Hiromi Inada

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.

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.

Mixing Real and Reciprocal Space

Traditionally, TEM has been considered either an imaging or diffraction based technique; images are obtained to elucidate microstructure while diffraction patterns are used to determine crystal structure. This traditional separation, however, is completely arbitrary. A brief review of electron microscopy shows that nearly all TEM techniques rely on the mixing of real (r-space) and reciprocal space (k-space) to some degree [1]. For example, bright- and dark-field imaging relies on the filtering of k-space to produce contrast; large-angle CBED patterns directly mix real and reciprocal space to allow the measurement of strain centres. For some years, electron backscatter diffraction (EBSD) imaging in the SEM has allowed the direct combination of real and reciprocal space. This backscatter geometry has both advantages and limitations; a particular limitation, spatial resolution, can be greatly improved by moving to the transmission geometry.

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

Aberration Corrected Analytical Scanning and Transmission Electron Microscope for High-Resolution Imaging and Analysis for Multi-User Facilities

In recent years the revolution in aberration correction technology has made ultrahigh resolution imaging and analysis routinely accessible on transmission electron microscope (TEM) and scanning transmission electron microscope (STEM). We have developed a new analytical 200 kV cold field emission TEM equipped with a probe-forming aberration corrector, the model is Hitachi HF5000 (Figure 1). The microscope is fully covered in a metal enclosure to reduce the influence from environmental acoustic noise and temperature variation. Remote operation through Ethernet communication is possible as a result of a new design individual microprocessor circuit. Regarding the atomically resolved analytical capability, one of the key demands is to achieve high performance at a multi-user facility. To meet this demand, the Hitachi HF5000 is designed to be user friendly and extensive sample capability covers most requirement s from users in the fields of material science, materials fabrication, and device industry. The HF5000 is capable of TEM imaging, STEM imaging with bright field (BF), annular dark field (DF) detectors, and secondary electron (SE) imaging. The probe-forming aberration corrector with automated correction of up to third order aberrations allows users to obtain aberration-free STEM illumination optics with minimized effort. Figure 2 gives an example after the aberration correction, the Ronchigram pattern of the amorphous specimen shows an approximately 32 mrad half angle flat region, corresponding to the optimal aperture condition for aberration-free STEM imaging. While TEM and STEM imaging probe the bulk structure of specimens, the SE imaging helps understanding the surface structure. It is important to note that the SE image can be acquired simultaneously with STEM image therefore both surface and bulk structures are revealed side-by-side at the same time, even at atomic resolution [1]. Such a triple imaging capability on one microscope column is very unique and critical in studying heterogeneous materials such as catalysts.

Development of Automatic Magnification Calibration Function for Scanning Transmission Electron Microscope

Scanning transmission electron microscopes (STEM) are ideally suited to address the growing demand for the metrology of features which range from a few nanometers to tens of nanometers in size. The metrology must be carried out with a high degree of accuracy and repeatability. For metrology with scanning electron microscopes (SEM) or critical dimension SEMs (CD-SEM) the magnification is calibrated by using a Hitachi’s standard micro scale specimen whose pitch is guaranteed by JQA (Japan Quality Assurance Organization). With a pitch of 240nm these micro scale specimens can be used to calibrate the magnifications used for many line-width applications. However, the metrology of smaller features such as gate oxides or ONO layers requires magnifications from ten to one hundred times higher than can be calibrated with the micro scale specimen. Calibrating the magnifications used to measure nanometer scale features has been difficult and researchers are still in need of the appropriate calibration specimen. A HD-2300 STEM (Hitachi High Technologies) guarantees a 0.2nm spot size [1]. This STEM is equipped with three detectors, for secondary electron (SE), dark field (DF) and bright field (BF) STEM imaging. Elemental mapping can be performed using EDX or by using the ELV-2000 Hitachi EELS mapping system. The HD-2300 can observe and analyze samples over a wide range of magnifications. Low magnifications (500-50kx) are useful for imaging the construction of transistors and their interconnects, and can be calibrated using the standard micro scale specimens. High magnification images (> 1000kx) are required for measuring gate oxides, and for observing lattice fringes. This report describes an automatic magnification calibration function for the high magnification range required to accurately measure features from a few to tens of nm in size. The automated magnification calibration function requires only that the user obtain a lattice image from Si or from some other material with known lattice constants. By relying on lattice images, no special calibration sample is required, and the operator can in many cases use the sample’s substrate. The function is carried out by processing the image using the Fast Fourier Transform (FFT) method, which makes calibration fast and easy on the PC which controls the HD2300. The FFT method averages lattice information from the whole image and distinguishes the desired pitch. Figure 2. shows a brief flow chart of this function, and the process is described as follows: First lattice fringe images of an Si crystal are captured at selected magnification (1). Next the images are moved into the magnification calibration GUI window of the HD-2300 (2). The FFT calculation is then carried out automatically to measure the magnification error of the equipment (3). Finally, the result of this error calculation is fed back to the HD-2300 controller to compensate for the magnification error (4). By using this automatic magnification calibration function, the magnification error is held to less than 1% and 3 sigma repeatability is 1.9% at a magnification of 1,500,000 times. High accuracy metrology for semiconductor and nanotechnology features ranging from a few nm to tens of nm in 752 Microsc Microanal 11(Suppl 2), 2005 Copyright 2005 Microscopy Society of America DOI: 10.1017/S1431927605500771