Kazutoshi Kaji

STEM/SEM, Chemical Analysis, Atomic Resolution and Surface Imaging At ≤ 30 kV with No Aberration Correction for Nanomaterials on Graphene Support

Full analytical capabilities considered standard for high-voltage STEM/TEMs at 30kV are expensive and typically require monochromators especially for Schottky emitter based instrumentation [1-3] due to the strangle-hold of the chromatic aberrations. In addition, the power supplies for lenses designed for 200/300kV contribute increasingly to the energy width of the electron beam at the low level currents needed for ≤ 30kV instrumentation. Therefore, enhancing an atomic resolution SEM with a cold field emission gun (cFEG) with STEM, EELS and diffraction capabilities provides an excellent platform for combining surface investigations typically for SEMs with high resolution and analysis capabilities of a typical STEM at comparatively low cost.

The development of a new windowless XEDS detector

A new windowless X-ray energy-dispersive spectroscopy (XEDS) detector has been developed for an analytical electron microscope (AEM). Different from the conventional XEDS detectors, the new detector does not contain an ultra-thin window (UTW) and a vacuum gate valve which are the major causes of low X-ray detection sensitivity and vibration problems for AEM imaging, respectively. The performance of the newly designed detector was examined at an AEM column vacuum level of 10⁻⁵ Pa. The X-ray detectability was improved considerably; in particular, the sensitivity for detecting nitrogen characteristic X-ray signal was three times higher than that of the conventional UTW detectors.

The newly installed aberration corrected dedicated STEM (Hitachi HD2700C) at Brookhaven National Laboratory

The Hitachi HD2700C was recently successfully installed at the newly established Center for Functional Nanomaterials, Brookhaven National Lab (BNL). It was the first commercial aberration corrected electron microscope manufactured by Hitachi. The instrument is based on HD2300, a dedicated STEM developed a few years ago to complete with the VG STEMs [1]. The BNL HD2700C has a cold-field-emission electron source with high brightness and small energy spread, ideal for atomically resolved STEM imaging and EELS. The microscope has two condenser lenses and an objective lens with a gap that is slightly smaller than that of the HD2300, but with the same ±30° sample tilts capability. The projector system consists of two lenses that provide more flexibility in choosing various camera lengths and collection angles for imaging and spectroscopy. There are seven fixed and retractable detectors in the microscope. Above the objective lens is the secondary electron detector to image surface morphology of the sample. Below are the Hitachi HAADF and BF detector for STEM, and a Hitachi TV rate (30frame/sec) CCD camera for fast observations and alignment. The Gatan 14bit 2.6k×2.6k CCD camera located further down is for diffraction (both convergent and parallel illumination) and Ronchigram analysis. The Gatan ADF detector and EELS spectrometer (a specially modified high energy resolution Enfina spectrometer incorporating full 2nd and dominant 3rd order corrected optics and low drift electronics, a 16bit 100×1340 pixel CCD) are located at the bottom of the instrument. The CEOS probe corrector has been modified and optimized for this instrument.

The development and characteristics of a high-speed EELS mapping system for a dedicated STEM

A new EELS (electron energy loss spectroscopy) real-time elemental mapping system has been developed for a dedicated scanning transmission electron microscope (STEM). The previous two-window-based jump-ratio system has been improved by a three-window-based system. It is shown here that the three-window imaging method has less artificial intensity in elemental maps than the two-window-based method. Using the new three-window system, the dependence of spatial resolution on the energy window width was studied experimentally and also compared with TEM-based EELS. Here it is shown experimentally that the spatial resolution of STEM-based EELS is independent of the energy window width in a range from 10 eV to 60 eV.

