Depth Profiling Of Thin Films Research Articles

Effects of Hydrogen Dissociation During Gas Flooding on Formation of Metal Hydride Cluster Ions in Secondary Ion Mass Spectrometry

The application of hydrogen flooding was recently shown to be a simple and effective approach for improved layer differentiation and interface determination during secondary ion mass spectrometry (SIMS) depth profiling of thin films, as well as an approach with potential in the field of quantitative SIMS analyses. To study the effects of hydrogen further, flooding of H2 molecules was compared to reactions with atomic H on samples of pure metals and their alloys. H2 was introduced into the analytical chamber via a capillary, which was heated to approximately 2200 K to achieve dissociation. Dissociation of H2 up to 30% resulted in a significant increase in the intensity of the metal hydride cluster secondary ions originating from the metallic samples. Comparison of the time scales of possible processes provided insight into the mechanism of hydride cluster secondary ion formation. Cluster ions presumably form during the recombination of the atoms and molecules from the sample and atoms and molecules adsorbed from the gas. This process occurs on the surface or just above it during the sputtering process. These findings coincide with those of previous mechanistic and computational studies.

Strain depth profiles in thin films extracted from in-plane X-ray diffraction

Thin films generally contain depth-dependent residual stress gradients, which influence their functional properties and stability in harsh environments. An understanding of these stress gradients and their influence is crucial for many applications. Standard methods for thin-film stress determination only provide average strain values, thus disregarding possible variation in strain/stress across the film thickness. This work introduces a new method to derive depth-dependent strain profiles in thin films with thicknesses in the submicrometre range by laboratory-based in-plane grazing X-ray diffraction, as applied to magnetron-sputtering-grown polycrystalline Cu thin films with different thicknesses. By performing in-plane grazing diffraction analysis at different incidence angles, the in-plane lattice constant depth profile of the thin film can be resolved through a dedicated robust data processing procedure. Owing to the underlying intrinsic difficulties related to the inverse Laplace transform of discrete experimental data sets, four complementary procedures are presented to reliably extract the strain depth profile of the films from the diffraction data. Surprisingly, the strain depth profile is not monotonic and possesses a complex shape: highly compressive close to the substrate interface, more tensile within the film and relaxed close to the film surface. The same strain profile is obtained by the four different data evaluation methods, confirming the validity of the derived depth-dependent strain profiles as a function of the film thickness. Comparison of the obtained results with the average in-plane stresses independently derived by the standard stress analysis method in the out-of-plane diffraction geometry validates the solidity of the proposed method.

High resolution in situ Li depth profiling of thin films stacked Li ion batteries under charging conditions by means of TERD and RBS techniques with 5 MeV He+2 ion beam

The experimental results are reported on Ni/Si/LiPON/LCO/Pt/Ti and Au/Si/LiPON/LCO/Au batteries prepared on SiN and Al self-supporting films (LiPON = Li3.3PO3.8N0.2, LCO = LiCoO2) which have been in situ measured simultaneously by Transmission ERD and RBS techniques with 5 MeV He+2 ion beam, in order to understand Li transport from positive electrode (LCO) to negative electrode (Si) under charging and over-charging. It has been found on over-charging that for the former battery Li ions over-flowed out of Si reacts with Ni electrode in the follow direction, whereas for the latter battery Li once charged in Si returns back through LiPON to LCO in the counter direction and reacts with Al through intermediate Au. Quite different two transport phenomena of Li on over-charging are discussed in terms of transport process of Li through LiPON, the difference in Fermi energies of the metal electrodes at both sides of LCO/LiPON/Si and in contact potentials of Si with the metal electrodes.

ZnO/TiO2 nanocomposite photoanode as an effective UV-vis responsive dye sensitized solar cell

