Preferential sputtering observed in Auger electron spectroscopy sputter depth profiling of AlAs/GaAs superlattice using low‐energy ions

Auger electron spectroscopy (AES) sputter depth profiling of an ISO reference material of the GaAs/AlAs superlattice was investigated using low‐energy Ar⁺ ions. Although a high depth resolution of ～1.0 nm was obtained at the GaAs/AlAs interface under 100 eV Ar⁺ ion irradiation, deterioration of the depth resolution was observed at the AlAs/GaAs interface. The Auger peak profile revealed that the enrichment of Al due to preferential sputtering occurred during sputter etching of the AlAs layer only under 100 eV Ar⁺ ion irradiation. In addition, a significant difference in the etching rates between the AlAs and GaAs layers was observed for low‐energy ion irradiation. Deterioration of the depth resolution under 100 eV Ar⁺ ion irradiation is attributed to the preferential sputtering and the difference in the etching rate. The present results suggest that the effects induced by the preferential sputtering and the significant difference in the etching rate should be taken into account to optimize ion etching conditions using the GaAs/AlAs reference material under low‐energy ion irradiation. Copyright © 2005 John Wiley & Sons, Ltd.     

Depth resolution and inhomogeneity of the sputtering dose with sample rotation and ion beam rastering

Sample rotation during sputter depth profiling can improve the measured depth resolution. We examine some of the practical issues of particular relevance to rotation conditions in SIMS. There are many ways of arranging the rotation for sputtering and, to illustrate the issues, one configuration is studied. Through simulations of the spatial distribution of ion dose using a rastered ion beam and sample rotation, we demonstrate that significant variations in the distribution of ion dose across the surface can occur during the profile. With rotational frequencies much lower than raster frequencies, as used for Auger and XPS, these variations are typically small and have the same periodicity as the rotation. If rotational frequencies are similar to, or larger than the raster frequency, then large spatial variations in dose can occur. In this case, extreme care must be taken to ensure that the two frequencies are not related by a simple rational number and that the sputtering ion beam size should be significantly broader than the line spacing of the raster, but significantly smaller than the raster size. Specific recommendations are provided for setting both the rotation frequency and the size of the sputtering ion beam in order to ensure that the relative standard deviation of the ion dose across the analysis area remains lower than 1%. However, the best method may be a stepwise rotation of 90° after each analysis and sputtering cycle. © Crown copyright 2011. Reproduced with the permission of Her Majesty's Stationery Office. Published by John Wiley & Sons, Ltd.     

VAMAS interlaboratory study on organic depth profiling

We present the results of a VAMAS (Versailles project on Advanced Materials and Standards) interlaboratory study on organic depth profiling, in which twenty laboratories submitted data from a multilayer organic reference material. Individual layers were identified using a range of different sputtering species (C₆₀ⁿ⁺, Cs⁺, SF₅⁺ and Xe⁺), but in this study only the C₆₀ⁿ⁺ ions were able to provide truly ‘molecular’ depth profiles from the reference samples. The repeatability of profiles carried out on three separate days by participants was shown to be excellent, with a number of laboratories obtaining better than 5% RSD (relative standard deviation) in depth resolution and sputtering yield, and better than 10% RSD in relative secondary ion intensities. Comparability between laboratories was also good in terms of depth resolution and sputtering yield, allowing useful relationships to be found between ion energy, sputtering yield and depth resolution. The study has shown that organic depth profiling results can, with care, be compared on a day‐to‐day basis and between laboratories. The study has also validated three approaches that significantly improve the quality of organic depth profiling: sample cooling, sample rotation and grazing angles of ion incidence. © Crown copyright 2010.     

A statistical approach to delta layer depth profiling

We present a simple statistical model describing the removal and relocation of material during a sputter depth profiling experiment. All input parameters are determined from low‐fluence molecular dynamics simulations, making the model de facto parameter free. The model can be used to extrapolate data from the molecular dynamics simulations to projectile fluences relevant to sputter depth profiling experiments. As a result, the erosion of the surface is calculated in terms of fluence‐dependent filling factors of different sample layers. Using input data determined for the 20‐keV C₆₀ cluster bombardment of silicon, it is found that a steady‐state erosion profile is reached after removal of approximately 20 monolayer equivalents of material. Plotting the contribution of particles from a specific layer to the instantaneous sputtered flux, one can directly determine the delta layer response function predicted from such a model. It is shown that this function can be parameterized by the semiempirical Dowsett response function, and the resulting fitting parameters are compared with published depth profile data. The model is then used to study the role of different processes influencing the observed depth resolution. We find that the statistical nature of the sputtering process suffices to explain many features of experimentally measured delta layer depth profiles. Copyright © 2012 John Wiley & Sons, Ltd.     

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.     

