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TEM imaging at 15 kV
Ultramicroscopy ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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Aberration-corrected STEM/TEM imaging at 15 kV Takeo Sasaki a,n, Hidetaka Sawada a, Fumio Hosokawa a, Yuta Sato b, Kazu Suenaga b a
EM Business Unit, JEOL Ltd., 3-1-2 Musashino, Akishima, Tokyo 196-8558, Japan Nanotube Research Center, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5, Higashi 1-1-1, Tsukuba, Ibaraki 305-8565, Japan
b
art ic l e i nf o
a b s t r a c t
Article history: Received 1 October 2013 Received in revised form 1 April 2014 Accepted 14 April 2014
The performance of aberration-corrected (scanning) transmission electron microscopy (S/TEM) at an accelerating voltage of 15 kV was evaluated in a low-voltage microscope equipped with a cold-field emission gun and a higher-order aberration corrector. Aberrations up to the fifth order were corrected by the aberration measurement and auto-correction system using the diffractogram tableau method in TEM and Ronchigram analysis in STEM. TEM observation of nanometer-sized particles demonstrated that aberrations up to an angle of 50 mrad were compensated. A TEM image of Si[110] exhibited lattice fringes with a spacing of 0.192 nm, and the power spectrum of the image showed spots corresponding to distances of 0.111 nm. An annular dark-field STEM image of Si[110] showed lattice fringes of (111) and (22̄ 0) planes corresponding to lattice distances of 0.314 nm and 0.192 nm, respectively. At an accelerating voltage of 15 kV, the developed low-voltage microscope achieved atomic-resolution imaging with a small chromatic aberration and a large uniform phase. & 2014 Elsevier B.V. All rights reserved.
Keywords: Low-voltage electron microscopy Aberration correction Scanning transmission electron microscopy Transmission electron microscopy
1. Introduction Non-destructive observation is quite important for soft materials and nanomaterials such as graphene, carbon nanotubes, and fullerenes, which collapse easily during observations at higher accelerating voltages [1–3]. Low-voltage aberration-corrected (scanning) transmission electron microscopy (STEM/TEM) is becoming a key instrumental tool because it enables us to observe materials with less knock-on damage and with high analytical sensitivity owing to the large ionization cross section [4–7]. Thus, we have developed a highsensitivity STEM/TEM system for use at 30–60 kV (Triple C #1) under the JST–CREST project [8]. Atomic-resolution imaging and single-atom chemical analysis of carbon-related materials were realized at an accelerating voltage of 60 kV, as shown by Suenaga et al. [1,3]. We also demonstrated 136 pm imaging of a Si–Si dumbbell using a silicon crystalline specimen in annular dark-field (ADF) STEM at 30 kV [8] using a cold-field emission gun (CFEG) and a higher-order aberration corrector. Chemical analysis of individual atoms in metallofullerene molecules was conducted without massive destruction at 30 kV [9]. Several studies have been reported on high-resolution low-voltage electron microscopy at 20–60 kV [1–6]. If a microscope could be operated at a lower voltage, imaging with higher sensitivity and analysis with smaller electron energy-loss spectroscopy (EELS) signal delocalization [10,11] without knock-on damage would be expected. n
Corresponding author. Tel.: þ 81 42 542 2227; fax: þ 81 42 546 8063. E-mail address: [email protected] (T. Sasaki).
Thus, we evaluate the resolution at a lower accelerating voltage using the developed microscope for STEM/TEM. In this paper, the operating voltage is set to 15 kV, at which the wavelength λ of an electron is about twice that at 60 kV: λ ¼9.9 pm at 15 kV and λ ¼4.9 pm at 60 kV. 2. Instrumentation (electron source) The Triple C #1 microscope is equipped with a CFEG with a W (310) emitter [12] designed specifically for low-voltage electron microscopy to reduce the effect of chromatic aberration of the condenser lens. The energy spread of the electron source (ΔE) was measured using a low-voltage GIF spectrometer (Quantum, GATAN) (Fig. 1). EEL spectra were acquired with an entrance aperture diameter of 1.5 mm, energy dispersion of 0.025 eV/ch, and exposure time of 2.0 s. ΔE was varied by changing the extractor voltage. The measured full width at half-maximum (FWHM) values of the zeroloss peak in the EEL spectra at 15 kV were 0.275 eV and 0.325 eV at emission currents of 0.7 μA and 2.2 μA, respectively. For TEM imaging, ΔE was set to less than 0.33 eV. 3. TEM imaging 3.1. Geometrical aberration correction at 15 kV We tuned up the microscope at an accelerating voltage of 15 kV. Delta-type Cs correctors for probe/image-forming systems
http://dx.doi.org/10.1016/j.ultramic.2014.04.006 0304-3991/& 2014 Elsevier B.V. All rights reserved.
Please cite this article as: T. Sasaki, et al., Aberration-corrected STEM/TEM imaging at 15 kV, Ultramicroscopy (2014), http://dx.doi.org/ 10.1016/j.ultramic.2014.04.006i
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Table 1 Measured aberration coefficients.
