bluuebunny/crossref_metadata_2025 huggingface dataset:
On the Mechanical Properties of a Poly(oxazolidone) Polymer for Encapsulant Applications
Authors: Manoharan M. , Chian K. S.
Published: April 2011

Epoxy based resins have been widely used as encapsulation materials in microelectronics components due to their good mechanical, thermal and electrical properties, outstanding adhesion to various substrates and the ability to be processed under a variety of conditions. These epoxy resins have a high glass transition temperature but low toughness and damage tolerance. As electronic devices continue to be miniaturized and the scale of integration increases, the encapsulation of these chips leads to mechanical and thermal stress related failures. Mechanical stresses could result from the mismatch in thermal expansion rates between the polymeric encapsulant, the silicon chip and the metal leadframe. The accommodation of these stresses may lead to cracking of the encapsulant or delamination. In this study, poly(oxazolidone) polymer was prepared by reacting 4,4′-Diphenyl-methanediisocyanate (MDI) with diglycidyl ether of bisphenol A (DGEBA) at elevated temperature. The mechanical properties of this polymer were investigated both as a stand-alone material as well as a bi-material sandwich with a brittle substrate. The degree of cracking in the substrate is shown to be a function of the thickness of the polymer layer. This result is analyzed based on the variation in the position of the neutral axis in this composite.
Thermal Control of Interfaces for Microelectronic Packaging
Authors: Marotta E. E. , Hana B.
Published: April 2011

The continuous miniaturization of electronic devices places an ever-increasing importance on the thermal management of electronic systems and its subcomponents. The increased power densities and heat generation, due to the miniaturization of the device line features, may lead to higher operating temperatures and greater warpage between the silicon device and its organic carrier. The higher operating temperature may result from the degradation of the overall thermal performance. These additive effects will also lead to an increasing number of thermally induced failures, which will be further magnified when future microelectronic packaging incorporates flip-chip technology.

The higher operating temperatures within microelectronic systems result from inadequate dissipation of the heat generated, while the warpage effect is caused by the mismatch between the thermal coefficients of expansion (ICE) induced by thermal stresses. Often these high temperatures result from the thermal resistance between subcomponents, such as between the contacting surfaces of laminated printed circuit boards, device/epoxy cement and heat spreader (i.e., finned heat sink or heat pipe), and any other metallic or non-metallic interstitial material employed between contacting interfaces.

Published experimental data of potential coatings, adhesives, and elastomeric gaskets is presented that can improve the thermal contact conductance of contacting surfaces within microelectronic systems. In addition, recommendations for future analytical and experimental studies of the mnechanistic principles, which control thermal performance of interstitial materials, are discussed for non-uniform pressure distribution.
The Effects of Interfaces on C-4 Solder Bump Reliability
Authors: Sullivan Michael J.
Published: April 2011

C-4 solder bump technology is used as an interconnect method between microelectronic chips and substrates because of four unique features; excellent electrical performance, large number of connections in area arrays, small size and superb reliability in properly fabricated terminals. C-4 terminals consist of chip metallization (BLM or UBM), solder, substrate metallization and underfill, if needed. The reliability of C-4 interconnections will be described in terms of the physical interfaces in the terminal which include insulator-metal, metal-intermetallic, grain boundary and, if needed, underfill-chip/substrate/solder interfaces. Some of these interfaces are present in the as-deposited materials while others form during the solder reflow operation. Reactive UBM's such as Ti/Cu/Au form layers of intermetallic compounds such as TiCu, TiCu2 and TiCu3 and react with hydrogen ambients to form TiH2. The solder reacts with the UBM copper to form Cu3Sn and Cu6Sn5. The solder also reacts with the substrate metallization (electroless Ni-Au as an example) to form Ni3Sn2 and Ni3Sn4. Reactions with Au also occur with the formation of TiAu4 and AuSn. These reactions can cause problems not because the intermetallics are brittle (intermetallics are relatively brittle compared to the solder but they are also very strong as compared to the solder), but because of the consequences of the reactions such as, stress increase, impurity snowplow, Kirkendall voids and intermetallic spalling. Non-reactive UBM's like Cr/Cu/Au require a “phased” region between the Cr and Cu to achieve mechanical adhesion because of the absence of reaction. Proper control of these reactions will eliminate early fails that are usually planar and occur at reaction interfaces. Long term wear-out will occur during power on-off cycling when creep/fatigue cracks nucleate and grow at the grain boundaries in the solder. If underfill is used, impurity assisted crack growth at underfill to chip/substrate/solder interfaces results in increased strain on the solder and failure. Reliability models for these various failure mechanisms will be discussed as well as methods of eliminating early interfacial failures.
The Effects of Substrate Preheating and Post-Deposition Annealing on CrMn/CoCrPt/CrMn/NiAI Films
Authors: Zou Jie , Laughlin David E. , Lambeth David N.
Published: April 2011

In this study, we examine the effects of introducing CrMn as an intermediate layer between the NiAI underlayer and the CoCrPt magnetic layer, and as an overlayer on top of the Co-alloy film. The effects of deposition temperature and post-deposition annealing on the texture and magnetic properties were studied. Post-deposition annealing effectively lowered the exchange coupling between grains, increased the coercivity by 38%, up to 4600 Oe, and moderately decreased the Ms by 15%. Interlayer diffusion of CrMn into the CoCrPt grain boundaries is believed to cause the change in magnetic properties. It was also found that granular exchange coupling seems to be correlated to the Co-alloy in-plane texture.
A Lens-Free and Mirror-Free Fiber Integrated Optical Magnetic Field Sensor Using Bi-Substituted Garnet
Authors: Minemoto H. , Itoh N. , Ishiko D. , Ishizuka S.
Published: April 2011

