High Resolution Secondary Electron Imaging Research Articles

Large-scale serial sectioning of environmentally assisted cracks in 7xxx Al alloys using femtosecond laser-PFIB

In order to better understand the mechanisms of environmentally-assisted cracking (EAC) of 7xxx aluminium aerospace alloys, large scale serial sectioning using the newly developed femtosecond laser plasma focused ion-beam (laser PFIB) has been performed on both AA7050 and AA7085 alloys containing environmentally-assisted cracks introduced during accelerated laboratory testing. The samples were firstly scanned using X-ray computed tomography in order to reveal the long-range crack morphology and allow for targeted lift out of the crack tip regions for serial sectioning. This paper describes the optimisation of the methodology for both the liftout procedure and subsequent serial sectioning using laser-PFIB. Electron backscatter diffraction (EBSD) mapping at each slice allowed for 3D reconstruction of statistically valid volumes of material, up to 0.5 mm3 with a fine voxel size (1 μm). High resolution secondary electron imaging at each slice allowed for 3D reconstruction of the crack volume, and the datasets were combined in order to investigate the interaction of the crack with the microstructure. Optimisation of both the liftout geometry and the slicing procedure resulted in significant reduction in laser-induced curtaining artefacts and higher quality data acquisition. For these alloys, laser slicing alone was sufficient to provide a deformation free surface allowing for EBSD indexing rates up to ~95% acquired at high speeds of ~2000 Hz leading to reasonable acquisition times of ~4–5 days for each dataset. The datasets have shown that the grains in these alloys are much larger and more complex than previously realised, and the interaction of these cracks with the microstructure have shown that complex-shaped recrystallised grains can lead to significant crack deflection in these alloys and so may help to explain the observed differences in EAC behaviour.

Time-of-flight secondary ion mass spectrometry in the helium ion microscope

A helium ion microscope, known for high resolution imaging and modification with helium or neon ions, has been equipped with a time-of-flight spectrometer for compositional analysis. Here we report on its design, implementation and show first results of this powerful add-on. Our design considerations were based on the results of detailed ion collision cascade simulations that focus on the physically achievable resolution for various detection limits. Different secondary ion extraction geometries and spectrometer types are considered and compared with respect to the demands and limitations of the microscope. As a result the development and evaluation of a secondary ion extraction optics and time-of-flight spectrometer that allows the parallel measurement of all secondary ion masses is reported. First experimental results demonstrate an excellent mass resolution as well as high-resolution secondary ion imaging capabilities with sub-8 nm lateral resolution. The combination of high resolution secondary electron images and mass-separated sputtered ion distributions have a high potential to answer open questions in microbiology, cell biology, earth sciences and materials research.

Co-Registered In Situ Secondary Electron and Mass Spectral Imaging on the Helium Ion Microscope Demonstrated Using Lithium Titanate and Magnesium Oxide Nanoparticles.

The development of a high resolution elemental imaging platform combining coregistered secondary ion mass spectrometry and high resolution secondary electron imaging is reported. The basic instrument setup and operation are discussed and in situ image correlation is demonstrated on a lithium titanate and magnesium oxide nanoparticle mixture. The instrument uses both helium and neon ion beams generated by a gas field ion source to irradiate the sample. Both secondary electrons and secondary ions may be detected. Secondary ion mass spectrometry (SIMS) is performed using an in-house developed double focusing magnetic sector spectrometer with parallel detection. Spatial resolutions of 10 nm have been obtained in SIMS mode. Both the secondary electron and SIMS image data are very surface sensitive and have approximately the same information depth. While the spatial resolutions are approximately a factor of 10 different, switching between the different images modes may be done in situ and extremely rapidly, allowing for simple imaging of the same region of interest and excellent coregistration of data sets. The ability to correlate mass spectral images on the 10 nm scale with secondary electron images on the nanometer scale in situ has the potential to provide a step change in our understanding of nanoscale phenomena in fields from materials science to life science.