Fig. 1. Zero-loss peaks taken with emission currents of 0.7 μA and 2.2 μA at 15 kV.
Fig. 2. Diffractogram tableau obtained using an amorphous carbon thin film at 15 kV.
were used to compensate for higher-order aberration of a six-fold astigmatism as well as spherical aberration [13,14]. The aberration coefficients were measured by the Zemlin diffractogram tableau method [15] in TEM at 15 kV, as shown in Fig. 2. Diffractograms were recorded with the incident beam tilted by up to 79 mrad; the measured aberration coefficients are presented in Table 1. The third-order spherical aberration (O4) was corrected to be 5.74 μm, and the six-fold astigmatism (A6) was compensated to be 78 μm. O4 had a negative value to counterbalance the fifthorder spherical aberration (O6) of 0.564 mm. Fig. 3(a) shows the result of Young's fringe test after geometrical aberration correction using a thin film of amorphous carbon with gold particles. The fringes on the diffractogram reached 0.2 nm, where the spatial frequency was calibrated using a spot of (0.235 nm) 1 from a gold nano-particle. The phase contrast transfer function was calculated at 15 kV using the experimental conditions. It extends to 0.2 nm, which agrees well with the result of Young's fringe test. 3.2. Observation of gold particles at 15 kV Fig. 4(a) shows a TEM image of gold particles on a carbon amorphous sample at 15 kV. The TEM images were taken with
Notation
Aberration
Amplitude
O2 A2 P3 A3 O4 Q4 A4 P5 R5 A5 O6 A6
Defocus 2-fold astigmatism Axial coma 3-fold astigmatism Spherical aberration Star aberration 4-fold astigmatism 4th-order axial coma Three lobe aberration 5-fold astigmatism 5th-order spherical aberration 6-fold astigmatism
305.4 nm 8.3 nm 29.0 nm 55.7 nm 5.74 µm 1.21 µm 0.44 µm 33.32 µm 11.76 µm 11.56 µm 0.564 mm 0.078 mm
Azimuth (deg)
1.92 56.33 21.25 84.20 16.19 14.01 23.48 10.11 17.89
exposure time of 2.0 s using a Gatan Ultra Scan 1000 with a highsensitivity scintillator. Lattice fringes of the {222} plane are seen in the particle on the left. Lattice fringes of the {220} and {400} planes with distances of 0.144 nm and 0.102 nm were imaged in the bottom particle. A simulated image by a multislice method [16] is inserted; the simulation indicates that lattice fringes are observable at 15 kV, which is consistent with the experimental image. The power spectrum in Fig. 4(b) shows spots corresponding to (0.102 nm) 1, (0.118 nm) 1, (0.123 nm) 1, and (0.144 nm) 1. Fig. 4(c)–(e) shows simulated images with several amounts of six-fold astigmatism around the edge of the (111) surface of a gold particle, where the thickness is set to be 1 nm. For a twohexapole-type Cs corrector, the value of A6 at 15 kV is supposed to be 2–10 mm, because A6 increases with the square of Cs (before correction) and 1/f, where f is the focal length of the objective lens [13]. For the simulations with the two-hexapole-type corrector, where A6 could not be compensated, the A6 values are set to be 10 mm for the semi-angle of π/4 limit of 30 mrad and to 1.8 mm for the semi-angle of π/4 limit of 40 mrad [Fig. 4(c)–(g)]. A larger A6 results in a smaller uniform phase. With the large six-fold astigmatisms of 10 mm and 1.8 mm, Fresnel fringes outside the particle are observed around the surface. For the simulation with the delta corrector, the measured value of 78 μm in Table 1 is used [Fig. 4(e) and (h)], and the semi-angle of π/4 limit extends to 50 mrad. With the small value of A6, Fresnel fringes due to the phase disturbance caused by aberration are not observed. For atomic-resolution imaging without image blurring on the surface at 15 kV, the π/4 limit needs to reach 50 mrad by compensating for the six fold astigmatism. The simulated images show that a larger uniform phase area is required at the extremely low accelerating voltage of 15 kV. Our experimental imaging showed a sharp edge on nanometer-order particles. This indicates experimentally that the residual aberrations were compensated well at 15 kV in this study. Sufficient aberration correction up to higher angle is mandatory at ultralow-voltage imaging at atomic resolution. 3.3. Observation of thin crystalline specimen at 15 kV Fig. 5(a) shows a TEM image of a Si[110] single crystal at 15 kV. Lattice fringes of the {002} and {22̄ 0} planes are visible, with lattice distances of 0.272 nm and 0.192 nm, respectively. Fig. 5 (b) shows the power spectrum of the image. Spots corresponding to (0.136 nm) 1, (0.125 nm) 1, and (0.111 nm) 1 are seen. Fig. 6(a) and (b) shows a TEM image and its power spectrum, respectively, for a MoS2[0001] (0.5% Re-doped) specimen with several atomic layers. In this case, lattice fringes of {11̄ 00} and {112̄ 0} planes with distances of 0.274 nm and 0.158 nm, respectively, were also imaged clearly. The superimposed simulated image agrees well with the experimental data. The power spectrum of the
Please cite this article as: T. Sasaki, et al., Aberration-corrected STEM/TEM imaging at 15 kV, Ultramicroscopy (2014), http://dx.doi.org/ 10.1016/j.ultramic.2014.04.006i
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Fig. 3. (a) Young's interference test at 15 kV. (b) Phase contrast transfer function calculated for Cc ¼0.52 mm, ΔE¼ 0.27 eV, O4 ¼ 5.74 μm, and O6 ¼ 0.564 mm.
