Hydrocarbon Contamination Layer Research Articles

Nanostructure of Surface Films on Ni18P Alloy in Sulfate Solutions by the Maximum Entropy Method.

NiP alloys are very often used in industry, due to their outstanding performance in corrosion and wear. Alloys with high phosphorus content (≥16 atom % P) are amorphous and show high corrosion resistance in both neutral and acidic solutions irrespective of the presence of chloride ions. The reason for this behavior is attributed to the formation of a “P-enriched surface layer” whose exact nature is still under debate. In this work, an iterative algorithm based on the application of maximum entropy method on nondestructive angle-resolved X-ray photoelectron spectroscopy data has been applied to the investigation of the surface layer grown on Ni18P alloys following mechanical polishing and anodic polarization in sulfate solutions. The results show that the outermost region of the examined alloy has a complex layered structure: (1) an uppermost hydrocarbon contamination layer about 1 nm thick, which includes also adsorbed water; (2) a nickel (poly)phosphate layer of about 1 nm; (3) a highly phosphorus-enriched interface being about 2 nm thick with a marked phosphorus concentration gradient, from 70 to 20 atom %; and (4) bulk alloy with the stoichiometric composition. These findings, together with the chemical state of the different phosphorus compounds, allow us to conclude that the high corrosion and wear resistance of NiP alloys might be ascribed to the presence of a thin, self-repairing nickel (poly)phosphate layer grown on a strongly P-enriched interface. Because the Auger parameter of P at the interface is similar to that of elemental P, it might be also concluded that the interface is enriched in elemental phosphorus.

Downstream Plasma Technology for Cleaning TEM Samples on Carbon Films

Introduction With the advent of modern scanning/transmission electron microscopy (S/TEM) capable of higher resolution, better contrast, and faster throughput, it is imperative to ensure the cleanliness of the TEM sample under the ultrahigh vacuum conditions of the microscopes [1–5]. It is well known that sample contamination can severely deteriorate the quality of electron microscopy analysis of materials, especially as the sample regions of interest decrease in size. The adverse effects of sample contamination include obscuring the area of the sample being analyzed by buildup of a carbonaceous layer, interfering with focusing and astigmatism correction, and generating unexpected microanalysis signals [2, 3]. A variety of cleaning methods, including electron beam flooding, heating and/or cooling, ultraviolet light exposure, and plasma cleaning, have been developed to minimize sample contamination [4, 5]. Among them, plasma cleaning is considered the most effective way to prepare samples for electron microscopy. As shown in Figure 1a, a plasma can be described as an ionized gaseous state created by direct current (DC), radio frequency (RF), or microwave glow discharge, in which electrons, ions, and radicals coexist. The interaction of these plasma species with a solid surface causes three basic phenomena that lead to surface cleaning: heating from the electron-specimen interaction, sputtering from the ion-specimen interaction, and etching from the radical-specimen interaction [2]. Although the combination of all three plasma species is efficient in terms of cleaning rates, it can cause irreversible surface modification and undesirable heating. More problematically, as the plasma cleaning process removes the hydrocarbon contamination layer, it removes other carbon structures at the same time. This can be an issue for TEM carbon film users because the carbon film is extensively used as a support on TEM grids for both materials science and biological applications. For example, our laboratory routinely loads from 10 to 50 ex-situ lifted-out TEM samples on a carbon film Cu-grid in order to support the high volume wafer-based manufacturing process [6]. If the carbon support film is damaged before the hydrocarbon contamination layer on the TEM samples is removed, valuable information in TEM samples will be lost. Therefore, it is essential to develop a method that removes only the hydrocarbon contamination layer while preserving the carbon support film. Downstream oxygen plasma is one technology that has been extensively used in modern semiconductor and

Simulation and Modeling of Self-Assembled Monolayers of Carboxylic Acid Thiols on Flat and Nanoparticle Gold Surfaces