We have developed a novel lens-free and mirror-free fiber integrated optical magnetic field sensor. The use of a bent optical fiber and chip optical components allows easy assembly with no alignment. The linearity error and temperature dependence of the sensitivity of this sensor were less than ±1% and +2%, respectively.
Solution Flow System for Hydrothermal-Electrochemical Synthesis: New Opportunities for Multilayered Oxide Films
Authors: Suchanek W. , Watanabe T. , Sakurai B. , Yoshimura M.
Published: April 2011

A solution flow system for hydrothermal-electrochemical synthesis has been constructed in our laboratory. This equipment can operate at 20°-200°C, under the pressure of 1-50 atm., at flow rate of 1-50 cm3/min. Applicability of the flow system for low-temperature, hydrothermalelectrochemical synthesis of single-layer and multilayered thin films has been demonstrated using the BaTiO3-SrTiO3 system as an example. Single phase thin films as well as double layers have been deposited at 150°C, current density of 1 mA/cm2, and flow rates of 1-50 cm3/min. The flow rate is an important parameter allowing additional control of the films' morphology by affecting the growth rate. The multilayers can be prepared in only one experiment by simply changing the flowing solution. Processing using the solution flow cell may serve as an inexpensive and environmentally friendly way of fabricating any multilayered thin films, including magneto-optic films.
Elastic Behavior of Nanoparticle Chain Aggregates: Proposed Mechanisms
Authors: Friedlander Sheldon K. , Jang Hee Dong , Ryu Kevin H.
Published: April 2011

Nanoparticle chain aggregates (NCA) of 7 nm titania particles stretch under tension, and contract when the tension is relaxed. The NCA were stretched across expanding holes produced in the carbon film on an electron micrograph grid. After stretching up to 90%, the NCA broke loose at one end and contracted to a tightly folded chain. This pattern was observed in repeated tests. Mechanisms for this behavior and reasons for its generality are proposed. Implications are discussed for the ductility of nanoparticle ceramics, and the improved properties of rubber due to nanoparticle additives.
Particularities of Powerful Ultrasound Action on Nanostructuired Powders
Authors: Khasanov O.L. , Pokholkov Yu.P. , Sokolov V.M. , Dvilis E.S. , Bikbaeva Z.G. , Polisadova V.V.
Published: April 2011

The conditions of in ultrasound (US) transmission through a nanodispersed powder (NP) and possible effects of US-action at compacting of NP ZrO2 − Y2O3 are analyzed. These effLcts include agglomerate crushing, NP particle activation under the conditions of acoustic flows; dense NP compacting with the preservation of a compact nanostructure in the absence of acoustic flows. It was shown that the result of 13S action was dcfined by the compacting pressure P and ItS intensity level I, the values of which were divided into the ranges by the critical parameters PC and IC NP transfers from the state of gas-dispersed medium to the state of a solid porous body at P = PC, and IC characterizes the threshold of acoustic flows appearance in the powder body.
Development and Scale-up of the Low Density Core Process for Ti-64
Authors: Schwartz D. S. , Shih D. S. , Lederich R. J. , Martin R. L. , Deuser D. A.
Published: April 2011

The Low Density Core (LDC) process is a method for making metallic structures with solid surfaces covering a porous core. The development of the LDC process for making Ti-64 sheet based structures is described. Sheets ∼ 2000 × 1200 × 4 mm3 have been made with ∼ 40 vol. % core porosity levels, and several forming techniques have been demonstrated. The LDC billet breakdown and forming behavior was found to be similar to conventional Ti-64 processes, so LDC Ti-64 sheet has the advantage that it can be fabricated into structures using well-established methods.
The Effect of GASAR Processing Parameters on Porosity and Properties in Aluminum Alloy
Authors: Paradies C. J. , Tobin A. , Wolla Jeff
Published: April 2011

The GASAR process involves the dissolution of hydrogen in a molten metal or alloy by controlling the hydrogen pressure and the temperature in a high pressure chamber. The difference between the hydrogen solubility in the melt and the solid is used to control the growth of hydrogen pores within the solidifying structure. Designed experiments have been performed to determine the relative effects of the saturation pressure, temperature, casting pressure, mold temperature and cooling rate on the total porosity, pore size, distribution and shape in pure nickel and aluminum alloys. The shape and distribution of pores was found to depend upon the structure of the primary solid during pore nucleation and growth. Reducing the casting pressure had the strongest effect on increasing the total porosity and pore size.
Evaluation and Comparison of 3.0 nm Gate-Stack Dielectrics for Tenth-Micron Technology NMOSFETs
Authors: Yee K. F. , Osburn C. M. , Masnari N. A. , Hauser J. R. , Parker C. G. , Lucovsky G. , Henson W. K. , Wortman. J. J. , Kippenberg T. , Kuerschner S.
Published: April 2011

