Sputter Depth Profiling Research Articles

Practical guides for x-ray photoelectron spectroscopy: Use of argon ion beams for sputter depth profiling and cleaning

Ion beams are used in x-ray photoelectron spectroscopy (XPS) to clean samples and perform compositional sputter depth profiles. The purpose of this article is to compile good practice, recommendations, and useful information related to the use of argon ion sources for inexperienced users of XPS instrumentation. The most used type of ion source generates monoatomic argon ions at a range of energies from a fixed direction relative to the instrument. The angle and direction of the ion beam with respect to the surface are normally altered by manipulating the sample, and this may involve tilting the sample to change the angle of incidence or rotating the sample to change the azimuthal incidence angle. Atomic argon ion beams cause damage to the structure of the material surface, which may exhibit itself as a change in stoichiometry or topography as well as the implantation of argon atoms. Therefore, caution is required in the interpretation of XPS depth profiles. Gas cluster ion sources offer new possibilities and choices to XPS users. Gas cluster sources enable the sputtering of organic materials with high yield in comparison to inorganic materials and offer the potential for nearly damage-free depth profiling of delicate organic materials as well as low damage cleaning of inorganic materials. It may be possible to use argon clusters to reduce damage during the depth profiling of inorganic materials, but there is currently insufficient evidence to make any general recommendations.

Principal investigation on the surface chemistry of phosphated zinc coatings treated by hot active plasma

The effect of hot active plasma (HAP) on phosphated zinc coated steel was studied in detail with x-ray photoelectron (XPS) and scanning Auger electron spectroscopy (AES) combined with electron microscopy. In order to extend the scope of the methods from the surface down to the bulk of the coating, Ar+ ion sputter depth profiling coupled with XPS was utilized. While a mild surface cleaning effect was observed for weak treatments, increased HAP intensity caused significant chemical and structural changes on the coating surface. Initially, a partial decomposition of zinc orthophosphate (Zn3(PO4)2), as the sole surface constituent of the untreated coating, led to a mixed zinc metaphosphate (Zn(PO3)x) and zinc oxide (ZnO) phase, highly localized in randomly distributed micrometer sized spots on the treated surface. Finally, the most intensive HAP treatment, leading to a temperature rise of the treated sample up to 700 °C as revealed by in-situ temperature measurements, caused a complete transformation of the zinc phosphate material into a porous and rough ZnO layer. Moreover, the heat contribution of the HAP treatment induced remarkable modifications within the bulk of the coating – mixing of zinc with the underlying steel substrate – an effect mimicking a galvannealing process.

Deposition of Fe2O3:Sn semiconducting thin films by reactive pulsed HiPIMS + ECWR co-sputtering from Fe and Sn targets

Semiconducting Fe2O3 thin films were deposited on SnO2:F (FTO) and Pt substrates by reactive high-power impulse magnetron sputtering combined with electron cyclotron wave resonance plasma (HiPIMS + ECWR). Fe2O3 films were fabricated either by sputtering from a single Fe target or by co-sputtering from an additional Sn target. Plasma parameters during co-sputtering were measured by an RF probe system enabling the comparison between HiPIMS + ECWR and only HiPIMS conditions used for the film deposition. As deposited Fe2O3 films were post-annealed in air at 450 °C and 650 °C, respectively. It was shown that as deposited Fe2O3 films were amorphous but became crystalline after post-annealing at 450 °C. Further increase of annealing temperature to 650 °C did not improve significantly the crystalline structure of the Fe2O3 film. All post-annealed Fe2O3 films exhibited photocurrents in the anodic region; generally, hematite films annealed at 650 °C exhibited higher photocurrents than those annealed at 450 °C. Films doped by Sn co-sputtering had higher photocurrents than films only doped by Sn diffusion during post-annealing from the FTO substrate. Hematite films on Pt substrate doped by Sn co-sputtering and post-annealed at 650 °C exhibited the highest photocurrents. It was verified by XPS analysis with ion sputtering depth profiling that Pt atoms also diffuse from the Pt substrate into the Fe2O3 film during the post-annealing at 650 °C and can, similar to Sn in the case of an FTO substrate, act as a dopant.

