M.M. El-Gomati

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.

Very-low-energy electron microscopy of doped semiconductors

Imaging of As- and B-doped silicon regions has been performed in a scanning electron microscope operated in the cathode lens mode, with incident electron energies (EP) as low as 15 eV. The doped regions of n+ (As, 2.5×1020 cm−3) and p+ (B, 8×1019 cm−3) on n-type silicon (∼1015 cm−3) show distinct contrast with electron energies of about 3 keV. The brightest region is n+ followed by p+, then the n-type substrate. The highest contrast for the p+ and n+ type regions is reached at about EP=300 and 15 eV, respectively. The contrast mechanisms are explained in terms of metal-semiconductor contact assuming an adventitious carbon film at the surface.

Low temperature reactive ion etching of silicon with SF6/O2 plasmas

An investigation of low temperature reactive ion etching of silicon to produce ultrasmall structures is presented. Etching was performed between 10–100 mTorr with SF6/O2 plasmas. The silicon samples were cooled using liquid nitrogen, allowing the temperature to be controlled over the range of +25 to −120 °C. With a flow rate of 1 sccm of SF6 and sample temperatures below −85 °C the etch produces nearly anisotropic etching in agreement with predictions of the reactive spot model. At higher flow rates etching produced facetted features indicating higher chemical reactivity of the plasma with the silicon. With the addition of O2 to the plasma the facetting became even more pronounced.

Why is it that differently doped regions in semiconductors are visible in low voltage SEM

Although doped regions in semiconductors have been shown to give a different secondary electron yield in low-voltage scanning electron microscopy, the basic interpretation of this contrast has been difficult. It is accepted that this contrast stem from electronic phenomenon rather than atomic number differences between differently doped regions. However, the question is whether variations in the patch fields above the sample surface, balancing variations in the inner potentials, or surface coatings and/or surface states are the mechanisms responsible for the observed contrast. The present study reports on comparative experiments of these two models and demonstrates that the image contrast can be controlled by the presence of thin-surface metallic coatings. These results are the first evidence of the adlayer contacts, i.e., the subsurface electric fields instead of the patch fields above the surface, being responsible for the secondary electron contrast of doped semiconductors imaged in low voltage scanning electron microscopes under standard vacuum conditions, and they pave the way for the routine use of this method in semiconductor research and industry.

Electron Backscattering from Real and In-Situ Treated Surfaces

Abstract. Significant differences in backscattered electron (BSE) yields exist between the surfaces cleaned by methods used in electron microscopy and spectroscopy. These differences have been observed for Au, Cu and Al specimens, and are interpreted on the basis of simulated BSE yields. Composition and thickness of the surface contamination layers, responsible for the differences, are estimated. The results (7 nm of carbon on Au or 3 nm of oxide on Al) remain within expectation and indicate that the BSE yield measurements and BSE images should be interpreted cautiously. Peculiar results are obtained for Cu, perhaps due to a different cleaning procedure. A new concept of an information depth for the BSE signal is introduced as a depth within which the total BSE yield can be modelled as composed of the yields of layers proportional to their thickness weighted by the escape depths. This concept proved satisfactory for thin surface layers and brought the information depth values 2 to 4 times smaller than first estimated, i.e. half the penetration depth.

The role of oxygen in secondary electron contrast in doped semiconductors using low voltage scanning electron microscopy

It has been known for many years that p-doped and n-doped regions in semiconductors reveal different contrasts (p-doped normally brighter than n-doped) when imaged in a scanning electron microscope (SEM). This effect could be very useful to the semiconductor industry to determine dopant concentrations at nanoscale dimensions if it could be understood and made quantifiable. Highly doped n+ and p+ samples were studied in a SEM with two different oxide thicknesses. The samples were initially studied without applying any initial treatment and then the oxide was removed by dipping in diluted HF. The samples were studied as a function of primary electron beam energy. It was found that at low primary beam energy (1–2 keV), the p-type material was brighter than the n-type for both oxide thicknesses. However, at higher primary beam energy, the contrast reversed for the sample with the thicker oxide above a primary beam energy of 2 keV. The role of the oxide in these contrast variations is explored and the conseque...

Enhanced angular current intensity from Schottky emitters.