Fig. 4. (a) TEM image of gold particles on amorphous carbon film. Simulated image with defocus (Δf) ¼ 50 nm and thickness (t) ¼ 4 nm is superimposed for particle at lower right. Multislice TEM simulations were performed with Cs(O4)¼ 5.74 μm, O6 ¼ 0.564 mm, A6 ¼78 μm, Cc ¼ 0.52 mm, and ΔE ¼0.33 eV. (b) Power spectrum of the image. (c)–(e) Simulated images of edge of (111) surface of 1-nm-thick gold crystalline specimen with several amounts of six-fold astigmatism (10 mm, 1.8 mm, and 78 μm at 15 kV. White arrows indicate a surface atomic plane. (f)–(h) Phase maps with several amounts of six-fold astigmatism at 15 kV. Circles indicate semi-angle of π/4 limit. Value of π⧸4 limit denotes the spatial frequency at which the phase shift becomes π⧸4 with the aberration from the center of the optical axis. Color map from black to black and from white to white corresponds to phase shift of 2π. Blue and green denote opposite signs of the phase. A6 values in phase maps correspond to the values used in the image simulations of the (111) surfaces in (c)–(e).
Please cite this article as: T. Sasaki, et al., Aberration-corrected STEM/TEM imaging at 15 kV, Ultramicroscopy (2014), http://dx.doi.org/ 10.1016/j.ultramic.2014.04.006i
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Fig. 5. (a) TEM image of Si[110] crystal. Area on right is amorphous area at edge of specimen. Simulated image (Δf¼ 0 nm, t ¼7 nm) is superimposed. (b) Power spectrum of the image showing spot corresponding to (0.111 nm) 1.
Fig. 6. (a) TEM image of MoS2[0001] showing lattice fringes of {112̄ 0} and {11̄ 00} planes. Simulated image (Δf¼ 0 nm, t¼ 8 nm) is superimposed. Black and gray circles represent Mo and S atoms, respectively. (b) Power spectrum of the image.
Fig. 7. (a) TEM image of h-BN[0001]. The numbers of layers are indicated in the image. (b) Power spectrum of the image. (c) Simulated images (Δf ¼0 nm) of each thickness. Black and gray circles represent B and N atoms, respectively.
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experimental image shows spots corresponding to (0.091 nm) 1 and (0.103 nm) 1. 3.4. Imaging of BN sheet at 15 kV Fig. 7(a) and (b) shows a TEM image and its power spectrum, respectively, for multiple sheets of a h-BN[0001] sample. Lattice fringes of the {11̄ 00} plane with a spacing of 0.217 nm were imaged. The power spectrum of the image shows spots corresponding to (0.217 nm) 1, (0.125 nm) 1, and (0.108 nm) 1. The number of BN layers is determined from the intensity of the image, and the results are shown in Fig. 7(a). Lattice fringes with a spacing of 0.217 nm were not clear in the area of the single and second layers, whereas they were visible in the areas with more than three layers. Simulated images of the corresponding thicknesses are shown in Fig. 7(c). Lattice fringes are not clear in the simulated image with a single layer (t¼0.35 nm), whereas they are visible in those with more than two layers (t ¼0.7–2.1 nm). The experimental images agree well with the simulated ones in the thickness of
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more than three layers. On the other hand, the experimental ones in the area of single and two layers could not show clear lattice fringes because of the lack of signal-to-noise ratio. When a specimen is thin, a single scattering occurs, resulting in atomic structure imaging by the well-known linear scattering component. The experimental result indicated that the linear component of the scattered electron from the specimen was not sufficiently transferred to the imaging plane, because the attenuation of the envelope in the phase transfer function from weakphase objects is extremely severe at higher spatial frequencies, as shown in Fig. 3(b), due to chromatic aberration in TEM imaging at 15 kV. Thus, the areas with less than a few atomic layers were not imaged with sufficient contrast. To achieve single-layer imaging with sufficient contrast, it would be necessary to decrease the chromatic aberration by using a Cc corrector. However, these experimental images and simulations indicated that the atomic-resolution lattice fringe was visualized in the areas with more than a few atomic layers because of the achromatic effects of the strong interference caused by multiple scattering [17] in this observation of BN sheets.
4. STEM imaging
Fig. 8. Ronchigram taken with STEM probe focused on amorphous Ge sample.
