Superalloys are widely applied as materials for components in the hot section of gas turbines. As superalloys have a limited oxidation life, the application of a coating is vital. The most commonly applied coatings in stationary gas turbines are MCrAlY coatings. Since the turbine components are exposed to high cyclic thermal stresses, MCrAlY coatings must also show a high thermal fatigue resistance. In this thesis, the effect of Cobalt and Rhenium on microstructure, oxidation and thermal fatigue of NiCoCrAlY coatings is presented. Additionally the condition of the coatings after testing in an industrial gas turbine is shown. The influence of Cobalt and Rhenium on coating microstructure was investigated by thermodynamic modelling and by metallography. It could be shown that both elements reduce the γ`-phase fraction and increase the β-phase fraction owing to an expansion of the γ+β field in the phase diagram. Modelling showed that Rhenium promotes the formation of α-Cr, which could be explained by a shift of the α-Cr solvus to higher temperatures and lower Cr concentrations. In the real coatings Re causes the precipitation of TCP-phase. The oxide scale growth rate is increased by Cobalt and Rhenium and it appears that Yttrium plays a significant role for that effect. Coating consumption due to simultaneous oxidation and interdiffusion could be decreased by the application of Cobalt and Rhenium. In thermal fatigue testing Rhenium reduces the time to crack initiation and increases crack propagation rate, although it could be shown that Rhenium increases the creep resistance of the coating. The effect could be explained by the influence of Rhenium on the microstructure, which increases creep resistance, but also reduces the ductility of the coating.

The electrical conductivity of transparent conducting oxides is well exploited in front surface electrodes for solar cells where high transmission is also important. Fluorine doped tin oxide (SnO2: F) is the most popular choice of front contacts for CdTe solar cells. In this thesis, Cd2SnO4 and Zn2SnO4 thin films are investigated focusing on their electrical and optical properties and used them in solar cells. Processing for these materials is optimized for optimum solar cell performance. Cd2SnO4 thin films are deposited by co-sputtering of CdO and SnO2 targets in Ar ambient at room temperature. Then films are subjected to high temperature annealing in He ambient. The films crystallize in inverse spinel structure. The average transmission of a Cd2SnO4 thin film with a thickness of 2500[angstrom] obtained in this study is 92%. The lowest resistivity obtained in this work for a Cd2SnO4 film with a thickness of 2500[angstrom] is 5.4 X10-4 cm. The effect of stoichiometry on structure, optical and electrical properties of Cd2SnO4 is studied by varying the amount of CdO and SnO2 in the Cd2SnO4 film. Zinc stannate thin films are deposited by co-sputtering of ZnO and SnO2 targets in Ar ambient at both room temperature and elevated temperatures. As deposited and high temperature annealed Zn2SnO4 thin films are highly resistive. The average transmission of a Zn2SnO4 thin film with a thickness of 2000[angstrom] and annealed at 600ʻC in He has been 94%. Zn2SnO4 thin films are incorporated as a buffer layers into CdTe solar cells. SnO2: F is used as a front contact in CdTe solar cells in conjunction with high resistive Zn2SnO4 buffer layer.The best SnO2:F /zinc stannate cell device performance for room temperature deposited zinc stannate film resulted for the device with Zn/Sn =2.1. It has an efficiency of 12.43% with VOC = 810mV, FF = 66.6% and JSC = 23.1 mA.

