SIMS depth profiling of semiconductor interfaces: Experimental study of depth resolution function

Reconstruction of original element distribution at semiconductor interfaces using experimental SIMS profiles encounters considerable difficulties because of the matrix effect, sputtering rate change at the interface, and also a sputtering‐induced broadening of original distributions. We performed a detailed depth profiling analysis of the Al step‐function distribution in GaAs/Al_xGa_1?xAs heterostructures by using Cs⁺ primary ion beam sputtering and CsM⁺ cluster ion monitoring (where M is the element of interest) to suppress the matrix effect. The experimental Depth Resolution Function (DRF) was obtained by differentiation of the Al step‐function profile and compared with the ‘reference’ DRF found from depth profiling of an Al delta layer. The difference between two experimental DRFs was explained by the sputtering rate change during the interface profiling. We experimentally studied the sputtering rate dependence on the Al_xGa_1?xAs layer composition and applied it for a reconstruction of the DRF found by differentiating the Al step‐function distribution: the ‘reconstructed’ and ‘reference’ DRFs were found to be in good agreement. This confirmed the correctness of the treatment elaborated. Copyright © 2010 John Wiley & Sons, Ltd.     

Comparison of depth resolution from a microscopically modulated Rh/C film after sputter profiling by SIMS and SNMS

Sputter depth profiles of an Rh/C microscopically modulated thin film (double layer thickness 7 nm) were obtained by low energy SIMS and SNMS. The depth resolution was obtained using the linear superposition of error functions at the interfaces. In optimal cases the depth resolution was shown to be of the order of 1–2 nm for both techniques.     

A new approach to express ToF SIMS depth profiling

We propose a new approach to express SIMS depth profiling on a TOF.SIMS‐5 time‐of‐flight mass spectrometer. The approach is based on the instrument capability to independently perform raster scans of sputter and probe ion beams. The probed area can be much smaller than the diameter of a sputter ion beam, like in the AES depth profiling method. This circumstance alleviates limitations on the sputter beam–raster size relation, which are critical in other types of SIMS, and enables analysis on a curved‐bottomed sputter crater. By considerably reducing the raster size, it is possible to increase the depth profiling speed by an order of magnitude without radically degrading the depth resolution. A technique is proposed for successive improvement of depth resolution through profile recovery with account for the developing curvature of the sputtered crater bottom in the probed area. Experimental study of the crater bottom form resulted in implementing a method to include contribution of the instrumental artifacts in a nonstationary depth resolution function within the Hofmann's mixing–roughness–information depth model. The real‐structure experiment has shown that the analysis technique combining reduction of a raster size with a successive nonstationary recovery ensures high speed of profiling at ~100 µm/h while maintaining the depth resolution of about 30 nm at a 5 µm depth. Copyright © 2015 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.     

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.     

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.     

Surface characterization and depth profiling of biological molecules by electrospray droplet impact/SIMS

Electrospray droplet impact (EDI) was applied to the analysis of peptides. The etching rate of bradykinin was estimated to be ~2 nm/min. This value is about one order of magnitude greater than the etching rate for SiO₂ (0.2 nm/min). Considering that the etching rate of argon cluster ions Ar₇₀₀⁺ for organic compounds is more than two orders of magnitude larger than that for inorganic materials, the rather small difference in etching rates of EDI for organic and inorganic materials is unique. When water/ethanol (1/1, vol%) solution of gramicidin S and arginine was dried in air, [gramicidin S + H]⁺ was observed as a predominant signal with little [Arg + H]⁺ right after the EDI irradiation, indicating that EDI is capable of detecting the analytes enriched on the sample surface. Copyright © 2011 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.     

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.     

Spatial determination of gold catalyst residue used in the production of ZnO nanowires by SIMS depth profiling analysis

In this paper we demonstrate how secondary ion mass spectrometry (SIMS) can be applied to ZnO nanowire structures for gold catalyst residue determination. Gold plays a significant role in determining the structural properties of such nanowires, with the location of the gold after growth being a strong indicator of the growth mechanism. For the material investigated here, we find that the gold remains at the substrate–nanowire interface. This was not anticipated as the usual growth mechanism associated with catalyst growth is of a vapour–liquid–solid (VLS) type. The results presented here favour a vapour–solid (VS) growth mechanism instead. Copyright © 2007 John Wiley & Sons, Ltd.     