ADF-STEM imaging was performed to evaluate the STEM performance at 15 kV. The segmented Ronchigram autocorrelation function matrix (SRAM) method [18] in STEM was used for the auto-tuning system. Fig. 8 shows the Ronchigram taken from an amorphous Ge film after aberration correction by the SRAM method for STEM at 15 kV. The flat contrast region corresponds to 50 mrad. The infinitely magnified shadow image of the specimen shows flat and featureless contrast, indicating that the probe is focused on the specimen. Fig. 9(a) shows an ADF-STEM image of a Si[110] crystal. The convergence semi-angle was 50 mrad, and the detection semi-angle for ADF-STEM imaging was from 60 mrad to 180 mrad. The image was taken at a scanning speed of 38 μs/pixel and a probe current of about 10 pA, with an energy spread of the source of 0.50 eV. Lattice fringes of {111}, {002}, and {22̄ 0} planes are imaged, with lattice distances of (0.314 nm) 1, (0.272 nm) 1, and (0.192 nm) 1, respectively. Fig. 9(c) shows the ADF image simulated under the experimental STEM conditions by
Fig. 9. (a) Raw ADF-STEM image of a Si[110] crystal. Contrast and brightness of each raw ADF image were adjusted to produce an appropriate background level and avoid excess background subtraction, which can give rise to artificial high-frequency spots in the power spectrum. (b) Power spectrum of the image. (c) Simulated image (Δf¼ 0 nm, t¼ 7 nm, ΔE¼ 0.50 eV, Cs ¼ 0 mm, Cc ¼0.52 mm, α¼ 50 mrad, Ip ¼ 10 pA, and Gauss image¼34 pm). The detection semi-angle is 60–180 mrad for the inner and outer regions of the ADF image. (d) Calculated probe shape under the same condition as (c). FWHM, D50, and D59 values were 0.111 nm, 0.322 nm, and 0.407 nm, respectively.
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a multislice method that takes dynamical scattering, thermal diffuse scattering, and chromatic aberration into account [16]. The probe shape was calculated under the same conditions, as shown in Fig. 9(d). The power spectrum of the image reveals a 11̄ 3 spot of (0.164 nm) 1 in Fig. 9(b). The value of 0.164 nm is equivalent to 16.5λ, where the wavelength of electrons at 15 kV is 9.94 pm. This normalized performance against λ at 15 kV is almost the same as that of 15.9λ at 30 kV [19]. 5. Summary The performance of TEM and STEM imaging at 15 kV was tested for the first time. In TEM, atomic-resolution images of gold particles, a Si crystalline specimen, and MoS2 layers were obtained with a higher-order geometrical aberration corrector and a CFEG. We demonstrated the correction capacity for geometrical aberration at 15 kV using simulation and experimental data. The observation of a single atomic layer of an h-BN[0001] specimen indicated that a linear component was not sufficiently transferred, whereas lattices in several layers were detected in TEM. STEM imaging at 15 kV revealed a crystalline lattice with a spacing of 0.192 nm in the silicon specimen. Acknowledgments This work was supported by the JST under the Research Acceleration Program (2012–2016). Appendix A. Supporting information
References [1] K. Suenaga, Y. Sato, Z. Liu, H. Kataura, T. Okazaki, K. Kimoto, H. Sawada, T. Sasaki, K. Omoto, T. Tomita, T. Kaneyama, Y. Kondo, Nat. Chem. 1 (2009) 415–418. [2] O.L. Krivanek, N. Dellby, M.F. Murfitt, M.F. Chisholm, T.J. Pennycook, K. Suenaga, V. Nicolosi, Ultramicroscopy 110 (2010) 935–945. [3] K. Suenaga, M. Koshino, Nature 468 (2010) 1088–1090. [4] O.L. Krivanek, M.F. Chisholm, V. Nicolosi, T.J. Pennycook, G.J. Corbin, N. Dellby, M.F. Murfitt, C.S. Own, Z.S. Szilagyi, M.P. Oxley, S.T. Pantelides, S.J. Pennycook, Nature 464 (2010) 571–574. [5] U. Kaiser, J. Biskupek, J.C. Meyer, J. Leschner, L. Lechner, H. Rose, M. Stö gerPollach, A.N. Khlobystov, P. Hartel, H. Müller, M. Haider, S. Eyhusen, G. Bennere, Ultramicroscopy 111 (2011) 1239–1246. [6] D.C. Bell, C.J. Russo, D.V. Kolmykov, Ultramicroscopy 114 (2012) 31–37. [7] K. Suenaga, K. Akiyama-Hasegawa, Y. Niimi, H. Kobayashi, M. Nakamura, Z. Liu, Y. Sato, M. Koshino, S. Iijima, J. Electron Microsc. 61 (2012) 285–291. [8] T. Sasaki, H. Sawada, F. Hosokawa, Y. Kohno, T. Tomita, T. Kaneyama, Y. Kondo, K. Kimoto, Y. Sato, K. Suenaga, J. Electron Microsc. 59 (2010) S7–S13. [9] K. Suenaga, Y. Iizumi, T. Okazaki, Eur. Phys. J. Appl. Phys. 54 (2011) 33508. [10] D.A. Muller, J. Silcox, Ultramicroscopy 59 (1995) 195–213. [11] M. Stö ger-Pollach, Micron 41 (2010) 577–584. [12] Y. Kohno, E. Okunishi, T. Tomita, I. Ishikawa, T. Kaneyama, Y. Ohkura, Y. Kondo, T. Isabell, Microsc. Anal. 24 (2010) S9–S13. [13] H. Sawada, T. Sasaki, F. Hosokawa, S. Yuasa, M. Terao, M. Kawazoe, T. Nakamichi, T. Kaneyama, Y. Kondo, K. Kimoto, K. Suenaga, J. Electron Microsc. 