High Speed and Sensitive X-ray Analysis System with Automated Aberration Correction Scanning Transmission Electron Microscope

In recent years, the aberration-correction technique has brought a revolution in analytical microscope by making atomic-resolution imaging and analysis routinely achievable in transmission electron microscope (TEM) and scanning transmission electron microscope (STEM). We have developed as a product an electron microscope the performance of which is dramatically increased by inclusion of a sphericalaberration-correction function (Inada et al., 2009a, 2009b). In addition, the application of new aberration-correction techniques, such as atomic-resolution secondary-electron (SE) imaging, is now being investigated (Zhu et al., 2009; Inada et al., 2011a, 2011b; Inada & Zhu, 2014). Scherzer (1947) proved that combinations of rotationally symmetrical electromagnetic lenses had convex lens effects only, and the spherical aberration coefficients were always positive. However, multipole lenses in the aberrationcorrection devices of TEMs and STEMs have resulted in concave lens effects, that is, lenses with negative spherical aberrations, and these are now in wide use for cancelling out the positive spherical aberrations of object lenses (Beck, 1979; Rose, 1981; Crewe, 1982; Rose, 1990; Haider et al., 1998). On the other hand, optics systems using multipole lenses give rise to various types of parasitic aberration due to the heterogeneity of the magnetic properties of the materials, and slight deviations from symmetry during machining. With the aberration-correction devices in previous use, for correcting multiple types of aberration, alignment was a difficult process, and users required considerable experience to be able to make

Searching ultimate nanometrology for AlOx thickness in magnetic tunnel junction by analytical electron microscopy and X-ray reflectometry.

Practical analyses of the structures of ultrathin multilayers in tunneling magneto resistance (TMR) and Magnetic Random Access Memory (MRAM) devices have been a challenging task because layers are very thin, just 1-2 nm thick. Particularly, the thinness (approximately 1 nm) and chemical properties of the AlOx barrier layer are critical to its magnetic tunneling property. We focused on evaluating the current TEM analytical methods by measuring the thickness and composition of an AlOx layer using several TEM instruments, that is, a round robin test, and cross-checked the thickness results with an X-ray reflectometry (XRR) method. The thickness measured by using HRTEM, HAADF-STEM, and zero-loss images was 1.1 nm, which agreed with the results from the XRR method. On the other hand, TEM-EELS measurements showed 1.8 nm for an oxygen 2D-EELS image and 3.0 nm for an oxygen spatially resolved EELS image, whereas the STEM-EDS line profile showed 2.5 nm in thickness. However, after improving the TEM-EELS measurements by acquiring time-resolved images, the measured thickness of the AlOx layer was improved from 1.8 nm to 1.4 nm for the oxygen 2D-EELS image and from 3.0 nm to 2.0 nm for the spatially resolved EELS image, respectively. Also the observed thickness from the EDS line profile was improved to 1.4 nm after more careful optimization of the experimental parameters. We found that EELS and EDS of one-dimensional line scans or two-dimensional elemental mapping gave a larger AlOx thickness even though much care was taken. The reasons for larger measured values can be found from several factors such as sample drift, beam damage, probe size, beam delocalization, and multiple scattering for the EDS images, and chromatic aberration, diffraction limit due to the aperture, delocalization, alignment between layered direction in samples, and energy dispersion direction in the EELS instrument for EELS images. In the case of STEM-EDS mapping with focused nanoprobes, it is always necessary to reduce beam damage and sample drift while trying to maintain the signal-to-noise (S/N) ratio as high as possible. Also we confirmed that the time-resolved TEM-EELS acquisition technique improves S/N ratios of elemental maps without blurring the images.

