Cornelia Rodenburg

High resolution quantitative two-dimensional dopant mapping using energy-filtered secondary electron imaging

We have quantified two-dimensional dopant profiles in Si with high accuracy using a commercial field emission gun scanning electron microscope with added secondary electron energy-filtering capabilities. Quantification was achieved for dopant concentrations of 2.8×1017–7.5×1019cm−3 with an accuracy of ±8.5%. Using energy filtering, we have measured the surface potential difference ΔΦs across a 3nm wide p-doped Si layer with respect to the n-doped substrate of a test structure to be 0.72±0.15V. Spatial resolution limits are discussed and are shown to be limited by the secondary electron escape depth. A technique for the rapid collection of calibrated energy-filtered images is also proposed.

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.

Sub-nanometre resolution imaging of polymer-fullerene photovoltaic blends using energy-filtered scanning electron microscopy

The resolution capability of the scanning electron microscope has increased immensely in recent years, and is now within the sub-nanometre range, at least for inorganic materials. An equivalent advance has not yet been achieved for imaging the morphologies of nanostructured organic materials, such as organic photovoltaic blends. Here we show that energy-selective secondary electron detection can be used to obtain high-contrast, material-specific images of an organic photovoltaic blend. We also find that we can differentiate mixed phases from pure material phases in our data. The lateral resolution demonstrated is twice that previously reported from secondary electron imaging. Our results suggest that our energy-filtered scanning electron microscopy approach will be able to make major inroads into the understanding of complex, nano-structured organic materials.

Imaging the bulk nanoscale morphology of organic solar cell blends using helium ion microscopy.

We use helium ion microscopy (HeIM) to image the nanostructure of poly(3-hexylthiophene)/[6,6]-phenyl-C(61)-butric acid methyl ester (P3HT/PCBM) blend thin-films. Specifically, we study a blend thin-film subject to a thermal anneal at 140 °C and use a plasma-etching technique to gain access to the bulk of the blend thin-films. We observe a domain structure within the bulk of the film that is not apparent at the film-surface and tentatively identify a network of slightly elongated PCBM domains having a spatial periodicity of (20 ± 4) nm a length of (12 ± 8) nm.

A comprehensive Monte Carlo calculation of dopant contrast in secondary-electron imaging

Two-dimensional dopant mapping using secondary electrons in a scanning electron microscope is a useful and rapid technique for studying dopant distributions with high spatial resolution in semiconductor materials and devices. However, it has not yet found widespread application because the quantification of dopant concentrations currently lacks a firm theoretical model. This paper addresses the issue by means of Monte Carlo modelling. We demonstrate that by taking account of the electron affinity in Monte Carlo simulations to calculate the secondary-electron emission from doped silicon the dopant contrast can be explained. This paper also provides a firm theoretical model about the existence of surface effects in dopant contrast imaging.

Site-specific dopant profiling in a scanning electron microscope using focused ion beam prepared specimens

Site-specific dopant profiling across a silicon p-n junction has been performed in the scanning electron microscope (SEM) on samples prepared using focused ion beam (FIB) milling. The results are compared with the dopant contrast obtained after using non-site-specific cleaving and tripod polishing of specimens. FIB milling generates a damaged surface layer which reduces the dopant contrast observed in the SEM using secondary electron imaging. These results show that two dimensional dopant contrast can be observed from FIB-prepared membranes, thereby extending the application of SEM dopant profiling to the examination of complex nanometer-scale device structures for which site selection is essential.

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.

Quantitative secondary electron imaging for work function extraction at atomic level and layer identification of graphene.

Two-dimensional (2D) materials usually have a layer-dependent work function, which require fast and accurate detection for the evaluation of their device performance. A detection technique with high throughput and high spatial resolution has not yet been explored. Using a scanning electron microscope, we have developed and implemented a quantitative analytical technique which allows effective extraction of the work function of graphene. This technique uses the secondary electron contrast and has nanometre-resolved layer information. The measurement of few-layer graphene flakes shows the variation of work function between graphene layers with a precision of less than 10 meV. It is expected that this technique will prove extremely useful for researchers in a broad range of fields due to its revolutionary throughput and accuracy.

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.