Femtosecond laser ablation (fs-LA) XPS – A novel XPS depth profiling technique for thin films, coatings and multi-layered structures

Sputter depth profiling has been employed for XPS/AESdepth profilingsincethe late 1960s. However, for many materials, ion beam induced damage distorts the chemical state information and chemical composition, limiting the value of the analysis. A novelmethodology is presented in which XPS depth profiles are generated using a 160 fs pulse length, 1030 nm peak wavelength femtosecond laser in place of the traditional ion gun. Femtosecond laser ablation (fs-LA) XPS depth profiles are compared with argon monatomic and cluster ion beam depth profilesfor different classes of materials, including ceramics, semiconductors, polymers and metals. In all cases, the XPS spectra recorded following femtosecond laser ablation are fully representative of the original chemical composition and chemical state, with no ablation induced damage. The technique is also shown to be very versatile with ablation rates of <20 nm per pulse being achieved for thin films but profiles to depths of >30 μm are also realisable in practical time scales.fs-LA XPS depth profiling promises to be a very exciting new methodology for the XPS community, complementing sputter depth profiling but avoiding the chemical damage induced by ion beams and offering valuable new depth profiling capabilities.

Nitrogen isotope marker experiments in austenitic stainless steel for identification of trapping/detrapping processes at different temperatures

Expanded austenite – austenitic stainless steel containing a colossal amount of nitrogen in solid solution – is still some kind of conundrum with the nitrogen atom transport from the surface into the bulk being enhanced for higher nitrogen content. At the same time, atomic ordering due to an enhanced affinity between chromium and nitrogen is observed.Here, nitrogen transport experiments are performed at different temperatures using 14N and 15N as isotopic tracers. The depth distribution of each of these isotopes as well as the total nitrogen content is investigated with time-of-flight secondary ion mass spectrometry (ToF-SIMS) and glow discharge optical emission spectroscopy (GDOES), respectively. In parallel, phase information is obtained using in situ X-ray diffraction (XRD) during nitriding (for time-resolved data) and during sputter depth profiling (for depth-resolved data).During nitriding, no delayed transport of nitrogen, i.e. trapping, is observed for the expanded austenite. However, as soon as CrN precipitates are formed, nitrogen trapping is a dominant factor with an almost perfect correlation between precipitation and trapping. Cluster ion analysis in SIMS confirms this effect with the abundance of Cr2+ ions significantly increased compared to FeCr+ ions in the presence of precipitation. As nitrogen transport and CrN formation occur in parallel at higher temperatures, further experiments to separate the CrN formation at higher temperatures from the nitrogen transport at lower temperatures have been performed.

Comment on “Restoration of the original depth distribution from the SIMS profile using the depth resolution function in framework of RMR model” [J. Vac. Sci. Technol. B 41, 024003 (2023)

This comment is a summary of scientifically evident shortcomings of the so-called recoil-mixing-roughness (RMR)-model proposed in several publications by Kudriavtsev et al. in order to describe the formation of sputter depth profiles of concentration.

XPS characterization of a PuGa-7 at. % alloy

A δ-Pu(Ga-7 at. %) alloy has been characterized using x-ray photoelectron spectroscopy in the as-received condition, during sputter depth profiling through the surface oxide and following oxidation either by exposure to Langmuir quantities of oxygen or by oxidation from the residual gases in the ultrahigh vacuum environment. To enable quantitative analysis, we develop a procedure to separate the C 1s and Pu 5p1/2 peak intensities, which occur at the same binding energy. Sputter yields for the various oxides and alloys were calculated. The plutonium, oxygen, and gallium chemical state plots are presented for the oxides and alloy measured here.

