Secondary Electron Spectra Research Articles

Microanalysis using secondary electrons in scanning electron microscopy.

A recent study of secondary electron (SE) spectra in an Auger spectrometer demonstrated unique features indicative of the chemical nature of the tested material. The scanning electron microscope (SEM) naturally generates SEs; therefore, in this paper, we combine the concept of using differential voltage contrast (DVC) with SE spectroscopy to identify the chemical nature of a material. It is demonstrated that this method reveals the uniqueness of electron energy distribution in the conduction band of a solid or, what is the same, the uniqueness of a build-up of the outer electron shell system, and avoids errors due to the changes in the angular distribution or yield of the SE in the SEM. A theory of this new microanalytical method is developed. The experimental limitations of the SEM for this type of study are examined as well.

Surface potential measurement with high spatial resolution using a scanning Auger electron microscope

The microscopic surface potential distributions were measured from the onset energies of secondary electron spectra using a scanning Auger electron microscope. The spatial resolution is several tens of nanometers and a sensitivity of the surface potential measurement is 0.05 eV. We demonstrated the calibration of the instrument for local surface potential analysis based on the onset energy measurement of the secondary electron spectrum. Several applications of this technique for Ni polycrystal grains of different orientations, and a potential profile along the p–n junction of a light-emitting diode were demonstrated.

Interface dosimetry: measurements and Monte Carlo simulations of low-energy photon beams

A comparison of measured and simulated dose perturbations at high- Z interfaces with Monte Carlo (MC) codes, EGS4, MCNP4B , and PENELOPE, having varied algorithms is presented. The measured dose perturbations strongly depend on the chamber design and are always lower than the MC data. The EGS4 data are closer to the ion chamber values. The other two codes, MCNP4B and PENELOPE, predict relatively higher magnitude. The simulated secondary electron spectra from high- Z interfaces are different but cannot explain the differences in magnitude. It is concluded that MC codes capable of handling low-energy transport and better boundary crossing algorithms are needed for interface effects.

A high-resolution add-on in-lens attachment for scanning electron microscopes.

A compact add-on objective lens for the scanning electron microscope (SEM) has been designed and tested. The lens is < 35 mm high and can be fitted on to the specimen stage as an easy-to-use attachment. Initial results show that it typically improves the spatial resolution of the SEM by a factor of three. The add-on unit is based upon a permanent magnet immersion lens design. Apart from the extra attachment to the specimen stage, the SEM with the add-on lens functions in the normal way. The in-lens unit can comfortably accommodate specimen heights up to 10 mm. The new add-on lens unit opens up the possibility of operating existing SEMs in the high-resolution in-lens mode. By using a deflector at the top of the add-on lens unit, it can also operate as a quantitative multichannel voltage contrast spectrometer, capable of recording the energy spectrum of the emitted secondary electrons. Initial experiments confirm that a significant amount of voltage contrast can be obtained.

Secondary electron emission yields from MgO deposited on carbon nanotubes

Enormously high secondary electron emission yields under electric field are observed from MgO deposited on carbon nanotubes. The yields reach a value as high as 15 000 and are strongly dependent upon the bias voltage applied to the sample. The creation of the electric field across the MgO film after bombardment of primary electrons is considered as one of key features, since positive charges are generated at the surface by departure of secondary electrons. Subsequent bombarding electrons produce other secondary electrons inside the MgO film, then the liberated secondaries are accelerated towards the surface under the strong field. Under this condition, the secondary electrons gain sufficient energy to create further electrons by impact ionization. The process continues until an equilibrium avalanche is established. To elucidate the earlier explanations, the kinetic energy spectra of secondary electrons are measured by an energy analyzer at various bias voltages in MgO/carbon nanotube samples. The analysis of spectral results with the energy band diagram gives us strong evidence for the suggested mechanism.