58 (2009) 341–347. [14] F. Hosokawa, H. Sawada, Y. Kondo, K. Takayanagi, K. Suenaga, Microscopy 62 (2013) 23–41. [15] F. Zemlin, K. Weiss, P. Schiske, W. Kunath, K.H. Herrmann, Ultramicroscopy 3 (1978) 49–60. [16] F. Hosokawa, unpublished, 2014. [17] L. Reimer, Transmission Electron Microscopy, 4thed., Springer, Verlag, Berlin, Heidelberg (1997) 233–237. [18] H. Sawada, T. Sannomiya, F. Hosokawa, T. Nakamichi, T. Kaneyama, T. Tomita, Y. Kondo, T. Tanaka, Y. Oshima, Y. Tanishiro, K. Takayanagi, Ultramicroscopy 108 (2008) 1467–1475. [19] T. Sasaki, H. Sawada, E. Okunishi, F. Hosokawa, T. Kaneyama, Y. Kondo, K. Kmoto, K. Suenaga, Micron 43 (2012) 551–556.
Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.ultramic.2014.04.006.
Please cite this article as: T. Sasaki, et al., Aberration-corrected STEM/TEM imaging at 15 kV, Ultramicroscopy (2014), http://dx.doi.org/ 10.1016/j.ultramic.2014.04.006i
Contents lists available at ScienceDirect
Ultramicroscopy journal homepage: www.elsevier.com/locate/ultramic
Aberration-corrected STEM/TEM imaging at 15 kV Takeo Sasaki a,n, Hidetaka Sawada a, Fumio Hosokawa a, Yuta Sato b, Kazu Suenaga b a
EM Business Unit, JEOL Ltd., 3-1-2 Musashino, Akishima, Tokyo 196-8558, Japan Nanotube Research Center, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5, Higashi 1-1-1, Tsukuba, Ibaraki 305-8565, Japan
b
art ic l e i nf o
a b s t r a c t
Article history: Received 1 October 2013 Received in revised form 1 April 2014 Accepted 14 April 2014
The performance of aberration-corrected (scanning) transmission electron microscopy (S/TEM) at an accelerating voltage of 15 kV was evaluated in a low-voltage microscope equipped with a cold-field emission gun and a higher-order aberration corrector. Aberrations up to the fifth order were corrected by the aberration measurement and auto-correction system using the diffractogram tableau method in TEM and Ronchigram analysis in STEM. TEM observation of nanometer-sized particles demonstrated that aberrations up to an angle of 50 mrad were compensated. A TEM image of Si[110] exhibited lattice fringes with a spacing of 0.192 nm, and the power spectrum of the image showed spots corresponding to distances of 0.111 nm. An annular dark-field STEM image of Si[110] showed lattice fringes of (111) and (22̄ 0) planes corresponding to lattice distances of 0.314 nm and 0.192 nm, respectively. At an accelerating voltage of 15 kV, the developed low-voltage microscope achieved atomic-resolution imaging with a small chromatic aberration and a large uniform phase. & 2014 Elsevier B.V. All rights reserved.
Keywords: Low-voltage electron microscopy Aberration correction Scanning transmission electron microscopy Transmission electron microscopy
1. Introduction Non-destructive observation is quite important for soft materials and nanomaterials such as graphene, carbon nanotubes, and fullerenes, which collapse easily during observations at higher accelerating voltages [1–3]. Low-voltage aberration-corrected (scanning) transmission electron microscopy (STEM/TEM) is becoming a key instrumental tool because it enables us to observe materials with less knock-on damage and with high analytical sensitivity owing to the large ionization cross section [4–7]. Thus, we have developed a highsensitivity STEM/TEM system for use at 30–60 kV (Triple C #1) under the JST–CREST project [8]. Atomic-resolution imaging and single-atom chemical analysis of carbon-related materials were realized at an accelerating voltage of 60 kV, as shown by Suenaga et al. [1,3]. We also demonstrated 136 pm imaging of a Si–Si dumbbell using a silicon crystalline specimen in annular dark-field (ADF) STEM at 30 kV [8] using a cold-field emission gun (CFEG) and a higher-order aberration corrector. Chemical analysis of individual atoms in metallofullerene molecules was conducted without massive destruction at 30 kV [9]. Several studies have been reported on high-resolution low-voltage electron microscopy at 20–60 kV [1–6]. If a microscope could be operated at a lower voltage, imaging with higher sensitivity and analysis with smaller electron energy-loss spectroscopy (EELS) signal delocalization [10,11] without knock-on damage would be expected. n
Corresponding author. Tel.: þ 81 42 542 2227; fax: þ 81 42 546 8063. E-mail address: [email protected] (T. Sasaki).
