Low-loss Electron Research Articles

Experimental and theoretical determination of low electron energy loss spectra of Ag and Ru

We show the experimental and calculated q -dependent low energy loss electron energy loss spectrum of Ru and Ag. The spectra were calculated within the time-dependent density-functional theory including local-field effects. For Ag, the momentum transfer was parallel to the (1 1 0) direction. For Ru the three main directions (0 1 0), (1 1 0) and (0 0 1) were investigated. The agreement between theory and experiment is very good for Ag and for momentum transfers parallel to the (0 0 1) direction of Ru. For momentum transfers parallel to the in-plane directions (1 1 0) and (0 1 0) the agreement for Ru is not satisfactory, which could be attributed to relativistic effects or to strong localization of the 4d states of Ru.

Time, energy, and spatially resolved TEM investigations of defects in InGaN

A novel sample preparation technique is reported to fabricate electron transparent samples from devices utilizing a FIB process with a successive wet etching step. The high quality of the obtained samples allows for band gap—and chemical composition measurements of In x Ga 1− x N quantum wells where electron beam induced damage can be controlled and shown to be negligible. The results reveal indium enrichment in nanoclusters and defects that cause fluctuations of the band gap energy and can be measured by low loss Electron Energy Spectroscopy with nm resolution. Comparing our time, energy, and spatially resolved measurements of band gap energies, chemical composition, and their related fluctuations with literature data, we find quantitative agreement if the band gap energy of InN is 1.5–2 eV.

Composition fluctuations in dilute nitride (Ga,In)(N,As)/GaAs heterostructures measured by low-loss electron energy-loss spectroscopy

We report on the investigation of composition fluctuations in epitaxially grown (Ga,In)(N,As) epilayers on GaAs(001) substrates by using electron energy-loss spectroscopy (EELS). The N and In concentrations are determined locally with a probe size of about 8nm from the low-loss EELS measurements. We demonstrate that the small amount of N incorporating in dilute nitride alloys can be measured quantitatively by the plasmon energy shift with respect to a GaAs reference, and that the In content is analyzed simultaneously from the In 4d transitions, which have been isolated from the overlapping Ga 3d transitions. Our spatially resolved EELS results are utilized to discuss the origin of the inherent composition fluctuations and their influences on the morphological instabilities during epitaxial growth.

Primary radicals production from water fragmentation by heavy ions

The importance of fragmentation of water molecules by heavy ions is discussed for several different physical environments, including tumor treatment, planetary sciences and fission reactors. The characteristics of the fragmentation yields by electrons, protons and photons are presented and compared with fragmentation yields of water molecules by C3+ and O5+ ions at energies corresponding to the Bragg peak. It is shown that for low energy electron loss and high energy electron capture, the water molecules essentially blow-out, releasing a much larger fraction of Oq+ ions as compared with electrons, protons and photons.

Decomposition in as-grown (Ga,In)(N,As) quantum wells

We report on the investigation of the local element distribution in as-grown (Ga,In)(N,As) quantum wells with high In and N contents by using low-loss electron energy-loss spectroscopy combined with dark-field transmission electron microscopy. The (Ga,In)(N,As) quantum wells were grown on GaAs(001) substrates at different growth temperatures by molecular-beam epitaxy. Lateral modulations on the nanometer scale were detected with reversal In and N distributions pointing to the existence of regions with a more favorable Ga–N and In–As bond configurations, respectively. These composition fluctuations are the driving force for the morphological instabilities at the interfaces. Lowering the growth temperature of the quantum well results in a more homogeneous element distribution of the quaternary compound. This result is discussed with regard to the influence of the epitaxial strain and cohesive bond energy on the alloy formation during epitaxial growth.

Analysis of extraterrestrial particles using monochromated electron energy-loss spectroscopy

A monochromated (scanning) transmission electron microscope was used to analyze individual sub-micron grains within interplanetary dust particles (IDP). Using low-loss and core-loss electron energy-loss spectroscopy, we analyzed fluid and gas inclusions within vesiculated alumosilicate grains. It is shown that nanometer-sized vesicles contain predominantly molecular oxygen (O 2) beside a small fraction of H 2O. Low-loss spectra reveal the Schumann–Runge continuum peaking at 8.6 eV and absorption bands reflecting vibrational excitation states of O 2 molecules between the first (12.1 eV) and second (16.1 eV) ionization energy. The presence of oxygen gas is supported by the corresponding oxygen K-edge fine structure. The valence state of Fe in iron-oxide within the IDP was also studied. Low-loss spectra provide qualitative information about the oxidation state of iron consistent with the Fe 2+/Fe 3+ ratio quantitatively derived from the Fe L 2,3 edge.