Quantitative analysis of the 16-mercaptohexadecanoic acid self-assembled monolayer (C16 COOH-SAM) layer thickness on gold nanoparticles (AuNPs) was performed using simulation of electron spectra for surface analysis (SESSA) software and X-ray photoelectron spectroscopy (XPS) experimental measurements. XPS measurements of C16 COOH-SAMs on flat gold surfaces were made at nine different photoelectron emission angles (5-85° in 10° increments), corrected using geometric weighting factors and then summed together to approximate spherical AuNPs. The SAM thickness and relative surface roughness (RSA) in SESSA were optimized to determine the best agreement between simulated and experimental surface composition. On the basis of the glancing-angle results, it was found that inclusion of a hydrocarbon-contamination layer on top the C16 COOH-SAM was necessary to improve the agreement between the SESSA and XPS results. For the 16 COOH-SAMs on flat Au surfaces, using a SAM thickness of 1.1 Å/CH(2) group, an RSA of 1.05, and a 1.5 Å CH(2)-contamination overlayer (total film thickness = 21.5 Å) for the SESSA calculations provided the best agreement with the experimental XPS data. After applying the appropriate geometric corrections and summing the SESSA flat-surface compositions, the best fit results for the 16 COOH-SAM thickness and surface roughness on the AuNPs indicated a slightly thinner overlayer with parameters of 0.9 Å/CH(2) group in the SAM, an RSA of 1.06 RSA, and a 1.5 Å CH(2)-contamination overlayer (total film thickness = 18.5 Å). The 3 Å difference in SAM thickness between the flat Au and AuNP surfaces suggests that the alkyl chains of the SAM are slightly more tilted or disordered on the AuNP surfaces.

Angle resolved XPS characterization of cationic polyacrylamides

AbstractIn many applications surfaces are modified using polymer films and the polymers used are often complex copolymers. In biomedical applications it is critical to determine the surface properties of a substrate as it is these that mediate the cellular interactions. The surface structure of copolymer films can only rarely be established from their bulk composition alone. In this study angle resolved XPS was used to build a model of the structure of copolymer films produced on glass substrates from a family of poly(acrylamide) copolymers containing cationic blocks. The thickness of the copolymer films was demonstrated to be dependent on the concentration of the polymer solution and the ratio of non‐cationic to cationic blocks in the copolymer. The data demonstrated that the cationic blocks of the copolymer preferentially segregated to the glass surface and the non‐cationic poly(acrylamide) blocks preferentially segregated to the air–vacuum interface. A low concentration of the cationic functional groups was present throughout the poly(acrylamide) layer and it was suggested that this resulted from a small fraction of the cationic blocks being pulled into the poly(acrylamide) layer at points along the polymer chain where the two blocks are connected. Evidence of a thin surface hydrocarbon contamination layer was also observed. Copyright © 2009 John Wiley & Sons, Ltd.

Evaluation of a simple correction for the hydrocarbon contamination layer in quantitative surface analysis by XPS

A simple method of correction of XPS data for the ubiquitous hydrocarbon contamination overlayer is described. The method uses only standard quantification algorithms and readily available reference data. The method is evaluated by comparison with the known stoichiometries of 11 simple oxygen-saturated inorganic compounds and one polymer prepared directly from the laboratory environment. Without correction, a mean error in quantification of ±8.2 relative % was found. After application of the correction, the mean error was reduced to ±5.6 relative %. An analysis of the results showed that after application of the correction, a significant proportion of the remaining error may have been due to thermal or radiation-induced oxygen loss from the compounds during measurement.

Spectroscopic ellipsometrical studies of UV irradiation effects on the surface properties of the ultra thin native oxide film on titanium

Under ultraviolet (UV) irradiation, an ultra thin titanium native oxides film covered by a hydrocarbon contamination (HC) layer on the as-deposited titanium film was investigated by using spectroscopic ellipsometry (SE), atomic force microscopy (AFM) and contact angle meter. Using a stable silicon dioxide sample as a reference provided an insight of the structural change of the titanium native oxide film under UV irradiation. The SE results indicate that UV irradiation can completely remove the adsorbed HC layer both on the Ti sample and on Si sample after 337 min and 4059 min respectively. The titanium native oxide accelerates the HC removing process. The pseudo-optical constants (pseudo-n and pseudo-k) of the Ti composite and the thickness of the surface layer on Ti decreased during UV irradiation, which was considered to be contributed to the changes of the surface properties of the titanium composite film. AFM images on the surface of UV-treated titanium native oxide films present a less root-mean-square (RMS) roughness and many isolated gains with a size of about 10 nm. [DOI: 10.1380/ejssnt.2005.284]