Even though the Schottky emitter is a high‐brightness source of choice for electron beam systems, its angular current intensity is substantially lower than that of thermionic cathodes, rendering the emitter impractical for applications that require high beam current. In this study, two strategies were attempted to enhance its angular intensity, and their experimental results are reported. The first scheme is to employ a higher extraction field for increasing the brightness. However, the tip shape transformation was found to induce undesirably elevated emission from the facet edges at high fields. The second scheme exploits the fact that the angular intensity is proportional to the square of the electron gun focal length [ Fujita, S. & Shimoyama, H. (2005) Theory of cathode trajectory characterization by canonical mapping transformation. J. Electron Microsc. 54, 331–343], which can be increased by scaling‐up the emitter tip radius. A high angular current intensity (JΩ∼ 1.5 mA sr−1) was obtained from a scaled‐up emitter. Preliminary performance tests were conducted on an electron probe‐forming column by substituting the new emitter for the original tungsten filament gun. The beam current up to a few microamperes was achieved with submicron spatial resolution.

Systematic trends in the transport mean free path with electron energy and atomic number

Abstract The inelastic mean free path, λ i , and the transport mean free path, λ tr , hold the key to understanding the effects of elastic scattering of electrons in electron spectroscopy techniques such as Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). Systematic trends in the variation of λ tr and χ = λ i / λ tr with atomic number, Z , and electron energy, E , are explored using the differential elastic scattering cross-section data of Czyzewski et al. (J. Appl. Phys., 68 (1990) 3066). At low Z , λ tr increases with energy in accord with the predictions of the Born approximation, but at higher Z , a more complex behaviour is revealed. In the first and second transition metal series, χ varies little with energy over much of the kinetic energy range probed in XPS in accord with the energy scaling ideas from the semi-classical scattering theory of Tilinin (Soviet Physics JETP, 67 (1992) 1570). In the third transition series, a pronounced minimum is found at around 200 eV, with χ for Au lower than for the corresponding transition metals. The results identify regions of the periodic table where elastic scattering effects are particularly pronounced.

On the Measurement of the Backscattering Coefficient for Low Energy Electrons

An electron detector for the measurement of backscattering and secondary electron coefficients based on that of Reimer and Tollkamp has been devised and built. The detector is used in ultra high vacuum and allows one to use in-situ ion bombardment of the sample to remove surface contaminants before data collection. New experimental measurements of the backscattering and secondary electron coefficients in the energy range 0.5-6keV are reported for C, Al, Si, Cu, Ge, Mo, Ag and Au. These are collected from both argon ion cleaned and as-inserted surfaces. The back- scattering coefficients of the argon ion cleaned surfaces monotonically increases for elements of atomic number Z (graterthan) 28 whilst it decreases for elements of Z (lessthan) 28 for all energies used. This is in contrast to the as-inserted surfaces that show a more complex pattern below 1 keV incident energies.

Toward Quantitative Scanning Electron Microscopy

Abstract The scanning electron microscope (SEM) has been a fundamental tool that has underpinned much advancement in research and engineering in various disciplines over many decades. However, it cannot be regarded yet as a true quantitative instrument, particularly with regard to the resulting image contrast and when imaging is carried out at the nanoscale level. Such a limitation is not applied to the SEM’s use as a measuring instrument, in which it performs exceptionally well as a critical dimension tool (CD-SEM). This lack of material quantification manifests itself further when the instrument is operated with low-energy electrons, in what is referred to as low-voltage SEM (LVSEM). This is due to the presence of carbonaceous deposits at the surface and a poor understanding of the emission of secondary electrons from the materials. In this chapter, a short review is given of some of the progress made in the efforts to improve the quantification of the SEM, with the emphasis on research carried out at York. Our results strongly suggest that the currently accepted theory, which explains why there is a correlation between the secondary electron yield and the work function of a metal, is incorrect. In addition, we show that the backscattering coefficient from materials can be strongly influenced by surface layers at low primary electron energy, and that a secondary electron contribution to the backscattering coefficient occurs at low primary beam energy. Finally, we present Auger electron spectra that have been in situ acquired from clean surfaces at high speeds in a high vacuum (10 -7 mbar), and thus represent a new way to determine the composition of nanostructures in an SEM.