Electron-solid Interactions Research Articles

Ultra-low voltage scanning electron microscopy

Although the benefits of operating the scanning electron microscope at low beam energies have been evident since the earliest days of the instrument, the successful utilization of the SEM under these conditions has required the development of high brightness field emission electron source, advanced lenses, and clean vacuums. As these technologies became available the level at which imaging became regarded as “low energy” has fallen from 10keV, first to 5keV, and more recently to 1keV. At this energy state of the art instruments can now provide an excellent balance between resolution – which becomes worse with decreasing energy – and desirable goals such as the minimization of sample charging and the reduction of macroscopic radiation damage – which tend to become more challenging as the energy is increased.An interesting new opportunity is to perform imaging in the ultra-low energy region between leV and 500eV. Over this energy range significant changes in the details of electron-solid interactions take place offering the chance of novel contrast modes, and the rapid fall in the electron beam range leads to the condition where the penetration of the incident beam into the sample is effectively limited to 1 or 2 nanometers. The practical problem is that of achieving useful levels of resolution and acceptable signal to noise ratios in the image. At energies below IkeV chromatic aberration dominates the probe formation in conventional instruments even when using an FEG source. However, the use of optimized retarding field optics essentially maintains chromatic aberration independent of landing energy down to very low values. Figure (1) shows an example of the performance that can be achieved on a commercial instrument – an Hitachi S-4500 – modified to operate in this mode, in this case at 50eV landing energy. The resolution of the image is judged from edge sharpness and detail to be significantly better than 0.1µm and, from experimental observation, this performance is apparently limited by residual astigmatism caused by uncorrected sample charging rather than by fundamental aberrations in the probe forming optics. Comparable, if somewhat lower resolution, images have been achieved on this, and other FEG SEMs, at energies as low as leV.

Quantitative EPMA of element depth distribution

A new model for the ionization depth distribution function has been proposed. Within the framework of this model full electron flux is considered to be divided into two fluxes propagating in forward and backward directions through a sample. The intensities of these fluxes can be derived on the basis of simple assumptions of electron-solid interactions. The approach can be effectively applied to a system with an arbitrary form of depth concentration profile. The obtained results are in reasonable agreement with Monte Carlo simulations and experimental results. Some mathematical techniques have been presented for quantitative analysis of thin films on substrates and element depth distribution.

Interaction of electrons and positrons with solids: from bulk to surface in thirty years

In 1963 electrons and positrons were regarded as probes of the bulk properties of solids via, e.g., energy-band theory, fast electron energy loss spectroscopy, and positronium decay. During the late 1960s it was recognized that at energies in the range 10 ⩽ E ⩽ 1000 eV,the inelastic collision mean free paths, λ, of electrons and positrons become comparable to atomic dimensions, i.e., λ ≈ 10 Å, so that the elastic reflection or emission of these particles from solids probes the properties of the uppermost few atomic layers. This recognition led to the development of core level spectroscopies (e.g., Auger electron spectroscopy, X-ray photoelectron spectroscopy) for surface compositional analysis in the 1960s, to the application of low-energy electron diffraction for quantitative surface structure determination in the 1970s, and to that of low-energy positron diffraction for surface structure analysis in the 1980s. In addition, in the 1960s it was recognized that resonant field emission through adsorbed species offers a quantitative spectroscopy of the single-electron excitation spectrum of these species: a discovery which presaged the use of photoemission spectroscopy for this purpose in the 1970s and the development of scanning tunneling spectroscopy in the 1980s. This article is an exposition of how the evolution of the understanding of the nature of electron (positron) solid interactions together with advances in ultrahigh vacuum instrumentation and semiconductor microelectronics led from the dawn of the surface science revolution in the 1950s and 1960s to the quantitative electron (positron) surface spectroscopies which are routinely deployed in the 1990s.

Stopping-power determination for compound by EELS

In Physics, Chemistry, Materials Science, Biology and Medicine, it is very important to have accurate information about the stopping power of various media for electrons, that is the average energy loss per unit pathlength due to inelastic Coulomb collisions with atomic electrons of the specimen along their trajectories. Techniques such as photoemission spectroscopy, Auger electron spectroscopy, and electron energy loss spectroscopy have been used in the measurements of electron-solid interaction. In this paper we present a comprehensive technique which combines experimental and theoretical work to determine the electron stopping power for various materials by electron energy loss spectroscopy (EELS ). As an example, we measured stopping power for Si, C, and their compound SiC. The method, results and discussion are described briefly as below.The stopping power calculation is based on the modified Bethe formula at low energy:where Neffand Ieffare the effective values of the mean ionization potential, and the number of electrons participating in the process respectively. Neffand Ieffcan be obtained from the sum rule relations as we discussed before3 using the energy loss function Im(−1/ε).