Ab initio calculations of plasmons and interband transitions in the low-loss electron energy-loss spectrum

In the low-loss region of the electron energy-loss (EEL) spectrum interband transitions and plasmon losses are observed and can provide information about composition, electronic structure and optical properties. Low-loss EEL spectra have been calculated using density-functional theory (DFT) for a large number of elements and some simple compounds. Good agreement with experimental spectra is observed, allowing interpretation of the features in the spectrum. The calculations of the plasmon energies are improved over free-electron calculations for most materials. The improvements that have become available in the past few years, both in energy spectrometers and in the calculation methods, should see increasing numbers of applications of low-loss EELS to measure electronic structure and bonding.

Top-down topography of deeply etched silicon in the scanning electron microscope

It is proposed to measure the cross sections of steep-sided etched lines and similar deep surface topography on partially completed silicon integrated circuit wafers using either the backscattered electron (BSE) or the low-loss electron (LLE) image in the scanning electron microscope (SEM). These images contain regions where the collected signal is zero because there is no direct line of sight between the landing point of the electron beam on the specimen and the BSE or LLE detector. It is proposed to use the boundary of such a region in the SEM image as a geometrical line to measure the surface topography. Or alternatively, a shadow can be seen in the distribution of either BSE or LLE with an image-forming detector system. The use of this shadow position on the detector to measure deep surface topography will be demonstrated.

Determination of the electron effective band mass in amorphous carbon from density-functional theory calculations

We generate a series of amorphous carbon materials of densities varying between 2.0 and $3.5\phantom{\rule{0.3em}{0ex}}\mathrm{g}∕{\mathrm{cm}}^{3}$. Their dielectric functions are calculated within the density-functional theory approach, and the low-loss electron energy-loss spectra are evaluated. The dependence of the $\ensuremath{\pi}+\ensuremath{\sigma}$ plasmon energy ${E}_{p}$ on the valence electron density $({n}_{e})$ is determined. We find that the free-electron model in which ${E}_{p}\ensuremath{\propto}{n}_{e}^{0.5}$ is valid. The coefficient of proportionality leads to a ratio of the electron-effective band mass ${m}^{*}$ to the free-electron mass $m$ of ${m}^{*}∕m=0.87$, which is the value obtained using measurements based on the grazing-angle x-ray reflectivity and electron energy-loss spectroscopy [see A. C. Ferrari et al., Phys. Rev. B 62, 11089 (2000)]. This result supports the reliability of amorphous carbon mass densities obtained from EELS data when an electron-effective mass of $0.87\phantom{\rule{0.3em}{0ex}}m$ is used.

Soot characterisation and diesel engine wear

AbstractThe hardness of various types of soot produced by heavy‐ and light‐duty diesel engines of European, Japanese, and North American designs was measured by low‐loss electron energy‐loss spectroscopy (EELS). No clear general trend can be established that shows heavy‐duty diesel engine soot is necessarily harder than light‐duty diesel engine soot. The variation in hardness among individual soot particles produced by the same diesel engine can be as large as differences between the hardest soot particles produced by heavy‐duty diesel engines and the softest soot particles produced by light‐duty diesel engines. There are heavy‐duty diesel engines that can produce soot that is softer than that produced by some light‐duty diesel engines and vice versa. Nevertheless, the hardness of all types of soot studied is close to the range of hardness of metal engine parts. Thus, the results indicate that soot is hard enough to abrade some metal engine parts.

Experimental investigation of phase contrast formed by inelastically scattered electrons

Phase contrast formed by inelastically scattered electrons in a crystal has been investigated using spatially resolved EELS, which enables simultaneous observation of lattice fringes formed by electrons of various energy losses. Lattice fringes produced by low-loss electrons overlap on an elastic TEM image like Fourier images. This means that the exit wave is preserved in low-loss scattering. Similar Fourier images occur for electrons suffering core-losses in the range 50–400 eV, which indicates delocalization and spatial coherence in those core-loss scattering events. The spatial coherence of inelastically scattered electrons is estimated from the focus dependence of energy-filtered lattice fringe contrast. Spatial coherence widths shorten with increasing energy-loss, and their energy-loss dependence is similar to diffraction errors derived from the characteristic angle for inelastic scattering.

