Sulfur Segregation Research Articles

Contact Technology for Strained nFinFETs With Silicon–Carbon Source/Drain Stressors Featuring Sulfur Implant and Segregation

In this work, strained n-channel FinFETs (nFinFETs) with silicon-carbon (Si:C) source/drain (S/D) stressors featuring NiSi:C contacts with segregated sulfur at the NiSi:C/Si:C interface are investigated in detail. The physical mechanism for the reduction in an effective Schottky barrier for electrons Φ <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Bn</i> due to presilicide sulfur ion implant and segregation is examined. The presence of sulfur near the NiSi:C/Si:C interface and its behavior as charged donor-like trap states was used to explain the enhancement of electron tunneling across the contact and the reduction in Φ <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Bn</i> down to 110 meV. New analysis using numerical simulation is presented. The results indicate that the presence of charged states near the interface plays a role in achieving low Φ <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Bn</i> . When the S-segregated NiSi:C contact was integrated in strained nFinFETs with Si:C S/D stressors, external series resistance is reduced, and the drive current is improved. The dependence of the drive current on fin width and gate length is also studied.

Quantification of grain boundary equilibrium segregation by NanoSIMS analysis of bulk samples

A technique for the quantification of equilibrium grain boundary segregation by high resolution secondary ion mass spectroscopy (NanoSIMS) on simple metallographically polished surfaces has been demonstrated for the model system of sulphur segregation to nickel grain boundaries. Samples of nickel containing 5.4 wt ppm of sulphur were annealed at different temperatures to achieve different equilibrium sulphur grain boundary concentrations, ranging from less than 1% to about 50% of a monolayer. Quantification was carried out from sulphur concentration profiles acquired across about 20 grain boundaries in each sample. An internal standard (nickel containing a known concentration of sulphur in solid solution) was used for calibration. It is found that, depending on the annealing temperature, the average grain boundary sulphur concentration ranges from 0.9 to 25.8 ng cm–2 (or 1.7 1013 to 4.8 1014 atoms cm–2), i.e. ~0.015 to ~0.43 monolayer. Thermodynamic analysis gives a segregation free energy of −97.8 kJ mol–1 and a grain boundary sulphur concentration at saturation of 26.7 ng cm–2 (or 5.0×1014 atoms cm–2), i.e. ~0.44 monolayer, in good agreement with previous measurements on this system. The limit of detection of the technique is shown to be as low as 0.24 ng cm–2 (or 4.5×1012 atoms cm–2), i.e. ~0.004 monolayer, with a counting time of only 10 min. Copyright © 2012 John Wiley &amp; Sons, Ltd.

Copper, Boron, and Cerium Additions in Type 347 Austenitic Steel to Improve Creep Rupture Strength

Type 347 austenitic stainless steel (18Cr-12Ni-Nb) was alloyed with copper (3 wt pct), boron (0.01 to 0.06 wt pct), and cerium (0.01 wt pct) with an aim to increase the creep rupture strength of the steel through the improved deformation and cavitation resistance. Short-term creep rupture strength was found to increase with the addition of copper in the 347 steel, but the long-term strength was inferior. Extensive creep cavitation deprived the steel of the beneficial effect of creep deformation resistance induced by nano-size copper particles. Boron and cerium additions in the copper-containing steel increased its creep rupture strength and ductility, which were more for higher boron content. Creep deformation, grain boundary sliding, and creep cavity nucleation and growth in the steel were found to be suppressed by microalloying the copper-containing steel with boron and cerium, and the suppression was more for higher boron content. An auger electron spectroscopic study revealed the segregation of boron instead of sulfur on the cavity surface of the boron- and cerium-microalloyed steel. Cerium acted as a scavenger for soluble sulfur in the steels through the precipitation of cerium sulfide (CeS). This inhibited the segregation of sulfur and facilitated the segregation of boron on cavity surface. Boron segregation on the nucleated cavity surface reduced its growth rate. Microalloying the copper-containing 347 steel with boron and cerium thus enabled to use the full extent of creep deformation resistance rendered by copper nano-size particle by increase in creep rupture strength and ductility.

Effect of impurities on the surface morphology of Fe(111)

The surface composition and morphology of Fe(111) have been examined through a combined analysis that includes low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), and scanning tunneling microscopy (STM). The preferential segregation of sulfur has been clearly identified by AES upon annealing. The STM images exhibit numerous triangular pits of various sizes, and the LEED patterns show diffused n × 1 spots. The triangular pits reveal a Sierpinski gasket fractal. For sulfur-free Fe(111), nitrogen segregates to the surface upon annealing, forming a 4√3 × 4√3 superstructure that is identified by LEED patterns and STM images. The STM images show nanoscale triangular clusters regularly aligned in a hexagonal 4√3 × 4√3 configuration. Ultra-thin chromium film deposited on a nitrogen-segregated Fe(111) surface with post-annealing induces further nitrogen segregation, resulting in the formation of triangular pyramid-shaped CrN nanoclusters.