As device dimensions continue to scale down into the deep submicrometer regime, there is an increasing challenge for fabricating robust gate dielectrics with low susceptibility to process-induced device degradation and a continuous motivation for the exploration of new options for thin gate dielectrics. This work assesses a variety of gate stack processing techniques as alternatives to conventionally furnace grown gate oxides in the context of a tenth-micron technology, which features LOCOS isolated, two-implant channel, NMOS transistors fabricated with a 3.0 nm thick gate dielectric, 0.15 μm thick polysilicon gate, implanted extension- and contact-junctions of 20 and 50 nm deep, respectively, and effective channel lengths down to 0.12 μm, operating at 1.2 volts. The alternative deposition and oxidation techniques include furnace oxynitride formation, rapid-thermal oxidation (RTO), rapid-thermal chemical vapor deposition (RTCVD) and plasma-assisted chemical vapor deposition (RPECVD). Compared to the 0.25- and 0.18-μm technological nodes, the thermal budgets associated with gate oxide formation are dramatically lower and their impact on channel dopant redistribution is not as strong as in previous technologies. Negligible polysilicon depletion effects were observed in the fabricated devices (Cinv/Cox = 97%). Drive currents and threshold voltage control comparable to furnace oxides were achieved by alternative gate-stack processing techniques.
Nitrogen Profile Engineering in Thin Gate Oxides
Authors: Kuehne J. , Hattangady S. , Piccirillo J. , Xing G. C. , Miner G. , Lopes D. , Tauber R.
Published: April 2011

In order to prevent boron penetration in PMOS transistors without degrading channel mobility, it is necessary to engineer the distribution of nitrogen introduced into the gate oxide. We have investigated methods of engineering this distribution using nitric oxide (NO) gas in an RTP system to thermally nitride ultra-thin gate oxides. In one approach, the gate oxide is simultaneously grown and nitrided in a mixture of nitric oxide and oxygen. For a 40 Å film, SIMS depth profiling shows that this process moves the nitrogen peak into the bulk of the oxide away from the oxide silicon interface. In another approach, an 11 Å chemical oxide produced by a standard pre-furnace wet clean is nitrided in NO at 800 deg. C. This film is subsequently reoxidized in either oxygen or steam. For an 1100 deg. C., 120 sec RTP reoxidation in oxygen, the final film thickness is 41 Å. The nitrogen has a peak concentration of 5 at. % and the peak is located in the oxide 25 Åfrom the oxide/silicon interface. Ramped voltage breakdown testing was carried out on MOS capacitors built using reoxidized NO nitrided films. They have breakdown characteristics that are equivalent to conventional furnace grown oxides. These films show considerable promise as gate dielectrics for CMOS technologies at geometries of 0.25um and below.
New Approaches for Formation of Ultra-Thin PtSi Layers for Infrared Applications
Authors: Donaton R. A. , Jin S. , Bender H. , Zagrebnov M. , Baert K. , Maex K. , Vantomme A. , Langouche G.
Published: April 2011

In this work we describe the formation of ultra-thin PtSi layers using sputtering for metal deposition and RTP for the silicidation. The problem associated with the controllability of deposition of ultra-thin metal layers can be circumvented by depositing a thick Pt layer followed by a 2-step RTP process with a selective etch step in between. Continuous and uniform 3 nm thick PtSi layers are formed with this technique. Moreover, in another approach similar to the previous one, but in which the first RTP step is omitted, a much smoother PtSi layer is formed. The importance of the interfacial Pt/Si layer formed during metal deposition is described. These processes are totally compatible with CMOS technologies, as shown below.
Transitions in Critical Island Size in Metal (100) and (111) Homoepitaxy: A Self-Consistent Rate Equation Approach
Authors: Popescu Mihail N. , Amar Jacques G. , Family Fereydoon
Published: April 2011

A self-consistent rate equation (RE) approach to submonolayer growth for a restricted pair-bond model that is relevant to low and intermediate temperature metal (100) and (111) homoepitaxy is introduced. In contrast to previous standard rate equation results, the transition temperature from i = 1 to a higher critical island size is well predicted along with the average island and monomer densities. It is shown that the method's implicit introduction of short-range correlations between attachment/detachment rates, together with a careful estimate of the escape rates for small clusters, are important factors for a good agreement with the kinetic Monte Carlo simulation results.
Stress Evolution During Growth of Sputtered Ni/Cu Multilayers
Authors: Ramaswamy Vidya , Clemens Bruce M. , Nix William D.
Published: April 2011

Results from in-situ measurements of stress during growth of (111)-textured Ni/Cu multilayers with small and large bilayer periods are presented. In multilayers with small bilayer periods, Ni is in uniform tension and Cu in uniform compression. This behavior is modeled as the growth of a coherent multilayer with alloying in the layers. In multilayers with large bilayer periods, stress relaxation is observed but the measured stresses are much lower than expected based on a Mathews-Blakeslee relaxation process. An alternative stress relaxation mechanism based on high defect densities is presented and discussed.
Accelerating the Dynamics of Infrequent Events: Combining Hyperdynamics and Parallel Replica Dynamics to Treat Epitaxial Layer Growth
Authors: Voter A. F. , Germann T. C.
Published: April 2011