The effects of intermetallic compound (IMC) formation on reliability of Pb-free Sn-based solders for flip chip and three-dimensional (3D) interconnects were studied. The dissertation is organized into four parts. In the first part, the effect of Sn grain orientation on electromigration (EM) reliability of Pb-free Sn-based flip chip solder joints was studied. The Sn grain microstructure in flip chip solder joints was characterized using the electron backscatter diffraction (EBSD) technique, and wa found to be closely related to the EM failure mechanims. The approach to grain structure optimization for improved EM reliability was also explored. In addition to the experimental work, a kinetic analysis was formulated to investigate the early EM degradation mechanism in Sn-based solder joints with Ni under-bump metallization (UMB). The aforementioned kinetic analysis, the intrinsic diffusion coefficients were not readily available in the literature. In the second part of the work, a Monte Carlo method known as simulated annealing was applied to estimate the unknown diffusion coefficients using a multi-parameter optimization method by fitting to experimental measurements. The intrinsic diffusion coefficients of Ni and Sn in Ni₃Sn₄ between 150 and 200°C, and those of Cu and Sn in Cu₃Sn and Cu₆Sn₅ between 120 and 200°C were estimatd. The activation energies for these diffusion coefficients were also determined. Together, this provides the diffusivity parameters to predict the intermetallic growth as a function of temperature. The third objective focused on the EM reliability of Sn-based microbump joints in 3D interconnects with through-silicon vias (TSVs). No EM-induced bump failure was observed, showing a robust EM reliability in microbumps. High temperature thermal annealing test was also performed on microbumps with three different metallizations in an effort to explore structural and process optimization. Finally, interfacial reaction induced stress in IMC microbumps was investigated. A numerial analysis was formulated to study the concurrent diffusion, phase transformation, and deformation in the process of IMC formation. Stress generation due to unbalanced diffusion rates and volumetric change upon phase transformation was considered. The coupled analysis was applied to investigate Ni₃Sn₄ growth in the Ni-Sn microbumping system. A simulation approach based on finite difference method with moving boundaries was employed to numerically solve stress evolution in Ni₃Sn₄. The equilibrium stress was also investigated using a modified model with a finite thickness of solder. Simulation predictions were found to be in good qualitative agreement with experimental observations. / text

The composition-dependent Curie temperature and bias-dependant dielectric permittivity of Barium Strontium Titanate (BST) makes it very attractive for tunable application in the RF/Microwave regime. In this research work, the performance of BST varactors fabricated on the conventional Pt/Ti/SiO2/Si bottom electrode stack were compared with those fabricated using chemical vapor deposited Nanocrystalline Diamond (NCD) as the diffusion barrier layer instead of SiO2. The varactors fabricated on NCD films displayed much better symmetry in capacitance-voltage behavior and better overall quality factors than varactors fabricated on SiO2. The improvement in performance can be attributed to existence of stable interfaces in the devices fabricated on NCD which reduced the bottom electrode losses at high frequencies. The SiO2 based BST varactors on the other hand displayed better reliability and breakdown fields. The main purpose of this research work is to develop a robust Metal Insulator Metal (MIM) structure to achieve better all round performance of BST varactors. In the second part of this research work, the prospect of developing diamond based layered Surface Acoustic Wave (SAW) devices using Ba0.8Sr0.2TiO3 as the piezoelectric layer is investigated. Structural characterization of BST thin films deposited on Si/NCD/Pt and Si/SiO2/Ti/Pt stack were performed using X-Ray Diffraction (XRD) and Atomic Force Microscopy (AFM). Cross-sectional studies on the two stacks were performed using Scanning Electron Microscopy (SEM). X-Ray Mapping (XRM) was then done to ascertain the quality of the interfaces and to check for interdiffusion between layers. MIM structures in the Coplanar Waveguide (CPW) configuration were fabricated using conventional lithography and etching techniques for high frequency measurements. The performance of the fabricated varactors was characterized from 100 MHz to 1 GHz. For the SAW application, structural characterization of Ba0.8Sr0.2TiO3 on Chemical Vapor Deposited (CVD) diamond was done and the deposition procedure was optimized to obtain thick BST films. SAW bandpass filters and resonators were designed wherein the device geometry was varied over a wide range in order to characterize the variation in device performance with geometry. Finally interdigital capacitor structures were fabricated and used for conducting Curie temperature measurements on the deposited BST films in order to determine the operation range of the deposited BST films.

Techn. Hochsch., Diss., 2002--Aachen.

Th.--Sci.--Bordeaux I, 1981. N°: 682.