Search for the plasmon in condensed water

Creation of plasmons in water by energetic particles in biological tissue is important because this form of energy can migrate from the place of excitation and be transferred to nearby molecules like DNA. We have searched for evidence of plasmons in solid water, using electron-energy-loss and secondary electron spectroscopies excited by 70--1000-eV electrons. We find that the energy of the main excitation, often attributed to plasmons, depends on the energy of the exciting electron, unlike the expected plasmon behavior. The energy spectrum of secondary electrons shows the presence of autoionization, but no evidence of plasmon decay.

Surface potential measurements of electron-irradiated insulators using backscattered and secondary electron spectra from an electrostatic toroidal spectrometer adapted for scanning electron microscope applications

A technique for the accurate determination of the surface potential US and its evolution during irradiation, is proposed. The technique is based on detecting both backscattered (BSE) and secondary electrons (SE) in a scanning electron microscope (SEM). The (BSE+SE) spectra are measured using a compact, highly sensitive electrostatic toroidal spectrometer (ETS), specially adapted for SEM applications. The use of an ETS analyzer set in a SEM for deducing the surface potential from (SE+BSE) spectra of electron irradiated insulators is introduced here. The surface potential is determined, either from the measured maximum energy of the secondary electron peak, or from its beginning. Various dielectric materials such as MgO, Al2O3, Y2O3, mica (potassium silicate aluminum), and Teflon were studied by this technique. Experimental investigations of the beam energy and current effects on the surface potential of bare insulators are reported. The change, due to this surface potential, in some physical quantities such as the amount of BSEs and the most probable energy of their spectral distribution is also studied. The results concerning coated and grounded insulators highlight the influence of the internal electric field on the BSEs energy distribution. An account of the various benefits of using the toroidal spectrometer in the surface potential measurement is also given.

The Energy Spectra of Secondary Electrons

Abstract It has been estimated that more than 90% of all scanning electron microscope (SEM) images ever published have been obtained using secondary electrons (SE) which are defined as those electrons emitted with energies between 0 and 50 eV. The properties of these secondary electrons are therefore of considerable interest and importance. However, although secondary electrons have been intensively studied since their discovery by Starke in 1901, the majority of the work has been aimed at determining the SE yield coefficient and its variation with energy for elements and compounds. The energy spectrum of secondary electrons has received far less attention although it is evident that the form of the spectrum must have an effect on the image contrast observed in the SEM because SE detectors are energy selective devices. The few studies that have been made have mostly concentrated on spectra obtained from clean samples observed under ultra-high vacuum conditions.

The Energy Spectra of Secondary Electrons

Abstract It has been estimated that more than 90% of all scanning electron microscope (SEM) images ever published have been obtained using secondary electrons (SE) which are defined as being those electrons emitted with energies between 0 and 50eV. The properties of these secondary electron are therefore of considerable interest and importance. However, although secondary electrons have been intensively studied since their discovery by Starke in 1901 the majority of the work has been aimed at determining the SE yield coefficient and its variation with energy for elements and compounds. The energy spectrum of secondary electrons has received far less attention although it is evident that the form of the spectrum must have an effect on the image contrast observed in the SEM because SE detectors are energy selective devices. The few studies that have been made have mostly concentrated on spectra obtained from clean samples observed under ultra-high vacuum conditions. This is understandable, because it is certain that the presence of a surface layer of contamination will change the SE spectrum to some degree or other, but it is unfortunate because all specimens in real SEMs are dirty and it is information about this situation that is required.

Determination of secondary electron spectra from insulators

A new technique for the determination of secondary electron (SE) spectra of insulators in a scanning electron microscope environment is presented. It is based on a capacitatively coupled charge measurement by subjecting the insulating film to a controlled pulsed electron beam. With the use of a planar grid analyzer configuration, an algorithm is used to estimate the SE spectrum based on normalized values of the S-curve obtained. Secondary electron spectra from several insulating materials employed in integrated circuit manufacturing, including silicon nitride (Si3N4), AZ 1350J photoresist, and pyralin polyimide, have been measured.

