Theory experiment comparison of the electron backscattering factor from solids at low electron energy (250-5,000 eV)

The electron backscattering factor was measured from 24 different elements at low primary beam energy (250-5,000 eV). The results were compared with Monte Carlo simulations from a variety of freely available programs and an in-house developed program. The results suggest that a thin film of oxide can modify the backscattering factor at low primary energy. In addition, a number of problems have been identified with the freely available programs.     

The secondary electron emission yield for 24 solid elements excited by primary electrons in the range 250-5000 ev: a theory/experiment comparison

The secondary electron (SE) yield, delta, was measured from 24 different elements at low primary beam energy (250-5,000 eV). Surface contamination affects the intensity of delta but not its variation with primary electron energy. The experiments suggest that the mean free path of SEs varies across the d bands of transition metals in agreement with theory. Monte Carlo simulations suggest that surface plasmons may need to be included for improved agreement with experiment.     

Monte Carlo modeling of cathodoluminescence generation using electron energy loss curves

This work demonstrates the validity of approximating cathodoluminescence generation throughout the electron interaction volume by the total electron energy loss profile. The energy loss profiles in multilayer specimens were accurately calculated using the Monte Carlo simulation CASINO. Resolution of cathodoluminescence images can be estimated from the electron beam spot diameter, the electron penetration range, and the minority carrier diffusion length.     

A Monte Carlo study of the thickness determination of ultra-thin films

A Monte Carlo method is utilized to simulate the energy spectra for low-energy electron backscattered from different thin films deposited on different bulk substrates. A method of thickness determination for ultra-thin films is presented, which is obtained from the analysis of the basic characteristics of the energy spectra of backscattered electrons. This method is predicted to be particularly suitable for determining the thickness of ultra-thin films, and to be useful for both cases of light film on heavy substrate and heavy film on light substrate. The thickness resolution is estimated to be as high as subnanometer scale, provided the resolution of the electron energy spectrometer used to be high enough.     

A computational test for the removal of plural scattering from angle-resolved energy loss spectra

A recently developed method based on matrix analysis for the removal of plural scattering from angle-resolved energy loss spectra is tested. A single loss function, Lorentzian in the energy and Gaussian in the angular variable is assumed as input for the test. Multiple scattering probabilities are simulated by summing up n-fold self-convolutions of the input function according to the Poisson distribution for incoherent n-fold scattering. The simulated profile serves as input for the retrieval algorithm, the result of which is compared with the original single-loss probability. It is concluded that the method is feasible, but not likely to be suited for routine investigations.     

A computer program to illustrate macro topography on electron backscattering

This paper describes a computer program which enables the trajectories of an electron to be followed inside a solid sample and its pathway if it is successful to exit the sample's surface, and to be considered a backscattered electron (BSE). The simulation aids to understand the backscattering process and the effect of the sample that has other than an ideal flat surface topography. Several types of surface for the sample are simulated: a flat surface which can be tilted, a rough surface represented by a sine wave, and a circular surface to represent filamentous structures. The target material is entered as its empirical formula so that its mean atomic number may be calculated. It is covered with a layer of conductive coating, and it may be structured with vertical or horizontal layers of a second-phase material. Where appropriate, the position of the electron beam impacting on the target may be moved in relation to the underlying structures. The passage of the electron through the substrate is displayed on the computer monitor in high-resolution graphics. Information about those electrons which return to the sample surface and are backscattered is stored for later analysis. The paths of these BSEs are replotted to show the volume of the sample from which the image-forming data are derived. Further graphic output provides histograms of the energy distribution and of the BSEs exit direction.     

Measuring the angular dependent energy distribution of backscattered electrons at variable geometry

An aluminium semisphere system with 120 points of entry and eight detection areas, assembled on a meridian covering 0.0026 steradian each, was put over a solid bulk sample (e.g., aluminium), which was mounted in the eucentric point so that the incident electron beam could be varied by a polar rotation of the sphere in steps of 11.25 degrees. The complete angular distribution of the backscattered electrons became available by a rotation in steps of 11.25 degrees azimuthally. For this particular setup, the signals from the detection areas as well as the signal from the rest of the semisphere were amplified by operational amplifiers (Burr-Brown OPA128LM). However the signal of the semisphere was not available at that time. Specimen current measurements made the total amount of electrons accessible, providing a possibility for normalization of the results and comparison with total backscattering coefficients. By use of counter voltage variable up to 10 kV inside the detection assembly, it was possible to measure an energy resolution of the backscattered electrons for each detection area at the same time. Details of the construction and calibration procedures, possible errors, and sources of systematic deviations as well as first test results are discussed.     

Mass loss rate in collodion is greatly reduced at liquid helium temperature

The mass thickness of collodion films has been monitored at several temperatures, under conditions typical of electron microscopy, through the use of an electron energy loss spectrometer. Compared to room temperature, only a five-fold reduction in the rate of mass loss was observed through the use of a commercial liquid nitrogen cooled stage; in contrast, the rate of mass loss was reduced more than one hundred fold when these films were held at liquid helium temperature.     

Thickness measurements with electron energy loss spectroscopy

Measurements of thickness using electron energy loss spectroscopy (EELS) are revised. Absolute thickness values can be quickly and accurately determined with the Kramers-Kronig sum method. The EELS data analysis is even much easier with the log-ratio method, however, absolute calibration of this method requires knowledge of the mean free path of inelastic electron scattering lambda. The latter has been measured here in a wide range of solids and a scaling law lambda approximately rho(-0.3) versus mass density rho has been revealed. EELS measurements critically depend on the excitation and collection angles. This dependence has been studied experimentally and theoretically and an efficient model has been formulated.     