Nonequilibrium Grain Boundary Segregation of Sulfur and Its Effect on Intergranular Corrosion for 304 Stainless Steel

The data obtained by bending tests for intergranular embrittlement after 45 h and 450 h exposure to Strauss solution have been reported for 304 stainless steel. The results show that an embrittlement peak appears at 650 °C for all samples quenched from 1260 °C and then sensitized for 150 h at 480, 565, 650, 730, 815 and 900 °C respectively. The temperature corresponding to the embrittlement peak is decreased to 565 °C when the sensitizing time is prolonged to 1500 h. In this paper, these data are analyzed with an isothermal kinetic model of nonequilibrium grain boundary segregation, indicating that the embrittlement peak is related to the critical time for nonequilibrium grain boundary segregation of sulfur.

Mechanism of film growth of pulsed electrodeposition of nanocrystalline copper in presence of thiourea

This article reports the effect of addition of small amount of thiourea on mechanism of film growth, levelling and grain refinement of pulsed electrodeposition of nanocrystalline copper on stainless steel substrate using simple aqueous acidic copper sulphate solution prepared from 0.25 M CuSO4·5H2O and 0.5 M H2SO4. The amount of thiourea used in the electrolyte is 0 and 36 mg/l. The results indicate the change in morphology of the deposits with addition of thiourea leading to dendrite free copper deposits. The growth mechanism of the copper deposition is found to change from Volmer–Weber type to Frank-Vander Merwe type making the deposits smoother when deposited with thiourea addition. A small amount (36 mg/l) of thiourea addition leads to decrease in the average grain size of copper from 1160 nm to 14 nm. The results clearly reveal the formation of nanocrystalline copper by addition of thiourea with three orders of magnitude reduction in grain size as compared to the sample deposited without thiourea. Preferential segregation of sulphur (present in thiourea) along the grain boundaries of nanocrystalline copper is shown by energy filter imaging using ultra high resolution transmission electron microscope (TEM), thereby restricting the growth of copper grains during pulsed electrodeposition.

Effect of Dy on oxide scale adhesion of NiAl coatings at 1200 °C

The cyclic oxidation behaviour of β-NiAlDy coatings produced by electron beam physical vapour deposition (EB-PVD) was investigated. For the Dy-free NiAl coating, numerous voids developed at the Al 2O 3 scale/NiAl interface and sulfur was found to segregate at the void surfaces, leading to early spallation of the scale. The addition of Dy prevented sulfur segregation and void formation at the scale/NiAl interface. As a result, the scale grown on the NiAlDy coating remained adherent and no spallation occurred even after 400 h cyclic oxidation. The 0.05 at.% Dy doped coating revealed lower oxidation rate than the more Dy doped coatings.

Temperature and dew point dependent segregation of phosphorus and sulfur in Fe–Mn–P–S model alloy

The dependence of surface segregation of P and S, on temperature and dew point of the annealing atmosphere was studied for an Fe–1Mn–0.02P–0.008S (all in wt.%) model alloy. The specimens were annealed for 1 min within a temperature window of 400 to 800 °C, in N 2–5%H 2 gas atmospheres with a wide range of dew points − 80 to 0 °C. Surface analyses were carried out by using XPS and FE-SEM. At the dew points − 80 and − 40 °C segregation of P was observed at 400 and 600 °C but S segregation occurred only at 800 °C. Phosphorus accumulation increases significantly with increase of dew point and, moreover, at the higher dew point, P replaces S at 800 °C.

Measuring grain boundary segregation using Wavelength Dispersive X-ray Spectroscopy: Further developments

A technique for the quantification of surface and grain boundary segregation using Wavelength Dispersive X-ray Spectroscopy (WDS) in the Scanning Electron Microscope (SEM) is proposed. As an example, the case of sulphur segregation in nickel is considered. The sulphur segregation can be quantified using a single voltage measurement (20 kV) of the sulphur Kα line intensity. The quantification is made from a simple proportionality equation derived from the XPP microanalysis model of correction. The result of segregation quantification by WDS is a surface concentration expressed in g.cm -2. Special attention was paid to the quantification of grain boundary segregation on fracture surfaces, taking into account the off-normal incidence of the electron beam on the analyzed surface. A simple technique that allows determination of the tilt angle from the specimen absorbed current is proposed. The influence of the azimuth angle of the analyzed surface with respect to WDS spectrometer is also discussed. The results of the WDS technique are shown to be repeatable within 5% with reasonable counting times (a few minutes per facet). As an example, the kinetics of sulphur grain boundary segregation in nickel at 750 °C was measured using WDS to document the quantitative capabilities of the technique. As already pointed out in a previous paper, it is confirmed that WDS is insensitive to surface contamination, which allows the analysis of ex-situ fractured samples.

