Sputter Profiling Research Articles

Response function during oxygen sputter profiling and its application to deconvolution of ultrashallow B depth profiles in Si

The secondary ion mass spectrometry (SIMS) response function to a B “δ surface layer” has been investigated. Using electron-gun evaporation combined with liquid nitrogen cooling of target, we are able to deposit an ultrathin B layer without detectable island formation. The B spatial distribution obtained from SIMS is exponentially decaying with a decay length approximately a linear function of the incident energy of the oxygen during the SIMS analysis. Deconvolution with the response function has been applied to reconstruct the spatial distribution of ultra-low-energy B implants. A correction to depth and yield scales due to transient sputtering near the Si surface region was also applied. Transient erosion shifts the profile shallower, but beam mixing shifts it deeper. These mutually compensating effects make the adjusted distribution almost the same as original data. The one significant difference is a buried B peak observed near the surface region.

The vanadium subsurface alloy on polycrystalline rhodium: formation and catalytic properties

Stable vanadium/rhodium subsurface alloys can be obtained by annealing vanadium metal overlayers on polycrystalline rhodium at elevated temperatures (⩾773 K). Their peculiarity is a purely rhodium-terminated surface with vanadium atoms positioned in the near-surface layers. Their structure and composition depend on the nominal vanadium coverage and on the sample temperature during preparation. In the present contribution the formation of the vanadium/rhodium subsurface alloy was studied in detail and the kinetics of CO hydrogenation on its surface was measured in situ at the different stages of alloy formation. In particular, the transition from “on-top” vanadium to the subsurface alloy state and finally to the bulk solution has been monitored with the help of Auger electron spectroscopy and argon ion sputter profiling. Submonolayers of vanadium on top of the rhodium surface promote initially the CO hydrogenation, but are rapidly deactivated by oxide and carbide formation. Subsurface alloy formation sets in upon annealing at 773 K and is connected with a significant increase of the catalytic activity, a change in the selectivity, and a reduced catalyst deactivation. Increasing the annealing temperature to 950 K leads to a state with the subsurface layer (second Rh surface layer) saturated with vanadium and excess vanadium dissolved into the bulk. The formation and the catalytic properties of the (sub)surface alloys obtained at different temperatures are discussed in light of additional experiments regarding their chemisorptive properties and of recent theoretical calculations.

Plasma diagnostics by detecting the ion flux profile to a biased-target

A new method for reactive plasma diagnostics is established. It is based on the electrostatic lens effect of the sheath evolving to a one-side biased-target that focuses charged particles to distinct regions of the surface. Thus, developing the ion sputtering profile one can obtain information about sheath thickness, negative ion density, and dust content. The applicability of the method is tested in electropositive and electronegative plasmas of Ar, O 2 and Ar/SF 6 gases.

Effect of interfacial and bulk organic contamination on the quality of thin silicon oxide

The effect of organic contamination of silicon (HF-last cleaned) and silicon dioxide (as-received) wafer surfaces on the quality of gate oxide was studied. Controlled contamination by model organics as well as cleanroom contamination conditions were investigated. Wafers were oxidized under oxidizing or inert ramp-up ambient to grow ultrathin thermal oxides (30 /spl Aring/). Surface and electrical characterization of the oxides was done by Auger sputter profiling, tunneling atomic force microscopy (TAFM) and gate oxide integrity (GOI) measurements. For oxides grown in an inert ambient during ramp-up, HF-last cleaned wafers had a large number of carbon-based defects as compared to as-received wafers. Oxygen in the ramp-up ambient oxidized and volatilized organics resulting in good quality thin gate oxides for HF-last cleaned wafers. However, for as-received wafers, the defect density was increased in an oxidizing ramp-up ambient. A probable mechanism for degradation of the gate oxide quality on HF-last wafers in an inert ramp-up ambient is investigated.

