Topography and field effects in the quantitative analysis of conductive surfaces using ToF-SIMS

doi:10.1016/j.apsusc.2008.05.164

Applied Surface Science

Volume 255, Issue 4, 15 December 2008, Pages 1560-1563

https://doi.org/10.1016/j.apsusc.2008.05.164 Get rights and content

Abstract

A detailed study is presented of the effects of surface topography on ToF-SIMS images, using experimental results from model conducting fibres, consisting of gold wires of diameters in the range of 33–125 μm mounted onto silicon wafers, and ion optics simulations using SIMION. Chemical analysis over the whole of the fibre surface is not possible, and typically only the central 15 μm of the fibre can be observed. Signals from the substrate are also shadowed by the presence of the fibre. Both effects are caused by the distortion of the extraction field around these conducting samples. We provide clear guidance to practical analysts for the reduction of the topographic field effects, by the use of a lower extractor voltage and an extraction delay. In particular, the effects of extraction delay on ion intensities, mass resolution and time-of-flight are studied in detail. In addition, for large incidence angles of ≥55°, a fraction of the Bi⁺ primary ions are scattered from the fibre target, emitting secondary ions from the substrate which are recorded at the location of the fibre. A method to diagnose this effect is given.

Introduction

Many innovative devices possess significant surface topography, including microfluidic systems, MEMS, fibres, composite materials, sensors and biomedical devices. The functionality and activity of these components is often critically dependent on their nanoscale surface chemistry and molecular interactions. However, quantitative chemical characterisation of surfaces with topography remains a significant challenge due to the lack of systematic and validated measurement methods. In particular, topography can cause many unwanted artefacts in ToF-SIMS spectra and images [1], [2]. There are many aspects to this problem, from theoretical considerations (e.g. sputtering yields vs. incidence angle), experimental practicalities (e.g. sample mounting and optimal set up) to the need for fast, robust and validated methods of image data analysis. In this initial study we combine an experimental approach using a simple model system, consisting of conducting gold wires mounted on a silicon substrate, with computer modelling using SIMION [3], to understand and quantify the key factors that give rise to unwanted topographical artefacts in ToF-SIMS images, and provide guidance to recognise and reduce these artefacts. Elsewhere [4] we extend our studies to insulating fibres with multi-component coatings, which are of critical importance, for example, to the personal care industry, and we optimise the use of multivariate methods such as principal component analysis (PCA) and multivariate curve resolution (MCR) for the rapid processing of high-resolution ToF-SIMS images for samples with topography.

Section snippets

Experimental

Gold wires, with nominal diameters of 33 μm, 60 μm and 125 μm, are obtained from Goodfellow and are used as received without additional cleaning. They are held under tension and mounted onto clean silicon wafers by the application of silver dag (Agar Scientific) at each end. Static SIMS analyses were made using an ION-TOF IV instrument (ION-TOF GmbH, Germany) of single-stage reflectron design [5] with an extraction gap of 1.5 mm and a typical extractor voltage of 2000 V. Positive ion images are

Topographic field effects

The total secondary ion images of a gold wire, with diameter of 125 μm, are shown in Fig. 1. The fibre is orientated with its axis parallel to the primary ion beam azimuth to minimise geometrical distortions, which are explained in reference [1]. It is clear that the topography critically limits the regions from which sputtered secondary ions may be detected, and chemical analysis over the whole area is difficult. Only a thin central region on the fibre can be detected with significant

Extractor voltage

We have already seen from Fig. 2 that using a lower extractor voltage can reduce the extent of topographic effects in ToF-SIMS imaging. However, this reduces the transmission of the analyser and the mass resolution. A lower extractor voltage also increases the acquisition time required for an image, since the cycle time required for measuring mass m using extractor voltage E is proportional to √(m/E). Where topographic effects are causing a significant issue for analysis, the extractor voltage

Conclusions and recommendations

We have conducted a detailed study of the key factors that give rise to topographic artefacts in ToF-SIMS images. Using this, we recommend the following: (1) if possible, samples should be analysed in an orientation that minimises geometrical distortions, for example, along rather than perpendicular to the fibre axis, (2) the sample should be in good contact with a flat conductive substrate to minimise distortions in the extraction field, (3) any primary ion scattering from one region of the

Acknowledgements

The authors would like to thank S.J. Spencer for sample preparation and mounting, and to Dr. A.G. Shard and F.M. Green for helpful comments. This work forms part of the Chemical and Biological Programme of the National Measurement System of the UK Department of Innovation, Universities and Skills (DIUS). This work also forms part of the DIUS Micro and Nano Technology programme.

