bluuebunny/crossref_metadata_2025 huggingface dataset:
Microstructural Effects on the Creep and Crack Propagation Behaviours of ³-Ti Aluminide Alloy
Authors: Lupinc V. , Onofrio G. , Nazmy M. , Staubli M.
Published: April 2011

Gamma titanium aluminides class of materials possess several unique physical and mechanical properties. These characteristics can be attractive for specific industrial applications. By applying different heat treatment schedules one can change the microstructural features of this class of materials. In the present investigation, two heat treatment schedules were used to produce two different microstructures, duplex (D) and nearly lamellar (NL) in the cast and HIP'ed Ti-47Al-2W- 0.5Si alloy.

The tensile strength and creep behaviour, in the 700–850°C temperature range, of this alloy have been determined and correlated to the corresponding microstructures. In addition, the fatigue crack propagation behaviour in this alloy has been studied at different temperatures. The results on the creep behaviour showed that the alloy with nearly lamellar microstructure has a strongly improved creep strength as compared with that of the duplex microstructure.
Characterizing Deformation Mechanisms in Ni3Ge-Fe3Ge Intermetallic Alloys
Authors: Balk T. J. , Kumar Mukul , Mryasov O. N. , Freeman A. J. , Hemker K. J.
Published: April 2011

The Ni3Ge-Fe3Ge model system provides us with a unique opportunity to characterize the mechanisms of deformation in both anomalous and normal L12 intermetallic alloys. The elastic moduli of alloys in this system have been measured and used as benchmarks for first principles calculations. At 77K, increasing the Fe content has been found to result in a dramatic increase in flow stress. The Ni-rich alloys exhibit a yield strength anomaly, but as Ni is replaced by Fe, the anomalous temperature dependence gradually disappears, and no yield strength anomaly is observed for alloys with more than 25 at% Fe. At low temperatures and Fe contents, the deformation microstructure has been found to be dominated by Kear-Wilsdorf locking; but a transition from octahedral glide and Kear-Wilsdorf locking to cube glide is observed as either Fe content or temperature is increased. This transition is related to changes that occur in the core structures of dissociated superdislocations and planar fault energies measured through computer simulations of weak-beam TEM images.
Dislocation Dynamics During the Deformation of Intermetallic Alloys and the Flow Stress Anomaly
Authors: Messerschmidt U. , Bartsch M. , Guder S. , Häussler D.
Published: April 2011

In situ straining experiments on NiAl, NiAl-0.2at% Ta, γ-TiAl, and MoSi2 in a high-voltage electron microscope showed a transition from the obstacle controlled dislocation motion or the Peierls mechanism at low temperatures to either an unstable or viscous motion at high temperatures. It is suggested that the viscous motion is due to the formation of point defect atmospheres around the dislocations, which cause additional drag and may be responsible for the flow stress anomaly in some of these materials. The atmospheres may be of an extrinsic or an intrinsic nature. A new model is proposed for the origin of intrinsic atmospheres assuming that the energy of a dislocation in an intermetallic alloy may be lowest if the dislocations contain a number of point defects in their core. The dragging of atmospheres may lead to an “inverse” dependence of the strain rate sensitivity on the strain rate, as observed experimentally. The macroscopic deformation data of the studied materials are discussed in terms of the model.
The Influence of Solid Solutions on Flow Behavior in -γ-TiAl
Authors: Woodward C. , Kajihara S. A. , Rao S. I. , Dimiduk D. M.
Published: April 2011

Modifications of alloy chemistry are often used to tailor the intrinsic flow behavior of structural materials. Models of solution strengthening, high temperature yield stress and creep must relate the effects of chemistry to the mechanisms that influence these material properties. In ordered alloys, additional information regarding the crystallographic site occupancy of ternary elements is required. In this study relaxed structures and energies for intrinsic and substitutional point defects are calculated using a first principles plane-wave-pseudopotential method. Calculated defect energies are used to predict the density and site preferences of solid-solutions (Si, Cr, Nb, Mo, Ta and W) in ³TiAl. Size and modulus misfit parameters are calculated and the interaction of these defects with a dissociated ordinary screw dislocation evaluated within anisotropic elasticity theory. The derived interaction strength is then related to solid-solution strengthening for these defect centers. Predicted solid-solution effects are in good agreement with experimental observations for the binary alloy.
Effect of Hot Rolling on the Microstructure and Mechanical Properties of Ti3Al Based Dual Phase Alloys
Authors: Wu Jiansheng , Zhang Lanting , Hua Wei , Qiu Guohua
Published: April 2011

Development of α2-Ti3A1 based dual phase alloys have shown some promising potentials in property improvement by introducing Ti5Si3sulicide phase into the matrix via Si alloying. However, the presence of coarse network of Ti5Si3phase formed by eutectic reaction in the as-cast state also embrittles the alloy. Both hot rolling and powder metallurgy are considered to be the possible ways to refine the Ti5Si3phase in the matrix. Two Ti-Al-Si-Nb alloys whose Si contents are 2 and 5 at. % respectively were arc melted into ingots and then hot rolled to sheets in this investigation. Optical metallographic examination correlates the microstructures of the as-cast and as-rolled alloys with the different rolling amounts, showing that the coarse silicide network is broken into small particles after hot rolling. Mechanical property testing from room temperature to 8000°C indicates that the strength and plastic elongation of the hot-rolled alloys are much higher than those of the as-cast ones. The data obtained in this investigation are comparable with those obtained in the P/M processed specimens. Fracture surfaces of the alloys are also examined.
Characterization of Laser-Deposited TiAl Alloys
Authors: Zhang X. D. , Brice C. , Grylls R. J. , Evans D. J. , Fraser H. L.
Published: April 2011