Calculated and experimental low-loss electron energy loss spectra of dislocationsin diamond and GaN

First-principles calculations of electron energy loss (EEL) spectra for bulk GaNand diamond are compared with experimental spectra acquired with a scanningtunnelling electron microscope offering ultra-high-energy resolution in low-lossenergy spectroscopy. The theoretical bulk low-loss EEL spectra, in theEg to10 eV range, are in good agreement with experimental data. Spatially resolvedspectra from dislocated regions in both materials are distinct from bulk spectra.The main effects are, however, confined to energy losses lying above the bandedge. The calculated spectra for low-energy dislocations in diamond are consistentwith the experimental observations, but difficulties remain in understanding thespectra of threading dislocations in GaN.

Imaging of Shallow Surface Topography by the Low-Loss Electron (LLE) Method in the Scanning Electron Microscope

Abstract The low-loss electron (LLE) method in the scanning electron microscope (SEM) was proposed by Dennis McMullan in 1953: “…the beam from the specimen could be restricted to the electrons which have lost only small amounts of energy and which have therefore travelled only short distances through the specimen.” Subsequent studies showed that the LLE method gives different image contrasts from the more familiar secondary electron (SE) method: (i) it is less affected by specimen charging; (ii) has a shallower information depth for a given beam energy; (iii) shows less serious penetration effects at sharp edges; (iv) shows stronger channeling contrast; and (v) Is better for showing shallow surface topography.

Discussion of Ways to Energy-Filter the Electron Backscattering Pattern (EBSP) in the Scanning Electron Microscope (SEM).

An electron backscattering pattern (EBSP) is formed by the electrons that are scattered with the highest energies from a crystalline target that is illuminated with an electron beam (EB) of kiloelectron-volt (keV) energy. In appearance it closely resembles the electron channeling pattern (ECP) that is formed in the scanning electron microscope (SEM) when the incident EB is rocked about a point. According to one possible explanation, it is assumed that in either case the pattern is formed by the electrons that leave the specimen with a minimum loss of energy in the reflected electron peak following a single wide-angle Rutherford-type scattering event (typically through about a right angle) close to the surface. Now if ECP and EBSP are reciprocal, they can be related by reversing the direction of the (zero-loss) electrons in the specimen. It can then be argued that these patterns must be formed because the probability of such a Rutherford-type wide-angle scattering event is modulated by both the incoming (ECP) and outgoing (EBSP) channeling conditions [1-6]. The slower scattered electrons undergo scattering, channeling and diffraction events that are in addition to this basic process. The related reflection high-energy electron diffraction (RHEED) pattern is formed with grazing incident and exit directions with electrons that are diffracted by Bragg planes parallel to the surface. Generally the EBSP is formed over a limited solid angle by placing a detecting screen in line-ofsight from the specimen or (in principle) with a retarding-field energy filter [6]. An alternative approach that is suggested here is to magnetically filter the scattered electrons by mounting the sample between the polepieces of a magnetic immersion lens, with the limitations: (a) The sample must be totally non-magnetic. (b) It will be irradiated more heavily than with the standard EBSP. Fig. 1 shows a solid sample mounted between the polepieces of a magnetic immersion lens. The magnetic field is adjusted so as to focus the incident EB onto the specimen. This same magnetic field also deflects the scattered electrons to follow spiral paths that periodically return to the lens axis. The fastest scattered electrons (reflected electron peak) can reach a limiting surface at the end of the first half-turn that cannot be reached by the slower scattered electrons. The slower scattered electrons can, however, go beyond this limiting surface on subsequent turns of the spiral path if not prevented from doing so [7]. It has been demonstrated that a detector that is placed at a short distance inside this limiting surface will collect only the fastest scattered electrons and can be used to give a magnetically filtered low-loss electron (LLE) image in the SEM [8-10].

Background Removal and Data Analysis for Low-Loss Transmission Electron Energy-Loss Spectroscopy

Background Removal and Data Analysis for Low-Loss Transmission Electron Energy-Loss Spectroscopy

Background subtraction for low-loss transmission electron energy-loss spectroscopy

We present a technique for removing the zero-loss background from electron energy-loss spectra at very low energies (down to ∼2 eV), generating results that are superior in a number of ways to the results of standard Fourier deconvolution techniques. Our technique is based on a separately measured background spectrum which is spline-interpolated and matched to the zero-loss peak in the low-loss spectrum using curve-fit techniques. The data points are weighted with the use of a semi-empirical model of the random error in the data produced by a spectrometer. We demonstrate in tests on real-world data that this model accounts for the random error within the energy range of interest. We discuss practical details of implementation and present detailed comparisons of the results of various algorithms on a piece of test data obtained from a carbon nanotube sample. Compared to the standard techniques, our algorithm tends to be more consistent, less dependent on arbitrary parameters, and better able to quantify spectral features with small signal-to-noise ratios, particularly those at very low energies.