A database on electron-solid interactions

Monte Carlo modeling of electron-solid interactions requires a detailed and accurate supply of experimental data on which to base its physics and against which to test its predictions. To meet this need, a collection of data—comprising measurements of secondary and backscattered electron yields, electron stopping powers, and x-ray ionization cross sections, as a function of energy—has been assembled from published sources. The quality and quantity of the compilation varies widely, with little or no data being available for the majority of elements in the periodic table, while results for complex materials of current technologic interest are also almost nonexistent. To meet the needs of Monte Carlo simulation in areas such as dimensional metrology or microanalysis, a program of systematic measurements is required.     

Accurate composition determination of Be-Ti alloys by electron energy loss spectroscopy

Accurate quantification of the Be content in Be-Ti alloys on a submicrometre scale can be accomplished with electron energy loss spectroscopy in an analytical electron microscope. The three major steps required to ensure the accuracy of the numerical results are analysed. The first step is the choice of the specimen thickness which should be such that the influence of the specimen surface effects can be ignored yet thin enough so that deconvolution of the spectra is unnecessary. The second step is the background extrapolation under the ionization edge of interest. In this study, a direct least-squares fit with a progressive weighting is used to avoid the drawbacks of the conventional linear least-squares fit. The third step is the calibration of the partial ionization cross-section ratio with the use of a standard specimen. Without this calibration step, the error in the final microanalysis result could be excessive, as demonstrated. With all these precautions taken into consideration, we are able to show that the intermetallic phase TiBe₁₂ exhibits a great deviation from its nominal stoichiometry.     

A monte carlo study of the position of phase boundaries in backscattered electron images

A Monte Carlo simulation of the contrast variation across phase boundaries in backscattered electron images of multiple phase composition systems has been used to develop a model for prediction of the position of the interface based solely on a knowledge of the signal intensity levels on either side of the interface. A wide range of average atomic numbers of the phases on either side of the boundary have been investigated at incident beam energies of 5, 10, 15, and 25 kV and a least-squares minimisation procedure used to optimise the parameters of a generalised model.     

Monte Carlo simulation of secondary electron and backscattered electron images in scanning electron microscopy for specimen with complex geometric structure

A new Monte Carlo technique for the simulation of secondary electron (SE) and backscattered electron (BSE) of scanning electron microscopy (SEM) images for an inhomogeneous specimen with a complex geometric structure has been developed. The simulation is based on structure construction modeling with simple geometric structures, as well as on the ray-tracing technique for correction of electron flight-step-length sampling when an electron trajectory crosses the interface of the inhomogeneous structures. This correction is important for the simulation of nanoscale structures of a size comparable with or even less than the electron scattering mean free paths. The physical model for electron transport in solids combines the use of the Mott cross section for electron elastic scattering and a dielectric function approach for electron inelastic scattering, and the cascade SE production is also included.     

Effects of the introduction of the discrete energy loss process into Monte Carlo simulation of electron scattering

First, the single scattering model is briefly described. Next, the hybrid model is explained, which takes into consideration a part of the discrete energy loss processes. The Vriens or the Gryzinski cross section is used for core electron ionizations, and the Moller cross section for free electron excitations. The model is applied to the calculations of the energy distribution of transmitted electrons through a thin film and the depth distribution of generated x-rays. From comparisons among the calculated results with and without energy straggling and experimental data, it is found that the Gryzinski cross section shows the best result.     

A simple backscattered electrons and specimen current detection device for the scanning electron microscope

A simple, low-investment device has been developed that allows the collection of backscattered electrons (BSEs) and specimen current (SC) signals for imaging purposes and current measurement. Originally, this system was designed for detection, measurement, and display of specimen current, with a video signal output whose level was modulated by this current. Eventually, a BSE detector was developed, using a graphite disk (about 8 cm in diameter) to collect the BSEs. The disk was mounted on a Philips SEM 5O5, attached and concentrically to the final lens aperture. This configuration gives a large solid angle of collection. The collected charge is further processed by the same electronics used in the aforementioned SC detection system. Electron channeling, topographic contrast with BSE, and material contrast with BSE and SC images can be obtained with reasonably good edge definition.     

Do SEII Electrons Really Degrade SEM Image Quality?

Generally, in scanning electron microscopy (SEM) imaging, it is desirable that a high‐resolution image be composed mainly of those secondary electrons (SEs) generated by the primary electron beam, denoted SE_I. However, in conventional SEM imaging, other, often unwanted, signal components consisting of backscattered electrons (BSEs), and their associated SEs, denoted SE_II, are present; these signal components contribute a random background signal that degrades contrast, and therefore signal‐to‐noise ratio and resolution. Ideally, the highest resolution SEM image would consist only of the SE_I component. In SEMs that use conventional pinhole lenses and their associated Everhart–Thornley detectors, the image is composed of several components, including SE_I, SE_II, and some BSE, depending on the geometry of the detector. Modern snorkel lens systems eliminate the BSEs, but not the SE_IIs. We present a microfabricated diaphragm for minimizing the unwanted SE_II signal components. We present evidence of improved imaging using a microlithographically generated pattern of Au, about 500 nm thick, that blocks most of the undesired signal components, leaving an image composed mostly of SE_Is. We refer to this structure as a “spatial backscatter diaphragm.” SCANNING 35:1‐6, 2013. © 2012 Wiley Periodicals, Inc.