Depth Profiling Method Research Articles

(Invited) Towards Spatially Resolved Chemical Analysis of Sn/Ag Solder Bumps By Means of Laser Ablation Ionization Mass Spectrometry (LIMS)

Following the ban of Sn/Pb solders from the markets eutectic Sn/Ag alloy has become the industrially most relevant option in the field of microchip fabrication1. Sn/Ag solder bumps are mostly prepared by means of through-mask electrodeposition. Due to the inherently different electrochemical properties of Sn and Ag, their efficient co-deposition requires optimization of plating parameters as well as the use of numerous organic plating additives2-5, rendering the development of high-performance plating bath compositions a very demanding task. Major criteria for assessing the quality of a plating formulation are the fraction of Ag and the homogeneity of Ag distribution within the as-deposited solder alloy, as these characteristics are crucially relevant for homogeneous melting and defect-free recrystallization during the soldering process1. It is of urgent interest to microchip industry to minimize embedment of the plating additives into the solder material, as these might cause voids during soldering and may lead to a reduced lifetime of the devices under working conditions. Investigation of these aspects requires sophisticated and highly spatially resolved depth profiling methods providing both lateral and vertical information on the solder composition and possible organic residues. To date, well-established composition analysis techniques for solid materials are only partly able to provide such data. In this contribution, we present depth profiling by means of femtosecond laser ablation ionization mass spectrometry (fs-LIMS) using the LMS instrument7, 8 as an approach to achieve these goals. For this purpose, we study fundamentals of fs-laser-matter interaction on Sn and Sn/Ag alloys with particular focus on the influence of different laser characteristics (IR and UV light, laser intensity) on ablation and reflow processes taking place on these materials. Furthermore, we compare different depth profiling approaches, i.e., spatially resolved one-dimensional depth profiling and two-dimensional raster matrices with different geometries. Moreover, we aim towards three-dimensionality with a sample-tailored raster approach that allows to investigate the electroplated deposit layer by layer or to even select specific regions of a given layer only. We demonstrate how this approach yields improved results especially in the analysis of laterally highly confined solder bump arrays. The thus obtained data allow for extraction of depth profiles of the major species Sn and Ag, as well as C, O, and S from the embedded additives within the alloy matrix. These are required to assess the homogeneity and the purity of Sn/Ag electroplated depositions from different bath formulations or under differential plating parameters. Correlation of different elemental profile data to each other might contribute to the evaluation of whether additive or electrolyte embedment takes place and thereby may crucially help to bring forward plating formulation development. M. Abtew and G. Selvaduray, Materials Science and Engineering: R: Reports, 2000, 27, 95-141.J. Y. Kim, J. Yu, J. H. Lee and T. Y. Lee, Journal of Electronic Materials, 2004, 33, 1459-1464.S. Joseph and G. J. Phatak, Surface and Coatings Technology, 2008, 202, 3023-3028.A. Hrussanova and I. Krastev, Journal of Applied Electrochemistry, 2009, 39, 989-994.C. Han, Q. Liu and D. G. Ivey, Materials Science and Engineering: B, 2009, 164, 172-179.A. Cedeño López, V. Grimaudo, P. Moreno-García, A. Riedo, M. Tulej, R. Wiesendanger, P. Wurz and P. Broekmann, Journal of Analytical Atomic Spectrometry, 2018, 33, 283-293.A. Riedo, M. Neuland, S. Meyer, M. Tulej and P. Wurz, Journal of Analytical Atomic Spectrometry, 2013, 28, 1256-1269.A. Riedo, V. Grimaudo, P. Moreno-García, M. B. Neuland, M. Tulej, P. Broekmann and P. Wurz, CHIMIA International Journal for Chemistry, 2016, 70, 268-273. Figure 1

Depth Profiling Using Reflected Electron Spectroscopy

Nondestructive depth profiling method based on reflected electron spectroscopy is presented. Large loss spectra of wide energy range is analyzed. Spectra analysis is performed using two approaches: boundary problem solution using invariant imbedding method and approximate interpretation based on Straight Line Approximation with correction coefficients, which depend on ration of evaporated layer thickness and transport mean free path. It is shown that the transport mean free path, not the inelastic mean free path, limits the information depth of the method in opposite of X-ray photoelectron spectroscopy, elastic peak electron spectroscopy. This fact significantly increase depths that can be analyzed using presented method. Thickness of the evaporated Nb layer on the Si substrate is determined.

