Boklae Cho

Study on fracture behavior of individual InAs nanowires using an electron-beam-drilled notch

The mechanical properties and fracture behavior of individual InAs nanowires (NWs) were investigated under uniaxial tensile loading in a transmission electron microscope. The InAs NWs exhibited elastic deformation during the tensile test till fracture and the experiments revealed a brittle fracture on the (111) plane of a zinc-blende structure. Hemi-ellipse-shaped notches with various radii of curvature, formed via a focused electron beam, were utilized to investigate the fracture behavior when there is a structural flaw at the nanometer scale. The dimensions of notches are controlled with proper understanding of electron beam irradiation on the InAs material system. The stress concentration phenomena with dependence on the notch size are demonstrated in the InAs NWs by analyzing the results from a finite element method simulation.

Note: O-ring stack system for electron gun alignment.

We present a reliable method for aligning an electron gun which consists of an electron source and lenses by controlling a stack of rubber O-rings in a vacuum condition. The beam direction angle is precisely tilted along two axes by adjusting the height difference of a stack of O-rings. In addition, the source position is shifted in each of three orthogonal directions. We show that the tilting angle and linear shift along the x and y axes as obtained from ten stacked O-rings are ±2.55° and ±2 mm, respectively. This study can easily be adapted to charged particle gun alignment and adjustments of the flange position in a vacuum, ensuring that its results can be useful with regard to electrical insulation between flanges with slight modifications.

Edge shadow projection method for measuring the brightness of electron guns

The performance of an electron gun is evaluated in terms of the gun brightness. The brightness of an electron gun is typically measured by dividing the angular current density by the virtual source area. An electron gun brightness measurement system was constructed without an electron lens. The system consists of movable apertures (∅ 30, 50, 100, 200 μm), a Faraday cup, and a phosphor screen. The Faraday cup is employed to measure the angular current density. The electron beam passes through an aperture and its shade is projected onto the phosphor screen. The virtual source position is determined by measuring the displacement of the aperture shade made by the movement of the aperture. The blurring width of the edge of the shadow on the screen is measured by a charged-coupled device camera to calculate the virtual source size. Brightness values of a tungsten filament electron gun were obtained and compared to reported values.

Low-energy scanning electron microscope using a monochromator with double-offset cylindrical lenses

The nanoelectronics industry demands continuous improvement in the performance of scanning electron microscopes (SEMs). Extremely low energies of less than 1 keV are required for SEM observations to allow the subsurface and nanoscale information of target specimens to be measured with minimized charge-up and beam damage depths because of the reduced interaction volumes. In this article, the authors propose a new monochromator (MC) structure and investigate its applicability to SEMs operating at such extra-low energies. The proposed MC, which uses double-offset cylindrical lenses, can perform energy filtering in its midsection and form a stigmatic and nonenergy dispersive image at the exit. The energy resolution is expected to be better than 10 meV for a pass energy of 4 keV. The MC has the additional advantage of a simple but robust structure, which is essential for industrial applications. Assuming the use of ideal and high-performance SEM optics, for which the spherical aberration coefficient and the ch...

Fabrication of a trimer/single atom tip for gas field ion sources by means of field evaporation without tip heating

A gas field ion source (GFIS) has many advantages that are suitable for ion microscope sources, such as high brightness and a small virtual source size, among others. In order to apply a tip-based GFIS to an ion microscope, it is better to create a trimer/single atom tip (TSAT), where the ion beam must be generated in several atoms of the tip apex. Here, unlike the conventional method which uses tip heating or a reactive gas, we show that the tip surface can be cleaned using only the field evaporation phenomenon and that the TSAT can also be fabricated using an insulating layer containing tungsten oxide, which remains after electrochemical etching. Using this method, we could get TSAT over 90% of yield.

Graphene-supporting films and low-voltage STEM in SEM toward imaging nanobio materials without staining: Observation of insulin amyloid fibrils

Utilization of graphene-supporting films and low-voltage scanning transmission electron microscopy (LV-STEM) in scanning electron microscopy (SEM) is shown to be an effective means of observing unstained nanobio materials. Insulin amyloid fibrils, which are implicated as a cause of type II diabetes, are formed in vitro and observed without staining at room temperature. An in-lens cold field-emission SEM, equipped with an additional homemade STEM detector, provides dark field (DF)-STEM images in the low energy range of 5-30keV, together with secondary electron (SE) images. Analysis based on Lenzs theory is used to interpret the experimental results. Graphene films, where the fibrils are deposited, reduce the background level of the STEM images compared with instances when conventional amorphous carbon films are used. Using 30keV, which is lower than that for conventional TEM (100-300keV), together with low detection angles (15-55mrad) enhances the signals from the fibrils. These factors improve image quality, which enables observation of thin fibrils with widths of 7-8nm. STEM imaging clearly reveals a twisted-ribbon structure of a fibril, and SE imaging shows an emphasized striped pattern of the fibril. The LV-STEM in SEM enables acquisition of two types of images of an identical fibril in a single instrument, which is useful for understanding the structure. This study expands the application of SEM to other systems of interest, which is beneficial to a large number of users. The method in this study can be applied to the observation of various nanobio materials and analysis of their native structures, thus contributing to research in materials and life sciences.

Blunted Tungsten Tip Cleaning by Nitrogen Gas Etching at Room Temperature without Tip Heating and Cooling

For a long time, the ultra-sharp tips are used for high resolution microscope, such as scanning tunneling microscope (STM), atomic force microscope (ATM), field emission scanning electron microscope (FE-SEM). In addition, helium ion microscope (HIM), which has sub-nanometer imaging resolution, employs the tip as gas field ionization source (GFIS) in recent years. There are a lot of required vital conditions of tip fabrication and source characterization for the high performance of microscopes. Among them, tip cleanliness and operation temperature are very essential for the stable and high charged particle beam current. The contaminants on the surface of tip will undoubtedly cause instabilities in the tunnel junction between tip and gases and abruptly increases fluctuation of the charged particle beam current, thus affecting the image quality and noise level. Also, its requirements for liquid nitrogen cooling make the microscope system complicated, expensive maintenance cost and price of apparatus. This study presents a simple and efficient way to clean the blunted tungsten tip using a field-assisted nitrogen etching at room temperature without tip heating and cooling methods.