Investigation of fatigue crack incubation and growth in cast MAR-M247 subjected to low cycle fatigue at room temperature

MC carbide particles (with Hafnium and/or Tantalum as constituent metallic element, M) were observed to crack extensively in a cast polycrystalline nickel-base superalloy, MAR-M247, when subjected to low-cycle fatigue loading at room temperature. High resolution secondary electron images taken on the surface of a double edge notch test specimen revealed that approximately half the carbide particles cracked in the highly-strained notch section of the specimen. These images further illustrated that the average surface area of cracked particles was approximately three times that of the uncracked particles. Additional analysis illustrated that the cracks within a large number of particles aligned nearly perpendicular to the loading direction. However, high aspect ratio particles (with aspect ratio $${>}3$$ ) were prone to incubate cracks aligned along its major axis, independent of the loading direction. Additionally, forward-scattered imaging often showed a high density of slip bands interaction with most of the particles which cracked. The life limiting crack growth in MAR-M247 was observed to be crystallographic in nature, as the crack grew along slip bands as measured by high-resolution electron backscatter diffraction, even after spanning many grains. Statistically representative microstructure models of MAR-M247 were generated and used in the crystal plasticity finite element simulations. As expected, there was a significant variation in the computed stress state among constituent carbide particles. The stress state of the carbide particles was found to be heavily influenced by the stress in surrounding grains and the orientation of the major axis of the particles with respect to applied load direction. For particles that intersect the free-surface, stress was found to be highly concentrated at the free surface and a positive correlation between the magnitude of free-surface area and the maximum principal stress was found. Additionally, high stress concentrations were observed in regions where carbide particles intersect grain boundaries.

High Energy BSE/SE/STEM Imaging of 8 um Thick Semiconductor Interconnects

High energy (100-400 keV) scanning electron microscope (SEM) imaging of samples can be performed in the transmission electron microscope (TEM)/scanning TEM (STEM) when the system has secondary electron (SE) and/or backscattered (BSE) electron detector(s) installed. Previously, very high resolution SE imaging has been demonstrated at 200 keV in a STEM with a spherical aberration corrected condenser lens [1]. Additionally, imaging of semiconductor Cu interconnects in SiO2 dielectric located at distances > 3um below the surface has been accomplished using incident beam energies of 100-400 keV and a backscattered electron (BSE) detector [2]. In the high energy BSE analysis case, multilevel Cu interconnects are ideal structures to study because: 1) there is a large BSE yield difference between Cu and SiO2 so Cu can be easily detected and 2) Cu lines exist at multiple levels below the surface so multiple structures can be seen from different depths in one BSE image. In this paper, a semiconductor chip with 9 interconnect levels spanning 8 um in height was studied using SE, BSE and STEM imaging at incident beam energies of 300-400 keV in order to understand image content and quality versus the depth of the structure below the surface.

A surface science study of the initial stages of hydrogen corrosion on uranium metal and the role played by grain microstructure

Controlled formation of UH3 on uranium metal surfaces was induced by a precisely limited uptake of hydrogen at 250°C and 320°C with 500mbarH2 pressure. The hydride (UH3) growth sites on the sample surfaces were studied using focused ion beam (FIB) milling with high resolution secondary electron imaging. Electron backscatter diffraction (EBSD) analysis was also used to examine grain orientation of the metal in the region of the corrosion sites.Results of the analysis indicated that the location of hydride sites was predominantly found to be associated with grain boundaries at the metal surface. In addition, FIB cross-sectioning of multiple hydride sites indicates the observed morphology of very small hydride growths to be consistent with nucleation at, and not beneath, the oxide–metal interface.

High-resolution SEM observation at the atomic level using a dedicated STEM with aberration correction

We have developed a high-resolution secondary electron (SE) imaging technique using a dedicated scanning transmission electron microscope (STEM) instrument with a high efficiency SE detector (Hitachi HD-2700) in order to investigate the structural characterization at the atomic level without thin film processing. The HD-2700 is equipped with a CEOS GmbH spherical aberration (Cs) corrector and can routinely achieve 1Å spatial resolution in SE and STEM imaging. We applied this technique for high accuracy critical dimension measurements of semiconductor materials using aberration corrected SE imaging. We have observed the cross sectional SE image of a semiconductor transistor along the Si[011] zone axis. The specimen thickness was 1μm and an accelerating voltage of 200 kV was used to observe the semiconductor structures, such as a metal gate. Under these conditions the high-k gate insulating layer, source, and drain structures can be clearly seen. The higher magnification SE images show that it is possible to resolve the Si111 plane, which has a spacing of 0.314 nm, and these images can be used for magnification calibration. These results indicate the ability to perform high precision measurements of crystal lattices for the structural characterization of semiconductor materials.