Microstructure of TiAl produced by Laser Engineered Net Shaping (LENS) has been characterized using optical microscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It has been shown that the substrate has a significant effect on the microstructure deposited. Depending on operational parameters, either equiaxed γ-TiAl/α2 -Ti3AI or single phase α 2 microstructure can be obtained. In-situ heating experiments reveal that nucleation of lamellae often starts at grain boundaries in this retained α 2grains. By careful control of post heat treatment, an ultra fine lamellar microstructure may be obtained, which may significantly improve tensile property in these alloys [3].
On the Formation of Silicide Precipitates in Niobium-Silicide Based Composites
Authors: Bewlay B. P. , Grylls R. J. , Fraser H. L.
Published: April 2011

Composites based on Nb-Si are attractive candidates for use as structural materials at the very high temperatures required for future aircraft engines. The composites described were produced by directional solidification, which gives a microstructure consisting of Nb dendrites with an Nb3Si-Nb eutectic. The aim of this paper is to provide a detailed characterization of precipitates observed in the Nb dendrites in both binary and higher-order alloys. The precipitates possess the Nb3Si stoichiometry, but not the stable Nb3Si structure. The precipitates form a metastable orthorhombic crystal structure which is related to the Nb matrix via a simple orientation relationship.
An Investigation on Dynamic Tensile Properties of TiAl Intermetallic Alloy
Authors: Wang Yu , Lin Dongliang , Zhou Yuanxing , Xia Yuanming , Law Chi C.
Published: April 2011

Room-temperature tensile properties of polycrystalline Ti–;47A1–2Mn–;2Nb alloy with nearly lamellar (NL) microstructures were investigated at the strain rates between 10-;5 and 1000 s-1 using a self-designed Split-Hopkinson tensile bar setup with a rotating disk and conventional testing machine. It was found that tensile ductility varies within a narrow range with the strain rate while dynamic strengths (σd) of the alloy are obviously higher than static strengths (σs). There exists a linear relationship between σs and the logarithm of the strain rate , and between σ d and the strain rate itself (). Fractography analysis indicated that the alloy fractured in a mixed mode of predominant transgranular cleavage and minor intergranular cracking under static and dynamic strain rates. Environmental effect was excluded from the main cause for the room-temperature brittleness of the investigated alloy.
The Influence of Second-Phase Dispersion on Environmental Embrittlement of Ni3(Si, Ti) Alloys
Authors: Takasugi T. , Hanada S.
Published: April 2011

Some quaternary Ni3(Si, Ti) alloyed with transition elements V, Nb, Zr and Hf was prepared beyond their maximum solubility limits to investigate the effect of second-phase dispersion on moisture-induced embrittlement. V-added Ni3(Si, Ti) alloy contained ductile fcc-type Ni solid solution as the second-phase, while Nb-, Zr- and Hf-added Ni3(Si, Ti) alloys contained hard dispersion compounds as the second-phase. V- and Nb-added Ni3(Si, Ti) alloys did not display reduced tensile elongation in air, indicating that their second phases have the effect of suppressing the moisture-induced embrittlement. Possible mechanisms for the beneficial effect by the second phase on the moisture-induced embrittlement of V- and Nb-added Ni 3(Si, Ti) alloys are discussed in association with hydrogen behavior and deformation property in the constituent phases or at matrix/second-phase interface.
Self-Diffusion of Both Components in the Intermetallic Laves Phase NbCo2
Authors: Denkinger M. , Mehrer H.
Published: April 2011

Self-diffusion of57Co and95Nb in the cubic Laves phase NbCo2 has been studied using the radiotracer method. The temperature dependences of the self-diffusivities are described for Nb31Co69 by

,

and for Nb29Co71 by

,

The diffusivity of 95Nb in Nb31Co69 is considerably lower than that of 57Co. The latter increases significantly with increasing Co content.

The results are discussed in terms of vacancy mediated diffusion in connection with structural aspects of the Laves phase together with the scarce information available for defects in NbCo2. The results are compared with interdiffusion data from our laboratory and with self-diffusion in the also close-packed L12 intermetallics.
Mechanical Properties of L12 Type Zn3Ti-Base Alloy
Authors: Hosoda Hideki , Hanada Shuji
Published: April 2011

An alloy composed of L12-type Zn3Ti was investigated in terms of phase stability and mechanical properties. Zn and Ti powders were mixed at a composition of Zn-25mol%Ti using a ball mill in Ar, and an ingot was made by melting the powders. Optical microscopy, X-ray diffraction analysis and thermogravimetry - differential thermal analysis were carried out. Mechanical properties were investigated by Vickers hardness tests at room temperature (RT) and compression tests from RT to 703K in Ar. It is found that (1) the alloy is mainly composed of L12 Zn3Ti, (2) the alloy has weak positive temperature dependence of strength, and (3) normalized strength by melting point is comparable to that of L12 Al3Ti-Cr alloys. L12 Zn3Ti has HV178 and is brittle at RT. Reaction temperatures of Zn-rich portion of the Zn-Ti phase diagram were also reinvestigated and a peritectic-reaction temperature between Zn3Ti and liquid + Zn2Ti is determined to be at 880K.
An Assessment of the Method Used to Determine Activation Parameters in L12 Alloys
Authors: Matterstock B. , Saada G. , Bonneville J. , Martin J. L
Published: April 2011

