Secondary Electron Image Contrast Research Articles

Effect of non-rare-earth element enrichment on in-the-column secondary electron image contrast of Nd-rich phases in Nd-Fe-B sintered magnets

In-the-column secondary electron (SE) imaging has demonstrated an important role in identifying various Nd-rich phases in Nd-Fe-B sintered magnets at a microscale field of view. It is well known that modifying the microstructure of Nd-Fe-B sintered magnets by adding non-rear-earth (NRE) elements dissolved into the intergranular Nd-rich phases during post-sinter annealing can effectively increase the magnet coercivity. However, the effect of NRE element enrichment on in-the-column SE image contrast of Nd-rich phases has not received much attention. In this study, we have combined a focused ion/electron dual beam system (FIB/SEM) and a scanning/transmission electron microscope (S/TEM) to investigate the in-the-column SE image contrast, structure, and element distribution of intergranular Nd-rich phases in commercial heavy-rare-earth (HRE) free Nd-Fe-B sintered magnets. In-lens SE imaging revealed that Cu, Co, or Al-enriched neodymium oxides with fcc, Ia3¯-type, or amorphous structures are brighter than the Nd2Fe14B matrix at an accelerating voltage of 2 kV and a working distance of 4 mm. Bright stripes inside the intergranular fcc NdOx were also visible in high-resolution through-the-column SE imaging, reflecting the nanoscale composition variation with Co or Cu enrichment. Contrarily, at the same imaging conditions, the nearly pure fcc NdOx and Ia3¯-Nd2O3 exhibit darker than the Nd2Fe14B matrix in the in-lens SE image. It has been demonstrated that the in-lens SE image contrast of Nd-rich phases with respect to the Nd2Fe14B matrix remains constant by systematically adjusting the accelerating voltage from 2 kV to 5 kV and the working distance from 4 mm to 6 mm. Our research results suggest that low-voltage in-the-column SE imaging could be used to track element diffusion in triple junction and grain boundary Nd-rich phases of Nd-Fe-B sintered magnets during post-sinter annealing by in-situ MEMS heating in SEM.

Competition between thermodynamics, kinetics and growth mode in the early-stage oxidation of an equimolar CoCrFeNi alloy

We demonstrate that the early-stage oxidation of a CoCrFeNi multi-principal element alloy depends upon a competition between kinetic and thermodynamic factors involving the relative diffusion rate of cations and oxygen, and how this couples to inward or outward oxide growth. The microstructures of oxide layers formed at temperatures from 500 to 800 °C for 0.5 h, as well as their chemical compositions, were investigated by transmission electron microscopy. A triple layer microstructure with an outer Fe-rich spinel oxide, an intermediate Cr-rich corundum structure, and a Ni-rich (Fe, Co, and Cr depleted) dealloyed region at the metal/oxide interface was observed. The dominant oxygen transport in corundum at 800 °C and below led to an inward growth of the corundum phase; the spinel oxide growth was dominated by cation diffusion, so it grew outward. The chromium was sequestered in the corundum layer, thereby favoring the formation of the chromium-free, Fe-rich spinel oxides with Co and Ni dopants. Since nickel cannot readily diffuse through corundum, it tends to remain in the alloy phase leading to the Ni-rich dealloyed region at the metal/oxide interface. Beyond the microstructure results, we exploit secondary electron image contrast to show the doping nature of the oxides, a p-type spinel and a n-type corundum growing on the metal surface.

Thirty per cent contrast in secondary-electron imaging by scanning field-emission microscopy.

We perform scanning tunnelling microscopy (STM) in a regime where primary electrons are field-emitted from the tip and excite secondary electrons out of the target—the scanning field-emission microscopy regime (SFM). In the SFM mode, a secondary-electron contrast as high as 30% is observed when imaging a monoatomic step between a clean W(110)- and an Fe-covered W(110)-terrace. This is a figure of contrast comparable to STM. The apparent width of the monoatomic step attains the 1 nm mark, i.e. it is only marginally worse than the corresponding width observed in STM. The origin of the unexpected strong contrast in SFM is the material dependence of the secondary-electron yield and not the dependence of the transported current on the tip–target distance, typical of STM: accordingly, we expect that a technology combining STM and SFM will highlight complementary aspects of a surface while simultaneously making electrons, selected with nanometre spatial precision, available to a macroscopic environment for further processing.