Origin of Degradation in Si‐Based All‐Solid‐State Li‐Ion Microbatteries

AbstractLike all rechargeable battery systems, conventional Li‐ion batteries (LIB) inevitably suffer from capacity losses during operation. This also holds for all‐solid‐state LIB. In this contribution an in operando neutron depth profiling method is developed to investigate the degradation mechanism of all‐solid‐state, thin film Si–Li3PO4–LiCoO2batteries. Important aspects of the long‐term degradation mechanisms are elucidated. It is found that the capacity losses in these thin film batteries are mainly related to lithium immobilization in the solid‐state electrolyte, starting to grow at the anode/electrolyte interface during initial charging. The Li‐immobilization layer in the electrolyte is induced by silicon penetration from the anode into the solid‐state electrolyte and continues to grow at a lower rate during subsequent cycling. X‐ray photoelectron spectroscopy depth profiling and transmission electron microscopy analyses confirm the formation of such immobilization layer, which favorably functions as an ionic conductor for lithium ions. As a result of the immobilization process, the amount of free moveable lithium ions is reduced, leading to the pronounced storage capacity decay. Insights gained from this research shed interesting light on the degradation mechanisms of thin film, all‐solid‐state LIB and facilitate potential interfacial modifications which finally will lead to substantially improved battery performance.

Oxidation-Induced Structure Transformation: Thin-Film Synthesis and Interface Investigations of Barium Disilicide toward Potential Photovoltaic Applications.

Barium disilicide (BaSi2) has been regarded as a promising absorber material for high-efficiency thin-film solar cells. However, it has confronted issues related to material synthesis and quality control. Here, we fabricate BaSi2 thin films via an industrially applicable sputtering process and uncovered the mechanism of structure transformation. Polycrystalline BaSi2 thin films are obtained through the sputtering process followed by a postannealing treatment. The crystalline quality and phase composition of sputtered BaSi2 are characterized by Raman spectroscopy and X-ray diffraction (XRD). A higher annealing temperature can promote crystallization of BaSi2, but also causes an intensive surface oxidation and BaSi2/SiO2 interfacial diffusion. As a consequence, an inhomogeneous and layered structure of BaSi2 is revealed by Auger electron spectroscopy (AES) and transmission electron microscopy (TEM). The thick oxide layer in such an inhomogeneous structure hinders further both optical and electrical characterizations of sputtered BaSi2. The structural transformation process of sputtered BaSi2 films then is studied by the Raman depth-profiling method, and all of the above observations come to an oxidation-induced structure transformation mechanism. It interprets interfacial phenomena including surface oxidation and BaSi2/SiO2 interdiffusion, which lead to the inhomogeneous and layered structure of sputtered BaSi2. The mechanism can also be extended to epitaxial and evaporated BaSi2 films. In addition, a glimpse toward future developments in both material and device levels is presented. Such fundamental knowledge on structural transformations and complex interfacial activities is significant for further quality control and interface engineering on BaSi2 films toward high-efficiency solar cells.

Improving the interfacial strength of carbon fiber/vinyl ester resin composite by self-migration of acrylamide: A molecular dynamics simulation

A molecular dynamics simulation was performed to investigate the migration of acrylamide (AM) to the carbon fiber (CF) surface from a liquid vinyl ester resin (VE), as well as the effect on the interfacial strength of CF/VE composite. The analysis of monomer concentration profile after the system equilibrium shows that an AM-rich interphase region was formed and the interfacial bond energy was increased. This can be explained by the greatest average atomic binding energy of AM with the CF surface. The migration of AM was confirmed experimentally by a depth profiling method using plasma etching and X-ray photoelectron spectroscopy. The AM molecules in interphase could chemically bond CF surface and VE matrix. As a result, the interface shear strength and interlaminar shear strength of CF/VE composite were enhanced by 126.98% and 81.86%, respectively.