During the growth of a surface, morphology-controlling diffusion events occur over time scales that far exceed those accessible to molecular dynamics (MD) simulation. Kinetic Monte Carlo offers a way to reach much longer times, but suffers from the fact that the dynamics are correct only if all possible diffusion events are specified in advance. This is difficult due to the concerted nature of many of the recently discovered surface diffusion mechanisms and the complex configurations that arise during real growth. Here we describe two new approaches for this type of problem. The first, hyperdynamics, is an accelerated MD method, in which the trajectory is run on a modified potential energy surface and time is accumulated as a statistical property. Relative to regular MD, hyperdynamics can give computational gains of more than 102. The second method offers a way to parallelize the dynamics efficiently for systems too small for conventional parallel MD algorithms. Both methods exploit the infrequent-event nature of the diffusion process. After an introductory description of these methods, we present preliminary results from simulations combining the two approaches to reach near-millisecond time scales on systems relevant to epitaxial metal growth.
Island Size and Environment Dependence of Adatom Capture: Cu/Co Islands on Ru(0001)
Authors: Bartelt M.C. , Evans J.W. , Schmid A.K. , Hwang R.Q.
Published: April 2011

The rate of capture by stable Co islands on Ru(0001) of additionally deposited Cu atoms is quantified using scanning tunneling microscopy, kinetic Monte Carlo simulations, and diffusion equation analyses. We find strong dependence of the capture rates on Co-island size, larger islands showing larger capture rates, qualitatively distinct from self-consistent mean-field predictions. The observed size dependence is shown to reflect larger island-free areas surrounding bigger islands, i.e., a strong correlation between island sizes and separations neglected in mean-field treatments.
Simulation of the Texture Evolution of Aluminium Alloys During Primary Static Recrystallization Using a Cellular Automaton Approach
Authors: Marx V. , Gottstein G.
Published: April 2011

A 3D model has been developed to simulate both primary static recrystallization and recovery of cold worked aluminium alloys. The model is based on a modified cellular automaton approach and incorporates the influence of crystallographic texture and microstructure in respect to both mechanisms mentioned above. The model takes into account oriented nucleation using an approach developed by Nes for aluminium alloys. The subsequent growth of the nuclei depends on the local stored energy of the deformed matrix (i.e. the driving pressure) and the misorientation between a growing nucleus and its surrounding matrix (i.e. the grain boundary mobility). This approach allows to model preferred growth of grains that exhibit maximum growth rate orientation relationship, e.g. for aluminium alloys a 40° <111> relationship with the surrounding matrix. The model simulates kinetics, microstructure and texture development during heat treatment, discrete in time and space.
A Finite Element Method for Simulating Interface Motion
Authors: Yu H.H. , Suo Z.
Published: April 2011

This paper describes our recent progress in developing a finite element method for simulating interface motion. Attention is focused on two mass transport mechanisms: interface migration and surface diffusion. A classical theory states that, for interface migration, the local normal velocity of an interface is proportional to the free energy reduction associated with a unit volume of atoms detach from one side of the interface and attach to the other side. We express this theory into a weak statement, in which the normal velocity and any arbitrary virtual motion of the interface relate to the free energy change associated with the virtual motion. An example with two degrees of freedom shows how the weak statement works. For a general case, we divide the interface into many elements, and use the positions of the nodes as the generalized coordinates. The variations of the free energy associated with the variations of the nodal positions define the generalized forces. The weak statement connects the velocity components at all the nodes to the generalized forces. A symmetric, positive-definite matrix appears, which we call the viscosity matrix. A set of nonlinear ordinary differential equations evolve the nodal positions. We then treat combined surface diffusion and evaporation-condensation in a similar method with generalized coordinates including both nodal positions and mass fluxes. Three numerical examples are included. The first example shows the capability of the method in dealing with anisotropic surface energy. The second example is pore-grain boundary separation in the final stage of ceramic sintering. The third example relates to the process of mass reflow in VLSI fabrication.
Microstructural Evolution and Metastability in Active Materials
Authors: Kinderlehrer David
Published: April 2011

Metastable systems pose significant problems in both analysis and simulation. We discuss here the evolution of microstructure in a shape-memory alloy where energetic contributions from disparate scales play determining roles. This is a challenge for modelling since the finest length scales cannot be ‘seen’ at macroscopic level. We then provide a mechanism for kinetics that gives a different notion metastability.
Unconditionally Gradient Stable Time Marching the Cahn-Hilliard Equation
Authors: Eyre David J.
Published: April 2011

Numerical methods for time stepping the Cahn-Hilliard equation are given and discussed. The methods are unconditionally gradient stable, and are uniquely solvable for all time steps. The schemes require the solution of ill-conditioned linear equations, and numerical methods to accurately solve these equations are also discussed.
Reliabilty of Graded-Index Polymer Optical Fibers
Authors: Blyler L. L. , Salamon T. , Ronaghan C. , Koeppen C. S.
Published: April 2011

Newly developed low loss, graded-index polymer optical fibers (GI-POF) are being investigated as high bandwidth media for data transmission in local area networks and for interconnection of large telecommunications and computer systems. These fibers are fabricated by distributing refractive index-raising dopants to form a core profile in a transparent glassy polymer fiber. The long term reliability of the GI-POF depends upon the degree to which the dopant remains immobilized in the polymer glass in service environments. This paper presents a method for assessing the diffusion kinetics of dopants in polymer optical fibers and for relating these measurements to GI-POF profile stability through numerical simulation. Simulated and measured index profile changes are compared to validate the approach.
Chemical Kinetics Models for the Fatigue Behavior of Fused Silica Optical Fiber
Authors: Matthewson M. J.
Published: April 2011