Study on Non-Equilibrium Grain-Boundary Segregation of Sulfur among Hastelloy X

Sulfur is the main element which caused Nickel-based alloy embrittlement. In this study, the sulfur in Hastelloy X superalloy was determinated with Auger Electron Spectroscopy (AES) for samples quenched from 1180 °C and aged at 500 °C for different time. Experiments results confirmed the non-equilibrium segregation characteristics of sulfur. The results showed that a segregation peak of sulfur is at about 20 min during ageing. This peak was satisfactorily elucidated by the theory of non-equilibrium grain-boundary segregation. By theoretical calculation, the critical time constant of impurities sulfur atom in the Hastelloy X δs= 357. At the same time, the result provides a theoretical basis for sulfur segregation mechanism.

Anomalous Sulfur Segregation Kinetics Governed by MnS Precipitation Reaction and {110} Annealing Texture Development in Al-free and 0.1% Mn–Added 3% Si–Fe Alloys

In 0.1% Mn–added 3% Si–Fe alloys, the total number of {110} grains increased with increasing bulk sulfur content and with decreasing heating rate. This is contrary to that in Mn-free alloys. During isothermal annealing, the segregation concentration of sulfur for the initial time range was lower in the alloy containing 95 ppm sulfur than that in the other alloy containing 25 ppm sulfur. Also the maximum segregation concentration of sulfur appeared at a later time in the former. Such an annealing texture development and a segregation behavior can be attributed to the MnS precipitation and the subsequent dissolution into the matrix.

Effect of rare earth element yttrium addition on microstructures and properties of a 21Cr–11Ni austenitic heat-resistant stainless steel

In this comparative study, the microstructure and the mechanical properties of a 21Cr–11Ni austenitic heat-resistant stainless steel with and without addition of rare earth (RE) element yttrium have been investigated. The results show that a number of fine spherical yttrium-rich oxide particles are not uniformly distributed in the matrix of steel with yttrium; instead, they are aligned along the rolling direction. The grains surrounding the alignment are nearly one order of magnitude smaller than those farther away from the alignment. The approximate calculation results indirectly show that the grain refinement may be mainly attributed to the stimulation for nucleation of recrystallization rather than to pinning by particles. Furthermore, the alignment has resulted in significant loss in transverse impact toughness and tensile elongation at room temperature. There is a trough in the hot ductility–temperature curve, which is located between 973 and 1173K. The ductility trough of steel with yttrium becomes shallow within a certain temperature range, especially around 1073K, indicating that improvement on hot ductility is achieved by yttrium addition. The results may be attributed to the increase of grain boundary cohesion indicated by the effective improvement on intergranular failure tendency, and the inhibitory effect of yttrium on sulfur segregation to grain boundaries is believed to be an important cause.

Hydrogen and moisture-induced scale spallation: Cathodic descaling of a single crystal superalloy

Moisture-induced delayed spallation (MIDS) of protective alumina scales at room temperature is a well known phenomenon. One mechanism proposes that water and Al alloy react, produce hydrogen at the scale–metal interface, and enable spallation. To test this mechanism, preoxidized samples of a single crystal superalloy, Rene’N5 + Y, were subjected to standard cathodic hydrogen charging treatments known to produce hydrogen embrittlement in bulk Ni and Ni 3Al alloys. Cathodic hydrogen charging, at <1 mA and an estimated −0.45 V SCE, stripped the scales at the oxide-metal interface, resulting in an initial loss of ∼3 mg/cm 2 and little additional change with time. This was supported by macro-photos and SEM of the spalled surface. On the other hand, anodic polarization at <1 mA produced less, but steady, linear weight loss (0.3 mg/cm 2), primarily by anodic dissolution of the metal. Hydrogen charging was thus shown to be detrimental to the alumina scale–metal bond, supporting the hydrogen factor in MIDS. These and other MIDS results show remarkable similarities to embrittlement of Ni subject to hydrogen charging at similar potentials and varying amounts of interfacial (grain boundary) sulfur segregation. The MIDS phenomenon is also discussed in terms of comparative static corrosion fatigue characteristics. It is not necessarily related to other diverse moisture effects occurring at high temperature.