abstract: In this dissertation, remote plasma interactions with the surfaces of low-k interlayer dielectric (ILD), Cu and Cu adhesion layers are investigated. The first part of the study focuses on the simultaneous plasma treatment of ILD and chemical mechanical polishing (CMP) Cu surfaces using N2/H2 plasma processes. H atoms and radicals in the plasma react with the carbon groups leading to carbon removal for the ILD films. Results indicate that an N2 plasma forms an amide-like layer on the surface which apparently leads to reduced carbon abstraction from an H2 plasma process. In addition, FTIR spectra indicate the formation of hydroxyl (Si-OH) groups following the plasma exposure. Increased temperature (380 °C) processing leads to a reduction of the hydroxyl group formation compared to ambient temperature processes, resulting in reduced changes of the dielectric constant. For CMP Cu surfaces, the carbonate contamination was removed by an H2 plasma process at elevated temperature while the C-C and C-H contamination was removed by an N2 plasma process at elevated temperature. The second part of this study examined oxide stability and cleaning of Ru surfaces as well as consequent Cu film thermal stability with the Ru layers. The ~2 monolayer native Ru oxide was reduced after H-plasma processing. The thermal stability or islanding of the Cu film on the Ru substrate was characterized by in-situ XPS. After plasma cleaning of the Ru adhesion layer, the deposited Cu exhibited full coverage. In contrast, for Cu deposition on the Ru native oxide substrate, Cu islanding was detected and was described in terms of grain boundary grooving and surface and interface energies. The thermal stability of 7 nm Ti, Pt and Ru ii interfacial adhesion layers between a Cu film (10 nm) and a Ta barrier layer (4 nm) have been investigated in the third part. The barrier properties and interfacial stability have been evaluated by Rutherford backscattering spectrometry (RBS). Atomic force microscopy (AFM) was used to measure the surfaces before and after annealing, and all the surfaces are relatively smooth excluding islanding or de-wetting phenomena as a cause of the instability. The RBS showed no discernible diffusion across the adhesion layer/Ta and Ta/Si interfaces which provides a stable underlying layer. For a Ti interfacial layer RBS indicates that during 400 °C annealing Ti interdiffuses through the Cu film and accumulates at the surface. For the Pt/Cu system Pt interdiffuion is detected which is less evident than Ti. Among the three adhesion layer candidates, Ru shows negligible diffusion into the Cu film indicating thermal stability at 400 °C. / Dissertation/Thesis / Ph.D. Physics 2012

Les pales de turbine Haute Pression (HP) sont soumises à des sollicitations cycliques, thermiques et mécaniques complèxes. L'objectif de ce travail est de caractériser l'influence du cyclage thermique sur les comportements des systèmes revêtements MCrAIY/superalliage à base de nickel, en surface et à coeur, et d'étudier la complexité des intéractions entre le substrat et le revêtement. Nos efforts ont été concentrés sur l'étude couplée de dégradations environnementales (oxydation, corrosion) et mécaniques (fluage) à haute température, qui sont représentatives de l'application pale de turbine. Les essais d'analyse thermogravimétrique cycliques menés à 1150°C ont permis de mettre en évidence et de quantifier l'écaillage important des couches d'oxydes sur le MC2 lors du refroidissement, et le faible écaillage des couches sur les matériaux revêtus pour les durées considérées (30h à 1150°C). Les résultats des essais d'oxydation et corrosion permettent de définir le comportement global des revêtements pour les températures et les dégradations rencontrées en service. Ces essais ont permis d'évaluer le pouvoir protecteur des revêtements en fonction de leur épaisseur et on mis en évidence l'importance de la notion de réservoir en aluminium et en chrome. Des diagrammes d'occurence de phases représentatifs de l'état de dégradation de ces revêtements ont été construits. La modification de la composition du revêtement NiCoCrAIYTa par un ajout de platine en surface a amélioré la résistance des révêtements aux phénomènes d'oxydation cyclique et de corrosion. La mise en place d'essais de fuage novateurs, caractérisés par le cyclage thermique des échantillons sous chargement constant à 1150°C/80MPa, a révélé la grande influence de ce cyclage sur la vitesse de déformation et sur la durée de vie en température des matériaux. Les différents paramètres qui caractérisent le cycle thermique (durée du palier en température, vitesses de chauffage et de refroidissement) ont un effet significatif sur la durée de vie à rupture. La répétition des cycles thermiques accélère l'endommagement de la structure à coeur.