There have been numerous studies of the fatigue and strength behavior of fused silica optical fibers. However, no coherent model has emerged that self-consistently describes the simultaneous effects of stress, temperature and activity of the corroding species (e.g. water). A power law degradation kinetics model (relating the crack growth rate to the applied stress intensity factor, KI) is widely used although various exponential forms based on chemical rate theory have also been proposed. The dependence of fatigue on parameters such as humidity, pH and temperature, has usually been treated in an empirical manner. Sometimes it is even ignored -for example, the service environment is often assumed to be the same as the proof test environment when making lifetime predictions, thus avoiding the need for understanding the humidity dependence; this assumption is often unjustified. This paper reviews the dependence of fatigue on environmental factors and highlights some of the inconsistencies in published data. It is then attempted to present a coherent kinetics model that simultaneously accounts for stress temperature, humidity, etc. Several possible forms of the model are compared to a range of experimental data of several different types. The comparison is made using fitting techniques that account for correlation between fit parameters. It is found that a simple exponential form of the degradation kinetics model gives the best overall description of the temperature, humidity and pH effects on static and dynamic fatigue. It should be noted that the exponential form predicts shorter lifetimes than the ubiquitous power law model. Therefore, under some circumstances, the predictions of “worst case” models based on power law kinetics are unduly optimistic.
Effect of Refractory Particles on the Strength of Optical Fibers
Authors: Wissuchek D. J.
Published: April 2011

Silica-clad optical fibers are proof tested in tension at levels of 700 MPa or greater to ensure product quality. The flaws passing proof testing generally are assumed to be ideal; however, some of these flaws are associated with refractory particles that are fully or partially submerged on the silica surface. Residual stresses are generated in the silica due to a thermal expansion mismatch between the particles and the host silica. These stresses can affect the crack growth characteristics and lifetime predictions for silica-clad optical fibers.

Residual stresses around refractory particles are quantified using analytical and finite element models. Zirconia particles generate the highest stresses at low aspect ratios. As aspect ratio increases, materials with higher elastic moduli, such as SiC, generate the highest stress levels. The residual stress fields are incorporated into crack stress intensity relations and their effect on optical fiber strength is evaluated for different refractory materials, particle sizes, and particle aspect ratios. The results are compared to the residual stress fields and stress intensity relations for model flaws created by indentation with diamond pyramids.
Strength and Reliability of Metal-Coated Optical Fibers at High Temperatures
Authors: Biriukov A. S. , Bogatyrjov V. A. , Lebedev V. F. , Sysolyatin A. A. , Khitun A. G.
Published: April 2011

Hermetically metal coated fibers are candidates for applications at high temperatures. To date, various metals with melting point up to 1400 °C have been used to coat fibers by the freezing technique. However, the major problems consist of ensuring mechanical reliability and acceptable optical losses in metal coated fibers at high temperatures. We present an experimental study of the temperature effects at 20 – 1050 °C on the performance of optical fibers with copper coating. Specific fiber degradation mechanism have been revealed
Highly Reliable Operation of InGaAlAs MESA-Waveguide Photodiodes in Humid Ambient
Authors: Nakamura Hitoshi
Published: April 2011

Reliability of lnGaAlAs mesa-waveguide photodiodes suitable for low-cost surface integration modules applied to access network systems is investigated. A passivation technique including silicone-based polymer encapsulation was used to ensure high reliability. Under a non-humidity condition of 200°C/10 V, a 9,600-hour stable operation was observed, and the estimated median lifetime is more than 100 years at the field service temperature of 85°C. A measured slight change in the dark current is explained in terms of hole accumulation at the semiconductor-insulator interface. Under a humid condition of 85°C/85% RH/10 V, there was no change in the dark current over 10,000 hours. The observed median lifetime at 135°C/85% RH/10 V of 1,500 hours would lead to an estimated median lifetime at 45°C/50% RH of about 50 years. These results suggest that the WG-PD is applicable to low-cost non-hermetic optical modules for optical access networks.
Utlra-Thermostable Long-Period Gratings Written in Nitrogen-Doped Silica Fibers
Authors: Karpov V. I. , Grekov M. V. , Dianov E. M. , Golant K. M. , Khrapko R. R.
Published: April 2011

Long-period gratings were written in nitrogen-doped-silica-core optical fibers by point heating. The periodic variations of the effective refractive index were induced via diffusion of nitrogen from the fiber core. It is shown experimentally that such gratings remain functional at temperatures of up to 1200 °C.
Influence of Active Nitrogen Species on the Nitridation Rate of Sapphire
Authors: Ptak A.J. , Ziemer K.S. , Millecchia M.R. , Stinespring C.D. , Myers T.H.
Published: April 2011

The operating regimes of two rf-plasma sources, an Oxford CARS-25 and an EPI Unibulb, have been extensively characterized. By changing the exit aperture configuration and using an electrostatic deflector, the Oxford source could produce either primarily atomic nitrogen, atomic nitrogen mixed with low energy ions, or a large flux of higher energy ions (>65eV) as the active species in a background of neutral molecular nitrogen. The EPI source produced a significant flux of metastable molecular nitrogen as the active species with a smaller atomic nitrogen component. Nitridation of sapphire using each source under the various operating conditions indicate that the reactivity was different for each type of active nitrogen. Boron contamination originating from the pyrolytic boron nitride plasma cell liner was observed.
Defects in GaN Pyramids Grown on Si(111) Substrates by Selective Lateral Overgrowth
Authors: Mao Zhigang , McKernan Stuart , Carter C. Barry , Yang Wei , McPherson Scott A.
Published: April 2011