Parallel simulation of electron-solid interactions: A rapid aid for electron microscope data interpretation

Monte Carlo electron trajectory simulations have been adapted to run on a parallel supercomputer. The increased speed achieved by parallelization of the Monte Carlo code results in the ability to model small probability events easily. The techniques for parallelization of the algorithm are presented here along with examples of applications. The applications include the calculation of the X-ray spatial resolution in thin films, backscattered electron imaging of voids in Al metallizations and X-ray production in thin-film specimens.

The line shape in projectile Auger electron spectra from highly charged ion surface interactions

Auger electron spectra from the neutralization of highly charged ions which impinge on a metal surface are influenced by ion-solid, electron-ion, and electron-solid interactions. Individual as well as combination effects between these interactions are studied by means of a Monte Carlo simulation of projectile Auger electron spectra. A new mechanism in spectrum formation is identified, i.e. Doppler line broadening due to post emission elastic scattering. This effect explains some shoulders or weak satellite lines which are seen in recent measurements on both sides of the peak. Known methods of background correction can treat this effect only in special circumstances.

Parallel simulation of electron-solid interactions for electron microscopy modeling

A parallel implementation of a Monte Carlo algorithm for modeling the scattering of electrons in solids and the resulting X-ray production is described. Two important issues for accurate and fast parallel simulation are discussed-random number generation and load-balancing. Timing results for the parallel simulation are given which show even modest-sized parallel machines can be competitive with conventional vector supercomputers for Monte Carlo trajectory simulations. Examples of parallel calculations performed to analyze specimen composition data and to characterize electron microscope performance are briefly highlighted.

Imaging Surface Atomic Structure by Means of Auger Electrons

Measurements of the complete angular distribution of Auger electrons emitted from well-defined platinum[111] single-crystal surfaces have led to the discovery that the distributions are composed of "silhouettes" of surface atoms "back lit" by emission from atoms deeper in the solid. Theoretical simulations of Auger electron angular distributions based upon atomic point emitters and spherical atomic scatterers of uniform cross section are in close agreement with these experimental results, but opposite to previous theoretical predictions. In view of the definitive results obtained and the straightforward agreement between theory and experiment, angular distribution Auger microscopy (ADAM) is useful for direct imaging of interfacial structure and investigation of electron-solid interactions in the physical and biological sciences and engineering. Applicability of ADAM is illustrated by images obtained for monolayers of silver and iodine on platinum[111].

A Monte Carlo simulation of electron beam lithography used to create 0.5-μm structures on GaAs

We have used a positive trilevel resist structure on GaAs to verify our general purpose calculations of electron beam lithography. The model uses a single-scattering Monte Carlo simulation of electron–solid interactions, a simple dissolution rate versus energy model for the resist, and a string model for resist development. The spot size of the electron beam was varied from 0.2 to 0.4 μm in diameter. Experimental results on scanning electron microscopy linewidths and wall profiles are in good agreement with the simulation. Using the model, we have determined the change in process latitude with increasing spot size. However, we find that the lithography process is still able to create 0.5-μm lines even with large (0.4-μm) spots.

