lighter elements such as silicon and carbon [2, 3]. The observation of atomic resolutionsecondary electron imaging is unprecedentedand may lead to new theories of signal gener-ation, along with a host of new applications –some of which are highlighted here.

Scanning Transmission Electron MicroscopyThis work was undertaken using the HitachiHD-2700 dedicated STEM installed at theBrookhaven National Laboratory in NY. ThisSTEM incorporates a probe aberration-correc-tor (CESCOR, from CEOS GmbH) (Figure 1) [4].The microscope also features a cold field-emis-sion gun with high brightness and a smallenergy spread, originally developed in consul-tation with Crewe in 1970s [5] and now fullyoptimised for stable observation and analysisin aberration-corrected microscopes. Coupledtogether these two technologies provide asmall electron beam diameter (<0.1 nm) withhigh current for analysis and high signal-noiseratio imaging. Figure 2 shows a schematic of the detector

configuration. The HD-2700 is equipped withan Everhart-Thornley type high efficiency sec-ondary electron detector situated above theupper pole-piece (the so-called through-the-lens design similar to that in a high-resolutionSEM). Retractable annular darkfield (ADF) andbrightfield (BF) STEM detectors are situated

B IOGRAPHYHiromi Inada has a mas-ter’s degress in engineer-ing from Nagaoka Uni-versity of Technology.Joining Hitachi Ltd. in1998 he focused on TEMand STEM development.From 2007 to 2009 he was stationed at TheCenter for Functional Nanomaterials inBrookhaven National Laboratory as a visit-ing researcher, working with Dr Yimei Zhuon aberration-corrected STEM studies. Hecurrently works in the Science and MedicalSystem Design Division of Hitachi High-Tech-nologies with responsibility for TEM/STEMdesign.

ABSTRACTAberration-corrected electron microscopyhas enabled sub-nanometre atomic scalecharacterisation to be applied to a widerange of materials although to date onlytransmitted electron signals have generallybeen studied. The very latest studies intoatomic resolution secondary electron (SE)imaging using an aberration-correctedSTEM are described. Following a series ofexperiments, we have succeeded in observ-ing isolated single atoms using secondaryelectron signals. In addition, we have clearlyobserved atomic resolution not only forheavy elements like uranium but also lighterelements such as silicon and carbon. Webelieve this new technique can provide sig-nificantly greater insights into many materi-als through the correlation of topographicand surface structures with conventionaltransmitted-electron imaging techniques,whilst also enabling atomic resolution evenon bulk specimens.

KEYWORDSscanning transmission electron microscopy,secondary electron imaging, cold field-emis-sion, aberration-correction, atomic resolu-tion

AUTHOR DETA I L SDr Hiromi Inada,Hitachi High-Technologies Corporation, Science and Medical Systems BusinessGroup, Ibaraki, 312-0057, JapanEmail: inada-hiromi@hitachi-hitec.com

Microscopy and Analysis 25(7):S5-S8 (AM), 2011

SE IMAGING IN STEM

I N TRODUCT ION

Realising the Potential of Atomic-ResolutionImagingIn recent years the aberration-correction tech-nique has brought a revolution in analyticalmicroscopy by making atomic-resolutionimaging and analysis routinely achievable inboth transmission and scanning transmissionelectron microscopy (TEM and STEM). Scanning electron microscopes (SEM), how-

ever, remain widely used in the observation ofspecimens at the micrometer and nanometerscales and are key to the development of manymaterials and devices. In the SEM, the sec-ondary electron (SE) imaging mode is com-monly used, enabling us to observe three-dimensional topographic images of the sam-ple surface. One of the most advanced com-mercially available SEMs (Hitachi’s SU9000 In-lens SEM/STEM) achieves a spatial resolutionof less than 0.4 nm. Whilst studies into evenhigher-resolution SE imaging have beenapproached, none of the research to date hasachieved atomic resolution [1]. We have now succeeded in observing sec-

ondary electron images of single uraniumatoms using an aberration-corrected 200 kVcold field-emission (CFE) STEM. We haveobserved atomic resolution images not onlyfor heavy elements like uranium but also for