The characterisation of dislocation mechanisms in connection with macroscopic mechanical properties are usually performed through transient tests, such as strain-rate jumps, load relaxations or creep experiments. The present paper includes a careful and complete theoretical analysis of the relaxation and the creep kinetics. We experimentally show that the plastic strain-rate is continuous at the transition between constant strain-rate conditions and both load relaxation and creep test. The product of the plastic strain-rate at the onset of the transient test () with the characteristic time (tk) of the transient is found to be independent of , as theoretically expected. This is a clear indication that the assumptions underlying the theoretical analysis are relevant.
Exafs Evidence For Atom Location In Icosahedral TiZrNi And Hydrogenated TiZrNi Quasicrystals
Authors: Sadoc A. , Kim J. K. , Kelton K.F.
Published: April 2011

We present results from the first extended X-ray absorption fine-structure studies in a Ti/Zrbased quasicrystal. The icosahedral Ti53Zr27Ni20 phase and an icosahedral Ti45Zr38Ni17 phase that also contains 1.7 hydrogen atoms for each metal atom were investigated. Experiments performed above the Ti, Ni and Zr K absorption edges have allowed to identify different types of atomic subshells around the Ti, Ni and Zr atoms. Significant differences are observed between these two quasicrystalline alloys, suggesting preferential sites for the hydrogen atoms.
Discerning Phason Coherency In Quasicrystalline Systems
Authors: Naumis G. G. , Wang Chumin , Barrio R. A.
Published: April 2011

Phason disorder is expected to modify the neutron and x-ray scattering spectra of quasicrystals. However, it is not clear yet if phasons are coherent modes in real space, or if they should be considered as local random defects. In this work, a comparative study of different sorts of phasons is performed, and their effects on the dynamical structure factor of Fibonacci chains are analyzed in detail. The results show that coherent and random phasons can be distinguished for high values of the momentum transfer. Finally, it is observed that a random phason produced in the quasicrystal's hyper-space leads to a coherent phason field in real quasicrystalline lattice.
A Decagonal A170Mn17Pd13 Quasicrystal Structure With A Non-Centrosymmetric 5D Space Group
Authors: Yamamoto Akiji , Weber Steffen
Published: April 2011

A single-crystal X-ray analysis revealed that a decagonal Al70Mn17Pd13 has a noncentrosymmetric structure with the 5-dimensional (5D) space group P105mc. The structure refinement based on a 5D cluster model gave Rw= 0.119 (R = 0.167) for 1428 independent reflections and a reasonable chemical composition of Al69.1Mn18.5Pd12 4.
Melting And Solidification Behaviour Of Lead Particles Embedded In Quasicrystalline Matrices
Authors: Singh Alok , Tsai A. P.
Published: April 2011

Composites of nanoparticles of lead were prepared by embedding into Al-Cu-V and Al- Cu-Fe icosahedral, and Al-Cu-Co decagonal phase alloys by means of rapid solidification. The microstructure of these composites, the orientation relationships and the interfaces formed between the particles and the matrix has been studied by transmission electron microscopy. The melting and solidification behaviour of the lead particles in these matrices has been studied by differential scanning calorimetry. The microstructure and the nature of the particle-matrix interface has been related to the melting/solidification behaviour of the particles in the matrix.
Phase Diagram Determination For Modifications Of The D Phase In The Al-AlCo-AINi System
Authors: Lück R. , Scheffer M. , Gödecke T. , Ritschj S. , Beelif C.
Published: April 2011

An extensive investigation into the At-AICo-AlNi ternary subsystem is presented. Observations have used the techniques of differential thermal analysis, magnetothermal analysis, dilatometry, metallography, X-ray diffraction, transmission electron microscopy, and high-resolution electron microscopy. Representative graphic documentation, as liquidus projection surface, isothermal sections, temperature-concentration section, and reaction scheme are presented. 11 phases from the binaries Al-Co and Al-Ni and the three ternary phases Y2 (Co2NiAl9), X and the decagonal phase D were found at room temperature. The decagonal phase is formed from the melt peritectically via a critical tie line and its primary formation area dominates at the liquidus projection surface. 45 three-phase regions are present according to the reaction scheme.

Several phase variants in the area of the decagonal phase were detected by transmission electron microscopy. Phase fields of the variants were determined from samples quenched from their respective temperatures. In-situ experiments on transformations of variants were performed by dilatometric measurements. The subdivision of the D phase area into the fields of the variants is discussed.
Leed Investigations Of A Cubic AL-PD-MN (110) Alloy
Authors: Heinzig M. W. , Warren O. L. , Shen Z. , Jenks C. J. , Lograsso T. A. , Thiel P. A.
Published: April 2011

An Al-Pd-Mn cubic alloy having a bulk chemical composition somewhat analogous to that of the icosahedral Al-Pd-Mn quasicrystal is studied. Our goal is to compare the surface structure and properties of the cubic alloy with those of the quasicrystalline alloy. In this paper, we report the first observations for the (110) surface of the cubic alloy using (primarily) low energy electron diffraction (LEED). The surface is prepared by sputtering and annealing in ultrahigh vacuum (UHV). In addition to the substrate LEED pattern, at least three superstructures evolve sequentially with annealing temperature.
Preparation And Properties Of Solar Selective Absorbers Based On AlCuFe AND AlCuFeCr Thin Films: Industrial Aspects
Authors: Eisenhammer T. , Nolte H. , Assmann W. , Dubois J. M.
Published: April 2011