Background and applications of electron beam test techniques

The use of electron beams for contactless testing of electrical functions and electrical integrity of different active devices in VLSI - chips - has been demonstrated over the past years. This method of testing electronic networks is based on an electron probe which is moved from point to point in the network and a current of secondary electrons emitted in response to the impingement of the probe is converted to a signal indicating the presence of a voltage or varying potential at the different points. Voltage contrast, electron beam induced current, dual potential approach, stroboscopic techniques and other methods have been developed and are used to detect different functional failures in devices.Very little attention in most of the applications has been paid to the electron optical environment, mostly SEM's were upgraded or converted to do the job of a “voltage contrast” machine. This by no means will satisfy all requirements and new thoughts will have to be given to aspects such as: low voltage electron guns, two lens systems, different means of detection, signal processing and storage are a few of them.Besides the VLSI application, the contactless testing of three dimensional conductor networks of a 10 cm × 10 cm × .8 cm multilayer ceramic poses a different and new application. The mechanism of electron-solid interaction, the secondary electron generation, the energy distribution, the secondary electron yield, are important parameters to understand for any electron beam test technique. According to the secondary electron yield curve in Figure 1, floating samples can be charged positively or negatively with respect to ground, depending on the primary electron energy.The surface potential of a sample in turn determines the impact energy of the primary electrons and also energy of the secondaries that leave the surface. These two effects generate different voltage contrasts to be used in electron beam testing. The test method for packages uses multiple electron beams to both generate and detect voltage differences between the surface terminations of conductors in order to identify opens and short in the package networks.

A Monte Carlo thin Film Simulation for Microcomputers

Monte Carlo simulations are now widely used as a means of studying the details of electron solid interactions (Heinrich et al 1976). The use of this technique has, however, been limited by the perception that a large computer, and a substantial investment in computing time are required to obtain useful results. This paper describes a Monte Carlo simulation, written in BASIC that is implemented on an APPLE micro-computer, and is readily adapted to most other "personal" computers.

The influence of electron exchange and relativistic corrections on elastic low energy electron diffraction from compound semiconductors

The model of the electron-solid interaction used for dynamical low energy electron diffraction (LEED) calculations is extended to include both an energy dependent local exchange interaction and relativistically computed ion-core charge densities. These extensions of earlier work based on non-relativistic, local exchange models are tested for the (110) surfaces of InP, ZnTe and InSb. Calculations reveal discernible differences between LEED intensities computed using the energy dependent exchange force and those obtained using energy independent local exchange forces. The replacement of the non-relativistic ion-core potential with a relativistic one produces smaller changes which are most apparent when the energy dependent exchange force is used for compound semiconductors containing one or more components from the fifth row or lower in the periodic table (e.g., Cd, In, Sb or Te).

Applications of Computers in Electron Microscopy

During most of the fifty years since the invention of the electron microscope, the primary hands-on operating role of the microscopist has been scanning specimens, selecting, and recording fields/features of particular interest. Now, microscopy is entering a new era. The advent of microcomputers and microprocessors is about to modify the customary role of microscopists.The new technology will enable them to respond to a growing need for more quantitative data than can be generated by conventional methods based on manual control of data acquisition and processing.During the last 20 years, attention was devoted to the physics of image formation in terms of electron-solid interactions, interference phenomena, and lens performance. During this same period, considerable effort was also put into elucidating the specific nature of many newly discovered structures including crystal defects, interfaces, membranes, second phase particles, and numerous other perturbations of the specimen “matrix.” Sophisticated mathematical descriptions of image contrast, i.e., intensity as a function of position, are now available to predict the appearance of most structures of interest.

Theory of spin-polarized low-energy electron diffraction from ferromagnetics

A theory of elastic scattering of low-energy electrons is outlined, in which the electron-solid interaction contains two spin-dependent contributions: an elastic exchange-correlation part in the real ion-core potential and an inelastic imaginary part describing electron-hole excitation. Numerical calculations for the Fe(110) surface predict, for an unpolarized primary beam, large spin polarization effects (up to 60%), which are highly sensitive to the magnetization of the topmost stomic layer.

3913. Electron-solid interactions: their nature and consequences. (USA)

A highly enantioselective route has been devised for the synthesis of the 3,5 cis-disubstituted valerolactone moiety (C-1 to C-9 segment) of rhizoxin via intramolecular radical cyclisation of suitably functionalised 6-heptenyl radical.