Application of the low-loss scanning electron microscope image to integrated circuit technology part II--chemically-mechanically planarized samples.

Chemical-mechanical planarization (CMP) is a process that gives a flat surface on a silicon wafer by removing material from above a chosen level. This flat surface must then be reviewed (typically using a laser) and inspected for scratches and other topographic defects. This inspection has been done using both the atomic force microscope (AFM) and the scanning electron microscope (SEM), each of which has its own advantages and disadvantages. In this study, the low-loss electron (LLE) method in the SEM was applied to CMP samples at close to a right angle to the beam. The LLEs show shallower topographic defects more clearly than it is possible with the secondary electron (SE) imaging method. These images were then calibrated and compared with those obtained using the AFM, showing the value of both methods. It is believed that the next step is to examine such samples at a right angle to the beam in the SEM using the magnetically filtered LLE imaging method.

Four-dimensional dielectric property image obtained from electron spectroscopic imaging series.

We have demonstrated a new quantitative method to characterize two-dimensional distributions of energy-dependent dielectric function of materials from low loss electron spectroscopic image (ESI) series. Two problems associated with extracted image-spectrum from the low-loss image series, under-sampling and loss of energy resolution, were overcome by using fast Fourier transformation (FFT) interpolation and maximum entropy deconvolution method. In this study, Black Diamond/Si3N4/SiO2/Si-substrate dielectric layer designed for copper metallization was used as the sample. We show that the reconstructed (FFT interpolated and maximum entropy deconvoluted) image-spectrum obtained from ESI series images can be quantified with the same accuracy as conventional electron energy-loss spectroscopy spectra. Since the analysis of the dielectric function is sensitive to the local thickness of the specimen using Kramers-Kronig analysis, we also developed a new method to quantitatively determine the dielectric constant for low-k materials. We have determined the thickness of the Black Diamond using the extrapolated thickness method from the materials of known dielectric constants. Using Kramers-Kronig formula, the dielectric function map can be deduced from two-dimensional reconstructed single scattering spectra with providing the information of thickness. We proposed a four-dimensional data presentation for revealing the uniformity of the energy dependent property. The accuracy of our methods depends on the thickness determination and on the quality of the reconstructed spectra from the image series.

Application of the low-loss scanning electron microscope image to integrated circuit technology. Part 1--Applications to accurate dimension measurements.

Scanning electron microscopes (SEMs) are the most extensively used tools for dimensional metrology and defect inspection for integrated circuit technologies with 180 nm and smaller features. Currently, almost all SEMs are designed to collect as many secondary and backscattered electrons as possible. These signals are mainly secondary electrons (SE1, SE2, and SE3) detected with various detection schemes. To facilitate the electron collection, very strong electric and magnetic fields are applied not just in the path of the primary electron beam but to the emerging electrons as well. These new systems provide strong signals, thus better signal-to-noise ratio, and thus resulting in higher throughput than older ones. On the other hand, the use of secondary electrons means that measurement results are much more prone to the detrimental effects of electron beam interactions, sample charging, and sample contamination than measurements with higher-energy backscattered electrons. The use of backscattered electrons, especially low-loss electrons (LLE), can provide better surface sensitivity, edge accuracy, and repeatability, possibly at the expense of measurement speed. This two-part study investigates the benefits and drawbacks of low-loss electron imaging to edge characterization for dimensional metrology and enhancement of fine surface features done through filtration or separation of the generated LLE signal and the use of energy-dependent signals. Part 1 reviews and illustrates the potential for accurate dimensional measurements at low accelerating voltage by LLE, and Part 2 will concentrate on the enhancement of surface features in chemical-mechanically planarized specimens with the use of a novel LLE detector.

Fourier images feature of lattice fringes formed by low-loss electrons as observed using spatially-resolved EELS technique.

The lattice fringes of inelastically scattered electrons were investigated using spatially-resolved electron energy-loss spectroscopy (EELS) technique. The intensity profiles of lattice fringes formed by low-loss electrons were observed, and their position and contrast were elucidated with high spatial resolution. It was found that the conventional transmission electron microscope image includes low-loss images, which are the integrated image of through-focus images weighted by the intensity of the EEL spectrum. The modulation feature in the lattice fringes of inelastically scattered electrons is similar to the Fourier images. It means that the exit wave field of elastically scattered electrons is almost preserved in that of low-loss electrons.