Thus, we evaluate the resolution at a lower accelerating voltage using the developed microscope for STEM/TEM. In this paper, the operating voltage is set to 15 kV, at which the wavelength λ of an electron is about twice that at 60 kV: λ ¼9.9 pm at 15 kV and λ ¼4.9 pm at 60 kV. 2. Instrumentation (electron source) The Triple C #1 microscope is equipped with a CFEG with a W (310) emitter [12] designed specifically for low-voltage electron microscopy to reduce the effect of chromatic aberration of the condenser lens. The energy spread of the electron source (ΔE) was measured using a low-voltage GIF spectrometer (Quantum, GATAN) (Fig. 1). EEL spectra were acquired with an entrance aperture diameter of 1.5 mm, energy dispersion of 0.025 eV/ch, and exposure time of 2.0 s. ΔE was varied by changing the extractor voltage. The measured full width at half-maximum (FWHM) values of the zeroloss peak in the EEL spectra at 15 kV were 0.275 eV and 0.325 eV at emission currents of 0.7 μA and 2.2 μA, respectively. For TEM imaging, ΔE was set to less than 0.33 eV. 3. TEM imaging 3.1. Geometrical aberration correction at 15 kV We tuned up the microscope at an accelerating voltage of 15 kV. Delta-type Cs correctors for probe/image-forming systems
http://dx.doi.org/10.1016/j.ultramic.2014.04.006 0304-3991/& 2014 Elsevier B.V. All rights reserved.
Please cite this article as: T. Sasaki, et al., Aberration-corrected STEM/TEM imaging at 15 kV, Ultramicroscopy (2014), http://dx.doi.org/ 10.1016/j.ultramic.2014.04.006i
T. Sasaki et al. / Ultramicroscopy ∎ (∎∎∎∎) ∎∎∎–∎∎∎
2
Table 1 Measured aberration coefficients.
Fig. 1. Zero-loss peaks taken with emission currents of 0.7 μA and 2.2 μA at 15 kV.
Fig. 2. Diffractogram tableau obtained using an amorphous carbon thin film at 15 kV.
were used to compensate for higher-order aberration of a six-fold astigmatism as well as spherical aberration [13,14]. The aberration coefficients were measured by the Zemlin diffractogram tableau method [15] in TEM at 15 kV, as shown in Fig. 2. Diffractograms were recorded with the incident beam tilted by up to 79 mrad; the measured aberration coefficients are presented in Table 1. The third-order spherical aberration (O4) was corrected to be 5.74 μm, and the six-fold astigmatism (A6) was compensated to be 78 μm. O4 had a negative value to counterbalance the fifthorder spherical aberration (O6) of 0.564 mm. Fig. 3(a) shows the result of Young's fringe test after geometrical aberration correction using a thin film of amorphous carbon with gold particles. The fringes on the diffractogram reached 0.2 nm, where the spatial frequency was calibrated using a spot of (0.235 nm) 1 from a gold nano-particle. The phase contrast transfer function was calculated at 15 kV using the experimental conditions. It extends to 0.2 nm, which agrees well with the result of Young's fringe test. 3.2. Observation of gold particles at 15 kV Fig. 4(a) shows a TEM image of gold particles on a carbon amorphous sample at 15 kV. The TEM images were taken with
Notation
Aberration
Amplitude
O2 A2 P3 A3 O4 Q4 A4 P5 R5 A5 O6 A6
Defocus 2-fold astigmatism Axial coma 3-fold astigmatism Spherical aberration Star aberration 4-fold astigmatism 4th-order axial coma Three lobe aberration 5-fold astigmatism 5th-order spherical aberration 6-fold astigmatism
305.4 nm 8.3 nm 29.0 nm 55.7 nm 5.74 µm 1.21 µm 0.44 µm 33.32 µm 11.76 µm 11.56 µm 0.564 mm 0.078 mm
Azimuth (deg)
1.92 56.33 21.25 84.20 16.19 14.01 23.48 10.11 17.89
exposure time of 2.0 s using a Gatan Ultra Scan 1000 with a highsensitivity scintillator. Lattice fringes of the {222} plane are seen in the particle on the left. Lattice fringes of the {220} and {400} planes with distances of 0.144 nm and 0.102 nm were imaged in the bottom particle. A simulated image by a multislice method [16] is inserted; the simulation indicates that lattice fringes are observable at 15 kV, which is consistent with the experimental image. The power spectrum in Fig. 4(b) shows spots corresponding to (0.102 nm) 1, (0.118 nm) 1, (0.123 nm) 1, and (0.144 nm) 1. Fig. 4(c)–(e) shows simulated images with several amounts of six-fold astigmatism around the edge of the (111) surface of a gold particle, where the thickness is set to be 1 nm. For a twohexapole-type Cs corrector, the value of A6 at 15 kV is supposed to be 2–10 mm, because