SIMS of organic layers with unknown matrix parameters: Locating the interface in dual beam argon gas cluster depth profiles

Four simple methods are evaluated to determine their accuracies for establishing the interface location in secondary ion mass spectrometry intensity depth profiles of organic layers where matrix effects have not been measured. Accurate location requires the separate measurement of each ion's matrix factor. This is often not possible, and so estimates using matrix-less methods are required. Six pure organic material interfaces are measured using many secondary ions to compare their locations from the four methods with those from full evaluation with matrix terms. For different secondary ions, matrix effects cause the apparent interface positions to vary over 20 nm. The shifts in the intensity profiles on going from a layer of P into a layer of Q are in the opposite direction to that for going from Q into P, so doubling layer thickness errors. The four methods are as follows: M1, use of the median interface position in the intensity profiles for the five lightest ions for 15 ≤ m/z ≤ 150; M2, extrapolation of the position for each ion to m/z = 0 for ions with m/z ≤ 150; M3, as M2 but for m/z ≤ 300; and M4, the extreme positions for all m/z ≤ 100. Comparison with the location using matrix terms shows their ranking, from best to worst, to be M4, M3, M1, and M2 with average errors of 10%, 12%, 14%, and 17%, respectively, of the profile interface full widths at half maximum. Use of pseudo-molecular ions is very much poorer, exceeding 50%, and should be avoided.     

Experimental shift allowance in the deconvolution of SIMS depth profiles

We study the deconvolution of the secondary ion mass spectrometry (SIMS) depth profiles of silicon and gallium arsenide structures with doped thin layers. Special attention is paid to allowance for the instrumental shift of experimental SIMS depth profiles. This effect is taken into account by using Hofmann's mixing‐roughness‐information depth model to determine the depth resolution function. The ill‐posed inverse problem is solved in the Fourier space using the Tikhonov regularization method. The proposed deconvolution algorithm has been tested on various simulated and real structures. It is shown that the algorithm can improve the SIMS depth profiling relevancy and depth resolution. The implemented shift allowance method avoids significant systematic errors of determination of the near‐surface delta‐doped layer position. Copyright © 2013 John Wiley & Sons, Ltd.     

Reduction of matrix effects in TOF‐SIMS analysis by metal‐assisted SIMS (MetA‐SIMS)

We investigated reduction of the matrix effect in time‐of‐flight secondary ion mass spectrometry (TOF‐SIMS) analysis by the deposition of a small amount of metal on the sample surfaces (metal‐assisted SIMS or MetA‐SIMS). The metal used was silver, and the substrates used were silicon wafers as electroconductive substrates and polypropylene (PP) plates as nonelectroconductive substrates. Irganox 1010 and silicone oil on these substrates were analyzed by TOF‐SIMS before and after silver deposition. Before silver deposition, the secondary ion yields from the substances on the silicon wafer and PP plate were quite different due to the matrix effect from each substrate. After silver deposition, however, both ion yields were enhanced, particularly the sample on the PP plate, and little difference was seen between the two substrates. It was therefore found that the deposition of a small amount of metal on the sample surface is useful for reduction of the matrix effect. By reducing the matrix effect using this technique, it is possible to evaluate from the ion intensities the order of magnitude of the quantities of organic materials on different substrates. In addition, this reduction technique has clear utility for the imaging of organic materials on nonuniform substrates such as metals and polymers. MetA‐SIMS is thus a useful analysis tool for solving problems with real‐world samples. Copyright © 2005 John Wiley & Sons, Ltd.     

TOF‐SIMS studies of yttria‐stabilised zirconia

The surface of an as‐polished and an as‐sintered yttria‐stabilised zirconia pellet was analysed with XPS and TOF‐SIMS (depth profiling and imaging) in order to study the distribution of impurities. The polished sample was slightly contaminated with Na, K, Mg and Ca. The sintered sample showed a thin surface film of segregated species, especially Na, Si and Al. Below the surface film, it was found that the grain boundaries were filled with impurities. The chemical compositions of the as‐polished and as‐sintered surfaces are very different and the surface state should be considered when performing electrochemical measurements. Copyright © 2006 John Wiley & Sons, Ltd.     

