Kuniyasu Nakamura

Chapter 4 – Hitachi's Development of Cold-Field Emission Scanning Transmission Electron Microscopes

Publisher Summary This chapter describes Hitachis efforts to develop cold-field emission (CFE) technology and scanning transmission electron microscope (STEMs). The chapter introduces cutting-edge application data obtained with the latest CFE-STEMs, highlights Hitachis contributions to field emission (FE) technology development, and shows how the knowledge gained has been passed from generation to generation at Hitachi. The chapter also shows how CFE scanning electron microscopes (SEM)/ STEM technologies were established. The fusion of TEM technology by Hitachi started before World War II, and the CFE technology introduced by Crewe played an important role in Hitachis development of FE-STEMs. These were not fortuities. The very strong enthusiasm of the researchers and designers at Hitachi met the challenge to develop the worlds highest performance STEMs. Their enthusiasm enabled Hitachi to develop various types of STEMs, from the 50-kV STEM in 1975 to the latest HD-2700 dedicated STEM. Along the way, M. Haider and H. Rose developed Cs-correction technology and it was introduced into the HD-2700. The result was elemental and chemical-bonding–state mapping and high-resolution STEM image observation at an atomic order. These microscopes contributed to the fine structure analysis of nano-materials, nano-electronic devices, and nano-biomaterials in various fields. CFESTEMs have become an essential analytical tool in many fields.

Application of 80-200 kV aberration corrected dedicated STEM with cold FEG

We have developed new STEM instrumentation with a cold field emission source (Hitachi HD-2700) in order to perform structural characterization and elemental mapping at the atomic level. The instrument utilises the CEOS GmbH (Germany, managing director: Dr. Max Haider) aberration corrector. The accelerating voltage range is between 80 kV and 200 kV. The cold field emission source proves to be the ideal emitter for analytical transmission electron microscopes due to its high brightness, high current density and small energy spread. In this study, we have examined low accelerating voltage conditions for obtaining high image contrast and high performance elemental analysis (in which FWHM of zero loss peaks are 0.3 eV for acquisition time of 1 second and 0.34 eV for acquisition time of 40 second by accelerating voltage of 80 kV, respectively). We have observed high contrast bright field STEM images of graphene carbon at an accelerating voltage of 80 kV, in which lattice fringes can be clearly seen.

Simulation Study of Noise Influence in 3-Dimensional Reconstruction using High-Angle Hollow-Cone Dark-Field Transmission Electron Microscope Images.

Three-dimensional reconstruction of crystal structures by filtered convolution back projection of high-angle hollow-cone dark-field transmission electron microscope images (HADF images) of a specimen is investigated. To determine the distribution of Cu particles in Si crystal from reconstructed images, the necessary conditions are estimated. Undistorted distribution of particles is obtained from a simulated reconstruction when the specimen is inclined between ±50°, the inclination axis direction error is ±2° and Poisson noise is 30 db. Under these conditions, the distribution of Cu particles in a Si crystal is experimentally reconstructed.

Atomic Species Analysis and Three-Dimensional Observation by High-Angle Hollow-Cone Dark-Field Transmission Electron Microscopy

Methods of determining atomic species and reconstructing three-dimensional images of the specimen structure were examined by high-angle hollow-cone dark-field transmission electron microscopy (HADF-TEM). The contrast of the HADF image systematically changed depending on the atomic number and the size of particles deposited on carbon films. The relationship between the contrast and the atomic number was well simulated using the theory of multiple electron scattering. The specimens, which were Cu particles precipitated in Si crystal, were observed with an inclination from -50 to 50° by steps of 2°. The images were processed using computed tomography. It became clear that the spatial distribution of the Cu particles differed depending on whether the precipitated position was a dislocation or a (111) stacking fault.