First-Principles Study on the Grain Boundary Embrittlement of Metals by Solute Segregation: Part I. Iron (Fe)-Solute (B, C, P, and S) Systems

The microscopic mechanism of grain boundary (GB) embrittlement in metals by solute segregation has been not well understood for many years. From first-principles calculations, we show here that the calculated cohesive energy (=2·surface energy − GB energy) of bcc Fe Σ3(111) symmetrical tilt grain boundary (STGB) is reduced by the segregation of sulfur (S) and phosphorous (P) while it is increased by the segregation of boron (B) and carbon (C). The rate of the decrease/increase in the cohesive energy is proportional to the experimental shift in the DBTT of high-purity iron with increasing segregation. This indicates that the change in the cohesive energy of GB plays a key role in the GB embrittlement of metals.

Failure investigation on copper-plated blind vias in PCB

Failures like open circuit are always encountered in PCBs (printed circuit boards) due to defects in the blind vias. In this paper, systematic analysis such as macro and micro observation was carried out on cracking blind vias in PCB used for mobile phones. The predominant cracking mechanism of sulfur segregation in form of Cu x S, which was generated by the excessive use of brightener additive, was proposed for the first time. Meanwhile, grain coarsening in copper deposition led by inappropriate current density was the other main cause for ductility decreasing in the plating layer. Complementarily, microcrack propagation was explained based on the finite element method (FEM) results of stress distribution after thermal cycling. Finally, suggestions and countermeasures were addressed, which were of importance for reference to the structural integrity and electrical reliability of blind vias in PCB during process and service.

Formula omitted] contacts with sub-0.1 eV effective Schottky barrier heights obtained by sulfur segregation

We demonstrate tuning of the Schottky barrier height, ϕ B of nickel–platinum alloy silicide ( Ni x Pt 1 - x Si ) contacts on n-type silicon by segregating sulfur at the silicide/Si interface. It is shown that while there is negligible effect of sulfur on the thermal stability and silicide resistance, extremely small barrier height values of 0.05–0.07 eV at the silicide/Si interface can be achieved by sulfur segregation.

Gettering of S in Ni from first principles

Sulphur segregation to grain boundaries, surfaces or interfaces seriously degrades the performance of many metallic systems. The addition of appropriate dopants may reduce the sulphur effect. Here we present a first-principles study to provide physical insights into Hf and Pt interactions with S in Ni. The pair affinities are assessed through total energetics, electron localization function and differential charge density analyses. Calculations show that Hf can effectively getter S and hence inhibit S segregation even at high temperatures, while Pt cannot.

Strain-Rate Sensitivity and Failure Peculiarities in Compression of the Nanocrystalline Ni-20 Pct Fe Alloy at Low Temperatures

The strain-rate sensitivity of the applied stress was measured, and the values of the activation volume of the process of the plastic deformation V along the deformation curve in compression of the nanocrystalline (NC) Ni-20 pct Fe alloy (with average grain size ~22 nm) in the temperature range of 300 to 77 K (27 to −196 °C) were calculated. It was found that the decrease of the temperature from 300 to 77 K leads to a decrease of the value of V from 20 to 8 b3, and values of V do not depend on the strain. Fractographic features of the failure surfaces were studied in the temperature range 300 to 4.2 K (27 to −269 °C). Observed traces of melting on these surfaces at temperatures below 300 K indicate the intense heating in the catastrophic shear band of the alloy in the moment of failure. Causes of low-temperature decohesion along grain boundaries are discussed in terms of the sulfur segregation influence.

Influences of sulfur segregation to permeability in Pd-D2 permeation system

The influence of segregated sulfur on palladium permeability was investigated using deuterium permeation through palladium membrane, which was pretreated by annealing in vacuum and/or in air. It has been found that the palladium membrane is almost impermeable after vacuum annealing, while it is permeable after air annealing. A combination of vacuum annealing of a Pd membrane followed by annealing in air leads to better and more stable D2 permeation because S has been depleted from the Pd bulk. In fact, cleaning S from Pd in spell out (UHV) normally proceeds via cycles of high T annealing to drive S to the surface, followed by burning off the S as SO2 by heating in an O2 environment. The results in this study well demonstrate the influence of sulfur on the deuterium-permeation rate and on the behavior of sulfur segregation from Pd bulk to the surface.

Comparison of the oxidation of high-sulfur Ni–25Cr–5Al alloys in as-cast and as-sputtered states

To understand the effect of sulfur on the oxidation of nanocrystalline (NC) alloys, a high-sulfur alloy having a chemical composition similar to a coarse-grained (CG) cast alloy of Ni–25Cr–5Al–1S (wt.%) was fabricated using magnetron sputtering. The oxidation of the two alloys in isothermal and cyclic conditions in air at 1000 °C shows that the alumina scale formed on the cast alloy was susceptible to spallation, whereas the alumina scale on the sputtered alloy was intrinsically adhesive. The result indicates that the nanocrystallization of alloys helps to eliminate the detrimental “sulfur effect” on oxidation, through minimizing the segregation of sulfur to the scale/alloy interface.