Measurement of sputtering yields and damage in C60 SIMS depth profiling of model organic materials

Sputter‐depth profiles of model organic thin films on silicon using C₆₀ primary ions have been employed to measure sputtering yields and depth resolution parameters. We demonstrate that some materials (polylactide, Irganox 1010) have a constant and high sputtering yield, which varies linearly with the primary ion energy, whereas another material (Alq₃) has lower, fluence‐dependent sputtering yields. Analysis of multi‐layered organic thin films reveals that the depth resolution is a function of both primary ion energy and depth, and the sputtering yield depends on the history of sputtering. We also show that ～30% of repeat units are damaged in the steady‐state regime during polylactide sputtering. Crown Copyright © 2006. Reproduced with the permission of Her Majesty's Stationery Office. Published by John Wiley & Sons, Ltd.     

TOF‐SIMS study of cystine and cholesterol stones

Two different human stones, cystine and cholesterol from the kidney and gall bladder, were examined by time‐of‐flight secondary ion mass spectrometry using Ga⁺ primary ions as bombarding particles. The mass spectra of kidney stone were compared with those measured for the standard compounds, cystine and cysteine. Similar spectra were obtained for the stone and cystine. The most important identification was based on the existence of the protonated molecules [M + H]⁺ and deprotonated molecules [M‐H]^‐. The presence of cystine salt was also revealed in the stone through the sodiated cystine [M + Na]⁺ and the associated fragments, which might be due to the patient treatment history. In the gallstone, the deprotonated molecules [M‐H]⁺ of cholesterol along with relatively intense characteristic fragments [M‐OH]⁺ were detected. Copyright © 2012 John Wiley & Sons, Ltd.     

An investigation of hydrogen depth profiling using ToF‐SIMS

Hydrogen depth distributions in silicon, zinc oxide, and glass are of great interest in material research and industry. Time‐of‐flight SIMS has been used for hydrogen depth profiling for many years. However, some critical information, such as optimal instrumental settings and detection limits, is not easily available from previous publications. In this work, optimal instrumental settings and detection limits of hydrogen in silicon, zinc oxide, and common glass were investigated. The recommended experimental settings for hydrogen depth profiling using time‐of‐flight SIMS are: (i) keeping pressure in the analysis chamber as low as possible, (ii) using a cesium beam for sputtering and monitoring the H^– signal, (iii) employing monatomic ion analysis beams with the highest currents, and (iv) using interlace mode. In addition, monatomic secondary ions from a matrix are recommended as references to normalize the H^– signal. Detection limits of hydrogen are limited by the pressure of residual gases in the analysis chamber. The base pressure of the analysis chamber (with samples) is about 7 × 10^?10 mbar in this study, and the corresponding detection limits of hydrogen in silicon, zinc oxide, and common glass are 1.3 × 10¹⁸ atoms/cm³, 1.8 × 10¹⁸ atoms/cm³, and 5.6 × 10¹⁸ atoms/cm³, respectively. Copyright © 2011 John Wiley & Sons, Ltd.     

High mass and spatial resolution mass spectrometry imaging of Nicolas Poussin painting cross section by cluster TOF‐SIMS

Time‐of‐flight secondary ion mass spectrometry (TOF‐SIMS) imaging using cluster primary ion beams is used for the identification of the pigments in the painting of Rebecca and Eliezer at the Well by Nicolas Poussin. The combination of the high mass resolution of the technique with a sub‐micrometer spatial resolution offered by a delayed extraction of the secondary ions, together with the possibility to simultaneously identifying both minerals and organics, has proved to be the method of choice for the study of the stratigraphy of a paint cross section. The chemical compositions of small grains are shown with the help of a thorough processing of the data, with images of specific ions, mass spectra extracted from small regions of interest, and profiles drawn along the different painting layers. Copyright © 2016 John Wiley & Sons, Ltd.     

