Bradley L. Thiel

An improved model for gaseous amplification in the environmental SEM

We present a new model for the gas amplification effect used in many environmental scanning electron microscopes, wherein molecular complexity is shown to be the critical factor. Monte Carlo simulations, based on experimental electron scattering cross‐sections, are used to deduce a predictive model for the amplification process that is superior to the Townsend gas capacitor model. These predictions are compared with experimentally obtained amplification curves. Significantly, it is shown that the ionization efficiency of the electrons changes dramatically over the gap distance, and a constant value cannot be assumed. Atomic and molecular excitations affect the amplification process in two ways: first, they serve to lower the average kinetic energy of the imaging electrons, thereby keeping a greater fraction near the ionization threshold energy. Second, molecular normal modes determine the effectiveness of positive gas ions in producing additional secondaries upon surface impact. Practical implications such as signal gain and fraction of useful signal as a function of operating conditions are discussed in the light of the new model. Finally, we speculate on potential new contrast mechanisms brought about by the presence of an imaging gas.

Secondary electron contrast in low-vacuum∕environmental scanning electron microscopy of dielectrics

Low vacuum scanning electron microscopy (SEM) is a high-resolution technique, with the ability to obtain secondary electron images of uncoated, nonconductive specimens. This feat is achieved by allowing a small pressure of gas in the specimen chamber. Gas molecules are ionized by primary electrons, as well as by those emitted from the specimen. These ions then assist in dissipating charge from the sample. However, the interactions between the ions, the specimen, and the secondary electrons give rise to contrast mechanisms that are unique to these instruments. This paper summarizes the central issues with charging and discusses how electrostatically stable, reproducible imaging conditions are achieved. Recent developments in understanding the physics of image formation are reviewed, with an emphasis on how local variations in electronic structure, dynamic charging processes, and interactions between ionized gas molecules and low-energy electrons at and near the sample surface give rise to useful contrast m...

Applications of environmental scanning electron microscopy to colloidal aggregation and film formation

Abstract Environmental scanning electron microscopy (ESEM) is a rather new form of electron microscopy, which permits the observation of hydrated samples in their native state, and also does not require that insulators are coated with a conducting layer. These two factors make it ideal for studying colloidal dispersions as they aggregate and/or film form. This paper describes the application of ESEM to three situations involving aggregating latices. Firstly the nature of fractal structures grown from aggregating acrylic latices is discussed, with a comparison given of the behaviour with and without added salt as the screening between particles is altered. Secondly the behaviour of vinyl latices is considered. The impact of the addition of starch, both modified and unmodified, upon the particle size distribution and ability to film form is examined. Finally, the structures which form when the hard inorganic component silica is added to acrylic latices are explored. Together these three examples illustrate some of the many strengths of the ESEM in the field of colloidal dispersions and aggregates.

Evolution of the nanostructure of deposits grown by electron beam induced deposition

Environmental scanning electron microscopy (ESEM) was used to perform electron beam induced deposition (EBID) using a WF6 precursor. The deposits consist of WO3 nanocrystals embedded in an amorphous matrix. Oxide formation is attributed to residual oxidizers present in the ESEM chamber during EBID. Under conditions of fixed low electron flux, the WO3 grain size and the degree of deposit crystallinity increase with time. These changes correlate with the degree of electron energy deposition into the material during growth, indicating that electron beam induced modification of as-grown material is significant in controlling the nanostructure and functionality of materials fabricated by EBID.

Secondary electron imaging of nonconductors with nanometer resolution

The resolution of secondary electron (SE) images in scanning electron microscopy (SEM) is limited by the SE diffusion length. However, most materials are poor electrical conductors and in practice, resolution and image information content are often limited by charging. We demonstrate how charging can be eliminated as the resolution-limiting factor using a gaseous SE detector for magnetic immersion electron lenses. Charging is stabilized by ions produced in a magnetic field-assisted gas ionization cascade. The charge control self-regulation process does not quench the SE imaging signal, thereby enabling high resolution image contrast mechanisms that are suppressed in high vacuum SEM.