Crystal orientation mapping via ion channeling: An alternative to EBSD

A new method, which we name ion CHanneling ORientation Determination (iCHORD), is proposed to obtain orientation maps on polycrystals via ion channeling. The iChord method exploits the dependence between grain orientation and ion beam induced secondary electron image contrast. At each position of the region of interest, intensity profiles are obtained from a series of images acquired with different orientations with respect to the ion beam. The profiles are then compared to a database of theoretical profiles of known orientation. The Euler triplet associated to the most similar theoretical profile gives the orientation at that position. The proof-of-concept is obtained on a titanium nitride sample. The potentialities of iCHORD as an alternative to EBSD are then discussed.

Orientation contrast of secondary electron images from electropolished metals

Orientation contrast obtained by an in-lens secondary electron detector in a scanning electron microscope from electropolished/etched metals is reported. The imaging conditions for obtaining such orientation contrast are defined. The mechanism responsible for the formation of the orientation contrast is explained, and an application example of this new imaging method is given.

Channeling in helium ion microscopy: Mapping of crystal orientation.

Background: The unique surface sensitivity and the high resolution that can be achieved with helium ion microscopy make it a competitive technique for modern materials characterization. As in other techniques that make use of a charged particle beam, channeling through the crystal structure of the bulk of the material can occur.Results: Here, we demonstrate how this bulk phenomenon affects secondary electron images that predominantly contain surface information. In addition, we will show how it can be used to obtain crystallographic information. We will discuss the origin of channeling contrast in secondary electron images, illustrate this with experiments, and develop a simple geometric model to predict channeling maxima.Conclusion: Channeling plays an important role in helium ion microscopy and has to be taken into account when trying to achieve maximum image quality in backscattered helium images as well as secondary electron images. Secondary electron images can be used to extract crystallographic information from bulk samples as well as from thin surface layers, in a straightforward manner.

Image simulation for atomic resolution secondary electron image

It has been demonstrated recently that an atomic resolution secondary electron (SE) image can be achieved with a scanning transmission electron microscope (STEM) equipped with a probe-aberration corrector. Its high sensitivity to the surface structure provides a powerful tool to simultaneously study both surface and bulk structure in the STEM, in the combination with the annular dark field (ADF) image. To quantitatively explain the atomic resolution SE image and retrieve surface-structure information, an image simulation is required. Here, we develop a method to simultaneously calculate, for the first time, the atomic resolution SE and ADF-STEM images, based on the multislice method with a frozen-phonon approximation. An object function for secondary electrons, derived from the inelastic scattering, is used to calculate the intensity distribution of the secondary electrons emitted from each slice. The simulations show that the SE image contrast is sensitive to the surface structure and the electron inelastic mean free path, but insensitive to specimen thickness when the thickness is more than 5 nm. The simulated SE images for SrTiO(3) crystal show good agreement with the experimental observations.

Resistance Change Caused by Electrochemically Induced Carrier Injection in NiO Films.

ABSTRACTIn our previous work, low resistance state (LRS) and high resistance state (HRS) areas on a nickel-oxide (NiO) film formed by applying a voltage using conductive atomic-force microscopy (C-AFM) was observed by scanning electron microscope (SEM). Comparing the observed secondary electron image (SEI) contrast to the report about the dopant-type dependence of SEI contrast reported on silicon, it was suggested that the LRS and HRS areas are, respectively, electrochemically induced p-type Ni1-xO (x > 0) and intrinsic (stoichiometric) or ntype Ni1-xO (x ≤ 0). In this paper, we verified that resistance change caused by C-AFM is due to electrochemically induced carrier injection. Reduction effect of H2annealing on the writing area, voltage dependence of depletion layer capacitance formed between the writing area and AFM-tip using scanning nonlinear dielectric microscopy (SNDM), and the effect of Schottky barrier formation between the writing area and thin metal layer on SEI contrast were investigated. Based on these results, it was clarified that the LRS and HRS areas are, respectively, p-type Ni1-xO (x > 0) and intrinsic (stoichiometric) or n-type Ni1-xO (x ≤ 0)