Reconstruction of the electric charge density in thin films from the contrast transfer function measurements

The radial distribution of the beam-induced charge in thin films is investigated using the contrast transfer properties of the transmission electron microscope. The phase shift due to charging is measured as the phase difference between the contrast transfer functions of two photos taken with and without film at the back focal plane. Solving the inverse Laplace problem with this input data recovers the charge density of the measured film. The electric potential function in the whole area is reconstructed using the boundary integral method and the analytical solution of the Laplace equation for the electric potential is induced from unit step-wise surface charge. The phase shift of electron waves is derived in a weak lens approximation. In this way, the radial dependence of the charge density and the magnitude of the electrostatic potential at the thin film are obtained. The surface charge density reaches quasi-equilibrium state after the first 30 min of the electron beam pre-irradiation. The hydrocarbon contamination layer on the surface of the film is considered to be the main source of charging. An explanation of the qualitative behavior of the charge density, based on the contamination diffusion theory, is proposed.

Electron Beam Assisted Chemical Etching of Single Crystal Diamond Substrates

Electron beam assisted chemical etching (EBACE) with oxygen gas is applicable to direct fine patterning of single crystal diamond substrates. A scanning electron microscope (SEM) combined with an oxygen gas introduction system was used for EBACE of diamond. In order to prevent surface charge-up during etching and SEM observation, a hydrocarbon contamination layer, which has conductivity and can be deposited during electron beam irradiation using oil vapor in a vacuum system, was used. Etching characteristics of single crystal diamond substrates by EBACE with oxgen gas were mainly investigated. It was found by in-situ SEM observation that hole, line and rectangular patterns with several µm2 area and sub-µm depth into the diamond substrates were successfully fabricated by EBACE utilizing spot, line and raster scanning modes of the SEM. The depths of holes and rectangular patterns were proportional to electron beam exposure times. Etched areas of line and rectangular patterns were larger than scanned area. An etching yield of 1.99 ×10-2 carbon atoms of diamond per electron has been observed for EBACE using oxygen gas.

Accuracy of the correction for a hydrocarbon contamination layer in routine quantitative XPS analysis of oxygen‐inert samples

AbstractA procedure for determination of the reduced thickness of the contamination layer for oxygen‐inert samples was implemented. The correction algorithm is valid for the reduced thickness of a contamination layer x ≥ 0.3 (x is expressed as the ratio of the layer thickness to the electron attenuation length in the contamination layer at energy 1 keV). It is shown that the linear background subtraction method traditionally used in XPS generally does not provide correct results. Application of an advanced experimental data treatment method with correction for the presence of a contamination layer to quantitative surface analysis of Al2O3, SiO2, GeO2, Fe2O3, NiO, CuO and NaF resulted in a decrease of the mean error of analysis from 27% using the elemental sensitivity factors approach to 5%.

Hydrogen Passivation of HF ‐ Last Cleaned (100) Silicon Surfaces Investigated by Multiple Internal Reflection Infrared Spectroscopy

The hydrogen passivation of (100) silicon surfaces is characterized as a function of the chemical treatment by means of multiple internal reflection infrared (IR) spectroscopy using a novel sample preparation method. The samples are etched in different solutions (aqueous , , , , and buffered ) and investigated with and without DI water rinsing. The stability of the surfaces against oxidation and the buildup of a hydrocarbon contamination layer during storage in cleanroom air are examined. The IR peak at 2123 cm−1 is related to a coupling mode between dihydrides and neighboring monohydrides.