Subnanometre depth resolution in sputter depth profiling of Si layers using grazing‐incident low‐energy O2+ and Ar+ ions

The sputter damage profiles of Si(100) by low‐energy O₂⁺ and Ar⁺ ion bombardment at various angles of incidence were measured using medium‐energy ion scattering spectroscopy. It was observed that the damaged Si surface layer can be minimized down to 0.5–0.6 nm with grazing‐incident 500 eV Ar⁺ and O₂⁺ ions at 80°. To illustrate how the damaged layer thickness can be decreased down to 0.5 nm, molecular dynamics simulations were used. The SIMS depth resolution estimated with trailing‐edge decay length for a Ga delta‐layer in Si with grazing‐incident 650 eV O₂⁺ was 0.9 nm, which is in good agreement with the measured damaged layer thickness. Copyright © 2004 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.     

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.     

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.     

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.     

Quantitative SIMS depth profiling of diffusion barrier gate-oxynitride structures in TFT-LCDs

Gate oxynitride structures of TFT-LCDs were investigated by SIMS, and successful solutions are demonstrated to overcome difficulties arising due to the charging effects of the multilayer systems, the matrix effect of the method, and the small pattern sizes of the samples. Because of the excellent reproducibility achieved by applying exponential relative sensitivity functions for quantitative analysis, minor differences in the barrier gate-oxynitride composition deposited on molybdenum capped aluminium-neodymium metallisation electrodes were determined between the centre and the edge of the TFT-LCD substrates. No differences were found for molybdenum-tungsten metallisations. Furthermore, at the edge of the glass substrates, aluminium, neodymium, and molybdenum SIMS depth profiles show an exponential trend. With TEM micrographs an inhomogeneous thickness of the molybdenum capping is revealed as the source of this effect, which influences the electrical behaviour of the device.The production process was improved after these results and the aging behaviour of TFT-LCDs was investigated in order to explain the change in control voltage occurring during the lifetime of the displays. SIMS and TEM show an enrichment of neodymium at the interface to the molybdenum layer, confirming good diffusion protection of the molybdenum barrier during accelerated aging. The reason for the shift of the control voltage was finally located by semi-quantitative depth profiling of the sodium diffusion originating from the glass substrate. Molybdenum-tungsten was a much better buffer for the highly-mobile charge carriers than aluminium-neodymium. Best results were achieved with PVD silicon oxynitride as diffusion barrier and gate insulator deposited on aluminium-neodymium metallisation layers.     

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.     

Surface topography effects on glow discharge depth profiling analysis

Glow discharge optical emission spectroscopy (GD‐OES) has been shown to be of immense value in elemental depth profiling of thin or thick films on conductive or non‐conductive substrates. For aluminium, GD‐OES has been employed to examine locations of markers and tracers in anodic films, thereby assisting understanding of transport phenomena. In order to investigate the influence of surface topography on depth profiling analysis, anodic aluminium oxide films of various thicknesses, with incorporated electrolyte species, were produced on superpure aluminium substrates of controlled roughnesses. The distributions of incorporated species in the films were subsequently probed. Surface topography modifications and consequent depth resolution degradation were examined during depth profiling analysis performed by GD‐OES. The results reveal that the sputtering process leads to the roughening or smoothing of the surface topography of the specimen for a ratio of the film thickness to the amplitude of the substrate texture less, or greater, than 1 respectively. As a consequence of the surface topography dependence of the ion bombardment, analysis of thin films over rough surfaces suffers from depth resolution limitations due to sputtering‐induced topography changes, thereby limiting quantification of the resultant spectra. Copyright © 2010 John Wiley & Sons, Ltd.     

Peak‐fitting of high resolution ToF‐SIMS spectra: a preliminary study

Peak‐fitting has been performed on a series of peaks obtained by time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) analysis in order to assess whether information may be obtained from this procedure on the samples' characteristics. A variety of samples were examined including a range of treatments for aluminium leading to different surface roughnesses, polymer films with a range of polydispersities, molecular weight (MW) and thicknesses as well as aluminium samples with adsorbed adhesion promoters on the surface. Variation of peak‐fitting was assessed by varying the peak intensity, full width at half maximum (FWHM) and peak asymmetry. Although further studies are needed it is possible to say that the peak width increases with roughness whereas peak asymmetry seems to be related to oxide thickness. Polymer characteristics do not seem to influence the width whereas the peak asymmetry increases either versus MW or polydispersity. A possible assumption is that the peak asymmetry relates to the ion formation processes. Additional work with varying polymer films thickness indicates that both FWHM and peak asymmetry may be related to sample charging and this could be used for assessment of film thicknesses. Finally, peak‐fitting was used to obtain a more reliable peak area when peaks are too close in mass to use current methods. Copyright © 2009 John Wiley & Sons, Ltd.