Model experiments with Auger electron and X-ray photoelectron spectroscopy at lithiated graphite material: Surface changes related to ion- and electron-beam effects

Li-based batteries as a key element for green energy economy should be permanently improved in their performance. Therefore, the complex electrochemical processes in such batteries have to be understood in detail. For their characterization beside (electro-)chemical methods also electron spectroscopic methods such as X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) are used to get more insight into the chemical processes at the surfaces. One crucial point is stability of the solid electrolyte interface (SEI) upon measurements, especially at the (graphite-)anodes. In this paper, we use highly oriented pyrolytic graphite (HOPG) as a model anode material, which was lithiated in a molten metal intercalation process. Strong beam damage effects at the lithiated HOPG under electron and ion beam radiation are observed and described. These effects are important for using of local electron beam investigations and sputter depth profiling, often applied also for SEI characterization.

Dynamic character of compositional sputter depth profiling by SIMS: A comparison of different models for quantitative profile evaluation

We are dealing with some new insights in the quantification of sputter depth profiles obtained by secondary ion mass spectroscopy, which can be easily extended to XPS or AES. Recent publications present a rather negative image of the mixing-roughness-information depth (MRI) model of quantitative sputter depth profile evaluation in conjunction with Dowsett’s up-and-down slope (UDS) model, at first we attempt to point out the merits of the MRI model. Since these publications come up with a new, alternative model [called roughness-mixing-recoil model (RMR)], we discuss in detail the flaws of both the UDS and the RMR models. In conclusion, we present some instructive examples that show the decisive validity of the MRI model in describing the nature of the process of sputter depth profiling.

Sputter yields of monoatomic solids by Ar and Ne ions near the threshold: A Bayesian analysis of the Yamamura Model

Semi-empirical formulae like the Yamamura model provide a quick reference of sputter yields for applications such as sputter depth profiling and secondary ion mass spectrometry. Fit parameters in such models are prone to errors which can propagate into the prediction of sputter yields. We compare experimental sputter yields to predictions of the Yamamura model using a Bayesian Markov Chain Monte Carlo (MCMC) algorithm. The model parameters Q (linear scaling) and s (power-law scaling) are explored. The results from MCMC are then compared to propositions of Seah (Surface and Interface Analysis, 2005) and extended to a collection of target materials by fitting simulated yields for argon and neon using TRIDYN. Q was found to be proportional to the threshold energy and a simple relation is proposed. The simplicity notwithstanding, Q is speculated to be dependent on a multitude of parameters such as density, energy transfer and orbital filling.

Formation of MoO3/Organic Interfaces

AbstractMolybdenum trioxide (MoO3) has been extensively used in numerous organic semiconductor devices for hole injection and extraction. In this paper, photoelectron spectroscopy combined with cleavage and sputter depth profile has been used to probe the structures of buried MoO3/organic semiconductor interfaces. Organics used in this work include: tris(4‐carbazoyl‐9‐ylphenyl)amine (TCTA), N,N′‐bis(naphthalene‐l‐yl)‐N,N′‐bis(phenyl)benzidine (NPB), 4,4′‐bis(carbazol‐9‐yl)‐2,2′‐biphenyl (CBP), and 1,3‐bis(N‐carbazolyl) benzene (mCP). It is found that there are two distinct types of interfaces: sharp interfaces (type‐I) where the oxide layer has limited or no diffusion when deposited on organics that have a high glass transition temperature such as TCTA and NPB; mixed interfaces (type‐II) where the formation of interfaces is followed by significant diffusion and reaction on organics having low glass transition temperatures such as CBP and mCP. The causes for these two types of interfaces are discussed.

TOF-SIMS for carbon hybridization state analysis

Time-of-flight secondary ion mass spectra measured for different carbon allotropes under Bi3+ cluster ion bombardment were acquired. Ion yield of homogeneous (Cn)- and heterogeneous metal-carbon (CsCn)- secondary cluster ions was examined. Qualitative explanation of the regularities was given based on the concept of nonlinear atomic collision cascades. It was suggested that the features of secondary cluster yield can be used for determination of hybridization state of the target material. Unlike most other techniques, secondary ion mass spectrometry is capable of the sputter depth profiling of the hybridization state.

