Mark A.E. Jepson

Energy selective scanning electron microscopy to reduce the effect of contamination layers on scanning electron microscope dopant mapping

We demonstrate that energy selective scanning electron microscopy can lead to substantial dopant contrast and resolution improvements (compared to standard SEM) when the energy selection is carried out based on Monte Carlo modelled secondary electron spectra in combination with detector transfer modelling.

Use of EBSD to characterise high temperature oxides formed on low alloy and stainless steels

Abstract Exposure of steel to high temperatures in air leads to the formation of an oxide scale, the composition and structure of which depends sensitively on the oxidation conditions and the alloying elements contained within the steel. In this paper, the oxide scale structures formed on low alloy and stainless steels are characterised using electron backscatter diffraction (EBSD). In low alloy steels, this crystallographic information obtained using EBSD enables both the phases within the scale (i.e. haematite, magnetite and wüstite) and orientation relationships between them to be established. This showed that both strong preferred growth within the phase layers and orientational relationships between phase layers, can occur depending on the composition and oxidation conditions. For the scales on stainless steels, the technique enabled the two crystallographic structures, corundum and spinel to be isolated. These structures can be easily differentiated using the EBSD data alone, but the individual phases within them can only be distinguished using the chemical data, collected simultaneously with the EBSD data, because of their crystallographic similarity. This technique revealed two discrete phases for each structure within the oxide scales. For the spinel structure, this consisted of a predominantly chromium and iron containing layer beside the substrate below a coarse grained phase composed of nickel and iron. Meanwhile, an iron rich (haematite) layer at the upper scale surface and a thin chromium rich phase that exists within the fine grained lower scale both possessed the corundum structure.

Dopant contrast in the helium ion microscope

Due to miniaturisation of semiconductor devices, there is an increasing need for nanoscale characterisation of dopant distributions. Scanning electron microscopy (SEM) has been identified as a potential technique to fulfil this need, providing that a small enough probe size (~ 0.1 nm) could be achieved. Probes of this size are not possible in a low-voltage scanning electron microscope but a He-ion beam can be focussed to probe sizes as small as 0.25 nm; a significant improvement over that attainable in the SEM. This paper presents results from the first use of helium ion microscopy (HeIM) to examine dopant contrast in semiconductor materials. It was found that the spatial resolution is improved when compared to SEM and that the contrast mechanism has similarities making HeIM an ideal candidate for future 2-dimensional nanoscale dopant mapping.

Effect of rejuvenation heat treatments on gamma prime distributions in a Ni based superalloy for power plant applications

Abstract The gamma prime (γ′) distribution in the Ni based superalloy, CMSX-4, has been examined in detail using a field emission gun scanning electron microscope and a transmission electron microscope after high temperature degradation and the subsequent application of a number of rejuvenation heat treatments. The observed microstructures have been studied in order to explain the observed mechanical properties of the material and also to assess the effectiveness of the heat treatment procedures and their effect on the subsequent mechanical performance. It is shown that there are significant differences in the rafting behaviour and the size of the channels between the γ′ particles and the size of the tertiary γ′ particles in each of the different microstructural conditions. The aim of this research is to provide greater understanding of the suitability of rejuvenation heat treatments and their role in the extension of component life in power plant applications.

The effect of oxide overlayers on secondary electron dopant mapping.

The International Technology Roadmap for Semiconductors ranks dopant profiling as one of the most difficult challenges for analysis of semiconductors. Dopant mapping in the scanning electron microscope (SEM) has the potential to provide a solution. This technique has not yet found widespread application, however, mainly due to the lack of a comprehensive theoretical model, uncertain quantification, and its inability to differentiate doping levels in n-type silicon. Although a Monte Carlo model was recently published that closely matched experimental data obtained in p-doped silicon to data obtained from the theoretical model, a large discrepancy between experimental data obtained for n-type silicon was found. Here we present a Monte Carlo model that provides close matches between experimental and calculated data in both n- and p-type silicon, paving the way for a widespread application of SEM dopant contrast.