Microstructure Change of B2 Precipitates in an Fe-Al-Ni Alloy due to Two-Step Heat-Treatment

We have been studying the microstructure change of B2 cubic precipitates into an A2+B2 complex structure in Fe-Al-Ni alloy. In this study, we carried out detailed observation using focused ion beam (FIB) and scanning transmission electron microscopy (STEM). First, Fe-14.3at%Al-10.3at%Ni solid solution was prepared. Secondly, the specimens were heated at 1173 K, at which they formed B2 cubic precipitates (ordered bcc) dispersed in an A2 matrix (disordered bcc). After that, the B2/A2 two-phase specimen was annealed at 973 K. Then we fabricated STEM specimens using FIB, followed by high-resolution secondary electron imaging. We repeated this slice-and-observation procedure to determine the detailed microstructure of this heat-treated alloy. At the early stage of the 973 K annealing, the A2 phase appeared in the original B2 precipitates and showed a spongelike structure, whereas small nanometer-order B2 particles appeared in the A2 matrix. The A2/B2 interface at this stage showed no anisotropic morphology. Therefore, the main driving force of this process may not be strain energy, but chemical and interface energies. Further annealing at 973 K decreased the number of small B2 particles in the A2 matrix, and these particles dissolved into the matrix eventually. The annealing also changed the A2/B2 spongelike structure, which was observed in the original B2 precipitates, into simple structures such as the A2 core and B2 crust. Then the B2 phase showed ordinal coarsening behavior. When B2 precipitates, which had hollow cubic morphology, were observed to be very close to each other, the face-centered area of the B2 crust tended to dissolve and only large B2 precipitates remained.

Microanalysis Techniques for the Investigation of Interphases Formed between Thermoset and Thermoplastic Polymers: Scanning Electron Microscopy and Energy Dispersive X-Ray Analysis

Prepreg resin systems are typically of complex composition and require very specific manufacturing conditions. These characteristics restrict the use of some commonly used micro analysis techniques. This paper investigates the use of low acceleration voltage scanning electron microscopy and energy dispersive x-ray analysis for the characterization of diffused polymer interfaces. It is shown that, by operating at the dynamic charge balance, high resolution secondary electron images of polymer interfaces can be obtained and that conductive coating is not required. In addition, the effect of acceleration voltage on the interaction volume in EDX analysis is discussed using Monte Carlo simulation. X-ray intensity measurements in combination with afore mentioned Monte Carlo simulation is used to define practically obtainable spatial resolution limits. It is shown that by reducing the acceleration voltage below 5kV spatial resolution higher the 500nm can be obtained.

High-Resolution Secondary Electron Imaging of a FIB Prepared Si Sample with an Aberration Corrected Electron Microscope

Extended abstract of a paper presented at Microscopy and Microanalysis 2010 in Portland, Oregon, USA, August 1 – August 5, 2010.

Amphiboles between the sheets: observations of interesting morphologies by TEM and FESEM

We observed interesting amphibole morphologies by transmission electron microscopy (TEM) in air samples collected by the United States Environmental Protection Agency (USEPA) in Libby, Montana. Individual amphibole particles were identified and photographed on approximately 200 randomly selected air samples. Transfer of the TEM grids and relocation of these amphiboles in the field emission scanning electron microscope (FESEM) produced high-resolution secondary electron images that allowed a closer examination of these particles. Some of the amphiboles were perfect euhedral single crystals (several micrometers long and 200–500 nanometers wide) showing the classic {110} forms, while other single crystals appeared to be attached to sheet silicates and yet others appeared to have “wings” or “fins,” often less than one micrometer in width, attached to their edges. Energy Dispersive X-Ray Spectroscopy (EDS) and selected area electron diffraction (SAED) from the TEM confirmed that these particles were intergrown amphiboles and sheet silicates. Soil samples collected by the USEPA in Libby, Montana were also analyzed. FESEM investigation revealed that the soil samples contained amphiboles with the same morphologies as observed in the air samples, and were also intergrown with sheet silicates. Thus, these amphiboles occur as single crystals between the layers of vermiculite and hydrobiotite.