Solar selective absorbers for thermal energy applications can be realized with films of quasicrystal forming alloys. Absorbers were produced by sputtering thin films of AlCuFe and AlCuFeCr. Alumina and floatglass were used as dielectric antireflective coatings, which are necessary for the required optical properties. For applications, several aspects are of importance: high solar absorptance and low thermal emittance have to be achieved. Moreover, the coatings have to show good adhesion and stability with respect to oxidation in air at elevated temperatures and exposure to humidity and condensation of water. The present study emphasizes these aspects, especially degradation stability, for coatings deposited on technical copper foils with large roughness. Oxidation stability is achieved easily, while humidity stability depends strongly on the choice of materials. The combination AlCuFeCr/floatglass turned out to be stable, while the combination AlCuFe/alumina is destroyed rapidly.
Electrically switchable reflection gratings in polymer dispersed liquid crystals
Authors: Natarajan L. V. , Sutherland R. L. , Tondiglia V. P. , Siwecki S. , Pogue R. , Schmitt M. , Brandelik D. , Epling B. , Berman G. , Wendel C. , Ritter M. , Stallings M. , Bunning T. J.
Published: April 2011

Electrically switchable volume reflection holograms were written by inhomogeneous illumination of a prepolymer syrup containing a nematic liquid crystal and a multifunctional acrylate monomer. Switchable holograms are diffractive optics structures and the diffraction efficiency can be controlled by the application of an electric field. Reflection gratings with grating spacing varying between 0.16-0.27 µm were made during the phase separation of liquid crystals from the fast curing prepolymer syrup. The reflection efficiency of the holograms were electrically modulated with the applied field of ∼10-15V/µm. Real time study of the grating formation revealed that the maximum efficiency is reached in ∼15 seconds. The shrinkage of the host polymer during grating formation resulted in the blue shift of the reflection notch. The response time of the grating in an electric field is ∼50 µs. Low voltage scanning electron microscope studies showed the presence of discrete nematic droplet domains of sizes 30-60 nm in liquid crystal rich region.
The Optical and Morphological Properties of Fluorinated Polymer Dispersed Liquid Crystals
Authors: Schulte M.D. , Clarson S.J. , Natarajan L.V. , Tondiglia V.P. , Bunning T.J.
Published: April 2011

Polymer dispersed liquid crystal (PDLC) films with a partially fluorinated polymer matrix were investigated. The optical and morphological properties of fluorinated PDLC's were different from non-fluorinated films. The incremental addition of a fluorine-substituted monofunctional methacrylate monomer to a pentaacrylate-based standard PDLC formulation resulted in significant improvement in contrast ratio. In addition, results suggest that fluorine decreased compatibility between the polymer and liquid crystal phases. Scanning electron microscopy (SEM) was employed to elucidate morphological differences between fluorinated host matrices and non-fluorinated control films.
New Wholly-Aromatic Thermotropic Polyesters with Controlled Flexibility
Authors: Nagvekar Devdatt S. , Mather Patrick T. , Jeon Hong G. , Tan Loon-Seng
Published: April 2011

In this work, we present the synthesis and characterization of a new series of wholly-aromatic copolyesters derived from the condensation of various weight fractions of 4,4'-(o- phenylenedioxy)dibenzoic acid (OPDB) and substituted terephthalic acid (BTA) with 2-phenylhydroquinone (PHQ). The Higashi method, involving tosyl chloride and pyridine as solvent, was employed to yield polymer with significant molecular weight. These polymers are intended to enable accessible clearing transition and to control the balance of stiffness and toughness in melt-spun fibers systematically. We report the synthetic details along with characterization of quiescent phase behavior and morphology.
Effect of Composition on The Morphology and Electro-Optical Properties of Physically Crosslinked Liquid Crystals
Authors: Schneider A. , Geppert S. , Spontak R.J. , Gronski W. , Finkelmann H.
Published: April 2011

Addition of an ABA triblock copolymer to a midblock-selective solvent can, depending on copolymer/blend composition and the magnitude of block-block and block-solvent interactions, result in the formation of a physical network that is stabilized by aggregates of the incompatible A-block. In this work, an ABA copolymer with a nematic side-chain liquid crystal midblock is added to a low-molar-mass nematic liquid crystal (LC) in an effort to produce a comparable copolymer network and bind the LC matrix. This nanostructured system, designated a physically crosslinked liquid crystal (PCLC), may not suffer as much from the constraint-induced LC anchoring problems associated with conventional polymer-dispersed liquid crystals (PDLCs). Results presented here demonstrate that hierarchical phase behavior must be carefully considered in the design of PCLCs.
Relation Between Molecular and Macroscopic Properties of Nematic Liquid Crystals
Authors: Ferris Kim F. , Exarhos Gregory J. , Risser Steven M.
Published: April 2011

While many liquid crystal studies have focused on static structural and electronic properties, dynamic factors such as molecular anisotropy, partial ordering and structural fluctuations have hindered descriptions linking liquid crystalline properties at the molecular level with the macroscopic level properties (as measured experimentally). In this paper, we examine the transition from molecular to bulk properties for the nematogen 5-alkyl-cyanobiphenyl (5CB), using both molecular dynamics and quantum mechanical calculations. We perform molecular dynamics simulations on a collection of molecules in the nematic phase, sample the trajectory to create a representative collection of structures, and perform electronic structure calculations on the sampled structures.
Light Transmission-Light Scattering Reverse Mode Switching of (Liquid Crystalline Polymer/Liquid Crystal)Composite System
Authors: Kikuchi Hirotsugu , Kibe Shigeru , Yamane Hirokazu , Kajiyama Tisato
Published: April 2011