Monte Carlo simulation of electron emission from solids

Electron emission and spectroscopy has been simulated by a new Monte Carlo program which has been adapted especially to low energy electron scattering. The underlying physic model in based on elastic Mott cross sections and inelastic losses with full dispersion Δ E=hω( q ) . Charge carrier multiplication by secondary electron creation and subsequent cascading processes have been included. Surface effects like surface plasmons and the quantum mechanical surface transmittivity have also been taken into account. Results are obtained for the materials Be, C, Al, Si, Ag, Au, and SiO 2 . They include energy spectra of secondary and characteristic electrons. We find that attenuation lengths and related escape depths approach the inelastic mean free path λ in only in higher electron energy regions; below 100 eV they drop down to roughly 20% of λ in . The results should find application in spectroscopic microscopy by means of low energy electrons in the sub-keV range.

Comparison and Assessment of Electron Cross Sections for Monte Carlo Track Structure Codes

The purpose of this study was to make an intercomparison and assessment of cross sections for electrons in water used in electron track structure codes. This study is intended to shed light on the extent to which the differences between the input data and physical and chemical assumptions influence the outcome in biophysical modeling of radiation effects. Ionization cross sections and spectra of secondary electrons were calculated by various theories. The analyses were carried out for water vapor cross sections, as these are more abundant and readily available. All suitable published experimental total ionization cross sections were fitted by an appropriate function and used for generation of electron tracks. Three sets of compiled data were used for comparison of total excitation cross sections and mean excitation energy. The tracks generated by a Monte Carlo track code, using various combinations of cross sections, were compared in terms of radial distributions of interactions and point kernels. The spectrum of secondary electrons emitted by the ionization process was found to be the factor that has the most influence on these quantities. A different set of cross sections for excitation and elastic scattering did not affect the electron track structure as much as did ionization cross sections. It is concluded that all codes, using different cross sections and in different phase, currently used for biophysical modeling exhibit close similarities for energy deposition in larger size targets while appreciable differences are observed in B-DNA-size targets. We recommend fitted functions to all available suitable experimental data for the total ionization and elastic cross sections. We conclude that most codes produce tracks in reasonable agreement with the macroscopic quantities such as total stopping power and total yield of strand breaks. However, we predict differences in frequencies of clustering in tracks from the different models.

Application of auxiliary signals in x-ray fluorescence and electron microprobe analysis for density evaluation

Information included in auxiliary signals, i,e. incoherently and coherently scattered signals and transmitted beam intensities in x-ray fluorescence analysis (XRF), a selected channel in the scattered white synchrotron radiation and in the contents of selected hremsstrahlung channels in electron microprobe analysis (EMPA), can be extracted and exploited to complete the results of analyses. Such additional information is much more detailed when the devices are operating in the microprobe mode; however, this is not a necessary condition. The theoretical basis for the use of the auxiliary signals, in the sense proposed in this paper, is discussed. In situations where the main component is of essentially constant chemical composition but with variable density through the sample (wood of living trees, petrified wood, stalactites, coral structures-in general natural periodic structures) or with inclusions inside, the application of two kind of signals is preferable in XRF: the transmitted and scattered signal. For light organic matrices (cellulose, plastics), the information included in the Rayleigh or Compton signal is reversed in relation to the information from the transmission signals. The selected hremsstrahlung signal or the signal from the secondary electron spectrum can serve the same purpose if EMPA analysis is performed. In general, these signals provide auxiliary knowledge about the density of objects. An especially interesting situation is when the object is a chemical negative of the original, such as the distribution of silica in some kinds of petrilied wood with respect to the distribution of the cellulose in the original wood. Many original results of analyses are shown. Intercomparisons of the transmission images with the reversed scattered scans in light matrices are still scarce, while the observation of bremsstrahlung or secondary electron scans (in EMPA) and their scattered or transmission analogues (in XRF) for objects and their chemical negatives is demonstrated for the first time. Comprehensive results of the analyses of petrified wood and stalactites (EMPA and XRF) are presented.

Dose enhancement by a thin foil of high-Z material: a Monte Carlo study.