A6 increases with the square of Cs (before correction) and 1/f, where f is the focal length of the objective lens [13]. For the simulations with the two-hexapole-type corrector, where A6 could not be compensated, the A6 values are set to be 10 mm for the semi-angle of π/4 limit of 30 mrad and to 1.8 mm for the semi-angle of π/4 limit of 40 mrad [Fig. 4(c)–(g)]. A larger A6 results in a smaller uniform phase. With the large six-fold astigmatisms of 10 mm and 1.8 mm, Fresnel fringes outside the particle are observed around the surface. For the simulation with the delta corrector, the measured value of 78 μm in Table 1 is used [Fig. 4(e) and (h)], and the semi-angle of π/4 limit extends to 50 mrad. With the small value of A6, Fresnel fringes due to the phase disturbance caused by aberration are not observed. For atomic-resolution imaging without image blurring on the surface at 15 kV, the π/4 limit needs to reach 50 mrad by compensating for the six fold astigmatism. The simulated images show that a larger uniform phase area is required at the extremely low accelerating voltage of 15 kV. Our experimental imaging showed a sharp edge on nanometer-order particles. This indicates experimentally that the residual aberrations were compensated well at 15 kV in this study. Sufficient aberration correction up to higher angle is mandatory at ultralow-voltage imaging at atomic resolution. 3.3. Observation of thin crystalline specimen at 15 kV Fig. 5(a) shows a TEM image of a Si[110] single crystal at 15 kV. Lattice fringes of the {002} and {22̄ 0} planes are visible, with lattice distances of 0.272 nm and 0.192 nm, respectively. Fig. 5 (b) shows the power spectrum of the image. Spots corresponding to (0.136 nm) 1, (0.125 nm) 1, and (0.111 nm) 1 are seen. Fig. 6(a) and (b) shows a TEM image and its power spectrum, respectively, for a MoS2[0001] (0.5% Re-doped) specimen with several atomic layers. In this case, lattice fringes of {11̄ 00} and {112̄ 0} planes with distances of 0.274 nm and 0.158 nm, respectively, were also imaged clearly. The superimposed simulated image agrees well with the experimental data. The power spectrum of the
Please cite this article as: T. Sasaki, et al., Aberration-corrected STEM/TEM imaging at 15 kV, Ultramicroscopy (2014), http://dx.doi.org/ 10.1016/j.ultramic.2014.04.006i
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Fig. 3. (a) Young's interference test at 15 kV. (b) Phase contrast transfer function calculated for Cc ¼0.52 mm, ΔE¼ 0.27 eV, O4 ¼ 5.74 μm, and O6 ¼ 0.564 mm.
Fig. 4. (a) TEM image of gold particles on amorphous carbon film. Simulated image with defocus (Δf) ¼ 50 nm and thickness (t) ¼ 4 nm is superimposed for particle at lower right. Multislice TEM simulations were performed with Cs(O4)¼ 5.74 μm, O6 ¼ 0.564 mm, A6 ¼78 μm, Cc ¼ 0.52 mm, and ΔE ¼0.33 eV. (b) Power spectrum of the image. (c)–(e) Simulated images of edge of (111) surface of 1-nm-thick gold crystalline specimen with several amounts of six-fold astigmatism (10 mm, 1.8 mm, and 78 μm at 15 kV. White arrows indicate a surface atomic plane. (f)–(h) Phase maps with several amounts of six-fold astigmatism at 15 kV. Circles indicate semi-angle of π/4 limit. Value of π⧸4 limit denotes the spatial frequency at which the phase shift becomes π⧸4 with the aberration from the center of the optical axis. Color map from black to black and from white to white corresponds to phase shift of 2π. Blue and green denote opposite signs of the phase. A6 values in phase maps correspond to the values used in the image simulations of the (111) surfaces in (c)–(e).
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Fig. 5. (a) TEM image of Si[110] crystal. Area on right is amorphous area at edge of specimen. Simulated image (Δf¼ 0 nm, t ¼7 nm) is superimposed. (b) Power spectrum of the image showing spot corresponding to (0.111 nm) 1.
Fig. 6. (a) TEM image of MoS2[0001] showing lattice fringes of {112̄ 0} and {11̄ 00} planes. Simulated image (Δf¼ 0 nm, t¼ 8 nm) is superimposed. Black and gray circles represent Mo and S atoms, respectively. (b) Power spectrum of the image.
Fig. 7. (a) TEM image of h-BN[0001]. The numbers of layers are indicated in the image. (b) Power spectrum of the image. (c) Simulated images (Δf ¼0 nm) of each thickness. Black and gray circles represent B and N atoms, respectively.