Correlation between resistance-change effect in transition-metal oxides and secondary-electron contrast of scanning electron microscope images

Conductive atomic-force microscopy (C-AFM) writing is attracting attention as a technique for clarifying the switching mechanism of resistive random-access memory by providing a wide area filled with filaments, which can be regarded as one filament with large radius. The writing area on a nickel-oxide (NiO) film formed by conductive atomic-force microscopy was observed by scanning electron microscope, and a correlation between the contrast in a secondary-electron image (SEI) and the resistance written by C-AFM was revealed. In addition, the dependence of the SEI contrast on the beam accelerating voltage (Vaccel) suggests that the resistance-change effect occurs near the surface of the NiO film. As for the effects of electron irradiation and vacuum annealing on the C-AFM writing area, it was shown that the resistance-change effect is caused by exchange of oxygen with the atmosphere at the surface of the NiO film. This result suggests that the low-resistance and high-resistance areas are, respectively, p-type Ni1+δO (δ < 0) and insulating (stoichiometric) or n-type Ni1+δO (δ ≥ 0).

Observation of epitaxially ordered twinned zinc aluminate “nanoblades” on c-sapphire

We report the observation of a novel nanostructured growth mode of the ceramic spinel zinc aluminate grown on c-sapphire in the form of epitaxially ordered twinned crystallites with pronounced vertically aligned “nanoblades” on top of these crystallites. The nanostructures are formed on bare c-sapphire substrates using a vapour phase transport method. Electron microscopy images reveal the nanostructure morphology and dimensions and allow direct and indirect observation of the twin boundary location in a number of samples. The nanoblade structure with sharply rising sidewalls gives rise to a distinctive bright contrast in secondary electron images in scanning electron microscopy measurements.

Memory Retention Characteristics of Data Storage Area Written in Transition Metal Oxide Films by Using Atomic Force Microscope.

ABSTRACTConductive atomic-force microscopy (C-AFM) writing is attracting attention as a technique for clarifying the switching mechanism of resistive random-access memory (ReRAM) by providing a wide area filled with filaments, which can be regarded as one filament with large radius. We observed a C-AFM writing area of NiO films using SEM, and revealed a correlation between the contrast in a secondary electron image (SEI) and the resistance written by C-AFM. In addition, the dependence of the SEI contrast on the beam accelerating voltage (Vaccel) suggests that the resistance-change effect occurs near the surface of the NiO film. As for the effect of electron irradiation on the C-AFM writing area, it was shown that the resistance change effect was caused by exchanging oxygen with the atmosphere at the surface of the NiO film. This result suggests that the low resistance and high resistance areas are, respectively, p-type Ni1+δO (δ< 0) and insulating (stoichiometric) or n-type Ni1+δO (δ ≥0).

The influence of beam energy and oxidation on quantitative carbide analysis in the scanning electron microscope

In this paper the origin of the carbide/matrix contrast in secondary electron images and its ramifications for spatial resolution are investigated. A profound influence of the backscattered electron yield and the oxidation of the steel matrix was found, which manifests itself in a contrast reversal at low primary electron beam energies. The authors established that low beam energies are necessary to obtain the required spatial resolution. Strategies for optimizing the contrast between carbide and matrix in secondary electron images without compromising the spatial resolution are presented. A simple method to estimate the thickness of thin oxide layers from low voltage secondary electron images is also described.

Origin of the domain contrast on a Si(001)-2×1 surface imaged by secondary electrons

Secondary electron (SE) imaging of alternating domains on Si(001)-2×1 surfaces was studied. We experimentally investigated the relationship between the domain contrast and the incident electron-beam angles, and found that the domain contrast between alternating 2×1 terraces is reversed by changing the incident-beam-angle conditions. And we theoretically show that the intensity of electron waves near the surface changes drastically and reverses according to the incident beam conditions. Both these experimental and theoretical results indicate that the domain contrast in SE images does not originate from anisotropic SE emission from the reconstructed surface but from the difference between the SE excitation on the 2×1 terrace and that on the 1×2 terrace.