References (6)

S. Sun et al.
J. Am. Soc. Mass Spectrom.
(2005)
S. Rangarajan et al.
J. Vac. Sci. Technol. A
(2006)
L.A. McDonnell et al.
Anal. Chem.
(2003)

There are more references available in the full text version of this article.

Cited by (43)

Improving the technological readiness of time of Flight-Secondary Ion Mass Spectrometry for enhancing fingermark recovery - towards operational deployment
2023, Science and Justice
The processes routinely used by police forces to visualise fingermarks in casework may not provide sufficient ridge pattern quality to aid an investigation. Time of Flight-Secondary Ion Mass Spectrometry (ToF-SIMS) has been proposed as a technique to enhance fingermark recovery. The technique is currently designated a Category C process in the Fingermark Visualisation Manual (FVM) as it shows potential for effective fingermark visualisation but has not yet been fully evaluated. Here the sensitivity of ToF-SIMS on three common exhibit-type surfaces - paper, polyethylene and stainless-steel was compared to standard processes. An adapted Home Office grading scale was used to evaluate the efficacy of fingerprint development by ToF-SIMS and to provide a framework for comparison with standard processes. ToF-SIMS was shown to visualise more fingerprints than the respective standard process, for all surfaces tested. In addition, ToF-SIMS was applied after the standard processes and successfully enhanced the fingerprint detail, even when the standard process failed to visualise ridge detail. This demonstrates the benefit for incorporating it into current operational fingermark development workflows. Multivariate analysis (MVA), using simsMVA, was additionally explored as a method to simplify the data analysis and image generation process.
Halide perovskite for photodetector applications
2022, Low-Dimensional Halide Perovskites: Structure, Synthesis, and Applications
Halide perovskite has dawned as a novel material for photovoltaic applications owing to its futuristic characteristics. Furthermore, its exceptional characteristics such as simplicity and affordability of fabrication, long diffusion lengths, adjustable spectral absorption range, and high carrier mobility render it one of the market-competitive and most exceptional materials beyond photovoltaics such as photodetectors, lasers, light-emitting diodes (LED), memory devices, etc. Furthermore, the perovskite versatility in device architecture supports in achieving relatively higher performance devices. This chapter explains why halide perovskite materials are becoming more popular for a wide range of applications by describing their distinctive qualities, such as electrical and optical properties. Finally, this chapter also discusses the challenges to be overcome for several applications and the gold standards of perovskite as next-generation optoelectronics devices.
Conversion coating distribution on rough substrates analyzed by combining surface analytical techniques
2021, Applied Surface Science
Citation Excerpt :
This particular approach on the type of analyzed system has never been demonstrated, to our knowledge. The so-called shadowing effect of the surface roughness on the ion beam can be described by simple geometry [63]. The significant roughness of the analyzed sample can result in the zones less accessible for sputtering with the ion beam oriented 45° to the surface and influence the shape of the crater.
In order to describe the chemical composition and the distribution of a thin trivalent chromium conversion coating on rough electrodeposited Zn-based alloy substrate, a methodology combining surface analytical and sputtering techniques was used. A sputtering by argon in glow discharge optical emission spectrometry (GD-OES) and by oxygen using time-of-flight secondary ion mass spectrometry (ToF-SIMS) were used to monitor the in-depth chemical composition evolution of the conversion coating and define the metal-conversion coating interface. Surface topography measurements were performed by high resolution 3D optical microscope. The chemical characterization of the conversion coating on the top surface and near the rough metal-oxide interface inside the craters was performed by X-ray photoelectron spectroscopy.
The local depth and composition of the conversion layer inside and outside the craters were performed by Auger electron spectroscopy nanoprobe equipped with coaxial detector to prevent from shadowing effect during measurement. The results indicated inhomogeneous sputtering inside the ToF-SIMS craters, which was attributed to the shadowing effect, caused by the roughness of the substrate and the 45° incidence angle ion gun. GD-OES sputtering appeared homogeneous in different areas inside the crater despite the roughness of the substrate, which was attributed to the perpendicular incidence angle of the sputtering.
Chemical recognition based on high-accuracy matching factors as per time-of-flight–secondary-ion mass spectrometry: Application to trace cosmetic residues in human forensics