Quantitative dopant contrast in the helium ion microscope

As semiconductor devices shrink in size, the challenge of characterisation of their dopant distributions intensifies. Scanning electron microscopy (SEM) has been proposed as a suitable technique to overcome this challenge. However, current low-voltage (LV) SEMs are incapable of the probe sizes required for nano-scale dopant mapping, but the recently commercialised helium ion microscope (HeIM) is capable of probe sizes of 0.25 nm; a significant improvement over LVSEM. This paper discusses the dopant contrast mechanism in the HeIM and is the first demonstration of nano-scale, quantitative dopant mapping in the HeIM.

Resolution Limits of Secondary Electron Dopant Contrast in Helium Ion and Scanning Electron Microscopy

As the miniaturization of semiconductor devices continues, characterization of dopant distribution within the structures becomes increasingly challenging. One potential solution is the use of the secondary electron signal produced in scanning electron (SEMs) or helium ion microscopes (HeIMs) to image the changes in electrical potential caused by the dopant atoms. In this article, the contrast mechanisms and resolution limits of secondary electron dopant contrast are explored. It is shown that the resolution of the technique is dependent on the extent of electrical potential present at a junction and that the resolution of dopant contrast can be improved in the HeIM after an in-situ plasma cleaning routine, which causes an oxide to form on the surface altering the contrast mechanism from electrical potential to material contrast.

The use of EBSD to study the microstructural development of oxide scales on 316 stainless steel

Abstract 316 stainless steel has been oxidised at 1200°C in air for varying times and with different cooling rates. The resulting scales were examined using optical and electron microscopy techniques including electron backscatter diffraction (EBSD) and energy dispersive X-ray spectroscopy (EDS). It was found that the scales on a sample oxidised for 4 hours consists of three layers; the lowest layer is a fine equiaxed region which has a uniform distribution of chromium which is similar to the base metal, followed by a larger equiaxed layer with very little chromium content but a high iron content, with a final layer of columnar grains of which some are rich in nickel. With a slower cooling rate a large amount of internal oxidation within the metallic substrate was observed which showed a chromium content higher than the oxidised metal.

Helium ion microscopy based wall thickness and surface roughness analysis of polymer foams obtained from high internal phase emulsion.

Due to their wide range of applications, porous polymers obtained from high internal phase emulsions have been widely studied using scanning electron microscopy. However, due to their lack of electrical conductivity, quantitative information of wall thicknesses and surface roughness, which are of particular interest to tissue engineering, has not been obtained. Here, Helium Ion Microscopy is used to examine uncoated polymer foams and some very strong but unexpected contrast is observed, the origin of which is established here. Based on this analysis, a method for the measurement of wall thickness variations and wall roughness measurements has been developed, based on the modeling of Helium ion transmission. The results presented here indicate that within the walls of the void structure there exist small features with height variations of ~30 nm and wall thickness variations from ~100 nm to larger 340 nm in regions surrounding interconnecting windows within the structure. The suggested imaging method is applicable to other porous carbon based structures with wall thicknesses in the range of 40-340 nm.

Microstructural and Chemical Rejuvenation of a Ni-Based Superalloy

The microstructural evolution of the Ni-based superalloy CMSX-4 including the change in gamma prime morphology, size, and distribution after high-temperature degradation and subsequent rejuvenation heat treatments has been examined using field emission gun scanning electron microscopy and transmission electron microscopy. In this paper, it is shown that there are significant differences in the size of the ‘channels’ between gamma prime particles, the degree of rafting, and the size of tertiary gamma prime particles in each of the different microstructural conditions studied. Chemical analysis has been carried out to compare rejuvenated and pre-service samples after the same subsequent degradation procedure. The results indicate that although the microstructures of pre-service and rejuvenated samples are similar, chemical differences are more pronounced in the rejuvenated samples, suggesting that chemical segregation from partitioning of the elements was not completely eliminated through the applied rejuvenation heat treatment. A number of modified rejuvenation heat treatment trials were carried out to reduce the chemical segregation prior to creep testing. The creep test results suggest that chemical segregation has an immeasurable influence on the short-term mechanical properties under the test conditions used here, indicating that further work is required to fully understand the suitability of specific rejuvenation heat treatments and their role in the extension of component life in power plant applications.