Ultrathin oxide interfaces on 6H–SiC formed by plasma hydrogenation: Ultra shallow depth profile study

Silicon oxide ultrathin films grown on silicon carbide (6H–SiC) by plasma hydrogenation have been studied using ultrashallow depth profiling with time–of–flight secondary ion mass spectrometry. Plasma hydrogenation gives rise to an epitaxial 3×3R30° silicate structure on 6H–SiC(0001) and 6H–SiC(0001̄). By selecting appropriate sputtering conditions, an ultrathin and atomically abrupt interface delineating the boundary between the silicate epilayer (SiO+,Si2O+, and SiO3−,SiO2−) and bulk silicon carbide (SiC+) was observed on both C(0001̄) and Si(0001) faces. Differences in the sputtering profile between the C and Si faces suggest an enrichment of the interface stoichiometry by Si and O on the Si face. Our results support the structural models of the silicate on the C and Si-face 6H–SiC(0001) proposed by Starke [Appl. Phys. Lett. 74, 1084 (1999); J. Vac. Sci. Technol. A 17, 688 (1999)].

Surface roughening of silicon under ultra-low-energy cesium bombardment

Ripple formation on silicon due to oblique (ultra-)low-energy cesium bombardment was studied by measuring the degradation in depth resolution during sputter profiling of an antimony doped delta layer sample (six Sb deltas with an interlayer spacing of 5 nm). Additional insight into the evolution of surface roughness was obtained by recording depth dependent changes in matrix negative-ion signals. The surface topography of the sputtered crater bottoms was investigated ex situ by scanning electron microscopy. At beam energies between 0.25 and 1 keV, ripple formation was found to take place at “critical” angles exceeding 50–60°. Depending on the impact angle θ, the onset of ripple growth could be observed at depths as small as about 2 nm, i.e. immediately after saturating the sample with cesium. The rate of ripple growth increased with increasing θ. Almost complete loss of resolution of the Sb deltas occurred at angles about 10° above the critical angle, at depths as low as 15–20 nm.

Examination of the sputtering profile of NiTi under target heating conditions

In this paper we have examined compositional shifts in NiTi film sputter-deposited from a heated and a cooled target. Substrates were radially placed at 18° intervals to capture the semicircular sputtering profile. The composition of the film was measured using Rutherford backscattering spectroscopy (RBS). Film thickness values were measured and the atomic flux captured was calculated. Results indicate that the target temperature significantly influences the composition of the sputtered film. It was found that, for cold targets, Ni and Ti sputter at different angles, producing a compositional shift in the film when compared with the target. For hot targets the sputtering angle is influenced to a lesser degree, such that the compositional shift is negligible. Therefore, target temperature appears to represent an approach to alter the stoichiometry of films deposited.

Establishing an accurate depth-scale calibration in the top few nanometers of an ultrashallow implant profile

A method to accurately determine the sputter yield of the matrix from the earliest stages of a sputter profile is described. Using the technique of medium-energy ion-scattering spectroscopy, this method provides data that enable a depth scale to be established from subnanometer depths onward. It may be adapted to samples containing a thin amorphous surface layer (e.g., a preamorphized shallow implant) or to crystalline surfaces containing a heavy-element marker layer. In this Brief Report we have used this method to interpret the near-surface profile using erosion-rate data obtained from a 1-keV boron implant into a germanium preamorphized silicon (001) surface.

Electron energy loss spectroscopy of interfacial layer formation in Gd2O3 films deposited directly on Si(001)

High-resolution transmission electron microscopy and electron energy loss spectroscopy (EELS) were used to study the interfacial layers formed in Gd2O3 films on Si(001) during rapid thermal annealing at 780 °C in an O2 ambient. Oxygen diffuses through the films and reacts with the substrate to form a SiO2 layer and an intermediate layer containing Gd2O3 and SiO2. Singular value decomposition was used to profile the Si, SiO2, and Gd2O3 components through the film from the characteristic spectra observed in the Si L2,3, Gd N4,5 and O K-edge EELS. The profiling results support the results of x-ray photoelectron spectroscopy sputter profiling measurements and contribute to a complete picture of the chemical bonding within the films. The ab initio multiple scattering method was used to simulate the observed EELS spectra at the Si L2,3 and O K edges and provide further insight into the chemical bonding of the film and the origin of spectral features. For SiO2, the Si L2,3 EELS is due only to the first-neighbor oxygen atoms. The O K-edge EELS for Gd2O3 is also due only to Gd first neighbors, while the SiO2 EELS is very sensitive to the number of O second neighbors as well as the Si first neighbors.