Contrast of Highly Dispersed Metal Nanoparticles in High-resolution Secondary Electron and Backscattered Electron Images of Supported Metal Catalysts.

Platinum nanoparticles finely dispersed in activated carbon powders have been observed with high contrast in secondary electron (SE) and backscattered electron (BE) images. The factors that contribute to the visibility of small metal particles in high-resolution BE and SE images are discussed. Monte Carlo simulations provided insight into the scattering of electrons by small, heavy-element particles located on, or embedded in, light-element supports. The visibility of Pt nanoparticles in BE images depends on many factors including the size and the location of the Pt particles, the energy of the incident electrons, and the size of the electron probe. The SE signals generated by backscattered electrons may not significantly contribute to the visibility of small Pt particles in high-resolution SE images of carbon supported Pt catalysts. Only those small Pt particles that are located on or very close to the surface of the carbon support can be revealed in high-resolution SE images.

Field Emission SEM with a Newly Developed FEGUN and Conical Strongly Excited Objective Lens

Abstract A newly developed cold FE-GUN which enables to us to obtain large probe current and low emission noise, and conical strongly excited objective lens has been installed on the JSM-6700F Scanning Electron Microscope (SEM). In the range of accelerating voltages from 0.5 to 15kV, this instrument has got much better resolution as compared with in-lens type SEM (Ohyama et al 1986)(Fig. 1). We can obtain high-resolution secondary electron images with large samples (ex. 150mm ϕ×10mmH). Our old type objective lens (Kazumori et al 1994) has the limitation of working distance (WD), but the new lens enables us to work at very short WD at accelerating voltage of 15kV. As a result the spherical (Cs) and chromatic (Cc) aberration constants are 1.9mm and 1.7mm respectively at a WD of 3mm. In order to get large probe current, we increased emission current and got near the distance between the t ip of emi tter and the pr inciple plane of condenser lens.

Contrast of Highly Dispersed Metal Nanoparticles in High-resolution Secondary Electron and Backscattered Electron Images of Supported Metal Catalysts

Abstract Platinum nanoparticles finely dispersed in activated carbon powders have been observed with high contrast in secondary electron (SE) and backscattered electron (BE) images. The factors that contribute to the visibility of small metal particles in high-resolution BE and SE images are discussed. Monte Carlo simulations provided insight into the scattering of electrons by small, heavy-element particles located on, or embedded in, light-element supports. The visibility of Pt nanoparticles in BE images depends on many factors including the size and the location of the Pt particles, the energy of the incident electrons, and the size of the electron probe. The SE signals generated by backscattered electrons may not significantly contribute to the visibility of small Pt particles in high-resolution SE images of carbon supported Pt catalysts. Only those small Pt particles that are located on or very close to the surface of the carbon support can be revealed in high-resolution SE images.

Contrast of Highly Dispersed Metal Nanoparticles in High-resolution Secondary Electron and Backscattered Electron Images of Supported Metal Catalysts

AbstractPlatinum nanoparticles finely dispersed in activated carbon powders have been observed with high contrast in secondary electron (SE) and backscattered electron (BE) images. The factors that contribute to the visibility of small metal particles in high-resolution BE and SE images are discussed. Monte Carlo simulations provided insight into the scattering of electrons by small, heavy-element particles located on, or embedded in, light-element supports. The visibility of Pt nanoparticles in BE images depends on many factors including the size and the location of the Pt particles, the energy of the incident electrons, and the size of the electron probe. The SE signals generated by backscattered electrons may not significantly contribute to the visibility of small Pt particles in high-resolution SE images of carbon supported Pt catalysts. Only those small Pt particles that are located on or very close to the surface of the carbon support can be revealed in high-resolution SE images.