Selective lateral growth of GaN is a promising technique for producing high quality material for microelectronic and optoelectronic devices. Single-crystal GaN/AIN layers have been grown on Si(111) substrates and subsequently used as the seeding layer for selective lateral overgrowth. GaN pyramids are formed above holes patterned in a Si3N4 mask. Transmission electron microscopy (TEM, which also denotes the microscope) of these structures shows that the GaN pyramid, GaN seed layer, and AIN buffer layer in the samples have the following epitactic relationship with respect to the Si substrate: [1120]GaN || [1120]AIN || [110]Si and (0001)GaN || (0001)AIN || (111)Si. In the core of the pyramid (at or above the seed windows), dislocations thread through the pyramid perpendicular to the interface plane with very high density. Some of these threading dislocations, which originate from the GaN/AIN seed layer, form 90° bends and half loops at the edge of the pyramid core. In the lateral growth part of the GaN pyramid, the dislocation density is relatively low. The majority of dislocations thread through the pyramid parallel to the interface plane. Planar defects, usually parallel to the interface plane, were observed near the interface. The defect density decreases with the distance away from the interface, so that the top several microns of material maybe completely defect free. The mechanism of the growth of GaN pyramids is discussed and related to this defect structure.
Optical Investigations of AlGaN on GaN Epitaxial Films
Authors: Steude G. , Christmann T. , Meyer B.K. , Goeldner A. , Hoffmann A. , Bertram F. , Christen J. , Amano H. , Akasaki I.
Published: April 2011

We have investigated AlxGal-xN/GaN heterostructures (0<x<0.22) grown by metal organic vapor phase epitaxy on sapphire with photoluminescence (PL), reflexion and cathodo-luminescence experiments. The energetic positions of the free A-exciton in GaN and AIGaN as a function of the alloy compositions are deduced from temperature dependent PL and from reflexion measurements. We obtain a small bowing parameter and no evidence for a Stokes shift between absorption and emission. The AlxGal-xN films induce additional compressive strain on the underlying GaN film. Compositional inhomogeneities are present, but the fluctuations are too small to be important for carrier localisation. The broadening of the luminescence line width in the alloy can be described by statistical disorder of a random alloy.
Pendeo-Epitaxy - A New Approach for Lateral Growth of Gallium Nitride Structures
Authors: Zheleva Tsvetanka S. , Smith Scott A. , Thomson Darren B. , Gehrke Thomas , Linthicum Kevin J. , Rajagopal Pradeep , Carlson Eric , Ashmawi Waeil M. , Davis Robert F.
Published: April 2011

A new process route for lateral growth of nearly defect free GaN structures via Pendeoepitaxy is discussed. Lateral growth of GaN films suspended from {1120} side walls of [0001] oriented GaN columns into and over adjacent etched wells has been achieved via MOVPE technique without the use of, or contact with, a supporting mask or substrate. Pendeo-epitaxy is proposed as the descriptive term for this growth technique. Selective growth was achieved using process parameters that promote lateral growth of the { 1120) planes of GaN and disallow nucleation of this phase on the exposed SiC substrate. Thus, the selectivity is provided by tailoring the shape of the underlying GaN layer itself consisting of a sequence of alternating trenches and columns, instead of selective growth through openings in SiO2 or SiNx mask, as in the conventional lateral epitaxial overgrowth (LEO).

Two modes of initiation of the pendeo-epitaxial GaN growth via MOVPE were observed: Mode A - promoting the lateral growth of the {1120} side facets into the wells faster than the vertical growth of the (0001) top facets; and Mode B - enabling the top (0001) faces to grow initially faster followed by the pendeo-epitaxial growth over the wells from the newly formed {1120} side facets. Four-to-five order decrease in the dislocation density was observed via transmission electron microscopy (TEM) in the pendeo-epitaxial GaN relative to that in the GaN columns. TEM observations revealed that in pendeo-epitaxial GaN films the dislocations do not propagate laterally from the GaN columns when the structure grows laterally from the sidewalls into and over the trenches. Scanning electron microscopy (SEM) studies revealed that the coalesced regions are either defect-free or sometimes exhibit voids. Above these voids the PEGaN layer is usually defect free.
Properties of Epitaxial ZnO Thin Films for GaN and Related Applications
Authors: Shen H. , Wraback M. , Pamulapati J. , Liang S. , Gorla C. , Lu Y.
Published: April 2011

In this paper, we present a detailed study of high quality (110) ZnO films, epitaxially grown on R-plane sapphire substrates by metal-organic chemical vapor deposition (MOCVD). The epitaxial relationships are (1120) ZnO//(0112) Al2O3 and {0001} ZnO//[0111] A12O3 as confirmed by X-ray diffraction (θ-2θ, and φ-scan) and high-resolution cross-sectional transmission electron microscopy (HR-TEM). Low temperature photoluminescence (PL) indicates the ZnO thin films are almost strain free. Optical absorption and reflection measurements with linearly polarized light indicate a strong optical anisotropy. The polarization rotation towards the C-axis associated with the optical anisotropy is utilized to demonstrate an optically addressed ultra-fast, ultraviolet light modulator.
GaN CVD Reactions: Hydrogen and Ammonia Decomposition and the Desorption of Gallium Michael
Authors: Bartram E. , Creighton J. Randall
Published: April 2011

