Shigeto Isakozawa

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.

Spherical-aberration-free observation of TEM images by defocus-modulation image processing

Abstract Defocus-modulation image processing has been applied to correct the spherical aberration in transmission electron microscopic images. The correction procedure is performed by off-line integration of through-focus images with bipolar weighting functions. An electron microscope with a field emission gun, the HF-2000, was used in this experiment. The Thon diagrams constructed by plotting the power spectra of the processed images as a function of virtual defocus value clearly demonstrate that the spherical aberration was corrected very well, leading to the marked improvement of the resolution from 3.7 to 5.8 nm -1 . As a novel approach to assess the information limit more accurately, we have represented the Thon diagrams for the amplitude and phase components separately. The presented results reveal that this approach is of practical use even for thicker samples which cannot be treated as weak phase objects.

An autofocus method using quasi-Gaussian fitting of image sharpness in ultra-high-voltage electron microscopy

An accurate method using image sharpness to determine the best focusing is proposed for ultra-high-voltage electron microscopy. This method maximizes image sharpness for adjusting the focus. Five images with different defocus values are used to calculate the image sharpness. To obtain the best focus value that produces greatest image sharpness, fitting the quasi-Gaussian function to five image sharpness is a suitable alternative. This method, which maximizes image sharpness, gives better accuracy than the wobbler method for the ultra-high-voltage electron microscope. The focusing area can be selected without moving the field of view, because the focusing area can be selected at almost any area in the image.

Transmission Electron Microscope Sample Shape Optimization for Energy Dispersive X-Ray Spectroscopy Using the Focused Ion Beam Technique

By utilizing the focused ion beam (FIB) technique, we evaluated signal intensity ratios of analyzed areas and the matrix for energy dispersive X-ray spectroscopy with a transmission electron microscope (TEM-EDX). In the case of conventional FIB samples (H-shape), electrons scattered from the thin film area irradiate the sidewalls of the matrix (Si substrate) under the thin film position and they generate a large number of X-rays. These matrix signals are reduced by making samples as U-shaped and removing the underlying sidewalls. The final matrix (Si) X-ray signal is reduced by 90% compared with the conventional H-shaped samples.

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.

High-precision image-drift-correction method for EM images with a low signal-to-noise ratio.

The phase correlation method (PCM) is well known for high-precision matching between images. However, if the signal-to-noise ratio of an image is low, the method is difficult to apply. To solve this problem, we developed an improved PCM that can match images automatically with sub-pixel matching precision. Using this method, a 0.2-nm crystal lattice spacing was clearly revealed after 10 blurred images were processed in a verification experiment; such a lattice could not be recognized or hardly be recognized in each individual image.

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.

Design of a 300-kV gas environmental transmission electron microscope equipped with a cold field emission gun

A new in situ environmental transmission electron microscope (ETEM) was developed based on a 300 kV TEM with a cold field emission gun (CFEG). Particular caution was taken in the ETEM design to assure uncompromised imaging and analytical performance of the TEM. Because of the improved pumping system between the gun and column, the vacuum of CFEG was largely improved and the probe current was sufficiently stabilized to operate without tip flashing for 2-3 h or longer. A high brightness of 2.5 × 10(9) A/cm(2) sr was measured at 300 kV, verifying the high quality of the CFEG electron beam. A specially designed gas injection-heating holder was used in the in situ TEM study at elevated temperatures with or without gas around the TEM specimen. Using this holder in a 10 Pa gas atmosphere and specimen temperatures up to 1000°C, high-resolution ETEM performance and analysis were achieved.