Quantification of AES depth profiling data of polycrystalline Al films with Gaussian and non‐Gaussian surface height distributions

Sputtering‐induced roughness is the main distortional factor on the depth resolution of measured depth profiles, in particular, for sputtering of polycrystalline metals. Frequently, the surface height distribution of the sputtering‐induced roughness exhibits an asymmetrical feature. In such a case, a non‐Gaussian height distribution function (HDF) has to be applied for the quantification of a measured depth profile. By replacing the usually applied Gaussian HDF with that of an asymmetrical triangle in the Mixing‐Roughness‐Information depth model, measured Auger electron spectroscopy depth profiling data of the interface of polycrystalline Al films on Si are perfectly fitted. The asymmetric triangle height distributions obtained from the best fit are a reasonable approximation of the height distributions measured by atomic force microscopy. Copyright © 2013 John Wiley & Sons, Ltd.     

Analytic function to describe interfaces and resolution consistency in sputter depth profiling

A simple analytical function is derived to describe the interface shapes measured in sputter depth profiling by using X‐ray photoelectron spectroscopy or secondary ion mass spectrometry. This function involves the convolution of a central Gaussian function, often taken to describe the roughness, together with an exponential tail to describe mixing and an exponential approach often taken to describe an information depth. This model is consistent with Hofmann's mixing‐roughness‐information model that does the same by numerical analysis, but we present a direct analytical function that is more transparent to the user. The differential of the function gives Dowsett's function for delta layers. Depending on which of the 3 base parameters are identified as sample related, the analyst can obtain the centroid of the underlying composition. These functions are used to show the extent that the common measure of depth resolution for step edges and delta functions diverge as the profile becomes less Gaussian.     

Quantitative nitrogen analysis by Auger electron spectrometry and glow discharge optical emission spectrometry

Nitrides of refractory metals are investigated as diffusion barriers for Cu metallization. The composition, thermal stability and inter diffusion in layered systems are characterized by depth profile analysis. For the quantification of depth profiles determination of sensitivity factors is essential. For nitrogen and other light elements matrix specific standards are often not available and compound standards are used for calibration. We have investigated the systems Ta–N and Ta–Si–N and for comparison Cr–N by means of Auger electron spectrometry (AES) and glow discharge optical emission spectrometry (GDOES). A non-linear calibration curve for the N/Cr intensity ratio was observed with GDOES in the Cr–N-system, probably caused by self-absorption of the Cr line.     

Implementing the electron backscattering factor in quantitative sputter depth profiling using AES

Sputter depth profiling using Auger electron spectroscopy (AES) is influenced by the electron backscattering contribution to the AES intensity. When approaching an interface between two components having a different backscattering factor, the shape of the profile is characteristically distorted. This distortion is taken into account in a modified version of the mixing‐roughness‐information depth (MRI) model. The modification is based on the simplified assumption that the influence of the backscattering effect of the component below the interface increases exponentially with decreasing distance of the actual surface to the interface. Application of the modified MRI model is shown to yield excellent results of profile calculation for AES depth profiling of Si/W, C/Ta, C/Ti, and Au/TiO₂ interfaces, with backscattering factor ratios close to those predicted by the Ichimura–Shimizu relation. A simple correction of the backscattering influence is proposed and discussed. 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.     

Behavior and process of background signal formation of hydrogen,carbon, nitrogen,and oxygen in silicon wafers during depth profiling using dual-beam TOF-SIMS

The behavior and mechanism of background signals during depth profiling of atmospheric elements using dual-beam time-of-flight secondary ion mass spectrometry (TOF-SIMS) have been experimentally investigated for silicon wafers. The background signals of atmospheric elements were found to be inversely proportional to the sputtering rate. Most of the background signals are largely attributable to the accumulation of components through adsorption and ion bombardment in the pre-equilibrium state. On the other hand, the contribution of real-time adsorption during the instant after the last sputtering in the equilibrium state is negligible under the present experimental conditions. H₂O is dominant in the background formation process of hydrogen and oxygen, which is supported by the higher adsorption coefficients. The background levels of carbon and nitrogen are lower than those of hydrogen and oxygen. Furthermore, the background signal of carbon with respect to the sputtering rate shows a different trend than the other elements. This could be attributed to accumulation in the pre-equilibrium state. These results indicate that the background levels can be lowered close to those of dynamic-SIMS by using an extremely high sputtering rate in dual-beam TOF-SIMS.     

Characterization of oxide layers on amorphous Mg-based alloys by Auger electron spectroscopy with sputter depth profiling

Amorphous ribbons of Mg-Y-TM-[Ag] (TM: Cu, Ni), prepared by melt spinning, were subjected to electrochemical investigations. Oxide layers formed anodically under potentiostatic control in different electrolytes were investigated by AES and sputter depth profiling. Problems and specific features of characterization of the composition of oxide layers and amorphous ternary or quaternary Mg-based alloys have been investigated. In the alloys the Mg(KL(23)L(23)) peak exhibits a different shape compared to that in the pure element. Analysis of the peak of elastically scattered electrons proved the absence of plasmon loss features, characteristic of pure Mg, in the alloy. A different loss feature emerges in Mg(KL(23)L(23)) and Cu(L(23)VV). The system Mg-Y-TM-[Ag] suffers preferential sputtering. Depletion of Mg and enrichment of TM and Y are found. This is attributed mainly to the preferential sputtering of Mg. Thickness and composition of the formed oxide layer depend on the electrochemical treatment. After removing the oxide by sputtering the concentration of the underlying alloy was found to be affected by the treatment.     