The purpose of this work is to study the dose enhancement by a thin foil (thickness of 0.2-4 mm) of high-Z material in a water phantom, irradiated by high-energy photon beams. EGS4 Monte Carlo technique was used. Perturbations on the beam spectra due to the presence of the foils, and dose enhancement dependence of photon-beam quality, beam incident angle, atomic number (Z), the thickness and size of the foil, and the depth of the foil situated in the phantom were studied. Analysis of photon and secondary-electron spectra indicates that the dose enhancement near an inhomogeneity interface is primarily due to secondary electrons. A calculation for 1-mm-thick planar lead foil in a water phantom shows that the dose enhancements at 0.25, 1, 2 and 3 mm away from the foil in the backward region were 58%, 37%, 24% and 17%, respectively, for a 15 MV beam. Calculations for a variety of planar foils and photon beams show that dose enhancement: (a) increases with Z; (b) decreases with decreasing foil thickness when the foils are thinner than a certain value (1 mm for lead foil for 15 MV); (c) decreases with decreasing incident photon-beam energies; (d) changes slightly for beam incident angles less than 45 degrees and more prominently for larger angles; (e) increases with size of foil; and (f) is almost independent of the depth at which the foil is situated when the foil is placed beyond the range of secondary electrons. The dose enhancement calculation is also performed for a cylindrically shaped lead foil irradiated by a four-field-box. The dose enhancement of 34%/13% was obtained at 0.25/2 mm away from the cylindrical outer interface for a 15 MV four-field-box.

Input electron optics for Mott detectors used in secondary electron magnetometry

An electron spin polarization detector to be used in thin film and surface magnetometry via secondary electron emission is described in detail. It consists of a Mott detector coupled to an input electron optic designed to improve the overall efficiency of the system. This is achieved by taking full advantage of the polarization increase for electrons emitted at low kinetic energy, generally observed in ferromagnets. The input electron optic is appositely designed so as to operate as an energy low pass filter, which strongly modifies the energy distribution of the secondary electrons collected by the Mott detector. Ray tracing analysis indicates that the system performances are improved by more than a factor of 3 with respect to a bare detector operating with an undistorted secondary electron spectrum.

The role of auto-ionization transitions in the formation of the extended fine structure of high-energy secondary electron spectra

Extended fine structure (EFS) of secondary electron (SE) spectra has been detected beyond the high-energy (∼720 and ∼840 eV) LVV Auger lines in iron and nickel. Two mechanisms of its formation are considered: 1) direct transitions of electrons to the final state p according to Fermi’s “golden rule” and 2) second-order processes of auto-ionization type, passing through excitation of a core electron to an intermediate state q of the continuum with subsequent filling of the hole formed during this process by a valence electron and transition of the electron from the intermediate state q to the final state p. Interference of the direct wave with the wave reflected from neighboring atoms generates the EFS both in the final (p) and in the intermediate (q) state with two different periods determined by the wave numbers p and q. Comparison of calculated extended fine structures with the experimentally observed ones leads to the conclusion that the structure is formed by second-order auto-ionization processes.

Contrast mechanisms in scanning ion microscope imaging for metals

The contrast of the secondary electron images in scanning ion microscopy (SIM) is compared with that in scanning electron microscopy (SEM) with ultrahigh vacuum for Al, Cu, Ag, and Au metals deposited on the Si(100) clean surface. The order of the secondary electron yields as a function of the atomic number (Z2) for ion bombardment is opposite to that for electron bombardment. The brightness of the secondary electron images observed by a focused Ga+ ion beam at 30 keV decreases with increasing Z2, while that by the electron beam increases with Z2. On the other hand, the order of the total secondary ion yields in SIM increases with Z2. The secondary electron image observed by a focused Ar+-ion beam at 3 keV shows the similar contrast to that of the Ga+-ion beam. The different Z2 dependence of the secondary electron yields between SEM and SIM was quantitatively confirmed by the total secondary electron spectra and is discussed based on the range profile below the surface, and it is concluded that the decrease of the secondary electron yields in SIM is attributed to the increase of the reflected ions at the surface with increasing Z2.