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experimental image shows spots corresponding to (0.091 nm) 1 and (0.103 nm) 1. 3.4. Imaging of BN sheet at 15 kV Fig. 7(a) and (b) shows a TEM image and its power spectrum, respectively, for multiple sheets of a h-BN[0001] sample. Lattice fringes of the {11̄ 00} plane with a spacing of 0.217 nm were imaged. The power spectrum of the image shows spots corresponding to (0.217 nm) 1, (0.125 nm) 1, and (0.108 nm) 1. The number of BN layers is determined from the intensity of the image, and the results are shown in Fig. 7(a). Lattice fringes with a spacing of 0.217 nm were not clear in the area of the single and second layers, whereas they were visible in the areas with more than three layers. Simulated images of the corresponding thicknesses are shown in Fig. 7(c). Lattice fringes are not clear in the simulated image with a single layer (t¼0.35 nm), whereas they are visible in those with more than two layers (t ¼0.7–2.1 nm). The experimental images agree well with the simulated ones in the thickness of
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more than three layers. On the other hand, the experimental ones in the area of single and two layers could not show clear lattice fringes because of the lack of signal-to-noise ratio. When a specimen is thin, a single scattering occurs, resulting in atomic structure imaging by the well-known linear scattering component. The experimental result indicated that the linear component of the scattered electron from the specimen was not sufficiently transferred to the imaging plane, because the attenuation of the envelope in the phase transfer function from weakphase objects is extremely severe at higher spatial frequencies, as shown in Fig. 3(b), due to chromatic aberration in TEM imaging at 15 kV. Thus, the areas with less than a few atomic layers were not imaged with sufficient contrast. To achieve single-layer imaging with sufficient contrast, it would be necessary to decrease the chromatic aberration by using a Cc corrector. However, these experimental images and simulations indicated that the atomic-resolution lattice fringe was visualized in the areas with more than a few atomic layers because of the achromatic effects of the strong interference caused by multiple scattering [17] in this observation of BN sheets.
4. STEM imaging
Fig. 8. Ronchigram taken with STEM probe focused on amorphous Ge sample.
ADF-STEM imaging was performed to evaluate the STEM performance at 15 kV. The segmented Ronchigram autocorrelation function matrix (SRAM) method [18] in STEM was used for the auto-tuning system. Fig. 8 shows the Ronchigram taken from an amorphous Ge film after aberration correction by the SRAM method for STEM at 15 kV. The flat contrast region corresponds to 50 mrad. The infinitely magnified shadow image of the specimen shows flat and featureless contrast, indicating that the probe is focused on the specimen. Fig. 9(a) shows an ADF-STEM image of a Si[110] crystal. The convergence semi-angle was 50 mrad, and the detection semi-angle for ADF-STEM imaging was from 60 mrad to 180 mrad. The image was taken at a scanning speed of 38 μs/pixel and a probe current of about 10 pA, with an energy spread of the source of 0.50 eV. Lattice fringes of {111}, {002}, and {22̄ 0} planes are imaged, with lattice distances of (0.314 nm) 1, (0.272 nm) 1, and (0.192 nm) 1, respectively. Fig. 9(c) shows the ADF image simulated under the experimental STEM conditions by
Fig. 9. (a) Raw ADF-STEM image of a Si[110] crystal. Contrast and brightness of each raw ADF image were adjusted to produce an appropriate background level and avoid excess background subtraction, which can give rise to artificial high-frequency spots in the power spectrum. (b) Power spectrum of the image. (c) Simulated image (Δf¼ 0 nm, t¼ 7 nm, ΔE¼ 0.50 eV, Cs ¼ 0 mm, Cc ¼0.52 mm, α¼ 50 mrad, Ip ¼ 10 pA, and Gauss image¼34 pm). The detection semi-angle is 60–180 mrad for the inner and outer regions of the ADF image. (d) Calculated probe shape under the same condition as (c). FWHM, D50, and D59 values were 0.111 nm, 0.322 nm, and 0.407 nm, respectively.
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a multislice method that takes dynamical scattering, thermal diffuse scattering, and chromatic aberration into account [16]. The probe shape was calculated under the same conditions, as shown in Fig. 9(d). The power spectrum of the image reveals a 11̄ 3 spot of (0.164 nm) 1 in Fig. 9(b). The value of 0.164 nm is equivalent to 16.5λ, where the wavelength of electrons at 15 kV is 9.94 pm. This normalized performance against λ at 15 kV is almost the same as that of 15.9λ at 30 kV [19]. 5. Summary The performance of TEM and STEM imaging at 15 kV was tested for the first time. In TEM, atomic-resolution images of gold particles, a Si crystalline specimen, and MoS2 layers were obtained with a higher-order geometrical aberration corrector and a CFEG. We demonstrated the correction capacity for geometrical aberration at 15 kV using simulation and experimental data. The observation of a single atomic layer of an h-BN[0001] specimen indicated that a linear component was not sufficiently transferred, whereas lattices in several layers were detected in TEM. STEM imaging at 15 kV revealed a crystalline lattice with a spacing of 0.192 nm in the silicon specimen. Acknowledgments This work was supported by the JST under the Research Acceleration Program (2012–2016). Appendix A. Supporting information
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Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.ultramic.2014.04.006.
Please cite this article as: T. Sasaki, et al., Aberration-corrected STEM/TEM imaging at 15 kV, Ultramicroscopy (2014), http://dx.doi.org/ 10.1016/j.ultramic.2014.04.006i