Dislocation reduction in InP layers grown on sawtooth-patterned GaAs substrates

Abstract We report on the crystalline improvement of InP layers grown on GaAs substrates by using sawtooth-patterned substrates. The period of sawtooth pattern was 1.2 μm and the layers were grown by low-pressure metalorganic chemical vapor deposition. Cross-sectional transmission electron microscope observation is applied to examine the effectiveness of the sawtooth-shaped hetero-interface for the reduction of threading dislocations in InP layers which accommodate lattice mismatch. A remarkable decrease in the full width at half-maximum values of the X-ray diffraction peaks was obtained, signifying a dramatic crystalline improvement due to the reduction of dislocations by the sawtooth-shaped interface. The unevenness of the layer surface which originates from the sawtooth shape of the substrate can be readily smoothed by thermal cycle annealing, which is accompanied by additional crystalline improvement. Moreover, an attempt to put a thin strained interlayer in the epitaxial layer as a defect filtering tool is also presented.

Development of Two Steradian EDX System for the HD-2700 FE-STEM Equipped with Dual X-MaxN 100 TLE Large Area Windowless SDDs

The model HD-2700 [1] 200 kV spherical aberration (Cs) corrected dedicated Scanning Transmission Electron Microscope (STEM) has been used for analyzing nanoto subnano-area targets in the fields of nanoscience and nanotechnology with Energy Dispersive X-ray spectrometry (EDX). The Cs corrector [2] enables the formation of sub-nanometer probe size with several hundred to a thousand pico amperes of probe current, but still EDX detectors with much higher sensitivity are desired. Recent adaptation of Silicon Drift Detector (SDD) technology [3] accelerated the counting rate of detection and enhancement of detector active area. These features are suitable to improve analytical sensitivity. Using a windowless high solid angle SDD, high sensitivity elemental analysis can be achieved [4].

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.

Three dimensional dislocation analysis of threading mixed dislocation using multi directional scanning transmission electron microscopy

The recently developed multi directional scanning transmission electron microscopy (MD-STEM) technique has been applied to exactly determine the Burgers vector (b) and dislocation vector (u) of a threading mixed dislocation in a silicon carbide (SiC) as-epitaxial wafer. This technique utilizes repeated focused ion beam (FIB) milling and STEM observation of the same dislocation from three orthogonal directions (cross-section, plan-view, cross-section). Cross section STEM observation in the [1-100] viewing direction showed that the burgers vector have a and c components. Subsequent plan view STEM observation in the [000-1] direction indicated that the b=[u -2uuw] (u≠0 and w≠0). Final cross section STEM observation in the [11-20] direction confirmed that the dislocation was an extended dislocation, with the Burgers vector experimentally found to be b = [1-210]a/3 + [0001]c which decomposes into two partial dislocations of bp1 = [0-110]a/3 + [0001]c/2 and bp2 = [1-100]a/3 + [0001]c/2. The dislocation vector u is [-12-10]a/3 + [0001]c. This technique is an effective method to analyze the dislocation characteristics of power electronics devices.

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

GaAs single crystal for 3 inch diameter wafers grown by horizontal zone melt technique

We have developed a new horizontal zone melter (HZM) for boat growth of 3 inch diameter GaAs wafers. A silicon carbide (SiC) ring heater was used in the GaAs melting zone to concentrate the heating power in the small zone, and to minimize the variation in the longitudinal carrier concentration in the crystal. In order to maintain a flat, or a slightly convex, solid-liquid interface in the direction of crystal growth, the axial temperature gradient of the GaAs melt was minimized and the melt convection currents were reduced only in a small part of the GaAs melt near the solid-liquid interface. A Si-doped GaAs single crystal some 10 kg in weight and 600 mm in length was grown with the HZM. The length of the melting zone was 200 mm long. The longitudinal carrier concentration was constant from the seed (g = 0.1) to the middle (g = 0.65). The average dislocation densities of the 3 inch diameter wafers were about 2000 cm -2 both from near the seed and the tail.