RESUMEN
We describe a feedback mechanism in the gas cascade amplification process used in magnetic immersion lens environmental scanning electron microcopy (ESEM). Feedback dominates gas gain under the conditions typically used for ultra-high-resolution ESEM and gives rise to novel dependencies of the imaging signal and noise on microscope operating parameters. It is ascribed tentatively to the generation of free electrons upon de-excitation of metastable species in the gas cascade. The results have implications for optimization of ESEM systems for applications such as critical dimension metrology and real-time imaging of nanostructure growth by gas mediated electron beam induced deposition.
RESUMEN
Electron beam induced etching (EBIE) and deposition (EBID) are promising fabrication techniques in which an electron beam is used to dissociate surface-adsorbed precursor molecules to achieve etching or deposition. Spatial resolution is normally limited by the electron flux distribution at the substrate surface. Here we present simultaneous EBIE and EBID (EBIED) as a method for surpassing this resolution limit by using adsorbate depletion to induce etching and deposition in adjacent regions within the electron flux profile. Our simulation results indicate the possibility of growth control of radially symmetric nanostructures at the sub-1 nm length scale on bulk substrates. The technique is well suited to the fabrication of ring-shaped nanostructures such as those employed in plasmonics, sensing devices, magneto-optics and magnetoelectronics.
RESUMEN
Aerogels (AGs) are ultralow-density nanoporous solids that have numerous potential applications. However, as most AGs are strong insulators with poor mechanical properties, direct studies of the complex nanoporous structure of AGs by methods such as atomic force and conventional scanning electron microscopy (SEM) have not proven feasible. Here, we use low-vacuum SEM to image directly the ligament and pore size and shape distributions of representative AGs over a wide range of length scales (approximately 100-105 nm). The structural information obtained is used for unambiguous, real-space interpretation of small-angle X-ray scattering curves for these complex nanoporous systems. Low-vacuum SEM permits imaging of both cross-sections and skin layers of AG monoliths. Images of skin layers reveal the presence of microcracks, which alter the properties of cast monolithic AGs.
RESUMEN
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.
RESUMEN
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.