Track structure in radiation biology: theory and applications

Purpose: A brief review is presented of the basic concepts in track structure and the relative merit of various theoretical approaches adopted in Monte-Carlo track-structure codes are examined. In the second part of the paper, a formal cluster analysis is introduced to calculate cluster-distance distributions. Method: Total experimental ionization cross-sections were leastsquare fitted and compared with the calculation by various theoretical methods. Monte-Carlo track-structure code Kurbuc was used to examine and compare the spectrum of the secondary electrons generated by using functions given by Born-Bethe, Jain-Khare, Gryzinsky, Kim-Rudd, Mott and Vriens' theories. The cluster analysis in track structure was carried out using the k-means method and Hartigan algorithm. Results: Data are presented on experimental and calculated total ionization cross-sections: inverse mean free path (IMFP) as a function of electron energy used in Monte-Carlo track-structure codes; the spectrum of secondary electrons generated by different functions for 500 eV primary electrons; cluster analysis for 4 MeV and 20 MeV alpha -particles in terms of the frequency of total cluster energy to the root-mean-square (rms) radius of the cluster and differential distance distributions for a pair of clusters; and finally relative frequency distribution for energy deposited in DNA, single-strand break and double-strand breaks for 10 MeV/u protons, alpha -particles and carbon ions. Conclusions: There are a number of Monte-Carlo track-structure codes that have been developed independently and the benchmarking presented in this paper allows a better choice of the theoretical method adopted in a track-structure code to be made. A systematic bench-marking of cross-sections and spectra of the secondary electrons shows differences between the codes at atomic level, but such differences are not significant in biophysical modelling at the macromolecular level. Clustered-damage evaluation shows: that a substantial proportion of dose (30%) is deposited by low-energy electrons; the majority of DNA damage lesions are of simple type; the complexity of damage increases with increased LET, while the total yield of strand breaks remains constant; and at high LET values nearly 70% of all double-strand breaks are of complex type.

Monte Carlo study of fluence perturbation effects on cavity dose response in clinical proton beams

Current protocols for clinical proton beam dosimetry have not implemented any chamber-dependent correction factors for absorbed dose determination. The present work initiates a Monte Carlo study of these factors with emphasis on proton fluence perturbation effects and preliminary calculations of perturbation effects from secondary electrons. The proton Monte Carlo code PTRAN was modified to allow simulation of proton transport in non-homogeneous geometries of both unmodulated and modulated beams. The dose to water derived from the dose calculated in an air cavity agrees well with results from analytical calculations assuming a displacement of the point of measurement. For unmodulated beams small differences, limited to 0.8%, could be partially attributed to proton multiple scattering. Effects of replacing water around the cavity with wall material are explained by the introduction of a water-equivalent wall thickness. For modulated beams no significant perturbation effects arise. Secondary electron spectra are calculated analytically. Preliminary electron transport calculations with EGS4 show that wall perturbations of the order of 1% could result. Perturbation effects caused by the energy transport of secondary particles from inelastic nuclear interactions have not been studied here. Inclusion of inelastic nuclear energy transfers in the cavity dose, assuming total local absorption, indicate that separate scaling of this contribution with the ratio of total inelastic nuclear cross sections could be important.

Momentum spectra of atmospheric pions, muons, electrons and positrons at balloon altitudes

Momentum spectra of pions, muons, electrons and secondary positrons have been measured at an atmospheric depth of with the same instrument. Data was collected by the Matter Antimatter Space Spectrometer of the New Mexico State University in a balloon flight in September 1991 at the rigidity cut-off of in Fort Sumner, New Mexico. The first measurement of the positive muon spectrum in the range to is reported in this paper. The spectral index above of the negative muon momentum spectrum of this measurement is in agreement with analytical cascade calculations which assume a primary proton kinetic energy spectrum with a slope of in the corresponding kinetic energy range. In the momentum interval 300 - , both negative and positive muon fluxes turn out to be larger than calculated fluxes by a factor of about 1.4. The measurement of the secondary electron and positron energy spectra allows a reliable subtraction of the atmospheric background from the primary electron and positron fluxes which are affected by large uncertainties in most of the experiments. The energy spectra of the secondary particles reported here have the same systematic errors implying a higher relative accuracy with respect to those measurements made in different flights.