Parameters and mechanisms governing image contrast in scanning electron microscopy of single-walled carbon nanotubes

Image formation of single-walled carbon nanotubes (SWNTs) in the scanning electron microscope (SEM) is peculiarly sensitive to primary electron landing energy, imaging history, sample/substrate geometry, electrical conductivity, sample contamination, and substrate charging. This sensitivity is probably due to the extremely small interaction volume of the SWNTs' monolayered, nanoscale structures with the electron beam. Traditional electron beam/bulk specimen interaction models appear unable to explain the contrast behavior when directly applied to SWNTs. We present one systematic case study of SWNT SEM imaging with special attention to the above parameters and propose some physical explanations for the effect of each. We also demonstrate that it is possible to employ voltage biasing to counteract this extrinsic behavior, gain better control of the image contrast, and facilitate the interpretation of SWNT images in the SEM.     

Primary considerations for image enhancement in high-pressure scanning electron microscopy

The mechanisms of electron beam scattering are examined to evaluate its effect on contrast and resolution in high-pressure scanning electron microscopy (SEM) techniques reported in the literature, such as moist-environment ambient-temperature SEM (MEATSEM) or environmental SEM (ESEM). The elastic and inelastic scattering cross-sections for nitrogen are calculated in the energy range 5–25 keV. The results for nitrogen are verified by measuring the ionization efficiency, and measurements are also made for water vapour. The effect of the scattered beam on the image contrast was assessed and checked experimentally for a step contrast function at 20 kV beam voltage. A considerable degree of beam scattering can be tolerated in high-pressure SEM operation without a significant degradation in resolution. The image formation and detection techniques in high-pressure SEM are considered in detail in the accompanying paper.     

High resolution electron beam fabrication: a brief review of experimental studies that began at Cambridge University in 1962

Electron beam fabrication studies with the scanning electron microscope began at Cambridge University in 1962. Early experiments demonstrated that structures could be made that were smaller than could be made by other methods, but it was not until several years later at IBM Research that useful devices with these smaller dimensions were made. It was even later (1981) before the resolution limits of electron beam lithography with conventional resist were measured. It is now understood that with conventional resists, electron/resist interaction effects limit the minimum structure size to about 10 nm. Newer methods that involve the direct sublimation of materials such as NaCl and LiF allow structures down to about 1 nm to be made.     

An integrated PC-based image analysis system for microtomography and quantitative analysis of inner micro-object structure

A new type of image processing system, including microtomographic analysis, is presented. The great majority of existing approaches to image processing and analysis can be achieved by means of software, with a minimal amount of hardware. The addition of microtomographic methods to an image processing system allows quantitative parameters of inner object microstructure to be obtained non-destructively.     

Charging in scanning electron microscopy "from inside and outside"

This paper is an attempt to analyse most of the complicated mechanisms involved in charging and discharging of insulators investigated by scanning electron microscopy (SEM). Fundamental concepts on the secondary electron emission (SEE) yield from insulators combined with electrostatics arguments permit to reconsider, first, the widespread opinion following which charging is minimised when the incident beam energy E0 is chosen to be equal to the critical energy E(o)2, where the nominal total yield delta(o) + eta(o) = 1. For bare insulators submitted to a defocused irradiation, it is suggested here that the critical energy under permanent irradiation EC2 corresponds to a range of primary electrons, R, and nearly equals the maximum escape depth of the secondary electrons, r. This suggestion is supported by a comparison between published data of the SEE yield delta(o) of insulators (short pulse experiments) and experimental results obtained from a permanent irradiation for EC2. New SEE effects are also predicted at the early beginning of irradiation when finely focused probes are used. Practical considerations are also developed, with specific attention given to the role of a contamination layer where a negative charging may occur at any beam energy. The role of the various time constants involved in charging and discharging is also investigated, with special attention given to the dielectric time constant, which explains the dose rate-dependent effects on the effective landing energy in the steady state. Numerical applications permit to give orders of magnitude of various effects, and several other practical consequences are deduced and illustrated. Some new mechanisms for the contrast reversal during irradiation or with the change of the primary electron (PE) energy are also suggested.