Atomic Resolution Secondary ElectronImaging inAberration Corrected STEMHiromi Inada,1 Mitsuru Konno,1 Keiji Tamura,1 Yuya Suzuki ,1 Kuniyasu Nakamura1 and Yimei Zhu2

1. Hitachi High-Technologies Corporation, Science and Medical Systems Business Group, Ibaraki, Japan2. Department of Condensed Matter Physics and Materials Science, Brookhaven National Laboratory, NY, USA

Figure 1: A Hitachi HD-2700 200 kV scanningtransmission electron microscope asinstalled at the Brookhaven NationalLaboratory in New York, USA. This STEM incorporates a probeaberration corrector (CESCOR) fromCEOS GmbH.

MICROSCOPY AND ANALYSIS NANOTECHNOLOGY SUPPLEMENT NOVEMBER 2011 S5

beneath the specimen, enabling the simulta-neous observation of secondary and transmit-ted electron images.

ATOMIC RESOLUT ION IMAG INGWITH S ECONDARY E L ECTRONS

Secondary Electron Imaging – A Complemen-tary Imaging SignalFigures 2a to 2c show a low magnificationcomparison of SE, ADF-STEM and BF-STEMimages of a core-shell catalyst specimen (pal-ladium core and platinum shell) at 200 kV andwith a 0.1 nm diameter electron probe [6]. Wenote that the SE image clearly demonstrates

the topographical structure, enabling the dis-tribution of particles with respect to the car-bon support matrix to be studied. Thus, at lowmagnification the SE image provides usefulstructural information in itself. SE imagingcapability, however, also plays an importantrole in locating a suitable region for electronenergy-loss spectroscopy (EELS) and energy-dispersive X-ray (EDX) analysis enablingunwanted background material to beavoided. Figures 2d to 2f show high resolution images

of a gold nanoparticle on a carbon film. Au(111) lattice fringes with 0.24 nm spacing aremarked, whilst particle morphology and

faceting can be clearly observed in the SEimage. Once again, the ability to simultane-ously observe SE and TE signals is of para-mount importance.

Atomic Resolution Secondary Electron Imag-ing of Single AtomsFigure 3 shows simultaneously acquired SE andADF-STEM images of isolated and clumpeduranium atoms on a 2-nm thick carbon film (at200 kV with a convergence angle of 28 mrad).The test specimen is a typical uranyl acetatenegatively stained tobacco mosaic virus (TMV)on a carbon thin film, enabling the compari-son of single atom imaging capability with

Figure 2: Right: Configuration of the detec-tors on the HD-2700 STEM. Left: (a-e) Comparison of secondaryelectron (SE) (a, d), annular dark-field scanning transmission electronmicroscopy (ADF-STEM) (b, e), andbrightfield (BF)-STEM (c, f) atomicresolution images of gold nano-particles on a carbon film. Accelerating voltage 200 kV.

Figure 3: Images of isolated single andclusters of uranium atomsacquired simultaneously by SEimaging (a) and ADF-STEMimaging (b). Accelerating voltage 200 kV,convergence angle 28 mrad.

MICROSCOPY AND ANALYSIS NANOTECHNOLOGY SUPPLEMENT NOVEMBER 2011S6

SE IMAGING IN STEM

conventional ADF-STEM Z-contrast imaging.Quantitative analysis of the images indicatesthe existence of uranium atom information inthe SE image. Across 50 possible atom posi-tions in the SE and ADF images, the averagedfull-width and half-maximum (FWHM) inten-sity profile is less than 0.1 nm.

Atomic Resolution Secondary Electron Imag-ing of Light ElementsOur results indicate that SE atomic resolutionis achievable for both heavy and light ele-ments, although the spatial resolution forlight elements may be reduced [2, 3]. We havesuccessfully observed atomic resolution SEimages on composite functional materials suchas SrTiO3 and YBa2Cu3O7. Figure 4 shows one example of one of these

Figure 4: High-resolution secondaryelectron (a) and brightfield-STEM (b) images of a platinumcatalyst sitting on a carbongraphite support. Accelerating voltage 200 kV,probe diameter 0.1 nm, probecurrent 200 pA, convergencesemi-angle 32 mrad.