A facile and simple sol-gel process has been used for the synthesis of TiO2 and ZnO nanomaterials. The varied amount of ZnO is incorporated onto surface of TiO2 by wet chemical impregnation method to fabricate ZnO/TiO2 hetero structures. These hetero systems are employed as photoanodes for dye sensitized solar cell in paste form. The loading of ZnO onto TiO2 surface suppressed the diffraction peaks of anatase phase of TiO2. The particle size is decreased in the composite structure. SEM images showed the spherical agglomerated structure in all samples. The band gap energy values have been reduced in case of ZnO/TiO2 heterojunctions up to concentration of 15.0% ZnO/TiO2. Zn–O–Ti bridging was responsible for low band gap and reduced in the recombination of electron hole. Fourier transforms infra-red spectroscopy confirms the Zn–O–Ti linkage. The depth profile of thin films was determined by Rutherford backscattering spectroscopy. The photoanodes of DSSC fabricated with ZnO/TiO2 nanocomposites were characterized by photocurrent-voltage measurements and exhibited the enhancement in photovoltaic properties as compared to bare TiO2. 15.0% ZnO/TiO2 heterojunction is optimum quantity which delivered 2.8% greater photovoltaic efficiency than bare TiO2 based DSSC. The enhancement is attributed to superior dye loading and reduction in recombination of charge carriers leading to long life time of free electrons that are beneficial for the overall efficiency of device.

Handling, transfer, storage, and shipping of commercial thin film hydride disk target samples

Thin film hydride (i.e., elements that can react with hydrogen or its isotopes to form a chemical compound) targets are important for many applications, including accelerator research, oil-well logging, cancer research, various neutron devices, nuclear waste assay, process control studies, contraband detection, and for many other novel uses. They are very sensitive to air-oxidation and easily contaminated by improper handling. Targets must be handled very carefully after processing to minimize contamination from physical handling, particulate contamination, and air-exposure, which oxidize sensitive groups IIIB, IVB, and rare earth thin film materials, thus reducing their operating characteristics such as neutron output, and can produce arcing if used in neutron devices. This paper will discuss the development of handling techniques, special vacuum transfer, and shipping containers for thin film hydride target samples from postprocessing to transfer and shipment to a customer. Initial work began at the General Electric Neutron Devices Department, in Largo, FL, in the mid-1970s and was refined at the Los Alamos National Laboratory in Los Alamos, NM, in the mid to late 1990s. Studies were performed to determine the best physical handling techniques, procedures for reducing particulate contamination and for reducing air-exposure and moisture from targets. Items studied were argon or dry nitrogen filled gloveboxes, desiccators, vacuum shipping containers, and a new design loader in a glovebox (in 1993), with an International Organization for Standardization class 4 clean room dry nitrogen environment, with particle and moisture removal and measurements. Test samples of hydride thin films were used to study surface oxide contamination as a function of handling, transfer, and storage times utilizing Auger argon sputter thin film depth profiling techniques. Results have shown that argon filled gloveboxes and dry nitrogen filled desiccators, along with techniques used to minimize target air-exposure, and various vacuum ∼≤1 × 10−3 Torr (1.33 × 10−1 Pa) internal environment shipping containers initially pumped down to ∼1 × 10−7 Torr (1.33 × 10−5 Pa) or less, were adequate to supply customers with thin film hydride targets with surface oxides from 70 to 475 Å or 7.0 to 47.5 nm, as it takes ∼>500 nm to affect neutron output in neutron devices. A special dry nitrogen glovebox controlled clean room loader with a high-efficiency particulate air filter/fan module, and oxygen and water vapor monitors was developed to produce a pristine super clean environment with very low particulate and film surface oxide contamination. The authors demonstrated that we can ship thin film hydride targets of optimum quality adequate for many applications and can supply pristine targets when requested which are in a condition very close to that as processed directly out of a non-air-exposed loader. In addition, our results showed the relative oxidation susceptibility for the four occluder thin films studied: Sc > Zr > Er > Ti.

Reducing the Matrix Effect in Organic Cluster SIMS Using Dynamic Reactive Ionization.

Dynamic reactive ionization (DRI) utilizes a reactive molecule, HCl, which is doped into an Ar cluster projectile and activated to produce protons at the bombardment site on the cold sample surface with the presence of water. The methodology has been shown to enhance the ionization of protonated molecular ions and to reduce salt suppression in complex biomatrices. In this study, we further examine the possibility of obtaining improved quantitation with DRI during depth profiling of thin films. Using a trehalose film as a model system, we are able to define optimal DRI conditions for depth profiling. Next, the strategy is applied to a multilayer system consisting of the polymer antioxidants Irganox 1098 and 1010. These binary mixtures have demonstrated large matrix effects, making quantitative SIMS measurement not feasible. Systematic comparisons of depth profiling of this multilayer film between directly using GCIB, and under DRI conditions, show that the latter enhances protonated ions for both components by 4- to ~15-fold, resulting in uniform depth profiling in positive ion mode and almost no matrix effect in negative ion mode. The methodology offers a new strategy to tackle the matrix effect and should lead to improved quantitative measurement using SIMS. Graphical Abstract ᅟ.