A very narrow resonance in 18O(p, α) 15N near 150 keV: Application to isotopic tracing. II. High resolution depth profiling of 18O

Various depth profiling experiments illustrating the high depth resolutions obtained are presented for Ta 2O 5 and SiO 2 layers with sharp 18O 16O interfaces in depth ranges up to some hundreds of angstrom. A further increase of depth resolution is reached by resorting to glancing geometry with respect to the beam. The depth sensitivity of this resonance is actually so high that it is the energy straggling induced by the ubiquitous hydrocarbon contamination layer on the sample surface that limits the near-surface resolution. The experimental curves are interpreted with the stochastic theory of energy loss. The results are compared to those obtained with the previously used resonances for 18O depth profiling. Applications of these techniques to the isotopic tracer study of the microscopic growth mechanisms of SiO 2 on silicon are shown. Operated in UHV this method may yield potentially nanometric or even subnanometric near surface depth sensitivities, as illustrated by detailed theoretical calculations corresponding to ideal but realistic experimental conditions. These results illustrate the usefulness for very high resolution depth profiling of low energy narrow resonances in general due to the corresponding large d E/d x values.

A comparison of thin and thick targets for energy calibration of heavy ion beams with resonances

The signal-to-noise ( S N ) ratio near the resonance energy is compared for thick and thin hydrogen containing targets. This is done with a view to assessing the precision of energy calibration methods for heavy ion beams based on resonances in nuclear reactions such as 1H( 19F, αγ) 16O and 1H 15N, αγ) 12C. In a previous work a thick ammonium chloride target was found to ha very attractive features for energy and yield calibration purposes although it was thought that the S N ratio would be inferior to of a thin target. Here the S N ratio for a thick ammonium chloride target is compared with that for a thin target in the form of a hydrocarbon contamination layer which is known to contain a high concentration of hydrogen. It is found that the thick ammonium chloride target can be expected to give a superior S N ratio in most cases. General expressions are obtained and the accuracies and the precisions of the energy calibration methods are discussed.

Adventitious carbon—the panacea for energy referencing?

AbstractA brief historical review has been made of the values assigned to the binding energy of C1s electrons from hydrocarbon contamination on sample surfaces. The particular energy calibration technique used for each electron spectrometer on which the C1s determination was made has been identified. There are notable variations in the C1s electron binding energy for a hydrocarbon contamination layer with respect to the substrate on which it was measured. The possible source and nature of adventitious carbon have been discussed, as have factors which can affect its C1s binding energy value. Sodium phosphate salts have been used to illustrate the relative merits of using an internal energy reference line rather than the C1s electrons from the adventitious carbon layers of these salts. Alternative energy referencing techniques, which include deposition of soot from a candle flame and deliberate ‘in situ’ condensation of a volatile organic compound onto the sample surface, have been compared with the method under review. Another possible method is the mixing of powdered samples with reference compounds (e.g. graphite). It is concluded that, although the use of C1s electrons from adventitious carbon layers is often a convenient method of energy referencing, interpretation of binding energy data obtained should be treated with caution.

XPS—Investigation of contamination layers

The ratio (d/Λ) of the average thickness of a contamination layer to the mean free path of the photoelectrons is measured by means of a method utilizing variable take-off geometry. A sample holder, which permits a stepwise variation of the take-off angle from 27.5° to 83.5°, is described. The sample investigated consisted of a thin metal film upon a glass substrate chosen in order to provide a smooth and flat surface upon which a hydrocarbon contamination layer was formed during the measurement. The evaluation of d/Λ was carried out using a graphical procedure suggested by Head1.

Calibration of electron spectra

Energy calibration procedures, which have been used in ESCA, are reviewed. Binding energies of calibration lines suitable for solid and for gaseous samples have been determined. Charging effects of nonconducting samples have been investigated for different X-radiation, sample thickness and material. A calibration relative to lines from some insulating standard compound mixed into the sample was shown to be unreliable, while experiments indicated that in calibrating relative to a surface layer, the carbon line from the hydrocarbon contamination layer can be used as well as a line from a thick layer of a noble metal on a part of the sample.