Top‐Down Approach to Study Chemical and Electronic Properties of Perovskite Solar Cells: Sputtered Depth Profiling Versus Tapered Cross‐Sectional Photoelectron Spectroscopies

A study of the chemical and electronic properties of various layers across perovskite solar cell (PSC) stacks is challenging. Depth‐profiling photoemission spectroscopy can be used to study the surface, interface, and bulk properties of different layers in PSCs, which influence the overall performance of these devices. Herein, sputter depth profiling (SDP) and tapered cross‐sectional (TCS) photoelectron spectroscopies (PESs) are used to study highly efficient mixed halide PSCs. It is found that the most used SDP‐PES technique degrades the organic and deforms the inorganic materials during sputtering of the PSCs while the TCS‐PES method is less destructive and can determine the chemical and electronic properties of all layers precisely. The SDP‐PES dissociates the chemical bonding in the spiro‐MeOTAD and perovskite layer and reduces the TiO2, which causes the chemical analysis to be unreliable. The TCS‐PES revealed a band bending only at the spiro‐MeOTAD/perovskite interface of about 0.7 eV. Both the TCS and SDP‐PES show that the perovskite layer is inhomogeneous and has a higher amount of bromine at the perovskite/TiO2 interface.

Parabolic oxidation kinetics of a plutonium alloy at room temperature

Auger electron spectroscopy (AES) was used to characterize the oxide scale formed on an ∼0.5 wt. % Ga plutonium metal alloy. Analysis of changes to the characteristic Auger transitions for Pu and O during sputter depth profiles were used in combination with previously measured sputter yields to determine the thickness of the oxide scale and provide insight into its chemistry. From this data, the parabolic kinetics for the Pu-Ga alloy were determined as a function of relative humidity, RH = 14–92%, and oxygen partial pressure, p(O2) = 31–157 Torr, at 25 °C. For each relative humidity investigated, the parabolic rate constant was found to be independent of oxygen partial pressure. Power-law fits to the relative humidity effect on the rate constant was found to have a power of n = 1.265, indicating the diffusion species which rate-limits the reaction isn’t molecular water, which subsequent dissociates at the oxide-metal interface, or hydroxyl from dissociation at the gas-surface interface.

Room temperature oxygen exchange and diffusion in nanometer-thick ZrO2 and MoO3 films

The diffusion of oxygen in thin films of ZrO2 and MoO3 was investigated with atomic 18O as a tracer using low energy ion scattering sputter depth profiling. 3 nm amorphous and 20 nm polycrystalline films were prepared by reactive magnetron sputtering and exposed to atomic oxygen species at room temperature. Exposure results in a fast diffusion of oxygen to a limited depth of ~1 nm and ~2.5 nm for ZrO2 and MoO3, respectively, and surface exchange limited to a maximum of 65% to 75%. The influence of the crystalline structure of the films on exchange and diffusion was negligible. We propose that the transport of oxygen in oxides at room temperature is dominated by a field-induced drift, generated by the chemisorption of reactive oxygen species. The maximum penetration of oxygen is limited by the oxide space charge region, determined by the oxide electrical properties. We applied a drift–diffusion model to extract values of surface potential and kinetic parameters of oxygen exchange and diffusion. The developed experimental analysis and modelling suggest that the electric field and consequent distribution of charged species are the main factors governing exchange rates and species diffusion in an oxide thin film at room temperature.

Partially Crystallized Ultrathin Interfaces between GaN and SiNx Grown by Low-Pressure Chemical Vapor Deposition and Interface Editing.