A wavelet‐PCA method saves high mass resolution information in data treatment of SIMS molecular depth profiles

A detailed depth characterization of multilayered polymeric systems is a very attractive topic. Currently, the use of cluster primary ion beams in time‐of‐flight secondary ion mass spectrometry allows molecular depth profiling of organic and polymeric materials. Because typical raw data may contain thousands of peaks, the amount of information to manage grows rapidly and widely, so that data reduction techniques become indispensable in order to extract the most significant information from the given dataset. Here, we show how the wavelet‐based signal processing technique can be applied to the compression of the giant raw data acquired during time‐of‐flight secondary ion mass spectrometry molecular depth‐profiling experiments. We tested the approach on data acquired by analyzing a model sample consisting of polyelectrolyte‐based multilayers spin‐cast on silicon. Numerous wavelet mother functions and several compression levels were investigated. We propose some estimators of the filtering quality in order to find the highest ‘safe’ approximation value in terms of peaks area modification, signal to noise ratio, and mass resolution retention. The compression procedure allowed to obtain a dataset straightforwardly ‘manageable’ without any peak‐picking procedure or detailed peak integration. Moreover, we show that multivariate analysis, namely, principal component analysis, can be successfully combined to the results of the wavelet‐filtering, providing a simple and reliable method for extracting the relevant information from raw datasets. Copyright © 2016 John Wiley & Sons, Ltd.     

Quantitative SIMS depth profiling of Al in AlGaN/AlN/GaN HEMT structures with nanometer‐thin layers

The possibilities of quantitative secondary ion mass spectrometry (SIMS) depth profiling of Al in AlxGa1 ? xN/AlN/GaN transistor heterostructures are shown. Using a series of test structures for a TOF.SIMS‐5 time‐of‐flight mass spectrometer, we obtained a refined linear calibration dependence of the secondary‐ion yield on the composition ×, namely, Y(CsAl+)/Y(CsGa+) = K × x/(1 ? x), with a high linear correlation coefficient, Rl = 0.9996, which permits quantitative SIMS analysis of relatively thick AlGaN barrier layers. The method of profile reconstruction with allowance for the main artifacts of ion sputtering has been first applied for the analysis of GaN/AlGaN/AlN/GaN high electron mobility transistor structure. This method permits to perform quantitative analysis of the thickness and composition of a nanometer‐thin AlN sublayer and to estimate the measurement error. For the structure being studied, the AlN sublayer is 1.2 ± 0.2 nm thick. Copyright © 2016 John Wiley & Sons, Ltd.     

Cluster TOF‐SIMS imaging of human skin remains: analysis of a South‐Andean mummy sample

A skin sample from a South‐Andean mummy dating back from the XI^th century was analyzed using time‐of‐flight secondary ion mass spectrometry imaging using cluster primary ion beams (cluster‐TOF‐SIMS). For the first time on a mummy, skin dermis and epidermis could be chemically differentiated using mass spectrometry imaging. Differences in amino‐acid composition between keratin and collagen, the two major proteins of skin tissue, could indeed be exploited. A surprising lipid composition of hypodermis was also revealed and seems to result from fatty acids damage by bacteria. Using cluster‐TOF‐SIMS imaging skills, traces of bio‐mineralization could be identified at the micrometer scale, especially formation of calcium phosphate at the skin surface. Mineral deposits at the surface were characterized using both scanning electron microscopy (SEM) in combination with energy‐dispersive X‐ray spectroscopy and mass spectrometry imaging. The stratigraphy of such a sample was revealed for the first time using this technique. More precise molecular maps were also recorded at higher spatial resolution, below 1 µm. This was achieved using a non‐bunched mode of the primary ion source, while keeping intact the mass resolution thanks to a delayed extraction of the secondary ions. Details from biological structure as can be seen on SEM images are observable on chemical maps at this sub‐micrometer scale. Thus, this work illustrates the interesting possibilities of chemical imaging by cluster‐TOF‐SIMS concerning ancient biological tissues. Copyright © 2012 John Wiley & Sons, Ltd.     

Evaluation of the sputtering rate variation in SIMS ultra‐shallow depth profiling using multiple short‐period delta layers

We have developed multiple short‐period delta layers as a reference material for SIMS ultra‐shallow depth profiling. Boron nitride delta layers and silicon spacer layers were sputter‐deposited alternately, with a silicon spacer thickness of 1–5 nm. These delta‐doped layers were used to measure the sputtering rate change in the initial stage of oxygen ion bombardment. A significant variation of sputtering rate was observed in the initial 3 nm or less. The sputtering rate in the initial 3 nm was estimated to be about four times larger than the steady‐state value for 1000 eV oxygen ions. Copyright © 2003 John Wiley & Sons, Ltd.     