Conventional (polymer/liquid crystal) composite films exhibit the normal mode electro-optical switching, that is, the light scattering-light transmission switching upon electric field OFF-ON states, respectively. The “reverse mode” electro-optical switching, that is, the light transmission-light scattering switching upon electric field OFF-ON states, respectively, has been investigated for a novel type of composite system being composed of a side chain type liquid crystalline polymer(LCP) and low molecular weight liquid crystals(LCs). The polarizing optical microscopic observation and X-ray diffraction study revealed that the two types of smectic-like short range ordering were present in the (LCP/LCs) composite system in a nematic state. It was confirmed from electric capacitance measurements of the homogeneous alignment cell that these phases with different smectic-like short range orderings each exhibited a different value of threshold voltage. Although the homogeneous alignment cell of the (LCP/LCs) composite system was in a light transmitting state in the absence of an electric voltage, the cell turned into a light scattering state upon the application of an electric voltage of 5-15 V for the cell of 14 µm thick (0.4−1.1 MV" m−1). The reversible change from the light scattering state to the transmission state was observed after removing the electric voltage. The light scattering state of the (LCP/LCs) composite system upon the application of electric voltage might be due to the appearance of optically heterogeneous structure induced by the different values of the threshold voltage between the two types of smectic-like phases with different short range orderings. These phases might be in a phase-separated state with an optical dimension (several hundred nm). The reversible “reverse mode” electro-optical switching was realized for the (LCP/LCs) composite system in a nematic state in which the two types of smectic-like short range orderings were separately formed in an optical size level.
X-Ray Diffraction and Reflectometry Investigation of Interdiffusion In Sputtered Niobium-Tungsten Bilayers
Authors: Welzel U. , Lamparter P. , Mittemeijer E. J.
Published: April 2011

Interdiffusion in sputtered niobium(45nm)-tungsten(45nm) bilayers upon annealing at low temperatures (T<1000K) has been investigated using X-ray reflectometry and diffraction. The accompanying changes in macrostress and microstructure have been characterized by applying the X-ray diffraction sin2ψ T method and by qualitative evaluation of the diffraction line broadening, respectively. Annealing causes, besides interdiffusion, changes of macrostress and decrease of microstructural imperfection. Concentration profiles corresponding to diffusion lengths of only a few nanometers were determined by simulation of the measured reflectivity patterns. The values obtained for the diffusion coefficients are compared with corresponding values obtained by extrapolation from published data for bulk materials at much higher temperatures.
Modeling of Microstructure Development in Nanoscale Layered Systems
Authors: Pompe W. , Ullrich A. , Bobeth M.
Published: April 2011

The composition evolution in nanoscale layered systems during annealing is considered within the framework of the Cahn-Hilliard theory. Starting from as-deposited multilayers with morphologically rough and chemically diffuse coherent interfaces, the sharpening of concentration profiles and the flattening of interphase boundaries have been simulated. Grain boundary grooving within multilayers was supposed to initiate break-throughs in individual layers and the subsequent morphological development has been analyzed. The simulations reveal a deterioration of the layered structure by the fusion of individual layers.
Microstructure Evolution During Solid-State Reactions in Polycrystalline Nb/Al and Ti/Ai Multilayer Thin-Films
Authors: Lucadamo G. , Barmak K. , Carpenter D. T. , Lavoie C. , Cabral C. , Michaelsen C. , Rickman J. M.
Published: April 2011

The microstructural changes that occur during the reaction of sputter-deposited Nb/Al and Ti/Al multilayer thin-films with bilayer thicknesses ranging from 10 nm to 333 nm have been studied. The films were deposited with an overall stoichiometry of XAl3 (X = Nb,Ti) and subsequently annealed to different stages of the reaction in a differential scanning calorimeter (DSC). Data obtained from cross-sectional transmission electron microscopy (XTEM), and in situ synchrotron X-ray diffraction (XRD) experiments have provided evidence for a two-stage reaction mechanism for the formation of NbAl3. Microscopy results from a film with a bilayer period of 333 nm showed a microstructure that was consistent with two-dimensional growth in the plane of the interface. A uniform, 10 nm thick continuous layer of the product phase was formed followed by growth normal to the interface that initially consisted of larger, faceted grains. By the end of the reaction, an equiaxed NbAl3 grain structure was observed. High resolution elemental mapping using a scanning transmission electron microscope (STEM) revealed penetration of Nb into the Al layer and enhanced growth in regions where Al grain boundaries intersected the interface. Characterization of microstructure evolution in the Ti/Al system was complicated by the formation of two metastable structures consisting of cubic Ll2 followed by tetragonal DO23, and finally the equilibrium, tetragonal DO22 structure. However, the metastable phase transition temperatures were clearly isolated using the in situ XRD technique.
Evaluation of Young's Modulus and Yield Strength of Thin Film Structural Material Using Nanoindentation Technique
Authors: Son Dongil , Lee Yun-Hee , Ahn Jeong-Hoon , Kwon Dongil
Published: April 2011