XPS analysis of sol-gel-generated mixed-oxide layers for biomedical application

The excellent biocompatibility of titanium and its alloys is associated with the properties of their dense TiO2 layer on the surface. The adsorption of proteins of the body fluid to implant surfaces depends on the properties of the surface oxide layer, especially the electronic structure. Therefore, tailoring of the oxide layer is a method for influencing protein adsorption. In this study, titanium platelets are coated by the sol–gel process with mixed oxides containing the biocompatible elements Ti, Nb, Zr and Ta. In order to verify the composition of the produced oxide layer, which can differ from the adjusted precursor composition in the sol because of different reactivities of the precursors, the layers were analysed with x-ray photoelectron spectroscopy by carrying out sputter profiles. It was proved that the composition of a mixed-oxide layer prepared by the sol–gel process can be tailored by the use of mixed precursors. Copyright © 2002 John Wiley & Sons, Ltd.

Profile reconstruction in sputter depth profiling

Quantification of measured sputtering profiles in terms of atomic monolayer accuracy can only be accomplished by applying appropriate methods of deconvolution or profile reconstruction. Of the different approaches developed to adequately describe the depth resolution function, the so-called mixing-roughness-information depth (MRI)-model has proved its usefulness in many applications including both secondary ion mass spectrometry (SIMS) and Auger electron spectroscopy (AES). The MRI model is governed by three fundamental and physically well-defined parameters: atomic mixing length, roughness and information depth. Application examples are presented for depth profiling of GaAs/AlAs delta layers and thicker layers showing an accuracy of 0.2 monolayers. First order non-linear effects like preferential sputtering can be corrected by a composition dependent relation between sputtering time and sputtered depth. A non-linear relationship between intensity and elemental concentration can be corrected by a composition-dependent sensitivity factor, as in secondary ion cluster profiling. However, segregation and compound formation effects causing inhomogeneous elemental distribution in the mixing zone are difficult to include and are the main reason for the present limitations in MRI model applications.

Real-time/in situ diffraction study of phase and microstructural evolution in sputtered β-Ta/Ta2O5 films

The evolution of texture, grain size, and phase content in sputtered β-Ta/Ta2O5 films was studied and controlled in situ using a laboratory-based real-time x-ray diffraction system. The films were dc magnetron sputter deposited using sputter gas (Ar) pressures ranging from 2 to 20 mTorr. To collect diffraction data during growth, a grazing incidence x-ray scattering (GIXS) configuration was necessary. Because this configuration complicated the interpretation of texturing data, complete Schultz geometry pole figures were also used to characterize the films after growth, and the relationship between pole figure and GIXS data was examined. The film composition was evaluated using sputter profile x-ray photoelectron spectroscopy and secondary ion mass spectroscopy. It was found that the degree and rate of (002) out-of-plane texturing was systematically stronger in films grown at lower Ar pressures (< 10 mTorr). These films were predominantly β-Ta in composition and had relatively large grains (∼50 nm in diameter). Films grown using progressively higher Ar pressures had increasing amounts of Ta2O5, developed a (002) out-of-plane texture more slowly, and were nanocrystalline/amorphous in nature. Data from depositions where the Ar pressure was changed during growth showed that the deposition environment at the instant of layer formation determined the phase content, degree of texture, and grain size in the developing film; the characteristics of previous layers had little bearing on subsequent growth. This shows that tailored multilayer β-Ta/Ta2O5 films may be created and monitored in situ.