Selenization of Cu2ZnSnS4 thin film using a Se metal-organic source for solar cell applications

The effect of selenization temperature on the properties of Cu2ZnSn(S,Se)4 (CZTSSe) films using a Se metal-organic source was investigated. The metal-organic compound di-tert-butyl-selenide (DTBSe) was used as a Se source to selenize the Cu2ZnSnS4 (CZTS) films deposited by magnetron sputtering and crystallized by post-annealing on soda-lime glass. The selenization was conducted in the metal-organic chemical vapor deposition system and was carried out at temperature ranging from 300 to 390 °C for 30 min to prevent element loss, such as Zn, Sn. CZTS film was transformed into CZTSSe with various Se contents depending on the selenization temperature for 30 min. The crystalline properties were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD) and Raman spectroscopy with a 514 nm laser. Energy-dispersive X-ray spectroscopy (EDS) was utilized for stoichiometric characterization. The depth profiling of thin films was measured by auger electron spectroscopy (AES) to confirm distribution of the elements. And the band gap of CZTSSe thin films measured by UV–Vis–NIR spectrometry varied from 1.5 to 0.95 eV along with content of Se.

An approach to solving an ill posed inverse problem of residual stress depth profiling in thin films and compact solid materials

The inverse problem of evaluating residual stresses σ(z) in real space using residual stresses σ(τ) in image space is discussed. This problem is ill posed and special solution methods are required in order to obtain a stable solution. Moreover, the real-space solution must be localized in reflecting layers only in multilayer systems. This requirement imposes strong restrictions on the solution methods and does not allow one to use methods based on the inverse Laplace transform employed for compact solid materials. Besides, in the case of solid materials, the use of the inverse Laplace transform often leads to extremely unstable solutions. The stable numerical solution of the discussed inverse problem can be found using a method based on the Tikhonov regularization. Given the measured data and their pointwise error estimation, this method provides stable approximate solutions for both solid materials and thin films in the form of piecewise functions defined solely in diffracting layers. The approximations are shown to converge to the exact function when the noise in the experimental data approaches zero. If the initial data satisfy certain constraints, the method provides a stable exact solution for the inverse problem. A freely available MATLAB package has been developed, and its efficiency was demonstrated in the numerical residual stress calculations carried out for solid materials and thin films.

Thin film depth profiling by ion beam analysis.

The analysis of thin films is of central importance for functional materials, including the very large and active field of nanomaterials. Quantitative elemental depth profiling is basic to analysis, and many techniques exist, but all have limitations and quantitation is always an issue. We here review recent significant advances in ion beam analysis (IBA) which now merit it a standard place in the analyst's toolbox. Rutherford backscattering spectrometry (RBS) has been in use for half a century to obtain elemental depth profiles non-destructively from the first fraction of a micron from the surface of materials: more generally, "IBA" refers to the cluster of methods including elastic scattering (RBS; elastic recoil detection, ERD; and non-Rutherford elastic backscattering, EBS), nuclear reaction analysis (NRA: including particle-induced gamma-ray emission, PIGE), and also particle-induced X-ray emission (PIXE). We have at last demonstrated what was long promised, that RBS can be used as a primary reference technique for the best traceable accuracy available for non-destructive model-free methods in thin films. Also, it has become clear over the last decade that we can effectively combine synergistically the quite different information available from the atomic (PIXE) and nuclear (RBS, EBS, ERD, NRA) methods. Although it is well known that RBS has severe limitations that curtail its usefulness for elemental depth profiling, these limitations are largely overcome when we make proper synergistic use of IBA methods. In this Tutorial Review we aim to briefly explain to analysts what IBA is and why it is now a general quantitative method of great power. Analysts have got used to the availability of the large synchrotron facilities for certain sorts of difficult problems, but there are many much more easily accessible mid-range IBA facilities also able to address (and often more quantitatively) a wide range of otherwise almost intractable thin film questions.