Characterization of size‐segregated aerosols using ToF‐SIMS imaging and depth profiling

Size‐segregated particles were collected with a ten‐stage micro‐orifice uniform deposit impactor from a busy walkway in a downtown area of Hong Kong. The surface chemical compositions of aerosol samples from each stage were analyzed using time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) operated in the static mode. The ToF‐SIMS spectra of particles from stage 2 (5.6–10 µm), stage 6 (0.56–1 µm), and stage 10 (0.056–0.1 µm) were compared, and the positive ion spectra from stage 2 to stage 10 were analyzed with principal component analysis (PCA). Both spectral analysis and PCA results show that the coarse‐mode particles were associated with inorganic ions, while the fine particles were associated with organic ions. PCA results further show that the particle surface compositions were size dependent. Particles from the same mode exhibited more similar surface features. Particles from stage 2 (5.6–10 µm), stage 6 (0.56–1 µm), and stage 10 (0.056–0.1 µm) were further selected as representatives of the three modes, and the chemical compositions of these modes of particles were examined using ToF‐SIMS imaging and depth profiling. The results reveal a non‐uniform chemical distribution from the outer to the inner layer of the particles. The coarse‐mode particles were shown to contain inorganic salts beneath the organics surface. The accumulation‐mode particles contained sulfate, nitrate, ammonium salts, and silicate in the regions below a thick surface layer of organic species. The nucleation‐mode particles consisted mainly of soot particles with a surface coated with sulfate, hydrocarbons, and, possibly, fullerenic carbon. The study demonstrated the capability of ToF‐SIMS depth profiling and imaging in characterizing both the surface and the region beneath the surface of aerosol particles. It also revealed the complex heterogeneity of chemical composition in size and depth distributions of atmospheric particles. Copyright © 2014 John Wiley & Sons, Ltd.     

Evaluation and comparison of layered titanate nanosheets using TOF‐SIMS and g‐ogram analysis

The evaluation of nanostructure is important to develop the highly controlled nanomaterials. In this study, two kinds of layered titanate nanosheets, which were produced by using hexylamine and laurylamine, respectively, as surfactants were investigated by Gentle Secondary Ion Mass Spectrometry Gentle‐SIMS (G‐SIMS) and g‐ogram, which is the latest Time‐of‐Flight Secondary Ion Mass Spectrometry (TOF‐SIMS) data analysis method for detecting more intact ions and obtaining the information on original chemical structures of samples precisely from complicated TOF‐SIMS spectra. As a result, molecular related ions of the surfactants were detected from each sample, and the structural information of samples was obtained. From both samples, surfactant molecular ions connected with hydrocarbon were detected as more intact ions rather than molecular ions of themselves. It was suggested that hydrophobic domains of their lamellar mesostructure are formed robustly by more than two surfactant molecules connected with each other linearly. After all, important information on the chemical structure of the layered titanate nanosheets, which would be difficult to be found by using typical structural analysis methods such as X‐ray diffraction and transmission electron microscopy, were obtained using G‐SIMS and g‐ogram. Therefore, it was shown that g‐ogram and G‐SIMS are helpful to evaluate the nanostructured materials. And it was also shown that g‐ogram is applicable to organic–inorganic materials which contain long hydrocarbon structures. Copyright © 2015 John Wiley & Sons, Ltd.     

Adsorption of genetically engineered proteins studied by time‐of‐flight secondary ion mass spectrometry (TOF‐SIMS). Part B: hierarchical cluster analysis (HCA)

In Part A, we adopted principal component analysis (PCA) for the analysis of TOF‐SIMS data to assess the binding specificity of GBP‐1 to metallic Au, Ag and Pd. Within a given set of data, PCA aids in the interpretation of the TOF‐SIMS spectra by capitalizing on the differences from one spectrum to another. In Part B, we introduce another multivariate statistical method called ‘hierarchical cluster analysis (HCA)’, where visualization of the similarity and difference in data is readily observed, from which a variety of adsorption conditions of GBP‐1 were characterized. Copyright © 2007 John Wiley & Sons, Ltd.     