Isotopic labeling experiments have revealed correlations between hydrogen reactions, Ga desorption, and ammonia decomposition in GaN CVD. Low energy electron diffraction (LEED) and temperature programmed desorption (TPD) were used to demonstrate that hydrogen atoms are available on the surface for reaction after exposing GaN(0001) to deuterium at elevated temperatures. Hydrogen reactions also lowered the temperature for Ga desorption significantly. Ammonia did not decompose on the surface before hydrogen exposure. However, after hydrogen reactions altered the surface, N15H3 did undergo both reversible and irreversible decomposition. This also resulted in the desorption of N2 of mixed isotopes below the onset of GaN sublimation. This suggests that the driving force of the high nitrogen-nitrogen bond strength (226 kcal/mol) can lead to the removal of nitrogen from the substrate when the surface is nitrogen rich. Overall, these findings indicate that hydrogen can influence GaN CVD significantly, being a common factor in the reactivity of the surface, the desorption of Ga, and the decomposition of ammonia.
Effect of Buffer Layer and III/V Ratio on the Surface Morphology of Gan Grown by MBE
Authors: Piquette E. C. , Bridger P. M. , Beach R. A. , McGill T. C.
Published: April 2011

The surface morphology of GaN is observed by atomic force microscopy for growth on GaN and AlN buffer layers and as a function of III/V flux ratio. Films are grown on sapphire substrates by molecular beam epitaxy using a radio frequency nitrogen plasma source. Growth using GaN buffer layers leads to N-polar films, with surfaces strongly dependent on the flux conditions used. Flat surfaces can be obtained by growing as Ga-rich as possible, although Ga droplets tend to form. Ga-polar films can be grown on AlN buffer layers, with the surface morphology determined by the conditions of buffer layer deposition as well as the III/V ratio for growth of the GaN layer. Near-stoichiometric buffer layer growth conditions appear to support the flattest surfaces in this case. Three defect types are typically observed in GaN films on AlN buffers, including large and small pits and “loop” defects. It is possible to produce surfaces free from large pit defects by growing thicker films under more Ga-rich conditions. In such cases the surface roughness can be reduced to less than l nm RMS.
Cubic GaN Heteroepitaxy on Thin-SiC-Covered Si(001)
Authors: Hiroyama Yuichi , Tamura Masao
Published: April 2011

We have investigated the growth conditions of cubic GaN (β-GaN) layers on very thin SiC-covered Si(001) by using gas-source molecular beam epitaxy as functions of SiC layer thickness, Ga-cell temperature and substrate temperature. Under the present SiC formation conditions on Si substrates by carbonization using C2H2, gas, the SiC layers with the thickness between 2.5 and 4 nm result in the epitaxial growth of β-GaN on thus SiC-formed Si substrates. At the highest GaN growth rate of 110 nm/h (a Ga-cell temperature of 950 °C), β-GaN layers grown at a substrate temperature of 700 °C show a nearly flat surface morphology and the fraction of included hexagonal GaN becomes negligible when compared to the results of β-GaN layers grown under other conditions of Ga-cell and substrate temperatures. Thus obtained β-GaN films have good performance in photoluminescence intensity although the FWHM of band-edge recombination peak is still wider (137 meV) than the reported values for the β-GaN on 3C-SiC and GaAs.
Behavior of Crack Growth Resistance in Toughened Silicon Nitride Ceramics
Authors: Kawamoto H. , Hiramatsu K. , Takigawa Y. , Okada A. , Usami H.
Published: April 2011

Testing methods for crack-growth resistance-curve (R-curve) behavior were investigated and developed to analyze the toughened mechanism in polycry stalline ceramics. These methods are a biaxial-flexure method for small-scale disc-shaped specimens with micro-indentation cracks and a single-edged notched beam flexural method with crack stabilizers. In both methods, the growing crack length is measured directly as a function of applied stress, using the system that consist of a microscope and a CCD camera. Applying these testing methods, R-curve behavior of a toughened silicon nitride with a preferred orientation of elongated grains was evaluated to characterize the toughened mechanism, comparing with the behavior in a commercially available silicon nitride. The behavior having these rising R-curves is discussed with emphasis on the effects of microstructure such as grain-growth and grain-orientation, and resultant grain-bridgings behind the crack-tip.
The Fracture of Composite Materials: A Chaotic Approach
Authors: Tonelli R. , Meloni F. , Aymerich F.
Published: April 2011

In the present work, we analyze the behaviour of graphite-reinforced composite materials under periodic loading. We reconstruct the phase space of these physical systems by means of the tools of nonlinear system theory. Experimental measurements are used to calculate chaotic parameters as fractal and embedding dimensions. We use these quantities to characterize differences in the physical samples and modifications in their internal structure. This analysis can provide useful indications about the fracture process.
Scanning Force Microscopy as a Tool for Fracture Studies
Authors: Thome F. , Göken M. , Große Gehling M. , Vehoff H.
Published: April 2011

Dynamic simulations of the fracture toughness as a function of the orientation and tem- perature were carried out and compared with experimental results obtained by in-situ loading pre-cracked NiAl single crystals inside a scanning force microscope (SFM). In order to compare the simulations with the experiments the problem of the short crack with dislocations was solved for general loading and arbitrary slip line directions. The stress and strain field obtained could be directly connected to FEM calculations which allowed the examination of the stability of micro cracks at notches. The effect of different fracture conditions for biaxial loading were studied in detail.