Grazing-incidence x-ray fluorescence analysis for non-destructive determination of In and Ga depth profiles in Cu(In,Ga)Se2 absorber films

Development of highly efficient thin film solar cells involves band gap engineering by tuning their elemental composition with depth. Here we show that grazing incidence X-ray fluorescence (GIXRF) analysis using monochromatic synchrotron radiation and well-characterized instrumentation is suitable for a non-destructive and reference-free analysis of compositional depth profiles in thin films. Variation of the incidence angle provides quantitative access to the in-depth distribution of the elements, which are retrieved from measured fluorescence intensities by modeling parameterized gradients and fitting calculated to measured fluorescence intensities. Our results show that double Ga gradients in Cu(In1−x,Gax)Se2 can be resolved by GIXRF.

Zero-mode TEM parallel-plate resonator for high-resolution thin film magnetic resonance imaging

A parallel-plate radio frequency resonator has been designed for high-resolution thin film magnetic resonance imaging. The B1 field produced by the resonator was evaluated through experiment and numerical simulation. The resonator, composed of two conductive plates sandwiching the sample, generates a uniform B1 field parallel to the plates. This feature renders the resonator largely immune to radio frequency screening from conductive layers parallel to the sample. The resonator is custom fabricated according to the dimensions of the sample, yielding a high filling factor. The radio frequency probe is shown to facilitate high-sensitivity depth profiling of thin films. Three variations on the basic design are examined. Since the intention is to employ these resonators for functional studies of membranes, we introduce perforations in the parallel plates to permit mass transfer in and out of the thin films under study. One-dimensional depth profiles of Nafion 117 are presented with and without the addition of additional plates of conductive material. One-dimensional depth profiles of Nafion 1110 and a two-dimensional image of Nafion 117 in an operating fuel cell, which was integrated into the radio frequency circuit, are also illustrated.

Ion Beam Analysis: A Century of Exploiting the Electronic and Nuclear Structure of the Atom for Materials Characterisation

Analysis using MeV ion beams is a thin film characterisation technique invented some 50 years ago which has recently had the benefit of a number of important advances. This review will cover damage profiling in crystals including studies of defects in semiconductors, surface studies, and depth profiling with sputtering. But it will concentrate on thin film depth profiling using Rutherford backscattering, particle induced X-ray emission and related techniques in the deliberately synergistic way that has only recently become possible. In this review of these new developments, we will show how this integrated approach, which we might call "total IBA", has given the technique great analytical power.

Low-energy heavy-ion TOF-ERDA setup for quantitative depth profiling of thin films

Low-energy heavy-ion time-of-flight elastic recoil detection analysis (TOF-ERDA) is becoming a mature technique for accurate characterization of thin films. In combination with a small tandem accelerator (∼2 MV terminal voltage) and beam energies below 20 MeV, it is suitable for routine analysis of key materials in semiconductor technology. In this paper we discuss advantages and drawbacks of low-energy ERDA, compared to high-energy ERDA, in terms of depth and mass resolution, detection efficiency for light elements, sample irradiation damage and quantification accuracy. The results presented are obtained with the time-of-flight telescope recently developed at IMEC. The time-of-flight is measured with timing gates based on electrostatic mirrors and is acquired in coincidence with the energy signal measured by a planar Si detector.

Neutron studies of magnetic oxide thin films

We describe the use of neutron scattering techniques such as reflectivity and diffractionfor the study of oxide thin films. We first describe how neutron reflectivity cancomplement x-ray reflectivity for the study of some oxide materials. We then emphasizemagnetic thin films which have become an important field of study in the 1990s,following the discovery of new phenomena in heterostructures: magnetic exchangecoupling, exchange bias coupling at antiferro/ferromagnetic interfaces, enhancedmagnetism in ultrathin films or tunnel magnetoresistance for example. We showhow neutron scattering can provide detailed quantitative information about themagnetization depth profiles of thin films and about the magnetic order in epitaxialfilms.

Metal clustering during depth profiling of thin films

AbstractThin metallic layers of Cu, Cr, or Ag were deposited on Si substrates and then sputtered as in a standard depth profiling mode. The etching was completed by using 1–5 keV Ar+ beam, scanned over the surface. The composition of the exposed etched surface was monitored by X‐ray photoelectron spectroscopy (XPS) or Auger electron spectroscopy (AES). When the metal‐silicon interface was approached, the etching was stopped and the sample surface was analyzed by scanning electron microscopy (SEM) and scanning Auger microscopy (SAM). We observed clusters of metal on the Si substrate. Their characteristic sizes were 30–60 nm for Cu, 60–90 nm for Cr, and 100–200 nm for Ag. The patterns were sensitive to the ion energy—at 1 keV the Cu network was well defined, whereas at 3–5 keV energy the patterns become fuzzy without clear boundaries. The clustering of thin films near the interface can affect the depth profiles in surface analytical techniques, and can be used as a tool to create clusters in nanotechnology. Copyright © 2006 John Wiley & Sons, Ltd.