Master curves for gas amplification in low vacuum and environmental scanning electron microscopy

The concept of universal amplification profiles for gas cascade amplification of signals in low vacuum and environmental scanning electron microscopes is demonstrated both experimentally and theoretically using water vapor. For a given gas, cascade amplification gain profiles can be plotted onto a single master curve where the independent reduced parameter is the ratio of pressure to amplification field strength. When plotted in this fashion, both desired secondary electron and spurious background signal components fall onto respective master curves, with the amplitude being a function of anode bias only. These master curves can be described by simple Townsend Gas Capacitor equations using only two gas-specific parameters. As long as single scattering conditions apply, this approach allows for simplified, direct comparison of the gain characteristics of different gases and allows more intelligent selection of imaging conditions. The utility of treating signal amplification in this manner is demonstrated through a series of images collected under a variety of conditions, but with the ratio of pressure to amplification field strength kept constant. In practice, the range of operational parameter space in which this description can be applied to imaging is limited, as images typically have a mixture of secondary and backscattered contributions.

Electron postgrowth irradiation of platinum-containing nanostructures grown by electron-beam-induced deposition from Pt(PF3)4

The material grown in a scanning electron microscope by electron beam-induced deposition (EBID) using Pt(PF3)4 precursor is shown to be electron beam sensitive. The effects of deposition time and postgrowth electron irradiation on the microstructure and resistivity of the deposits were assessed by transmission electron microscopy, selected area diffraction, and four-point probe resistivity measurements. The microstructure, notably the platinum nanocrystallite grain size, is shown to evolve with electron fluence in a controllable manner. The resistivity was observed to decrease as a result of postgrowth electron irradiation, with the lowest observed value of 215±15????cm. The authors demonstrate that electron beam-induced changes in microstructure can be caused using electron fluences similar to those used during the course of EBID and suggest that the observed effects can be used to tailor the microstructure and functionality of deposits grown by EBID in situ without breaking vacuum.

Interfacial mixing and internal structure of Pt-containing nanocomposites grown by room temperature electron beam induced deposition

Material grown by room temperature electron beam induced deposition (EBID) using (CH3)3CH3C5H4Pt precursor consists of platinum nanocrystals embedded in an amorphous matrix. The crystallites are shown to intermix with the amorphous oxide on a Si substrate. The extent of intermixing scales with the electron energy density delivered to the material during growth. Dependencies on electron flux, fluence, and exposure time indicate that the intermixing process is athermal, electron-activated, and rate limited by mass transport inside the solid. Furthermore, the degree of deposit crystallinity is shown to scale with the electron flux and fluence used for EBID. We discuss mechanisms behind the observed changes in nanostructure and implications for the growth of functional materials by EBID.

Charging processes in low vacuum scanning electron microscopy.

A framework is presented for understanding charging processes in low vacuum scanning electron microscopy. We consider the effects of electric fields generated above and below the specimen surface and their effects on various processes taking place in the system. These processes include the formation of an ionic space charge, field-enhanced electron emission, charge trapping and dissipation, and electron-ion recombination. The physical mechanisms behind each of these processes are discussed, as are the microscope operating conditions under which each process is most effective. Readily observable effects on gas gain curves, secondary electron images, and X-ray spectra are discussed.

The study of water in heterogeneous media using environmental scanning electron microscopy

Abstract The Environmental scannning electron microscope has opened the door to examining a whole new range of water- and other liquid-containing specimens at unprecedented resolution and depth of field. In particular, the technique provides the opportunity to examine the nature and distribution of water in heterogeneous systems in both static and dynamic experiments. Obtaining useful information from these experiments, though, requires careful experimental design and a thorough understanding both of the chamber environment and of the many interactions between the imaging electrons, the specimen, and the chamber gas. Here, we present an introduction to the technique, followed by a selection of examples of experiments wherein the distribution and behaviour of all three thermodynamic phases of water may be of interest. It is our hope that this paper should inspire others to consider the merits of developing this technique.