Multicomponent metal alloys play an essential role in many technologies and their properties must be optimized by rational selection of the alloy’s components and its fractional composition of each. High-throughput materials synthesis allows us to prepare Composition Spread Alloy Films (CSAFs), sample libraries that contains all possible compositions of a binary or ternary alloy. In our lab, a Rotatable Shadow Mask (RSM) – CSAF deposition tool has been developed for the creation of CSAFs. Such CSAFs can be prepared with composition gradients and/or thickness gradients in arbitrarily controlled directions and on a variety of substrates. Once prepared, the CSAF libraries can be characterized thoroughly using a variety of highthroughput spectroscopic methods. Their bulk composition is mapped across the library using Energy Dispersive X-ray spectroscopy (EDX). The near-surface compositions are mapped across composition space using X-ray Photoemission Spectroscopy (XPS). Finally, the electronic structure can be mapped using UV photoemission spectroscopy (UPS) and valence band XPS. Once characterized, these CSAFs are being used for high-throughput studies of alloy catalysis and thermal properties of the alloys and of alloy-substrate interfaces. First of all, PdzCu1-z CSAF was prepared to show that alloy nanoparticles (aNPs) and thin films can adopt phases that differ from those of the corresponding bulk alloy. The mapping of XPS-derived core level binding energy shifts across PdzCu1-z SCSNaP library shows a promising result that the FCC phase can be dimensionally stabilized over the composition range where B2 phase exists in the bulk. This observation can potentially improve the performance of PdzCu1-z NP catalysts in H2 separation. Secondly, the relationship between catalyst activity-electronic structure-composition has been investigated. A high throughput characterization of electronic structure (valence band energy) of binary PdxAg1-x and ternary PdxCuyAu1-x-y CSAFs were performed by XPS. This XPS-derived valence band center is compared with UPS-derived data across PdxCuyAu1-x-y CSAFs. In addition, H2-D2 exchange reaction was studied on PdxAg1-x CASF. A higher HD formation rate is experimentally observed but cannot be predicted by the Langmuir-Hinshelwood model when the surface coverage is saturated. However, the proposed H2-D2 exchange mechanism (breakthrough model) involved with surface and subsurface hydrogen reaction is investigated to produce a same reaction order as Langmuir-Hinshelwood mechanism, which cannot explain the experimental observation. Furthermore, the thermal interface conductance (G) was studied as a function of metal alloy composition. A high-throughput approach to preparation, characterization, and measurement of G was also demonstrated to study the thermal property of alloyed materials. Our result in studying the G across the AuxY1-x (Y = Pd and Cu) CSAFs-dielectric interfaces has shown a linear relationship with alloy composition, which monotonically increases with decreasing Au (at. %). Lastly, the effect of interdiffusion in metal films on G at metal-dielectric interface was also examined. The XPS depth profiling was designed to experimentally determine the temperature effect on compositional profiles in the Au-Cu system, and how to further influence G. This study provides fundamental understanding of stability of adhesion layer of Cu and the effect of interdiffusion in Cu-Au alloy on the heat dissipation. All in all, the key value to these CSAF libraries is that they enable measurement of important alloy properties across entire binary or ternary alloy composition spaces, without the need to prepare and characterize numerous discrete composition samples.

Superalloys are widely applied as materials for components in the hot section of gas turbines. As superalloys have a limited oxidation life, the application of a coating is vital. The most commonly applied coatings in stationary gas turbines are MCrAlY coatings. Since the turbine components are exposed to high cyclic thermal stresses, MCrAlY coatings must also show a high thermal fatigue resistance. In this thesis, the effect of Cobalt and Rhenium on microstructure, oxidation and thermal fatigue of NiCoCrAlY coatings is presented. Additionally the condition of the coatings after testing in an industrial gas turbine is shown. The influence of Cobalt and Rhenium on coating microstructure was investigated by thermodynamic modelling and by metallography. It could be shown that both elements reduce the γ`-phase fraction and increase the β-phase fraction owing to an expansion of the γ+β field in the phase diagram. Modelling showed that Rhenium promotes the formation of α-Cr, which could be explained by a shift of the α-Cr solvus to higher temperatures and lower Cr concentrations. In the real coatings Re causes the precipitation of TCP-phase. The oxide scale growth rate is increased by Cobalt and Rhenium and it appears that Yttrium plays a significant role for that effect. Coating consumption due to simultaneous oxidation and interdiffusion could be decreased by the application of Cobalt and Rhenium. In thermal fatigue testing Rhenium reduces the time to crack initiation and increases crack propagation rate, although it could be shown that Rhenium increases the creep resistance of the coating. The effect could be explained by the influence of Rhenium on the microstructure, which increases creep resistance, but also reduces the ductility of the coating.