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Over the past fifty years the scanning electron microscope (SEM) has established itself as the most versatile and productive tool for imaging and microanalysis in many areas of science and technology, and some seventy-thousand instruments generate millions of micrographs every day. Scanning electron microscopes do, however, have one fundamental limitation in that the only experimental variable available to the operator is the choice of the accelerating voltage. Although the ability to vary beam energy is both necessary and important, it is an unfortunate fact that changing the beam energy also alters many aspects of performance: imaging resolution, relative strength of different signal components, depth of beam penetration, capabilities of the various analytical systems, and the severity of charging and beam-induced damage. This makes it difficult or impossible to optimize the interaction of interest.

Because the ability to perform some form of chemical microanalysis has become an essential feature for any microscope, it is necessary to investigate what options are available in the new “ORION” helium ion microscope (HIM). The HIM has the ability to visualize local variations in specimen chemistry in both the ion induced secondary electron and the Rutherford backscattered imaging modes, but this provides only limited and qualitative information. Quantitative, elementally specific, microanalysis could be performed in the HIM using secondary electron spectroscopy, Rutherford backscattered ion spectroscopy, or secondary ion mass spectroscopy, but while each of these options has promise, none of them can presently guarantee either reliable element identification or quantitative analysis across the periodic table.

The scanning helium ion microscope has been used in transmission mode to investigate both the feasibility of this approach and the utility of the signal content and the image information available. Operating at 40 keV the penetration of the ion beam, and the imaging resolution achieved, in MgO crystals was found to be in good agreement with values predicted by Monte Carlo modeling. The bright-field and annular dark-field signals displayed the anticipated contrasts associated with beam absorption and scattering. In addition, the diffraction of the He ion beam within the sample gave rise to crystallographic contrast effects in the form of thickness fringes and dislocation images. Scanning transmission He ion microscopy thus achieves useful sample penetration and provides nanometer scale resolution, high contrast images of crystalline materials and crystal defects even at modest beam energies.

The phase-contrast imaging of atomic lattices has now become commonplace for both Transmission Electron Microscopes (TEM) and Scanning Transmission Electron Microscopes (STEMs). Recently, however, bright-field STEM images of multi-wall carbon nanotubes (MWCNTs) recorded from an ultra-high resolution (UHR) in-lens field-emission scanning electron microscope (FE-SEM) operating at 30keV have also demonstrated lattice fringe resolution. One example of such an image containing multiple examples of fringe detail is shown in figure 1. The carbon lattice fringes were analyzed and their origin confirmed by the application of the FFT algorithms in the SMART image analysis program. The resulting power spectrum after thresholding to remove background noise (Figure 2) confirms that phase detail in the image extends down to about 5 Angstroms (0.5nm) and that well defined diffraction spots corresponding to a spacing of 3.4 Angstroms (0.34nm) generated by the (002) basal plane spacing of the graphite lattice are present.

Specimens that charge under electron beam irradiation in the scanning electron microscope (SEM) can be stabilized by choosing the beam energy to be such a value that the sum of the secondary and backscatter electron yields is unity, as this establishes a dynamic charge balance. We show here that for pure elements, the energies El and E2, for which charge balance occurs, are related directly to the atomic number of the material. Although generally there is no comparable relation for compounds, we also show that for polymers, the E2 energy is related both to the ratio of the number of valence electrons to molecular weight and to the electro-negativity of the monomer units that form the polymer.

Dynamic charging is the condition in which the magnitude of the charge on the surface of a non-conductor, such as a polymer, under electron irradiation appears to change with the incident current density. Measurements confirm that this effect does occur and show that the magnitude of the effect depends on the relationship between the incident beam energy and the energy (E2) at which the sample has a total electron yield of unity. The effect arises from changes in the leakage resistance of the sample, and two possible explanations for this behavior are suggested.

Ion induced secondary electrons (iSE) can produce high-resolution images ranging from a few eV to 100keV over a wide range of materials. The interpretation of such images requires knowledge of the secondary electron yields (iSE δ) for each of the elements and materials present and as a function of the incident beam energy. Experimental data for helium ions are currently limited to 40 elements and six compounds while other ions are not well represented. To overcome this limitation, we propose a simple procedure based on the comprehensive work of Berger et al. Here we show that between the energy range of 10-100keV the Berger et al. data for elements and compounds can be accurately represented by a single universal curve. The agreement between the limited experimental data that is available and the predictive model is good, and has been found to provide reliable yield data for a wide range of elements and compounds.

Extended abstract of a paper presented at Microscopy and Microanalysis 2013 in Indianapolis, Indiana, USA, August 4 – August 8, 2013.

Extended abstract of a paper presented at Microscopy and Microanalysis 2013 in Indianapolis, Indiana, USA, August 4 – August 8, 2013.

Extended abstract of a paper presented at Microscopy and Microanalysis 2013 in Indianapolis, Indiana, USA, August 4 – August 8, 2013.

Extended abstract of a paper presented at Microscopy and Microanalysis 2012 in Phoenix, Arizona, USA, July 29 – August 2, 2012.

Extended abstract of a paper presented at Microscopy and Microanalysis 2012 in Phoenix, Arizona, USA, July 29 – August 2, 2012.

Extended abstract of a paper presented at Microscopy and Microanalysis 2011 in Nashville, Tennessee, USA, August 7–August 11, 2011.

Extended abstract of a paper presented at Microscopy and Microanalysis 2011 in Nashville, Tennessee, USA, August 7–August 11, 2011.

Extended abstract of a paper presented at Microscopy and Microanalysis 2011 in Nashville, Tennessee, USA, August 7–August 11, 2011.

Extended abstract of a paper presented at Microscopy and Microanalysis 2011 in Nashville, Tennessee, USA, August 7–August 11, 2011.

Extended abstract of a paper presented at Microscopy and Microanalysis 2010 in Portland, Oregon, USA, August 1 – August 5, 2010.

Extended abstract of a paper presented at Microscopy and Microanalysis 2010 in Portland, Oregon, USA, August 1 – August 5, 2010.

Extended abstract of a paper presented at Microscopy and Microanalysis 2010 in Portland, Oregon, USA, August 1 – August 5, 2010.

Extended abstract of a paper presented at Microscopy and Microanalysis 2010 in Portland, Oregon, USA, August 1 – August 5, 2010.

Extended abstract of a paper presented at Microscopy and Microanalysis 2010 in Portland, Oregon, USA, August 1 – August 5, 2010.

Extended abstract of a paper presented at Microscopy and Microanalysis 2009 in Richmond, Virginia, USA, July 26 – July 30, 2009

Extended abstract of a paper presented at Microscopy and Microanalysis 2009 in Richmond, Virginia, USA, July 26 – July 30, 2009

Extended abstract of a paper presented at Microscopy and Microanalysis 2009 in Richmond, Virginia, USA, July 26 – July 30, 2009

Extended abstract of a paper presented at Microscopy and Microanalysis 2009 in Richmond, Virginia, USA, July 26 – July 30, 2009

Extended abstract of a paper presented at Microscopy and Microanalysis 2008 in Albuquerque, New Mexico, USA, August 3 – August 7, 2008