Aluminum films have wide applications in micromechanical devices such as micro sensors and actuators. Therefore, their mechanical properties are very important for reliability evaluation. However, there is no standardized method to evaluate the mechanical properties of the materials used in MEMS(microelectromechanical system) devices since the measured mechanical properties are influenced by many factors such as the surface condition of materials, intrinsic limit of the measurement device, etc. Hence, it was intended to evaluate the mechanical properties of thin film, which is important in its mechanical operation. Because MEMS devices are usually operated in the elastic range, Young's modulus and yield strength were evaluated by using a microcantilever beam technique. First, A1 cantilever beams were fabricated using the silicon bulk micromachining technology to have various film thicknesses. The load-displacement curves during beam bending by nanoindentation method were then obtained. The linear relationship of the curve in elastic range was utilized in deriving Young's modulus of the A1 film, which gave reproducible results regardless of film thickness. In the high load range, the deviation from the linear relation was detected, so that yield strength of A1 film could be evaluated. It was found that the yield strength increases with decreasing film thickness. By applying the misfit dislocation theory and the Hall-Petch relationship, the theoretical estimation could predict the trend of yield strength.
Influence of Cu Seed Deposition Temperature on Electroplated Cu Texture Formation in Damascene Structures
Authors: Jiang Qing-Tang , Mikkola Robert , Ortega Richard , Blaschke Volker
Published: April 2011

The deposition temperature of the PVD Cu seed layer has a critical impact on the subsequent electroplated Cu film. Sheet resistance transformation of electroplated Cu on 50°C seeded Cu was more than twice faster than on 150°C seeded Cu. X-ray pole figure analysis on a 3 mm × 3 mm dense array of 0.35 μm Cu damascene lines at spacing of 0.4 μm revealed significant grain orientation differences between directions parallel and perpendicular to the Cu lines. It was observed that for both seed process temperatures, the (111) pole figures showed a sharper texture parallel to the trench direction than to the perpendicular direction. After annealing at 450°C for 30 minutes, a (511) secondary orientation emerged and the (111) texture along the trench direction became even sharper. Perpendicular to the trench, the (111) texture split into two peaks after anneal, exhibiting near fiber texture. The deviation of the two splits from normal was 2.4° tilt towards the trench wall indicating strong interaction between trench sidewall and electrodeposited Cu inside the trench. The (111) pole figure analysis also revealed a 20% contribution of Cu growth directly from the side walls. Although the pole figure pattern of 150°C seeded sample resembles that of the 50°C seeded sample, the (111) fiber texture of a 50°C seeded sample was always stronger and sharper.
Surface Microstructure and Magnetic Properties of Laser-Processed Granular Co-Ag Films
Authors: Agostinelli E. , Fiorani D. , Foglia S. , Kaciulis S. , Testa A. M. , Vittori-Antisari M.
Published: April 2011

Co-Ag films prepared by Pulsed Laser Deposition and showing GMR in the as deposited state, have been irradiated by excimer laser: the chemical and structural modification induced by such treatment have been studied by XPS and by atomic force and electron microscopies. The pulsed laser irradiation favors the nucleation of nanosized Co grains at Ag grain boundaries, resulting in an enhancement of the magnetoresistivity at low fields and low temperature. Indeed, after laser treatment, a doubled field sensitivity (δGMR/δH) at T=I0K, with respect to the untreated samples, has been observed. The correlation with the observed magnetic properties variations, is discussed.
Effects of Magnetic and Non-Magnetic Impurity Addition on Magnetoresistance Behavior of Lanthanum Manganite Thin Films
Authors: Pietambaram Srinivas V. , Kumar D. , Singh Rajiv K. , Lee C. B.
Published: April 2011

In this paper, we report our studies on the growth, structural, magnetotransport and magnetic properties of La1−x,CaxMnO3: Co/Ag. Thin films of these materials were grown in situ on (100) LaAlO3 substrates using a pulsed laser deposition technique. Microstructural characterization carried out on these films show that the films are single phase, textured and smooth. The temperature dependence of resistance of the films in the 10 – 300 K range was examined in zero and applied field using four-probe technique and magnetization measurements were carried out using the quantum design superconducting quantum interference device (SQUID) magnetometer. The MR ratios of La0.7Ca0.3MMnO3, Ag added La0.7CaM0.3MnO3 and Co added La0.7CaM0.3MnO3 films were found to be 200%, 3.4 × 104% and 660% in 5 T and 21%, 435% and 30% in 0.1 T respectively. The enhanced MR ratio in the case of Ag added La0.7Ca0.3MMnO3 films is attributed to the increase in the oxygen content and in the case of Co added La0.7Ca0.3MMnO3, it can be attributed to the Co supplying some additional magnetic field.
Tungsten Chemical Mechanical Polishing Endpoint Detection
Authors: Sue Larry , Lültzen Jörn , Gonzales Simon
Published: April 2011

Conventional methods of controlling tungsten Chemical Mechanical Polishing (WCMP) processes have included blanket wafer polish rate and platen temperature monitoring. In order to enhance process control of WCMP, a second generation motor-current sense endpoint system was chosen as the evaluation tool. The endpoint detection system (Luxtron 9300) was installed on a WCMP polisher (IPEC 472) at the Motorola MOS6 facility. Endpoint algorithms were evaluated under various polishing conditions during time periods between pad changes. The data collected over 3 months led to a reliable endpoint recipe for both the contact and via WCMP process. The endpoint data compared with the conventional timed recipe, indicated that a time reduction at WCMP could be achieved, with a 10% increase in throughput. In addition, contact and via W-plug endpoint data showed a strong signal versus pattern density for given device types. In summary, the motor current endpoint system tested verified reliable endpoint, process control, and characterization capabilities.
Determination of the Planarization Distance for Copper CMP Process
Authors: Hymes S , Smekalin K. , Brown T. , Yeung H. , Joffe M. , Banet M. , Park T , Tugbawa T. , Boning D. , Nguyen J. , West T. , Sands W.
Published: April 2011