The dynamic simulation yielded predictions of K1C, slip line length and dislocation distri- butions as a function of loading rate, temperature and orientation. These predictions were tested by in-situ loading NiAl single crystals inside a SFM at various temperatures. The local COD, slip line length and apparent dislocation distribution at the surface were measured as a function of the applied load and the temperature. The experiments clearly demonstrated that dislocations emit from the crack tip before unstable crack jumps occur. The local COD could be directly related to the number of dislocations emitted from the crack tip. With increasing temperature the number of dislocations and the local COD increased before unstable crack jumps or final fracture occurred.
Barium Strontium Titanate Capacitors for Embedded Dram
Authors: Zurcher Peter , Tracy C.J. , Jones R.E. , Alluri P. , Chu P.Y. , Jiang B. , Kim M. , Melnick B.M. , Raymond M.V. , Roberts D. , Remmel T.P. , Tsai T.L. , White B.E. , Zafar S. , Gillespie S.J.
Published: April 2011

Long recognized as the best potential solution to the continued scaling of the onetransistor/one-capacitor standalone dynamic random access memory (DRAM) beyond a gigabit, barium strontium titanate (BST) and other high permittivity dielectrics are fast becoming enablers to embedding large amounts of memory into a high performance logic process. System requirements such as granularity, bandwidth, fill frequency, and power pose major challenges to the use of high density standalone DRAM, leading to the current push for embedded solutions where very wide buses are possible. As a result, projected embedded memory sizes are rapidly approaching that of the standalone products, and with the high wafer cost of the combined logic plus memory process, bit cell scaling is critical. The BST memory cell, with its low thermal budget processing, very high charge storage density, and high conductivity metal electrodes has the potential to be efficiently embedded with traditional logic flows if the materials and integration challenges of the required three dimensional (3D) bit cell capacitors can be solved. BST materials properties such as dielectric relaxation, interface capacitance, and resistance degradation and their impact on capacitor scaling will be reviewed along with the electrode materials issues associated with certain 3D capacitor designs. The scaling limits of BST bit cells in the deep sub-micron regime will be discussed.
Defect Identification in (La,Sr)CoO3−δ Using Positron Annhiiilation Spectroscopy
Authors: Friessnegg T. , Nielsen B. , Ghosh V. J. , Moodenbaugh A.R. , Madhukar S. , Aggarwal S. , Keeble D.J. , Poindexter E.H. , Mascher P. , Ramesh R.
Published: April 2011

Vacancy type defects in bulk La1−xSrxCoO3−δ samples were investigated by positron lifetime spectroscopy. The effects of Sr-doping as well as the effect of oxygen deficiency were determined. Comparing the resolved lifetimes with calculated values permits defect identification.
Preparation and Properties of Barium Incorporated Strontium Bismuth Tantalate Ferroelectric Thin Films
Authors: Lu Chung-Hsin , Wen Cheng-Yen
Published: April 2011

SrBi2Ta2O9thin films doped with barium ions were studied, in which Ba/(Sr+Ba) = 0.3 and 0.5, meanwhile the content of (Sr+Ba) remains unity to keep the stoichiometry of SrBi2Ta2O9. Films were deposited using metalorganic decomposition method with spin-on coating. Crystallinity, surface morphology, and ferroelectric properties of prepared thin films were investigated. From X-ray diffraction (XRD) analysis, barium ions substituted strontium ions in the SrBi2Ta2O9 lattice. Shift of diffraction peaks was observed, indicating a slight distortion of the lattice while barium ions incorporated into. The observation of prepared films indicated that the grain size of films annealed at 750 °C was about 0.7∼0.8 μm. Such barium incorporated SrBi2Ta2O9 thin films exhibited higher remanent polarization than the intrinsic SrBi2Ta2O9thin films.
Raman Study of SrBi2Ta2O9 Bi3TiNbO9 Thin Films
Authors: Pérez W. , Ching-Prado E , Dobal P.S. , Reynés-Figueroa A. , Katiyar R.S. , Tirumala S. , Desu S. B.
Published: April 2011

Thin films of ferroelectric (SrBi2Ta2O9)x(Bi3TiNbO9)1−x layered structure were prepared for x = 0.0, 0.2, 0.4, 0.6, 0.8, and 1.0, by metal organic solution deposition method on Pt/TiO2/SiO2/Si substrates. The micro-Raman studies reveal that the samples are mostly homogeneous. The Raman sprectrum for the film with x = 0 shows bands around 60, 170, 232, 256, 337, 569, and 839 cm−1, which indicate Bi3TiNbO9 (BTN) formation. The prominent band around 839 cm−1, which is an A1g mode of the orthorombic symmetry, corresponds to symmetric stretching of the BO6 octahedra. The frequency of this band is found to shift significantly as SrBi2Ta2O9(SBT) material is added to the BTN compound. In this paper, the evolution of the Raman bands with the inclusion of SBT material are presented and discussed. Finally, The Raman spectra of SBT- BTN films are compared with those obtained in SBT-BTN bulk ceramics.