The formation mechanism of the partially crystallized ultrathin layer at the interface between GaN and SiNx grown by low-pressure chemical vapor deposition was analyzed based on the chemical components of reactants and products detected by high-resolution sputter depth profile analysis by X-ray photoelectron spectroscopy. A reasonable mass action equation for the formation of Si2N2O was proposed from the feasibility analysis of the Gibbs free energy changes of the reaction. The high-energy-activated Ga2O on the surface likely assists in the synthesis of the crystallized components. A well-defined 1ML θ-Ga2O3 transition interface was inserted into Si2N2O/GaN pure interface supercell slabs to edit the unsaturated state of the bonds. Low-density states can be achieved when the effective charges of the unsaturated atoms are adjusted to a certain interval.

Preferential sputtering in quantitative sputter depth profiling of multi-element thin films

Preferential sputtering plays an important role in the quantification of measured depth profiles. The distortion of the measured depth profile as compared to the original one has to be evaluated upon preferential sputtering in depth profiling. In the framework of the mixing-roughness-information depth (MRI) model, the influences of preferential sputtering on the depth profile of multi-element thin films are quantitatively evaluated as demonstrated for a layered structure of three elements with different sputtering rates and subsequent interfaces as A/BC/A. Moreover, the mass conservation upon Auger electron spectroscopy/X-ray photoelectron spectroscopy depth profiling of multi-element structure is discussed with respect to the preferential effect. Finally, as an example, the measured glow discharge optical emission spectroscope depth profiling data of 60x(3.0 nm Mo/0.3 nm B4C/3.7 nm Si)/Si(111) multilayer structure are quantitatively evaluated using the extended MRI model.

Probing pollutant reactions at the iron surface: a case study on selenite reactions with nanoscale zero-valent iron

The speciation and distribution of Se in the oxide layer of nZVI were studied using high-resolution X-ray photoelectron spectroscopy with sputter depth profiling, and the speciation and distribution of Se were proved to be depth dependent.

A study of the interfacial chemistry between polymeric methylene diphenyl di‐isocyanate and a Fe–Cr alloy

The interactions of polymeric methylene diphenyl di‐isocyanate (pMDI) and a model Fe–Cr alloy have been studied by X‐ray photoelectron spectroscopy (XPS) and time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS). Films of two different thicknesses have been investigated: one with an extremely thin pMDI layer in which the interfacial chemistry can be probed directly and a thicker one in which sputter profiling using cluster ions is necessary to expose the interface chemistry for direct analysis. Multivariate analysis (MVA), using principal component analysis (PCA) and nonnegative matrix factorisation (NMF), has been used to identify specific ions associated with the interfacial region of the ToF‐SIMS sputter depth profile and chemical species from the XPS sputter depth profile. As an unsupervised method, this avoids an unconscious bias on the part of the analyst. Specific ions associated with pMDI interactions with both Fe and Cr allow the proposal of two complementary reaction mechanisms, supported by the XPS data. A range of cluster ions is used in this investigation, but the bulk of the work used argon clusters for the XPS depth profiles and Buckminster Fullerene projectiles for the ToF‐SIMS analyses. To ensure that such data were directly comparable, the ToF‐SIMS sputter profiles were repeated in a different system of the same type using argon cluster ions.

Optimization of the two parameters in the deconvolution procedure for obtaining the original in-depth distribution of composition from measured sputter depth profile by genetic algorithm

The deconvolution method with the TV (Total Variation)-Tikhonov algorithm and the MRI (atomic Mixing-Roughness-Information depth) depth resolution function was recently proposed for reconstructing the original in-depth distribution of composition directly from measured depth profiling data. In this paper, it shows that the two parameters of α and β in the TV-Tikhonov algorithm play an important role in determining the deconvolution result with high precision and strong anti-noise performance. Because the measured depth profiling data often have some kinds of noise and/or insufficient data points, the optimization of α and β values is necessary and is carried on by the genetic algorithm. The convergence of the genetic simulation and the influences of these two parameters on the deconvolution result are evaluated quantitatively. Finally, as an example, the original layer structure of a Ni/Ag multilayer is deconvoluted directly from the measured AES depth profiling data and is agreed well with the TEM measurement.