Bevel depth profiling by high-spatial-resolution sputtered neutral mass spectrometry with laser postionization

Secondary-ion mass spectrometry (SIMS) sputter depth profiling is used for the quantitative depth profile analysis of impurities. However, SIMS suffers from a large quantitative uncertainty and depth-scale uncertainty at the interfaces of heteromultilayers and in the near-surface region, because the secondary ion yield and sputtering yield are significantly influenced by matrix effects and accumulation effects of the primary ion. In this paper, the authors report on the development of a new depth profiling method with good depth-scale accuracy and low matrix effects to overcome these problems. This was achieved through the combination of high-spatial-resolution bevel depth profiling and sputtered neutral mass spectrometry with laser postionization (laser-SNMS). The sample used to evaluate this new bevel depth profiling method was a silicon on insulator wafer obtained using the separation by implantation of oxygen technique and implanted with boron. Depth profiles were obtained using both SIMS and laser-SNMS and evaluated by comparison with the stopping and range of ions in matter (SRIM) simulation. Although both methods afforded quite good depth resolutions, in SIMS the secondary ion signal intensity for boron was amplified by the influence of the matrix effect and showed a discontinuous profile shape at the interfaces, whereas the profile for boron obtained using laser-SNMS was consistent with the SRIM results and exhibited high continuity. By using a combination of the bevel depth profiling method and laser-SNMS method, it was confirmed that an easy-to-analyze depth profile could be obtained for the dopant concentration in multilayer samples, which is difficult to obtain using the conventional SIMS method.

Influence of oxygen partial pressure on the oxygen diffusion and surface exchange coefficients in mixed conductors

The performances of mixed ionic and electronic conductors is used in many applications, such as oxygen transport membranes or electrodes in solid oxide fuel cells. The performances of these systems depend mainly on two fundamental parameters including oxygen diffusion (DO) and the oxygen exchange coefficient (k). This work focuses on the impact of the oxygen partial pressure on oxygen diffusion and surface exchange coefficients of mixed conducting materials, as reported studies are scarce in the literature. In this way, two different mixed conducting materials are studied, namely, La0.6Sr0.4Fe0.6Ga0.4O3-δ, a perovskite-type material, and the nickelate La2NiO4+δ. The DO and k coefficients are determined by a specific oxygen permeation measurement and by the isotopic exchange depth profile method.

Depth profiling of alumina thin films using laser induced breakdown spectroscopy: Structural and morphological dependence

Laser induced breakdown spectroscopy (LIBS) is a relatively new technique that is being applied to thin films characterization as an alternative to the current conventional depth profiling methods. As it is well known, the laser-induced plasma is a micro source of light that can be analyzed spectroscopically in order to obtain the characteristic spectral lines of the elements in the sample surface. In depth profiling, focused laser pulses are fired repetitively onto the same spot of the sample surface, and the depth is related to the recorded consecutive spectra. In this study we are presenting the results of experiments carried out on the depth profile analysis of alumina coated silicon substrate using LIBS technique. The aluminum and silicon spectral emission lines were measured in the LIBS spectra using the fundamental wavelength (1064 nm) of the Nd:YAG laser. The concepts of the determination of the coating thickness and chemical composition are presented and the effect of thin films structure and surface roughness has been discussed.

In-depth component distribution in electrodeposited alloys and multilayers

It is shown in this overview that modern composition depth profiling methods like secondary neutral mass spectroscopy (SNMS) and glow-discharge – time-of-flight mass spectrometry (GD-ToFMS) can be used to gain highly specific composition depth profile information on electrodeposited alloys. In some cases, cross-sectional transmission electron microscopy was also used for gaining complementary information; nevertheless, the basic component distribution derived with each method exhibited the same basic features. When applying the reverse sputtering direction to SNMS analysis, the near-substrate composition evolution can be revealed with unprecedented precision. Results are presented for several specific cases of electrodeposited alloys and mulitlayers. It is shown that upon d.c. plating from an unstirred solution, the preferentially deposited metal accumulates in the near-substrate zone, and the steady-state alloy composition sets in at about 150-200 nm deposit thickness only. If there is more than one preferentially deposited metal in the alloy, the accumulation zones of these metals occur in the order of the deposition preference. This accumulation zone can be eliminated by well-controlled hydrodynamic conditions (like the application of rotating disc electrodes) or by pulse plating where the systematic decrease in the duty cycle provides a gradual transition from a graded to a uniform composition depth profile. The application of composition depth profile measurements enabled detecting the coincidence in the occurrence of some components in the deposits down to the impurity level. This was exemplified by the GD-ToFMS measurements of Ni-Cu/Cu multilayers where all detected impurities accumulated in the Cu layer. The wealth of information obtained by these methods provides a much more detailed picture than the results normally obtained with bulk analysis through conventional integral depth profiling and help in the elucidation of the side reactions taking place during the plating processes.

