same as for a 17 RZP spectrometer. 5. Conclusions
A newly designed soft X-ray spectrometer attached to a conventional scanning electron microscope has a broad detection range from 50 eV to 1116 eV by using a total external reflection zone plate array. This spectrometer enables us to detect Li K emission from an identified LiF specimen area by scanning electron microscopy. Thus, it is successfully shown that this WDS instrument provides new possibilities for chemical analysis, as well as methods for chemical analysis for a wide variety of new functional materials and basic research of compounds. Acknowledgments
The authors acknowledge support by ZIM-SOLO financed by BMWi. This work was also financially supported by the BMBF project No. 05K12CB4. We are grateful to A. Hafner for his support in the experimental data evaluation.
#210775 - $15.00 USD (C) 2014 OSA
Received 23 Apr 2014; revised 23 May 2014; accepted 26 May 2014; published 2 Jul 2014 14 July 2014 | Vol. 22, No. 14 | DOI:10.1364/OE.22.016897 | OPTICS EXPRESS 16902