Adsorption of oxygen on ultrathin Cu/Pt(111) films

Adsorption of oxygen on ultrathin Cu/Pt(111) films was studied using Auger electron spectroscopy, low-energy electron diffraction, and quadrupole mass spectroscopy. Comparing the oxygen-saturated copper films, the density of adsorbed oxygen is doubled as the copper coverage is reduced from 2 to 0.4 monolayers. In the submonolayer range, the more edge sites at the copper islands and larger Cu–Cu distances due to pseudomorphic growth provide a higher possibility for the adsorption of oxygen. A different adsorption rate of oxygen was resolved for the exposure of copper films to oxygen. As oxygen starts to incorporate into the film, the adsorption rate decreases. For a film prepared by alternate steps of copper deposition and oxygen adsorption, the oxygen distribution is not homogeneous from sputter profiling measurements. The amount of oxygen at both interfaces is higher than within the film. From the study of the annealing effects for Cu/Pt(111), the onset of diffusion for Cu adatoms into the Pt(111) substrate is slightly influenced by oxygen.

Thermal oxides of In0.5Ga0.5P and In0.5Al0.5P

An investigation of the chemical composition of wet thermal oxides grown on In0.5Ga0.5P and In0.5Al0.5P is reported. The oxides were grown in the temperature range 500–650 °C. An estimate of the expected oxide composition was obtained by the construction of three-dimensional phase diagrams of the In–Ga–P–O and In–Al–P–O systems. These diagrams indicate that under thermodynamic equilibrium, the oxide layers should be composed primarily of mixtures of InPO4 and GaPO4 on In0.5Ga0.5P and InPO4 and AlPO4 on In0.5Al0.5P. X-ray photoemission spectroscopy (XPS) sputter profiles were used to determine the distribution of elements in the oxide layers and to identify the chemical compounds. The binding energy shifts observed in the XPS data suggests that the oxides are composed primarily of metal phosphates with low concentrations of the metal trioxides. At lower growth temperatures, the oxides composition is uniform with depth, but there is an increasing depletion of In near the substrate interface as the growth temperature increases.

The Ti LMM Auger Spectrum of a Titanium Hydride Film

We present Auger spectra of titanium hydride (TiHy) and titanium films, covered by ultrathin gold layers. The TiHy films were obtained due to hydrogen interaction with the Ti film deposited in situ onto a Pyrex glass substrate under UHV conditions. Both Ti and TiHy films were then covered in situ by a nanoscale Au layer. The Auger analyses were performed in separate systems. Analysis of the features of the Ti Auger spectra during the sputter profile measurements allow the TiHy to be distinguished and characterized in the bulk region of the Au/TiHy layer.

On the chemical nature of boundary lubrication of stainless steel by chlorine- and sulfur-containing EP-additives

The nature of the extreme pressure (EP) effect of the dialkylpolysulfides and chlorinated paraffins during the ironing of stainless steel AISI 304 has been studied. A strip reduction test was used in combination with differential thermal analysis (DTA), profilometry, X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy analysis (AES). Strip reduction tests, in which chlorinated paraffin and dialkyl sulfide were compared as EP-additives, have shown significantly stronger EP-effect of the chlorine containing lubricant, than for the sulfur containing lubricant. DTA analysis of the Me-powder (where Me=iron, chromium or nickel)–dialkylpolysulfide (or chlorinated paraffin) mixtures have shown that dialkylpolysulfide was chemically active with iron and nickel (the highest activity was with nickel). Chromium was practically nonactive with dialkylpolysulfide. Chlorinated paraffin was equally active with iron, chromium and nickel. The better lubrication performance demonstrated by chlorinated paraffin compared to dialkylpolysulfides was attributed to the chemical activity of the chlorinated paraffin with all the main components of stainless steel. The depth profiles of the stainless steel strips were examined before and after strip reduction by use of AES sputter profiling. Results imply that the chlorine containing lubricant enforces the formation of a thick oxide layer.