Characterisation of nonlinear thin films using logarithmic Hilbert transform

A new technique based on logarithmic Hilbert transform processing of Maker-fringe curves to characterise second-order optical nonlinear depth profile of thin films is described and demonstrated experimentally. This technique, being analytical, is faster than its iterative alternative. This is the first time that a Hilbert transform based analytical tool has been applied to characterise nonlinear thin films.

Depth profiling of thin films of binary metal oxides

AbstractOur previous results on the XPS depth profiling of binary metal oxides are compared with the new experimental data obtained for the films of SnW, MoTi and MoSn oxides. The samples have been prepared by sol–gel and magnetron sputtering techniques. XPS depth profiling of the samples has been carried out by using cyclic Ar+ sputtering at the energies of 0.5–2.0 keV. In order to compare the chemical states of constituent elements at different sampling depths, angle‐resolved XPS depth profiling has been performed on selected samples.Starting from the stoichiometric oxides on the surface and going deeper into the volume of the films, the depth profiling revealed there the presence of reduced Mo and W ions. The amount of reduced species and their distribution in depth was found to be independent of the sputtering energy and photoelectron collection angle. These metal ions in lower oxidation states were attributed to the sample preparation processes rather than to the detrimental effect of ion sputtering, dominating at higher ion energies. Copyright © 2004 John Wiley & Sons, Ltd.

Artificial neural network algorithm for analysis of rutherford backscattering data

Rutherford backscattering (RBS) is a nondestructive, fully quantitative technique for accurately determining the compositional depth profile of thin films. The inverse RBS problem, which is to determine from the data the corresponding sample structure, is, however, in general ill posed. Skilled analysts use their knowledge and experience to recognize recurring features in the data and relate them to features in the sample structure. This is then followed by a detailed quantitative analysis. We have developed an artificial neural network (ANN) for the same purpose, applied to the specific case of Ge-implanted Si. The ANN was trained with thousands of constructed spectra of samples for which the structure is known. It thus learns how to interpret the spectrum of a given sample, without any knowledge of the physics involved. The ANN was then applied to experimental data from samples of unknown structure. The quantitative results obtained were compared with those given by traditional analysis methods and are excellent. The major advantage of ANNs over those other methods is that, after the time-consuming training phase, the analysis is instantaneous, which opens the door to automated on-line data analysis. Furthermore, the ANN was able to distinguish two different classes of data which are experimentally difficult to analyze. This opens the door to automated on-line optimization of the experimental conditions.

Depth Profiling of Optical Absorption in Thin Films via the Mirage Effect and a New Inverse Scattering Theory. Part II: Experimental Reconstructions on Well-Characterized Materials

Mirage effect spectrometry is experimentally evaluated in this work as a technique of optical depth profiling in thin films where no prior information is available about the sample properties. An apparatus suitable for performing quantitative measurements is described. High-precision experimental alignment procedures are introduced along with a new method for precise optical correction of the detector signal for experimental frequency response nonuniformities. Reconstructions were made of the heat source density and absorption coefficient depth profile in materials with known depth dependence. These included samples approximating weighted delta function arrays, and depth-continuous media known to obey Beer's law to a good approximation. The properties of these samples were examined independently by using a technique of depth-sensitive light microscopy. Mirage effect depth profiles reconstructed on samples containing discrete absorbers were effectively regularization limited, indicating that resolution is limited by random error in the data rather than experimental bias. Depth profiles obtained in continuously absorbing media show a good agreement with those obtained by reference methods.

Depth Profiling of Thin Films with Pulsed Glow Discharge Atomic Emission Spectrometry

The application of microsecond pulsed Grimm glow discharge atomic emission spectrometry for depth profiling of thin films is examined. The effects of pulsed conditions including pulse voltage, pulse frequency, pulse width, and Ar pressure on depth profiling performance were characterized for Zn and Cu coatings on steel. Using optimized conditions, linear calibration curves of coating thickness for Zn (6.1-26.9 μm) and Cu (50-500 nm) on steel were achieved. A precision of 2-5% relative standard deviation was determined. An ultrathin coating of Cu (10 nm) on steel was also measured by this technique.