Figure 5: Intensity measurement ofuranium stained tobaccomosaic virus clusters andcarbon film by changingthe specimen applied biasvoltage from 0 to +25 V toanalyse the contrast effectof secondary electronimaging quantitatively. The image gray level wasadjusted to be 0 as mini-mum and 35000 as maxi-mum.

studies on graphitic carbon (Z=6) with Pt cata-lyst particles. The carbon graphite lattice(shown by arrows) corresponding to 0.34 nm isobserved in Figure 4a. In comparison withSTEM imaging, the SE imaging techniqueappears more sensitive to light elements,yielding higher intensity and contrast as wellas topographical information. The observationof atomic images of light elements was unex-pected, and certainly challenges the tradi-tional understanding of the mechanism of SEimage formation.

Signal Generation: Are the Images really formed by Secondary Electrons?An important aspect of this study was tounderstand whether the electrons emergingfrom the surface were indeed secondary or

whether they were backscattered electrons.By applying a positive bias voltage to the spec-imen, the low energy SEs (<50 eV) aresupressed from escaping from the surface andonly higher energy backscattered electronscan reach the detector. Figure 5 is an image comparison of SE inten-

sity as a function of specimen positive bias.The minimum grey level is set to zero and themaximum grey level to 35,000 for the pur-poses of image comparison. The specimen wasbiased at 0, 2, 5, 10, 15 and 25 eV, respectively.The signal intensity decreases rapidly withincreasing positive bias and the signal is satu-rated at biases above 20 eV. Studies show thatfor 200 keV primary electrons the SEs repre-sent 90% of the total SE+BSE signal. Addi-tionally, using electron trajectory simulations

MICROSCOPY AND ANALYSIS NANOTECHNOLOGY SUPPLEMENT NOVEMBER 2011 S7

Figure 6: (a) Secondary electron images of silicon dumbbells with a specimen thickness of 50 nm.(b) Atomic resolution image of silicon gate oxide layer interface of a semiconductor device with a thickness of 1000 nm.(c) Corresponding schematic model of the semiconductor device.

of the optical configuration used in this exper-iment, it can be shown that only 0.002% of theSE3 generated by BSEs are passed through theupper pole-piece hole. Thus the signal contentis shown to be predominantly SE.In respect of the atomic resolution observed

it is also necessary to consider the beam-sam-ple interaction volume. Monte-Carlo simula-tions suggest that the electron interaction vol-ume at 200 keV is 1/100th of that at 20 keV.The traditional mechanism of secondary elec-tron imaging is attributed to the inelastic scat-tering and delocalised decay of collective elec-tron excitation. An atomic-scale SE imagemechanism is proposed in which contrastdepends on the width of the point spreadfunction, arising from inelastic scattering withlarge momentum transfer.

Atomically Resolved Imaging even on BulkSpecimens: Example ApplicationsIn the industrial field, SEM is widely used forroutine inspection and measurement as well asmaterials development. The routine metrol-ogy of semiconductors for process control hasbecome increasingly challenging as thedesign-size of devices has reduced. Conven-tionally the routine metrology of semiconduc-tors has been performed using high resolutionSEM, whilst the very latest semiconductordevices at the 45-nm node require TEM orSTEM imaging on thin specimens in order toprovide sufficient resolution for reliable assess-ment of gate oxide thickness and other keyparameters. The atomic resolution SE imagingtechnique may prove invaluable to semicon-ductor metrologists by ensuring atomic reso-lution is available without the complexity ofthin specimen preparation. Figure 6a shows high-resolution SE images