ESEM Imaging of P-N Junctions Using a Gaseous Secondary Electron Detector

Contrast in secondary electron images of p-n junctions has been observed using a FESEM operating at 1kV. This contrast has been attributed to energy band-bending effects and dopant level variations. In this paper, we report that contrast from the depletion layer of a p-n junction can be obtained in an ESEM using a gaseous secondary electron detector (GSED). The contrast is caused by a signal induced in the GSED by charging and discharging of the device.A 1N4002 p+pn silicon power diode was cross-sectioned along the lead axis to expose the Ag ohmic contacts and the semiconductor device. The section was mechanically polished to an optical finish using diamond abrasives and mounted in a holder which allowed electrical connection to both the p and n sides of the diode. Imaging experiments were performed in a Philips Electroscan XL30 ESEM and a JEOL 6300F below-lens FESEM. Current images were obtained with a GW Electronics 103 specimen current amplifier.

Contrast mechanisms of secondary electron images in scanning electron and ion microscopy

The contrast of secondary electron images of scanning electron microscopy (SEM) are compared with that of scanning ion microscopy (SIM) for metals. The dependence of the secondary electron yields on atomic number in SEM is opposite to that in SIM. The secondary electron yields for electron bombardment increases with atomic number for metals, while those for ion bombardment decreases with increasing atomic number. The origins of those phenomena are discussed on the basis of the range profiles of both particles respect to the surface.

Contrast of scanning ion microscope images compared with scanning electron microscope images for metals?

Y. Sakai, T. Y. T. S. T. I., T. Y. T. S. T. I., T. Suzuki and T. Ichinokawa, J. Anal. At. Spectrom., 1999, 14, 419 DOI: 10.1039/A806833J

Magnification and scanning time dependence of secondary electron image contrast of ferroelectric domains in aged triglycine sulphate crystal

Magnification and scanning time dependence of secondary electron image contrast of ferroelectric domains in aged triglycine sulphate crystal

Scanning Electron Microscopy of Dopants in Semiconductors

Abstract It has been demonstrated that doped regions in semiconductors yield contrast in secondary electron images taken under appropriate imaging conditions. Perovic et al. attributed this "electronic" contrast to surface potentials arising from the bending of the energy bands at the surface of the semiconductor. This band-bending results from the presence of surface states which pin the Fermi level. These effects have found application to an important problem in the electronics industry -- the profiling of dopant distributions in two-dimensions. However, such dopant distributions are rarely uniform, i.e., the dopant concentration may change over many orders of magnitude within distances of only a few tens to hundreds of nanometers. Therefore, it is necessary to understand how the observed contrast from the doped regions varies for non-uniform dopant distributions to apply the technique to real electronic devices.

Scattering corrections in small particle imaging

Various imaging modes in both STEM and SEM which are useful for characterising small particles are described with direct evidence that some correction of the direct image signal is often necessary because of additional scattering effects. Specific examples chosen are the effect of substrate scattering on annular dark field images, the influence of elastic or quasi-elastic scattering on energy-selected transmission imaging and the role of local variations in backscattering and stopping power in secondary electron image contrast.

Quantitative two-dimensional dopant profiles obtained directly from secondary electron images

Doping-dependent contrast in secondary electron images of p/n junctions in silicon obtained in a field-emission scanning electron microscope was observed and characterized. The optimum experimental conditions for observing this ‘‘electronic’’ contrast were established by investigating the effect of microscope and material parameters on the magnitude of the contrast. The contrast between the bright p-type areas and the darker n-type areas was maximized at an accelerating voltage of ∼1 kV, and when a through-the-lens detector configuration was employed. Secondary electron contrast profiles of boron doped p+/n junctions in silicon showed a good correlation with secondary ion mass spectroscopy depth profiles of the atomic concentration down to the 1017 cm−3 level. However, similar results were not obtainable for n+/p junctions. It is demonstrated that this contrast effect may be exploited for obtaining two-dimensional dopant profiles directly from secondary electron images of p+/n junctions provided that the technique is empirically calibrated against a one-dimensional dopant profiling method.