A planarization monitor has been applied to the copper system to investigate pattern dependencies during copper overburden planarization. Conventional profilometry and a noncontact, acousto-optic measurement tool, the Insite 300, are utilized to quantify the planarization performance in terms of the defined step-height-reduction-ratio (SHRR). Illustrative results as a function of slurry, pad type and process conditions are presented. For a typical stiff-pad copper CMP process, we determined the planarization distance to be approximately 2mm.
Modeling the Dependence of the Gate Current on Ge Content in Ultrathin Gate Dielectric Pmos Devices with Poly-Si1−Gex Gate Material
Authors: Shanware A. , Massoud H. Z. , Acker A. , Li V. Z. Q. , Mirabedini M. R. , Henson K. , Hauser J. R. , Wortman J. J.
Published: April 2011

The performance of CMOS devices improves due to the addition of Ge in their poly-Si gate material. The presence of Ge in the gate increases the current drive due to the reduction of the flatband voltage. The change in the flatband voltage is due to a shift in the valence-band energy level in the gate. This shift results in a change in the barrier height for electrons tunneling from the gate. Thus, the presence of Ge in the gate increases the tunneling current in the gate. This increase may result in a limitation in the use of SiGe gates in future generations of MOSFETs with ultrathin gate dielectrics. The purpose of this work is to investigate the effect of Ge content on the tunneling current in CMOS devices with ultrathin gate dielectrics.
Surface Morphology of Ultrathin Oxide Formed on Si(100)
Authors: Hattori T. , Fujimura M. , Nohira H.
Published: April 2011

The atomic-scale surface roughness of ultrathin thermal oxides formed on Si(100) were studied as a function of oxide film thickness up to the thickness of 2.0 nm. The height deviation on oxide surface is limited within single atomic-step height of 0.135 nm on Si(100) surface below the oxide film thickness of about 1 nm, but above this thickness the height deviation increases with the increase in thickness at 700°C. This increase in height deviation with thickness must be produced by the relaxation of oxidation–induced stress in bulk SiO2. Furthermore, the oscillation in surface roughness with constant amplitude and its oscillating period in oxide film thickness of 0.19 nm were found.
Electrical Characterization of a Double Barrier Direct Tunneling Diode Structure
Authors: Dons E. M. , Skowronski C. S. , Farmer K. R.
Published: April 2011

We report the electrical characterization of a direct tunneling diode structure that incorporates a multilayer dielectric. The dielectric consists of a stack of two thermally grown, ultrathin SiO2 layers, each ∼3.5 rin thick, separated by a deposited, continuous, undoped, ultrathin nanocrystalline Si layer ∼5.0 nm thick. Electrical measurements of this structure are reported for both n-type and p-type Si substrates. We find that the room temperature transport through this structure is accounted for by describing the intermediate Si layer as a quantum well with a continuum of states, and by otherwise assuming bulk properties for the ultrathin layers, such as the existence of a bandgap in the Si well and the usual Si-SiO2 interface potential barrier height at all interfaces. This structure is expected to be useful as the active dielectric in nonvolatile memory devices.
Relationship between Interfacial Roughness and Dielectric Reliability for Silicon Oxynitride Gate Dielectrics Processed with Nitric Oxide
Authors: Sapjeta J. , Green M. L. , Chang J. P. , Silverman P. J. , Sorsch T. W. , Weir B. E. , Gladden W. , Ma Y. , Sung C. Y. , Lennard W. N.
Published: April 2011

The greatest benefits of nitrogen incorporation into gate dielectrics may be obtained by placing nitrogen preferentially at the interfacial regions of the dielectric film. One method of distributing nitrogen in this manner is by using a three-step thermal process consisting of 1.) oxynitridation in NO, 2.) subsequent reoxidation in O2, and 3.) a final NO anneal. This study investigates the effect of NO processing on substrate/dielectric interface roughness and correlates that roughness with dielectric reliability. The initial NO-containing step can roughen the interface, as can subsequent reoxidation. Increased NO exposure yields a greater nitrogen content and a concomitant increase in interface roughness. These films show a degradation in charge to breakdown (Qbd) of at least an order of magnitude when compared with similarly prepared O2-oxide films. An O2/NO process produces films with interface roughness and Qbd comparable to that of pure SiO2, independent of nitrogen content. The oxynitride reliability depends on the exact scheme for incorporating nitrogen into SiO2.
Triggering and Suppression of Soft Breakdown During Mercury-Probe Assessment of Thin Gate Oxide Quality
Authors: Evseev S. , Cacciato A.
Published: April 2011

The breakdown of ultra-thin gate oxide layers is investigated using fast-feedback Hg-probe measurements to perform Exponentially Ramped Current Stress (ERCS) tests. Several parameters have been varied in the ERCS test: oxide thickness (4nm, 5nm, 6nm and 7nm), capacitor area (0.12cm2 and 0.023cm2) and initial injected current (5×10−5 A and 5×10−4 A). Soft breakdown is detected only in case of oxides thinner than 5 nm. It is found that the fraction of points on the wafer on which soft breakdown occurs reduces by increasing the value of the injected current at the beginning of the ERCS test or completely disappears by decreasing the capacitor area. Consequences of current results for correct routine assessment of gate oxide integrity in microelectronic manufacturing are discussed.
Breakdown Characteristics of Ultra-Thin Gate Oxides Caused by Plasma Charging
Authors: Chen Chi-Chun , Lin Horng-Chih , Chang Chun-Yen , Chien Chao-Hsin , Huang Tiao-Yuan
Published: April 2011