of a single crystal Si thin specimen (50 nm) [7].In the thin specimen, silicon dumbbells (0.136nm) and the corresponding power spectrumspot (004) can be observed. Most significantly,even with a 1-µm thick specimen, high-resolu-tion SE images still present lattice fringe infor-mation in the Si(111) plane with a resolutionof 0.314 nm (Figure 6b). This result suggeststhat: 1. measurement can can be calibrated‘on-the-fly’ using the lattice fringe in the sameregion of interest with high accuracy; and 2.the specimen for atomically resolved SE imag-ing can be much thicker than that for ADF andBF-STEM. We have confirmed that automaticmetrology software based on the contrastmeasurement can be applied with this SEimage.We believe the atomic-resolution SE imag-

ing technique can be applied to a variety ofmaterials and systems where thin specimenpreparation is either challenging due to mate-rial properties or impractical due to through-put requirements.

CONCLUS IONSHigh-resolution SE imaging enables the sur-face characteristics of specimens to beobserved simultaneously with the acquisitionof transmitted electron signals. We havedemonstrated atomic resolution secondaryelectron imaging for low and high-Z materials

using the Hitachi HD-2700. We have shown the results of atomic resolu-

tion SE imaging and we can conclude thatthere are three key parameters to be satisfied:1. An ultrahigh efficiency through the lens SEdetector is required.2. An aberration-corrected electron beam witha diameter of around 0.1 nm used simultane-ously with high current is required.3. A high-energy primary electron beamresults in a small interaction volume in thespecimen.The atomically resolved SE imaging tech-

nique is already being applied to a wide vari-ety of materials, although the full potential ofthe technique is yet to be realised. The abilityto correlate surface and structural properties iskey to understanding many material systems.The ability to perform atomic resolution imag-ing on bulk materials may revolutionise high-throughput electron microscopy, particularlyin semiconductor metrology or in applicationswhere thin specimen preparation is difficult orimpractical.

ACKNOWLEDGEMENTSThe author is grateful to Dr Yimei Zhu, DrJoseph Wall, Dr Lijun Wu and Dr Dong Su ofThe Brookhaven National Laboratory, Dr RayEgerton of The University of Alberta and DrJim Ciston of The Lawrence Berkley NationalLaboratory for collaborative work on theexperiments and for fruitful discussions on the

contrast mechanism. The author also thanksDr Michael Dixon of Hitachi High-TechnologiesEurope for support.

REFERENCES1. Howie, A. Recent developments in secondary electron

imaging. J. Microscopy 180:192-203, 1995.2. Zhu, Y., Inada,H., Nakamura, K., Wall, J. Imaging single

atoms using secondary electrons with an aberration-corrected electron microscope. Nature Materials 8:808,2009.

3. Inada, H., Su, D., Egerton, R., Konno, M., Wu, L., Ciston, J.,Wall, J., Zhu, Y. Atomic imaging using secondary electronsin a scanning transmission electron microscope:Experimental observations and possible mechanisms.Ultramicroscopy 111:865-876, 2011.

4. Inada, H., Wu, L., Wall, J., Su, D., Zhu, Y. Performance andimage analysis of the aberration-corrected Hitachi HD-2700C STEM. J. Electron Microscopy 58:111-122, 2009.

5. Inada, H., Kakibayashi, H., Isakozawa, S., Hashimoto, T.,Yaguchi, T., Nakamura, K. Hitachi's development of cold-field emission scanning transmission electron microscope.Advances in Imaging and Electron Physics, edited by P. W.Hawkes 159:123-186, 2009.

6. Wang, J., Inada, H., Wu, L., Zhu, Y., Choi, Y., Liu, P., Zhou, W.,Adzic, R. Oxygen reduction on well-defined core-shellnanocatalysts: Particle size, facet, and Pt shell thicknessEffects. J. Am. Chem. Soc. 131:17298-17302, 2009.

7. Konno, M., Suzuki, Y., Inada, H., Nakamura, K. High-resolution SEM imaging with aberration correction forhighly precise measurement of semiconductors. Microscopyand Microanalysis 17:930-931, 2011.

©2011 John Wiley & Sons, Ltd

MICROSCOPY AND ANALYSIS NANOTECHNOLOGY SUPPLEMENT NOVEMBER 2011S8

c