Investigation of rate-determining step of LaNi0.6Co0.4O3-δ film electrode

The rate-determining step (RDS) in the oxygen reduction process on a LaNi0.6Co0.4O3-δ film electrode was analyzed using electrochemical impedance spectroscopy in varied temperatures (873–1073 K), oxygen partial pressures (1–10−3 bar), and DC bias. From the evaluation of the area-specific interfacial conductivity, σE, and chemical capacitance, CE, it is suggested that the RDS is surface reaction under low p(O2) (10−2–10−3 bar) but there appears a contribution of bulk diffusion under high p(O2) in the film electrode at any given temperature. The change of the RDS was also confirmed using isotope exchange depth profile method where a significant change in the oxygen profile in the film electrode was observed. This is attributed to the rate of the bulk diffusion of O2− which is almost constant when the oxygen nonstoichiometry of LaNi0.6Co0.4O3-δ is small, whereas the surface reaction rate strongly depends on p(O2). At low p(O2), the surface reaction is slower than the bulk diffusion, so that the potential drop on the surface of the electrode film is large. In contrast, at high p(O2), the surface reaction is faster than the bulk diffusion where the potential decay is sluggish inside the film electrode.

Nanoscale residual stress depth profiling by Focused Ion Beam milling and eigenstrain analysis

Residual stresses play a crucial role in determining material properties and behaviour, in terms of structural integrity under monotonic and cyclic loading, and for functional performance, in terms of capacitance, conductivity, band gap, and other characteristics. The methods for experimental residual stress analysis at the macro- and micro-scales are well established, but residual stress evaluation at the nanoscale faces major challenges, e.g. the need for sample sectioning to prepare thin lamellae, by its very nature introducing major modifications to the quantity being evaluated.Residual stress analysis by micro-ring core Focused Ion Beam milling directly at sample surface offers lateral resolution better than 1μm, and encodes information about residual stress depth variation. We report a new method for residual stress depth profiling at the resolution better than 50nm by the application of a mathematically straightforward and robust approach based on the concept of eigenstrain. The results are validated by direct comparison with measurements by nano-focus synchrotron X-ray diffraction.

Improvement of ionic conductivity in A-site lithium doped sodium bismuth titanate

Oxide-ion conductors play a significant role in various applications such as solid oxide fuel cells (SOFCs), oxygen separation membranes and sensors. Recently, high ionic conductivity (~1×10−4Scm−1 at 600°C) was found in sodium bismuth titanate (NBT), which originates from oxygen vacancies compensating the introduced Bi-deficiency. By providing pathways with low diffusion barriers, the highly polarizable Bi3+ ions with 6s2 lone pair electrons and weak BiO bonds are also beneficial for the migration of oxygen ions. Here we report the influence of lithium doping on the electrical properties of NBT. The optimal doping level of 4at% Li on the Bi-site improves the ionic conductivity by one order of magnitude to ~7×10−3Scm−1 at 600°C without changing the conduction mechanism, which could be attributed to an increase in the oxygen vacancy concentration based on an acceptor doping mechanism. A further increase in Li content does not improve the total conductivity. Oxygen diffusion data were acquired by the Isotope Exchange Depth Profile (IEDP) method in combination with Secondary Ion-Mass Spectrometry (SIMS). The oxygen self-diffusion coefficients (e.g. 7.04×10−9cm2s−1 at 600°C) are in excellent agreement with the values derived from impedance spectroscopy data, suggesting that the oxygen ions are the main charge carriers in the system. Furthermore, a degradation test was performed for 100h under a variety of atmospheres, showing only a slight decrease in conductivity in both air and oxygen atmospheres attributed to the loss of material from the A-site. Comparison with other oxide-ion conductors indicates that Li-doped NBT materials are promising candidates for intermediate temperature SOFC applications.

Ti oxidation states in Zn(Ti) coating of hot-dip galvanized steels

Chemical composition analysis of coloured surface layers of hot-dip galvanized steel sheets was studied. Samples were produced in laboratory scale and studied by mass spectrometry and electron spectroscopy methods. Zinc bath alloyed with 0.15wt% titanium content resulted in surfaces with different colours depending on the bath temperature. We proved that surface colour was determined by the thickness of surface titanium-oxide layer created during sample cooling from the bath temperature. The bath temperature dependence of the titanium-oxide layer thickness gives information about the activation energy of its formation, which is essentially the activation energy of Ti diffusion through the oxide layer. Surfaces of hot-dip galvanized samples were studied by X-ray Photoelectron Spectroscopy. Different oxidation states of titanium were identified in the oxide layer. By our measurements we could prove that depth profiling methods using low energy ion beams for surface sputtering can be applicable to determination of chemical states without modifying them. Finally, we provided experimental evidence that the surface colour was determined by light interference.