Breakdown characteristics of ultra-thin gate oxides caused by plasma charging were studied in this work. It is observed that as oxide thickness is scaled down to 4 nm, some traditional monitor parameters may lose their sensitivity for detecting oxide degradation induced by plasma charging damage, due to insignificant trap generation. Even the gate leakage current, although sensitive for 4 nm oxide, may no longer be sensitive enough for even thinner oxide (e.g., 2.6 nm), due to the existence of large tunneling current. Moreover, several soft-breakdown events were found to occur in ultrathin oxide before the final onset of a catastrophic hard-breakdown. Finally, an equivalent local oxide thickness is calculated using local oxide thinning model to estimate the stepwise increase of gate current after soft-breakdown event.
Deposition and Treatment of TiO2 as an Alternative for Ultrathin Gate Dielectrics
Authors: Ma Yanjun , Ono Yoshi , Hsu Sheng Teng
Published: April 2011

We investigated the use of TiO2 as an alternate gate dielectric for future CMOS applications. To reduce the leakage current, different post deposition treatments were investigated. It was found that for very thin TiO2 films, ozone plasma exposure is an effective way in lowering thermal budget of the post deposition annealing. Doping TiO2with Si and/or Al can also be effective in substantially reducing the leakage current. In addition, the doped TiO2 has the desirable property of remaining amorphous even after anneal at 850°C. We also report the fabrication of submicron MOSFETs with TiO2 gate dielectrics equivalent to 2 nm of SiO2 and TiN/Cu gate electrodes.
Atomic Layer Deposition of Ta2O5 Films Using Ta(OC2H5)5 and Nh3
Authors: Song Hyun-Jung , Koh Wonyong , Kang Sang-Won
Published: April 2011

Tantalum oxide films were grown by chemical vapor deposition using an alternating supply of tantalum pentaethoxide and ammonia. The supply of one source was followed by a purge with argon gas before introducing the other source onto the substrate in order to prevent gas-phase reactions. At substrate temperature between 250-275 °C the film growth depended only on the number of source supply cycles (0.15 nm/cycle) and did not depend on the substrate temperature nor supply time of the sources. As-deposited films were amorphous, however, were crystallized after annealing at 800 °C in oxygen atmosphere by rapid thermal process. Annealed films showed increased dielectric constant and decreased leakage current density, which were 13.3 and 6.6 μA/cm2 at 1 MV/cm, respectively, for a 15-nm-thick film after annealing at 800 °C for 10 minutes.
Comparison of Valence-Band Tunneling in Pure SiO2, Composite SiO2 /Ta2O5, and Pure Ta2O5, in Mosfets with 1.0 nm-Thick SiO2-Equivalent Gate Dielectrics
Authors: Shanware A. , Massoud H. Z. , Vogel E. , Henson K. , Hauser J. R. , Wortman J. J.
Published: April 2011

The gate tunneling current in ultrathin gate dielectric NMOSFETs with positive gate bias is due to the tunneling of electrons from the conduction and valence bands of the substrate. Valence-band electrons tunnel from the substrate of NMOS devices when the valence-band edge in the substrate rises above the conduction-band edge in the substrate. This paper reports experimental trends in the contribution of valence-band electrons tunneling to the gate current of NMOSFETs with gate oxides composed of pure SiO2. The large gate tunneling current can be reduced by replacing the conventional SiO2 gate dielectric with alternative dielectrics with larger dielectric constants. This paper reports the effect of replacing SiO2 with alternative dielectrics on the contribution of valence-band electron tunneling to the gate current. Simulations are carried out for composite SiO2/Ta2O5 gate dielectric structures.
Large-Area Pulsed Laser Deposition of Tantalum Oxide Films
Authors: Boughaba S. , Islam M. U. , Sproule G. I. , Graham M. J.
Published: April 2011

A large-area pulsed laser deposition (PLD) technique was used to uniformly grow tantalum pentoxide (Ta2O5) films on silicon wafers of 75 mm diameter. A KrF excimer laser beam was focused onto a 90 mm diameter Ta2O5 target to induce its ablation in oxygen gas ambient. The large-area coverage was obtained by rastering the laser beam over the radius of the rotating target, while the substrate was rotated simultaneously.

The tantalum oxide films were characterized in terms of uniformity of thickness, composition, structure, and optical properties across the substrate. Average deviations typically below 1% were achieved in thickness, composition, indices of refraction and optical energy band-gap. Identical X-ray diffraction spectra were obtained at the center, middle of radius and edge of the wafers.
Sensitivity of Electrically Active Defect Spectra to Processing Conditions in Mev Heavy Ion Implanted Silicon
Authors: Mohapatra Y.N. , Giri P.K.
Published: April 2011

We have carried out electrical characterization of defects in heavily damaged silicon, where damage is created by MeV heavy ions at doses near but below amorphization threshold and samples are subjected to different annealing conditions. Defect characterization is carried out using combination of deep level transient spectroscopy (DLTS) and isothermal time analyzed spectroscopy (TATS). The defect spectrum is observed to be sensitive to I) furnace annealing between 400–600°C, ii) low temperature oven annealing at 160°C, and iii) forward injection current. The observed changes in spectra are indicators of extreme sensitivity of dominant defects to relaxation of the disordered medium, and recombination enhanced reactions. Surprisingly the defect spectra is usually dominated by a single peak with unbroadened lineshape characteristic of discrete energy level in the bandgap, though electrical signatures keep varying for different processing conditions. In the light of these measurements, we discuss the nature of stability and metastability of the defects believed to be due to intrinsic defect clusters.