Environmental gas impact on the emission volume of X-rays near the interface in the variable pressure scanning electron microscopy.

In this paper, our previously developed model to account for the secondary X-ray fluorescence and absorption effects near the interface of two adjacent materials in a high-vacuum scanning electron microscope (Zoukel & Khouchaf, 2014) is adapted and extended to experimental conditions of low-vacuum mode (in the presence of a gaseous environment in the SEM analysis chamber). The position shifting effect of the two Gaussian peaks issued from the first derivative equation that can fit the experimental low-vacuum EDS profiles is investigated. The impact of the medium gas on the emission volume of secondary X-rays near the interface is qualitatively discussed. Water vapour and helium are successively used as gas environment, in order to link the resolution of microanalysis profiles with the effects of the X-ray fluorescence and absorption phenomenon. A close agreement between Monte Carlo simulation and experimental results is found.

Gas effect on the emission and detection of the backscattered electrons in a VP-SEM at low energy.

The effect of the electron beam skirting on the emission and detection of the backscattered electrons (BSE) in a low vacuum scanning electron microscope is investigated at low energy regime. Monte Carlo computed dependencies of the BSE distribution on the water vapor and air pressure shown a significant increase of the extent of the BSE exit zone. The pressure variation has however a little effect when helium gas is used. A new approach based on the comparison between the sizes of the skirt and the BSE exit zone on the specimen surface provides a useful tool to determine the operating pressure range that ensures minimal degradation of the lateral resolution in BSE imaging mode.

The secondary X-ray fluorescence and absorption near the interface of multi-material: case of EDS microanalysis.

A simple model is proposed to take into account secondary X-ray fluorescence and absorption effects near the interface. This model is based on the investigation of the shape change of the first derivative equation that can fit the sigmoidal EDS profile obtained when a high vacuum electron beam passes through the interface of two adjacent materials. The contribution of the photoelectric absorption of primary X-rays (characteristic and Bremsstrahlung) and the secondary fluorescence on the degradation of the X-ray spatial resolution can be easily quantified. The close agreement between the simulated (Monte Carlo simulation using the Casino software) and the experimental data serves to assess the reliability of this developed model.

A new approach to reach the best resolution of X-ray microanalysis in the variable pressure SEM.

A validation of our recent new approach is presented here in order to better interpret the EDS analysis results in low vacuum SEM. This approach is based on correlation between two concepts: the electron beam skirt radius in the gas characterized by RS and the X-ray emission volume radius in the material characterized by RX. If RS≤RX; then the skirt impact on the analysis is null and the best possible X-ray lateral resolution within the limitations imposed by gas scattering is obtained. In order to follow the relationship between RS and RX, two aluminum foils with different thickness (2 µm and 20 µm) embedded separately in epoxy resin were used. The results showed the existence of the optimal experimental conditions depending on the pressure and the energy that verify the condition of RS≤RX. The experimental and simulated results show the great consistency of this approach.

Skirting effects in the variable pressure scanning electron microscope: limitations and improvements.

A new approach has been initiated to improve the spatial lateral resolution of the X-ray microanalysis and the backscattered electrons modes in variable pressure or environmental scanning electron microscope (VP-ESEM). This approach is based on correlation between two concepts: the electron beam skirt radius in the gas (R(S)) and the generation volume radius (R(X)) of X-ray signals and the generation volume radius (R(BSE)) of backscattered electrons in the material. In order to follow the relationship between R(S), R(X) and R(BSE), PMMA polymer, silicon oxide and aluminium are used. The results of the simulation show the existence of the best lateral resolution conditions named R (P, E) depending on the pressure and the energy for each material. This approach will enable us to propose some optimal experimental conditions to characterize different materials.

Evidence of depolymerisation of amorphous silica at medium- and short-range order: XANES, NMR and CP-SEM contributions.

This work reports medium- and short-range order of changes of amorphous silica submitted to chemical degradation. Structural changes were studied, using X-ray absorption near edge structure (XANES), nuclear magnetic resonance (29)Si NMR-MAS and controlled pressure scanning electron microscope (CP-SEM). The depolymerisation of amorphous SiO(2) compounds mainly induces the formation of Q(3) species and alkali-rich domains. The XANES Si K-edge spectra demonstrate the presence of different environments of silicon: one with four oxygen atoms and the other with a number of oxygen lower than four in agreement with previous studies.