2020, Microchemical Journal
Citation Excerpt :
The unknown spectrum commonly originates from a ToF-SIMS data file, which can be from a single mass spectral scan, an average of scans, or image mapping. Each search produces a hit list of library mass spectra, which is ordered by a factor of similarity, to match the target spectrum in accordance with a computed MF value [27]. Ideally, the MF should reflect the probability that the unknown and reference mass spectra originate from the same compound.
Recognizing the chemical composition of samples or specimens from time-of-flight secondary-ion mass spectrometry (ToF-SIMS) is particularly challenging. Although ToF-SIMS is a powerful tool for obtaining high-quality mass spectra, and produces a large raw dataset with rich chemical information, analysis and interpretation of the results are rather complex. In particular, chemical identification of multi-component mass spectra is difficult because of the large number of peaks. Applications of chemometric and statistical methods are essential. In this work, we used a probability-based matching factor (MF) in combination with principal component analysis (PCA) to recognize the most probable compound from various forensic specimens. We used PCA to classify cosmetics as a function of category and then highlighted the possible origins of various residues. In combination with PCA, we compared the fragmentation pattern of unknown residues with reference mass spectra in a library by using an MF algorithm, and produced a list of the most probable identities based on the best-matching values. We focused our investigation on popular Korean cosmetics and the most common types of forensic. We established a large database of references from different brands among various categories of cosmetic products. We collected both positive and negative ion mass spectra to resolve the chemical compositions, depending on the most appropriate polarity present in ToF-SIMS datasets. This new method offers a unique approach to identify the cosmetic residues present in forensic specimens, and compliments conventional methods for exploring the origin of residues or validating forensic evidence. The algorithm combined with PCA will benefit diverse chemical characterization fields.¹
Secondary ion mass spectrometry: The application in the analysis of atmospheric particulate matter
2017, Analytica Chimica Acta
Currently, considerable attention has been paid to atmospheric particulate matter (PM) investigation due to its importance in human health and global climate change. Surface characterization, single particle analysis and depth profiling of PM is important for a better understanding of its formation processes and predicting its impact on the environment and human being. Secondary ion mass spectrometry (SIMS) is a surface technique with high surface sensitivity, high spatial resolution chemical imaging and unique depth profiling capabilities. Recent research shows that SIMS has great potential in analyzing both surface and bulk chemical information of PM. In this review, we give a brief introduction of SIMS working principle and survey recent applications of SIMS in PM characterization. Particularly, analyses from different types of PM sources by various SIMS techniques were discussed concerning their advantages and limitations. The future development and needs of SIMS in atmospheric aerosol measurement are proposed with a perspective in broader environmental sciences.
A novel PFIB sample preparation protocol for correlative 3D X-ray CNT and FIB-TOF-SIMS tomography
2017, Ultramicroscopy
Citation Excerpt :
Such a configuration is not suitable for FIB-TOF-SIMS measurements as it leads to a very low ion detection due to the unfavourable distribution of the electric field lines. The influence of sample topology on the quality of TOF-SIMS signal (capability of detecting ions, presence of artefacts) has been studied based on metallic wires attached to flat surfaces [7,8]. The occurrence of shadows and technique limitations in ion extraction have been observed when imaging already tens of micrometre large objects.
We present a novel sample preparation method that allows correlative 3D X-ray Computed Nano-Tomography (CNT) and Focused Ion Beam Time-Of-Flight Secondary Ion Mass Spectrometry (FIB-TOF-SIMS) tomography to be performed on the same sample. In addition, our invention ensures that samples stay unmodified structurally and chemically between the subsequent experiments. The main principle is based on modifying the topography of the X-ray CNT experimental setup before FIB-TOF-SIMS measurements by incorporating a square washer around the sample. This affects the distribution of extraction field lines and therefore influences the trajectories of secondary ions that are now guided more efficiently towards the detector. As the result, secondary ion detection is significantly improved and higher, i.e. statistically better, signals are obtained.

View all citing articles on Scopus

View full text