Static TOF‐SIMS — a VAMAS interlaboratory study. Part I. Repeatability and reproducibility of spectra

An interlaboratory study involving 32 Time‐of‐Flight Static SIMS instruments from 12 countries has been conducted. Analysts were supplied, by NPL, with a protocol for analysis together with three reference materials; a thin layer of polycarbonate (PC) on a silicon wafer, a thin layer of polystyrene (PS) oligomers on etched silver and poly(tetrafluoroethylene) (PTFE). The study involved static SIMS analysis of each reference material for both positive and negative polarity secondary ions. The option to test instrument suitability for G‐SIMS was also provided. The results of this study show that over 84% of instruments have excellent repeatabilities of better than 1.9%. Repeatabilities can be as good as 0.4%. A relative instrument spectral response (RISR) is calculated for each instrument for each reference material and ion polarity. The RISR is used to evaluate variations in spectral response between different generic types of SIMS instruments. Use of the RISR allows the identification of contamination, charge stabilisation problems and incorrectly functioning ion detectors. The high quality of the data presented here allows the RISR to reveal differences in individual operation of each instrument such as the use of apertures to remove metastables from the spectra and the use of different post‐acceleration voltages for ion detection. Spectral reproducibility can be measured, here, by the equivalence of RISRs between materials and ion polarities. It is found that reproducibilities are on average 10% but can be as good as 4% for the best instruments. This figure shows the consistency between instruments in measuring spectra from different samples. This study sets out the basic framework to develop static secondary ion mass spectrometry (SSIMS) as a reliable measurement method. © Crown Copyright 2005. Reproduced with the permission of Her Majestry's Stationery Office. Published by John Wiley & Sons, Ltd.     

Investigating the chain conformations of spin‐coated polymer thin films by ToF‐SIMS depth profiling

Thin films of bromine‐terminated poly(bisphenol A octane ether) (BA‐C10) were prepared using 1,2‐dichlorobenzene (ODCB) as the solvent. The organization of the chains in these amorphous polymer films was evaluated using time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) depth profiling. For the thin films, the bifunctional polymer chains were folded and anchored to the substrate via their two Br end groups and a polymer brush of chain loops was formed on the substrate. As the film thickness increased, polymer chains in a random coil conformation were found to reside on the top of the polymer brush. Depth profiling revealed that the polymer chains were densely packed at the interface. Moreover, the polymer films showed thermal stability, implying strong interactions between the end groups and the substrate. Copyright © 2015 John Wiley & Sons, Ltd.     

TOF‐SIMS characterization of an aluminovanadate oxide catalyst

Time‐of‐flight Secondary Ion Mass Spectrometry (TOF‐SIMS) has been employed to characterize the surface physical and chemical state of aluminovanadate oxide catalyst precursors (‘V? Al? O’) precipitated at different pH values in the range of 5.5…10. The reference oxide V₂O₅ has also been studied for comparison purposes. It is shown that the analysis of molecular ion emission yields valuable information on the surface elemental and phase composition. Increasing pH values while precipitating from aqueous precursor solutions are found to result in a monotonic variation of the surface composition, in a progressive hydroxylation of aluminium and vanadium and in an increasing dispersion of vanadium oxide species. SIMS data evaluated on the basis of Plog's valence model of molecular ion emission reveal reduced V⁴⁺ states, the fraction of which is dependent on the pH value. The SIMS results are supported by XPS data. The enhancement of the catalytic activity in oxidative propane dehydrogenation over V? Al? O prepared at high precipitation pH is in good correlation with the measured surface characteristics. Copyright © 2007 John Wiley & Sons, Ltd.     

An effect of measurement conditions on the depth resolution for low‐energy dual‐beam depth profiling using TOF‐SIMS

An effect of measurement conditions on the depth resolution was investigated for dual‐beam time of flight‐secondary ion mass spectrometry depth profiling of delta‐doped‐boron multi‐layers in silicon with a low‐energy sputter ion (200 eV – 2 keV O₂⁺) and with a high‐energy primary ion (30 keV Bi⁺). The depth resolution was evaluated by the intensity ratio of the first peak and the subsequent valley in B⁺ depth profile for each measurement condition. In the case of sputtering with the low energy of 250 eV, the depth resolution was found to be affected by the damage with the high‐energy primary ion (Bi⁺) and was found to be correlated to the ratio of current density of sputter ion to primary ion. From the depth profiles of implanted Bi⁺ primary ion remaining at the analysis area, it was proposed that the influence of high‐energy primary ion to the depth resolution can be explained with a damage accumulation model. Copyright © 2013 John Wiley & Sons, Ltd.