Deuterium supersaturation in low-energy plasma-loaded tungsten surfaces

Fundamental understanding of hydrogen–metal interactions is challenging due to a lack of knowledge on defect production and/or evolution upon hydrogen ingression, especially for metals undergoing hydrogen irradiation with ion energy below the displacement thresholds reported in literature. Here, applying a novel low-energy argon-sputter depth profiling method with significantly improved depth resolution for tungsten (W) surfaces exposed to deuterium (D) plasma at 300 K, we show the existence of a 10 nm thick D-supersaturated surface layer (DSSL) with an unexpectedly high D concentration of ~10 at.% after irradiation with ion energy of 215 eV. Electron back-scatter diffraction reveals that the W lattice within this DSSL is highly distorted, thus strongly blurring the Kikuchi pattern. We explain this strong damage by the synergistic interaction of energetic D ions and solute D atoms with the W lattice. Solute D atoms prevent the recombination of vacancies with interstitial W atoms, which are produced by collisions of energetic D ions with W lattice atoms (Frenkel pairs). This proposed damaging mechanism could also be active on other hydrogen-irradiated metal surfaces. The present work provides deep insight into hydrogen-induced lattice distortion at plasma–metal interfaces and sheds light on its modelling work.

Depth Profiling of Dark and Light Green Bacan: Construction of Material Characters Models from Elemental Analysis and Mineralogical Characterization

We have demonstrated the evolutional depth profiling methods for local minerals of Bacan in order to establish the sold price and maintenance of minerals sector in Indonesia. The depth profiling methods was performed by elemental analysis and mineralogical characterisation using X-ray fluorescence (XRF) and X-ray diffraction (XRD). We refined materials parameters then constructed the materials models to describe the difference of materials characters. These results described that LG Bacan has crystals phase of Dioptase (CuSiO 3 ), Liebenbergite (Ni 2 SiO 4 ), and variant of Calderonite (Ca 2 CuP 2 O 9 ). DG Bacan has crystals phase of Fayalite (Fe 2 SiO 4 ), and variant of Ferriannite (KCaFe 3 Si 3 O 12 ). All crystals phase in LG Bacan has growth orientation on [001] direction, and has more structured crystallite size with small range of its distribution. All phase in DG Bacan has the preferred growth so, except Fayalite crystal phase. DG Bacan less structured than LG Bacan, with wider range of crystallite size distribution, but has harder structure than LG Bacan. Quartz structure in LG Bacan was more polar than DG Bacan.

Quantification of Hydrogen Concentrations in Surface and Interface Layers and Bulk Materials through Depth Profiling with Nuclear Reaction Analysis

Nuclear reaction analysis (NRA) via the resonant 1H(15N,αγ)12C reaction is a highly effective method of depth profiling that quantitatively and non-destructively reveals the hydrogen density distribution at surfaces, at interfaces, and in the volume of solid materials with high depth resolution. The technique applies a 15N ion beam of 6.385 MeV provided by an electrostatic accelerator and specifically detects the 1H isotope in depths up to about 2 μm from the target surface. Surface H coverages are measured with a sensitivity in the order of ~1013 cm-2 (~1% of a typical atomic monolayer density) and H volume concentrations with a detection limit of ~1018 cm-3 (~100 at. ppm). The near-surface depth resolution is 2-5 nm for surface-normal 15N ion incidence onto the target and can be enhanced to values below 1 nm for very flat targets by adopting a surface-grazing incidence geometry. The method is versatile and readily applied to any high vacuum compatible homogeneous material with a smooth surface (no pores). Electrically conductive targets usually tolerate the ion beam irradiation with negligible degradation. Hydrogen quantitation and correct depth analysis require knowledge of the elementary composition (besides hydrogen) and mass density of the target material. Especially in combination with ultra-high vacuum methods for in-situ target preparation and characterization, 1H(15N,αγ)12C NRA is ideally suited for hydrogen analysis at atomically controlled surfaces and nanostructured interfaces. We exemplarily demonstrate here the application of 15N NRA at the MALT Tandem accelerator facility of the University of Tokyo to (1) quantitatively measure the surface coverage and the bulk concentration of hydrogen in the near-surface region of a H2 exposed Pd(110) single crystal, and (2) to determine the depth location and layer density of hydrogen near the interfaces of thin SiO2 films on Si(100).