Observations on X‐ray enhanced sputter rates in argon cluster ion sputter depth profiling of polymers

Traditionally polymer depth profiling by X‐ray photoelectron spectroscopy (XPS) has been dominated by the damage introduced by the ion beam rather than the X‐rays. With the introduction of polyatomic and especially argon gas cluster ion‐beam (GCIB) sources for XPS instruments, this is no longer the case, and either source of damage may be important (or dominate) under particular conditions. Importantly, while ion‐beam damage is a near‐surface effect, X‐ray damage may extend micrometres into the bulk of the sample, so that the accumulation of X‐ray damage during long depth profiles may be very significant. We have observed craters of similar dimensions to the X‐ray spot well within the perimeter of sputter craters, indicating that X‐rays can assist GCIB sputtering very significantly. We have measured experimentally sputter craters in 13 different polymers. The results show that X‐ray exposure can introduce much more topography than might previously have been expected, through both thermal and direct X‐ray degradation. This can increase the depth of a crater by a remarkable factor, up to three in the case of poly‐L‐lactic acid and polychlorotrifluorothylene under reasonably normal XPS conditions. This may be a major source of the loss of depth resolution in sputter depth profiles of polymers. Copyright © 2012 John Wiley & Sons, Ltd.     

New horizons in sputter depth profiling inorganics with giant gas cluster sources: Niobium oxide thin films

X‐ray photoelectron spectroscopy is used to study a wide variety of material systems as a function of depth (“depth profiling”). Historically, Ar⁺ has been the primary ion of choice, but even at low kinetic energies, Ar⁺ ion beams can damage materials by creating, for example, nonstoichiometric oxides. Here, we show that the depth profiles of inorganic oxides can be greatly improved using Ar giant gas cluster beams. For NbO_x thin films, we demonstrate that using Ar_x⁺ (x = 1000‐2500) gas cluster beams with kinetic energies per projectile atom from 5 to 20 eV, there is significantly less preferential oxygen sputtering than 500 eV Ar⁺ sputtering leading to improvements in the measured steady state O/Nb ratio. However, there is significant sputter‐induced sample roughness. Depending on the experimental conditions, the surface roughness is up to 20× that of the initial NbO_x surface. In general, higher kinetic energies per rojectile atom (E/n) lead to higher sputter yields (Y/n) and less sputter‐induced roughness and consequently better quality depth profiles. We demonstrate that the best‐quality depth profiles are obtained by increasing the sample temperature; the chemical damage and the crater rms roughness is reduced. The best experimental conditions for depth profiling were found to be using a 20 keV Ar₂₅₀₀⁺ primary ion beam at a sample temperature of 44°C. At this temperature, there is no, or very little, reduction of the niobium oxide layer and the crater rms roughness is close to that of the original surface.     

Variation of the electron inelastic mean free path during depth profiling of the Fe/Si interface as determined by quantitative REELS

The aim of this work is to determine the dependence of the electron inelastic mean free path (IMFP) at the Fe/Si interface during depth profiling by sputtering with 3 keV Ar⁺ ions. In order to estimate the variation of the IMFP at the interface, reflection electron energy‐loss spectroscopy (REELS) measurements were performed after different sputtering times at the Fe/Si interface with three different primary electron energies (i.e. 0.5, 1 and 1.5 keV). Even though it is highly likely that a compound (i.e. Fe_xSi) is formed at the interface, all the experimental REELS spectra could be analysed as a linear combination of those corresponding to pure Si and Fe. Using the model developed by Yubero and Tougaard for quantitative analysis of these REELS spectra we could estimate the IMFP values along the depth profile at the interface. The resulting IMFPs are observed to vary linearly with the average composition (as determined by REELS) at the Fe/Si interface as it is sputter depth profiled. The energy dependence of the IMFP for different compositions is presented and discussed. For completeness, we have determined the energy‐loss functions as well as the IMFPs of the pure elements (i.e. Fe and Si). Copyright © 2004 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.     

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.     

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.     

A simple erosion dynamics model of molecular sputter depth profiling

A simple model which describes the essential features commonly observed in a molecular sputter depth profile is presented. General predictions of the dependence of measured molecular ion signals on the primary ion fluence are derived for the specific case where a mass spectrometric technique such as SIMS or secondary neutral mass spectrometry (SNMS) is used to analyze the momentary surface. The results are compared with recent experimental data on molecular depth profiles obtained by cluster‐ion‐initiated SIMS of organic overlayers. Copyright © 2008 John Wiley & Sons, Ltd.