Electron–solid interactions: Their nature and consequences

During the past two decades electron scattering and emission spectroscopies have become utilized almost universally to characterize the properties of solid surfaces. Two major trends have created this situation: The increasing availability of suitable, convenient experimental apparatus, and the recognition and exploitation of those features of electron–solid interactions which render these measurements sensitive to surface properties. Whereas prior Welch Award lectures have dealt with the former trend, the latter is examined herein. Three concepts are central to an understanding of the interaction of ’’low-energy’’, 10≲E≲103-eV electrons with the constituents of solids. First, their strong Coulombic interaction with the valence electrons of the solid renders their mean free paths for inelastic collisions of atomic dimensions, i.e., 2≲λ≲20 Å. Thus, elastically scattered or emitted electrons in this energy range must have emanated from the uppermost few atomic layers of the solid. Second, the interactions of these electrons with the core electrons are comparable to those with the valence electrons. Therefore, multiple elastic scattering as well as rapid energy loss characterizes their scattering and emission. Third, resonant processes associated with electronic states localized at a surface can be as important as inelastic processes in establishing surface sensitivity. Examples of this phenomenon are found in resonant field emission through adsorbates and resonant photoemission associated with the excitation of transitions characteristic of adsorbed complexes. The major portion of this article is devoted to an indication of how exploitations of these three concepts have led to quantitative surface analysis methodologies for metals and covalent solids. In the case of polymers and molecular solids, however, one further concept is required: the localization of electronic states caused by fluctuations in relaxation energy. These fluctuations lead to localized molecular ion (or exciton) states rather than extended band states. The final portion of this article is devoted to exploring the consequences of this additional concept and how it led to the invention of yet another electron spectroscopy, contact charge exchange spectroscopy, for the examination of molecular ion states in polymers.

Theory of (e, 2e) reaction near solid surfaces: Application to Si

The use of the (e, 2e) technique in surface state spectroscopy is discussed. A general formulation based on Green function formalism is developed, that allows to take into account the electron-solid interaction. The case of the Si(111) surface is considered in detail. It is shown that the (e, 2e) cross section strongly depends on surface conditions; hence this technique could be an efficient tool for testing reconstruction models.

Electron beam scattering in an organic specimen

The understanding of electron-solid interactions is of prime importance to both conventional transmission and scanning electron microscopists. Monte Carlo simulation of electron beam scattering in various target samples has made fundamental contributions to this understanding, especially in scanning electron microscopy where primary electron beams of a few to many kilovolts are utilized. A significant example is the understanding of energy dissipation patterns of incident electrons in an organic sample (polymethylmethacrylate-PMMA) which is very useful in electron beam lithography (1). Furthermore, with the close similarity between the organic sample and a biological specimen, a Monte Carlo approach is also very useful for the study of energy dissipation in biological specimens (2).To enhance our knowledge of these dissipation processes, a more comprehensive Monte Carlo simulation program has been developed. The program is based on the semi-direct technique of simulating individual inelastic scattering and elastic scattering with probabilistic weightings.

A simple Monte Carlo method for simulating electron-solid interactions and its application to electron probe microanalysis

A Monte Carlo method based on Curgenven and Duncumb's treatment (1971) is described. It is shown that the predictions of the model agree well with experimental backscattered electron energy distributions and phi ( rho z) curves. The model is used to generate phi ( rho z) curves for a wide range of analytical conditions. Based on these data an examination is carried out of the way in which the mean depth of X-ray production varies with electron accelerating voltage, overvoltage and atomic number of the target. An expression for the mean depth is deduced the form of this expression is compared with that predicted by the Philibert model (1963).

Determination of the structure of adsorbed monolayers by the energy-averaging method: Al(100)

The energy-averaging technique proposed by Tucker and Duke for the determination of the unit-cell geometry of adsorbed monolayers is verified for a model of strong scatterers on a substrate described by the lattice parameters and muffin-tin lattice potential characteristic of Al(100). The analysis is based on the inelastic-collision model in which three phase shifts (associated with s, p, and d-waves) are used to describe elastic electron interactions with the atomic scatterers, and an inelastic collision penetration depth, λ ee, determines the consequences on the elastic scattering of inelastic collisions of the incident electron. The technique is tested by comparing certain averages of scattered intensities calculated using the complete (dynamical) model with the predictions of a kinematical calculation based on the same model electron-solid interaction. Within this framework, for λ ee ⩽ 4 Å, the energy-averaging technique provides an adequate determination of the monolayer unitcell geometry to within an accuracy of ± 0.10 Å independent of the values of other parameters in the model electron-solid force law. The technique remains qualitatively valid but yields geometrical parameters with reduced accuracy for values of λ ee up to about 8–10 Å. To within this accuracy, it also may be applied to determine either the geometry of the upper layer of a clean surface for given atomic scattering factors or the atomic scattering factors themselves if the surface layer geometry is known a priori (as in the case of monatomic metals).