Quantification of Hydrogen Concentrations in Surface and Interface Layers and Bulk Materials through Depth Profiling with Nuclear Reaction Analysis.

Nuclear reaction analysis (NRA) via the resonant (1)H((15)N,αγ)(12)C reaction is a highly effective method of depth profiling that quantitatively and non-destructively reveals the hydrogen density distribution at surfaces, at interfaces, and in the volume of solid materials with high depth resolution. The technique applies a (15)N ion beam of 6.385 MeV provided by an electrostatic accelerator and specifically detects the (1)H isotope in depths up to about 2 μm from the target surface. Surface H coverages are measured with a sensitivity in the order of ~10(13) cm(-2) (~1% of a typical atomic monolayer density) and H volume concentrations with a detection limit of ~10(18) cm(-3) (~100 at. ppm). The near-surface depth resolution is 2-5 nm for surface-normal (15)N ion incidence onto the target and can be enhanced to values below 1 nm for very flat targets by adopting a surface-grazing incidence geometry. The method is versatile and readily applied to any high vacuum compatible homogeneous material with a smooth surface (no pores). Electrically conductive targets usually tolerate the ion beam irradiation with negligible degradation. Hydrogen quantitation and correct depth analysis require knowledge of the elementary composition (besides hydrogen) and mass density of the target material. Especially in combination with ultra-high vacuum methods for in-situ target preparation and characterization, (1)H((15)N,αγ)(12)C NRA is ideally suited for hydrogen analysis at atomically controlled surfaces and nanostructured interfaces. We exemplarily demonstrate here the application of (15)N NRA at the MALT Tandem accelerator facility of the University of Tokyo to (1) quantitatively measure the surface coverage and the bulk concentration of hydrogen in the near-surface region of a H2 exposed Pd(110) single crystal, and (2) to determine the depth location and layer density of hydrogen near the interfaces of thin SiO2 films on Si(100).

Extracting thermal histories from the near-rim zoning in titanite using coupled U-Pb and trace-element depth profiles by single-shot laser-ablation split stream (SS-LASS) ICP-MS

A method for depth profiling using single-shot laser-ablation split stream (SS-LASS) ICP-MS is developed to simultaneously measure U-Pb age and trace-element concentrations in titanite. Simple semi-infinite, 1-D half-space diffusion models were applied to near-rim, trace-element zoned domains in titanite to distinguish between cooling and (re)crystallization ages and investigate the potential for preservation of thermally mediated diffusive loss profiles. These data illustrate the need to measure multiple trace elements with varying diffusivities to interpret a mineral's thermal history resulting from the non-unique nature of 1-D diffusion models where both temperature and time are unknown. A case study of titanites from two Pamir Plateau gneiss domes indicates they underwent ≥25 Myr of amphibolite and granulite facies metamorphism yet did not experience significant volume diffusion modification following (re)crystallization. The interpretation of prolonged (re)crystallization rather than diffusion allows for high-resolution, near-rim temperature-time histories to be extracted using U-Pb dates and Zr4+ apparent temperatures by SS-LASS.

Interaction of deuterium plasma with sputter-deposited tungsten nitride films

Magnetron-sputtered tungsten nitride (WNx) films were used as a model system to study the behaviour of re-deposited WNx layers which could form in fusion devices with tungsten (W) wall during nitrogen seeding. The interaction of such WNx layers with deuterium (D) plasmas was investigated in dedicated laboratory experiments. D retention and N removal due to D plasma exposure (D flux: 9.9 × 1019 D m−2 s−1, ion energy 215 eV) at different temperatures were measured with ion beam analysis (IBA). Low-energy argon sputtering followed by IBA was applied to resolve the D distribution in the top-most surface of WNx with significantly improved depth resolution compared with the standard D depth profiling method by nuclear reaction analysis. Experimentally determined thicknesses for the penetration of D in WNx were compared with the penetration depth for D calculated in SDTrimSP simulations. Results show that D is only retained within the ion penetration range for samples exposed at 300 K. In contrast to the 300 K case, D diffuses beyond the implantation depth in a sample exposed at 600 K. However, the D penetration depth is much lower than in pure W at comparable conditions. The total amount of retained D in WNx at 600 K is by 50% lower than for implantation at 300 K with the same D fluence. Nitrogen is removed only within the D ion range.