Ultimate depth resolution and profile reconstruction in sputter profiling with AES and SIMS

New developments in quantitative sputter depth profiling during the past 10 years are reviewed, with special emphasis on the experimental achievement of ultrahigh depth resolution (<2 nm for sputtered depths of >10 nm). In this region, the depth resolution function generally is asymmetric (i.e. non-Gaussian) and it is governed by three fundamental parameters: atomic mixing length, roughness and information depth. The so-called mixing–roughness–information depth (MRI) model and its application to quantitative reconstruction of the in-depth distribution of composition, with a typical accuracy of one monolayer and better, is demonstrated for SIMS and AES depth profiles. Based on the combination of the above three parameters, the ultimate depth resolution is predicted to be ∼0.7–1.0 nm. Up to now, values of 1.4–1.6 nm have been reported and the use of low-energy molecular ions, e.g. by using sulphur hexafluoride as sputtering gas, has recently pushed the mixing length down to 0.4–0.6 nm. However, particularly for the depth profiling of multilayers, it can be shown that minimizing the roughness parameter, including straggling of the mixing length, is most important to achieve the ultimate depth resolution. Copyright © 2000 John Wiley & Sons, Ltd.

XPS and SEM studies of chromium oxide films chemically formed on stainless steel 316 L

Abstract The structure and composition of chromium oxide films formed on stainless steel by immersion in a chromium electrolyte have been studied by SEM and XPS. Cr2O3 crystallites in the range 30–150 nm are fully developed and cover the whole surface. The chemical composition in the depth and the thickness of the oxide layer have been determined by XPS sputter profiles. The oxide film can be described within the framework of a double layer consisting of a thin outer hydrated layer and an inner layer of Cr2O3.

Characterization of titanium hydride films covered by nanoscale evaporated Au layers: ToF‐SIMS, XPS and AES depth profile analysis

Thin titanium hydride (TiHy) films, covered by ultrathin gold layers, have been compared with the corresponding titanium films after analysis using a combination of time-of-flight SIMS (ToF-SIMS), XPS and AES. The TiHy layers were prepared under UHV conditions by precisely controlled hydrogen sorption at 298 K on Ti film evaporated onto a glass substrate. Both Ti and TiHy films were then covered in situ by a nanoscale Au layer. Analyses were performed in separate systems after long-term exposure of the films to air. The thin gold layers covering the Ti and TiHy surfaces prevent any extensive air interaction with both films, allowing characterization of the bulk Ti and TiHy layers ex situ, even after a long-term application in air. The chemical nature of the TiHy layers has been analysed after sputtering of the Au top layer. The high-mass-resolution positive-ion ToF-SIMS spectra disclosed only one peak at mass 49 (49Ti+) for the Ti and two peaks at mass 49 (49Ti+ and 48TiH+) for the TiHy film, reflecting a difference in hydrogen concentration. Analysis of the features of the Ti Auger spectra during the sputter profile measurements allows the TiHy to be distinguished and characterized in the bulk region of the Au/TiHy layer. Besides TiHy, TiO and TiOH were detected by XPS to be the main chemical compounds in the interface region of the Au/TiHy film. Copyright © 2000 John Wiley & Sons, Ltd.

Characterization of titanium hydride films covered by nanoscale evaporated Au layers: ToF-SIMS, XPS and AES depth profile analysis

Thin titanium hydride (TiHy) films, covered by ultrathin gold layers, have been compared with the corresponding titanium films after analysis using a combination of time-of-flight SIMS (ToF-SIMS), XPS and AES. The TiHy layers were prepared under UHV conditions by precisely controlled hydrogen sorption at 298 K on Ti film evaporated onto a glass substrate. Both Ti and TiHy films were then covered in situ by a nanoscale Au layer. Analyses were performed in separate systems after long-term exposure of the films to air. The thin gold layers covering the Ti and TiHy surfaces prevent any extensive air interaction with both films, allowing characterization of the bulk Ti and TiHy layers ex situ, even after a long-term application in air. The chemical nature of the TiHy layers has been analysed after sputtering of the Au top layer. The high-mass-resolution positive-ion ToF-SIMS spectra disclosed only one peak at mass 49 (49Ti+) for the Ti and two peaks at mass 49 (49Ti+ and 48TiH+) for the TiHy film, reflecting a difference in hydrogen concentration. Analysis of the features of the Ti Auger spectra during the sputter profile measurements allows the TiHy to be distinguished and characterized in the bulk region of the Au/TiHy layer. Besides TiHy, TiO and TiOH were detected by XPS to be the main chemical compounds in the interface region of the Au/TiHy film. Copyright © 2000 John Wiley & Sons, Ltd.