Image Formation With Upper and Lower Secondary Electron Detectors in the Low Voltage Field-Emission SEM

Abstract High-resolution secondary electron (SE) imaging was first demonstrated at 100 kV in the STEM a decade ago. High-resolution SE imaging is now routinely obtainable in field-emission SEMs. Although nanometer-scale surface features can be examined at low incident beam voltages we still do not fully understand the factors that affect the contrast of low voltage SE images. At high incident beam voltages, SE1 (SEs generated by the incident probe) and SE2 (SEs generated by backscattered electrons at the sample surface) can be spatially separated. SE1 carries high-resolution detail while SE2 contributes to background. At low incident beam voltages, however, the interaction volume of the incident electrons shrinks rapidly with decreasing incident beam voltage. Thus, both the SE1 and SE2 signals carry high-resolution information. At low incident beam voltages, SE3 (SEs generated by backscattered electrons impinging on the sample chamber, pole pieces and etc.) also carries high-resolution detail and contributes significantly to the collected signal, especially for high atomic number materials and at short working distances.

Development Of a Semi-In-Lens Digital FE-SEM

Observation of new materials such as semiconductor, polymer and compounds, with a Field Emission Scanning Microscope (FE-SEM) has been increasingly important in recent years. As the integration scale of semiconductor device as well as nanometer fabrication scale of the new materials are increasing, a high resolution FE-SEM became indispensable for research and development.Since these new materials are often non-conducting and charging, metal coating, minimum electron dose technique or rapid scanning must be used to observe the surface morphology. Furthermore, all the operational sequences have been required to be drastically simplified.We have developed a semi-in-lens fully digital FE-SEM which can obtain high resolution secondary electron images (SEI) with minimum influence of charging at the specimen. An outline of this new FE-SEM is shown in Fig. 1. The electron optical system (EOS) is based on the conventional FESEM (H. Kazumori et al, 1994) with a cold field emission gun (C-FEG), giving a guaranteed resolution of 2.5 nm at an accelerating voltage of lkV. It is possible to select ten scan speeds from 0.28s to 121s in the vertical direction with 1280 × 1024 pixels and average and integrate the SEI signals at the ambient scan speed.

Explanation of the high resolution backscattered electron image in the scanning electron microscope by the twin-population theory historical review

Several different “twin-population” theories have been proposed to explain the high resolution backscattered electron (BSE) image in the scanning electron microscope (SEM). In all of these, the BSE are divided into local BSE that give an image having a resolution that is significantly better than the electron penetration depth R and diffused BSE that give a less sharp image. Other theories attach more importance to the diffused BSE.The BSE image was originally proposed by von Ardenne in 1940 with the specimen at right angles to the incident electron beam (Fig. 1). This has been summarised by McMullan. Von Ardenne predicted high resolution secondary electron (SE) imaging by local SE (now called SE-I) and pointed out the advantages of low voltage operation, but failed to realise that there was a high resolution BSE signal also. McMullan distinguished between the BSE image as defined above (Fig. 2(b)) where the principal contrast mechanism is compositional and the reflected electron image in which the sample is tilted by typically 45° and forward-scattered BSE are collected (Fig. 2(a)).

Si(100) Surface Corrosion by NH4F Studied Using High Spatial Resolution Secondary Electron Imaging in a UHV - STEM

Si(100) Surface Corrosion by NH4F Studied Using High Spatial Resolution Secondary Electron Imaging in a UHV - STEM

Applications and techniques of thickness measurement for sections and thin films

The thickness of a specimen to be examined in the electron microscope is a significant parameter for many applications. However, due to the lack of ideal methodology for absolute thickness measurement, most quantitative analyses are performed using approaches which are more convenient. For example, theoretical factors, indirect relative thickness or ratio methods are used to eliminate the necessity to obtain the absolute thickness. These alternatives are theoretically valid. However, experimental verification using standards with known thicknesses are still imperative. Since absolute thickness remains indispensable, continuous development and improvement of techniques for thickness measurement are still much needed. The purpose of this study is to critically survey the importance of absolute thickness measurement and assess the available methods for thickness determination.Knowledge of specimen thickness can be applied for both qualitative and quantitative purposes. Specimen thickness can be used to study imaging properties in different electron microscopy modes. Monitoring of film thickness is, for example, essential when coating specimens for high resolution secondary electron imaging, preparing multilayer specimens in material sciences or preparing ultrathin support films.