3D Composition Imaging Using a Dedicated FIB/STEM System

A technique for forming a sub-micron-square pillar specimen and then observing the sample from different directions has been developed using a focused ion beam (FIB) / scanning transmission electron microscopy (STEM) system. The system employs an FIB/STEM compatible specimen holder with a specially designed rotation mechanism, which allows the specimen to be rotated 360 degrees [1]. This technique was used for the three dimensional (3D) elemental mapping of a Ni-base superalloy specimen which contains fine γ’ precipitates. Fig.1(a) shows a bright-field (BF)-STEM image of the cone shaped Ni-base superalloy specimen. Energy dispersive X-ray spectrometry (EDX) compositional maps of Ni-K, Cr-K and Ti-K (shown in Fig.1 (b), (c) and (d), respectively) were obtained from the same field of view as Fig.1 (a). Spectrum imaging combined with multivariate statistical analysis (MSA) [2] was used to enhance the weak X-ray signals of the 10 to 20 nm precipitates, which contain a low concentration of Ti-K. A rendering technique was also applied to the composition maps of Ni-K, Cr-K and Ti-K for 3D reconstruction of fine precipitates [3]. In a second study, this technique was used for the observation of a Si device. A specimen which included a contact plug of the Si device was trimmed into a micro-pillar with a 400 nm cross-section and a 5 μm length. Analysis was performed with a 200 kV HD-2300 STEM equipped with the EDAX genesis system. EDX spectrum images were acquired for a dwell time of 64ms per pixel with the incident beam size of 1 nm and the current of 0.9 nA. The sizes of spectrum images are 128 x 100 pixels with 1024 channels. Fig.2 shows a BF-STEM image (a) and a high angle annular dark-field (HAADF)-STEM image (b) of the contact plug. Fig.3 shows original EDX elemental maps of Si-K (a), Ti-K(b), N-K(c) and As-K(d). The elemental maps reconstructed from the maps shown in Fig.3 by MSA are shown in Fig.4 (a), (b), (c) and (d), respectively. The distributions of elements, especially the dopant As, were successfully enhanced by MSA. In these acquisition conditions, 36 tilt series of EDX spectrum images were acquired with a 5 degree step and then the 3D distributions of those elements were observed.

Energy Filtered STEM Imaging at 30kV and Below - A New Window into the Nano-World?

Energy filtered imaging has been available for many years for TEMs. Both, the in-column filter by Carl Zeiss [1] and the Gatan Imaging Filter [2] have proven useful and are well documented. Compared to TEM, energy filtering combined with BF-STEM has seen less publicity. This is surprising because BFSTEM by itself has an advantage over TEM: inelastically scattered electrons have less of an effect on the final image quality when compared to TEM. Furthermore, EF (energy-filtered) BF-STEM imaging has been reported as a promising approach [3]. However, EF BF-STEM imaging is handicapped by the requirement of a fast EELS detector: for acquiring a 512 × 512 pixel image in under 1s, an EELS detector capable of 512 × 512 = 262,144 EELS spectra /s is required.

Expanding the Depth of Field for Imaging with Low keV Electrons: High Resolution Surface Observations of Nanostructured LaB6 Using Low keV Secondary and Backscattered Electrons

1. Hitachi High-Technologies Corp., Application Development Department, Ibaraki, Japan. 2. Hitachi High-Technologies Corp., Electron Microscope Systems Design 1st Dept., Ibaraki, Japan. 3. Hitachi High Technologies America, NSD, Clarksburg, Maryland, USA. 4. University of Georgia, Department of Chemistry, Athens, Georgia, USA. 5. University of Georgia, Georgia Electron Microscopy, Athens, Georgia, USA.

The First Results of the Low Voltage Cold-FE SEM/STEM System Equipped With EELS

For various fields of research and development such as nano-materials, bio-materials or semiconductor devices, a role of electron microscope is becoming more important for not only image observation but also chemical characteristics analysis with high spatial resolution of nm or sub-nm level [1-3]. Since nano materials such as carbon nano-tube and graphene sheet are not strong against electrons accelerated to higher than 100 kV, it is one of effective approaches for reducing damages to lower accelerating voltage. Field-emission scanning electron microscope, Hitachi SU9000, performs to give not only secondary electron images including backscattered electron signals but also BFand DF-STEM images at 30 kV or lower voltage. BF-STEM lattice images at 30kV are detected for Si[222] lattice fringe, corresponding to 0.157 nm[4]. In present study, we have developed the prototypes of an electron energy loss spectrometer and a diffraction camera unit for SU9000 in order to acquire not only element distribution and chemical information but also crystal structures at 30 kV or lower voltage.