Confidence Limits in the Oxygen Transport Parameters of the (La0.8Sr0.2)(Cr0.2Fe0.8)O3-δ Determined By the Isotopic Exchange, Depth Profiling Method

The polycrystalline oxide (La0.8Sr0.2)(Cr0.2Fe0.8)O3- d or LSCrF is a mixed ionic and electronic conductor (MIEC) of a type called “acceptor-doped transition metal oxide” that has been explored for application as an electrode material in SOFC and other electrochemical devices working at intermediate to high temperatures (500°C to 900°C) [1]. The strontium acceptor doping on the A-site of this perovskite ensures high oxygen ion conductivity whilst the chromium substitution on the B-site ensures the stability of the perovskite structure for a wide range of oxygen partial pressures at the elevated temperatures. The suitability of this material as an electrode material is determined through the measurement of its ionic conductivity for oxygen ions and the rate of oxygen incorporation into the oxide at elevated temperatures using stable oxygen isotopic methods together with secondary ion mass spectrometry (SIMS). The isotopic exchange, depth profiling method (IEDP) method allows the bulk oxygen transport parameters, the diffusion coefficient, D(T) and surface reaction coefficient, k(T) both functions of temperature, T, to be determined once an appropriate solution of the diffusion equation [2] has been identified and assuming compliance with the boundary conditions of that solution. The IEDP results dataset, normalised isotopic fraction measured at N depth intervals, (xi, Cxi ) for i=1 to N is matched by the ‘least squares’ process that determines the most probable values of the two parameters, DFit and kFit , that minimises the ‘sum-of-squares’ of the residual differences, rxi between the measured profile data Cxi and the values calculated from the solution, Cxi Fit , where rxi = Cxi -Cxi Est . Ideally the conditions, ∂/∂D(∑(rxi )2)=0 and ∂/∂k(∑(rxi )2)=0 are satisfied. This process assumes that the variance associated with the depth measurement set, Vx is small compared to the variance of the measured isotopic fraction set, VCxFit . VDFit , the variance in DFit is calculated as a product, ( ∂D/∂rx)2 VCxFit . where the partial differential, (∂rx /∂D) , is the gradient of the sum of the residuals squared along the D axis for a fixed value of kFit in D and k space. The variance in kFit is calculated from a similar product, VkFit =( ∂k/∂rx)2 VCxFit . This statistical quantification of the variance of D and k for each IEDP measurement of the LSCrF oxide is reported in this contribution for three different exchange anneal ambients with an oxygen activity range between 1 and 1x10-14 [3]. All the calculations are done by numerical evaluation so the effect of the finite element size, (ΔD,Δk) and termination conditions for the ‘best-fit’ is also reported. . A comparison is made with the ‘goodness-of-fit’ tests using the often quoted “R-squared” and “chi” values from their probability distributions. The variances in DFit and kFit are known to be sensitive to the surface isotopic fraction, Cx=0 , which for the semi-infinite solution becomes, Cx=0 =1-exp(h' 2).erfc(h') where h'=k(tanneal /D)1/2 and tanneal is the time for the oxygen-18 exchange anneal. Values of h’ around 0.2 are preferred for similar variances in DFit and kFit whilst values of 0.02<h’<2 lead to higher errors with dissimilar upper and lower limits. This effect is shown from the results set for LSCrF with two cases where h’ is 7.3 in one dry oxygen exchange and h’is 0.073 in a steam and forming gas exchange reaction. The variance of D and k for each IEDP measurement is used to weight the plotted points in the Arrhenius plot and in the ‘best straight-line fit’ for an estimate of the activation energy for both diffusion and the surface exchange reaction. In this situation with often just a few points it is essential to apply the F-test or t-test significance tests for the null-hypothesis. The results for LSCrF are plotted with the standard 2-theta confidence limits derived from the ‘best-fit’ so that a reasonable assessment can be made with data in the literature that has been measured under similar conditions [4]. 1. Mizusaki, J., Solid State Ionics, 1992. 52(1-3): p. 79-91. 2. Crank, J., The mathematics of diffusion. 2nd ed. 1975, Oxford: Clarendon Press. 3. Chater, R.J. Ph.D. Thesis, Imperial College London, 2014 4. Ramos, T., Solid State Ionics, 2004 170(3-4): p. 275-286