Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1982. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE / Includes bibliographical references. / by Gretchen Lynn Kalonji. / Ph.D.

Neste trabalho apresentamos medidas de fotocondutividade, decaimento da fotocondutividade persistente, resistência em função da temperatura em amostras de AlxGa1-xAs de gap direto e indireto, dopadas com Si. Comparamos as teorias de Brooks-Herring e Takimoto, ambas referentes ao espalhamento por impurezas ionizadas, e sua aplicabilidade para nosso material. Interpretamos a presença de um estado de energia intermediário nos cálculos da energia de ativação baseado nos resultados de concentração de elétrons livres em função da temperatura .como devido ao defeito D-. Nos resultados de decaimento da fotocondutividade persistente no intervalo de 80 - 100K, contamos com a contribuição do espalhamento por dipolos e propomos o par d+ - VAS- como os responsáveis pela formação destes dipolos e conseqüente melhoria do ajuste da simulação numérica. / In this work, we show results of photoconductivity, decay of persistent photoconductivity, resistance x temperature in Si doped direct and indirect bandgap AlxGa1-xAs. We compare Brooks-Herring and Takimoto theories, both in reference to ionized impurity scattering applied to our material. We interpret the intermediate state in our calculation of activation energy as a D- defect. In the numerical simulation of decay of persistent photoconductivity in the range 80-100 K, we propose the dipole pair d+ - VAS- responsible for the fitting improvement, when the dipole scattering is taken into account.

Les cellules déficientes pour la recombinaison homologue (RH) présentent un ralentissement des fourches de réplication, un nombre aberrant de centrosomes et une aneuploïdie même en absence de stress exogène (Bertrand P 2003, Daboussi F 2005, 2008, Deng 1999, 2002, Griffin 2000, Kraakman-van der Zwet 2002). De plus, la fréquence des mitoses présentant des chromosomes surnuméraires est plus élevée dans ces cellules.L’ensemble des ces résultats suggéraient que le ralentissement des fourches de réplication dans les cellules déficientes pour la RH pourrait avoir un impact direct sur la formation de centrosomes surnuméraires. De plus, nous voulions savoir si cela pouvait également influencer la ségrégation des chromosomes au cours de la mitose. Les résultats que nous avons obtenus sont rassemblés dans l’article intitulé : “Homologous Recombination protects against mitotic defects and unbalanced chromosome segregation caused by spontaneous replication stress”.Le traitement des cellules compétentes pour la RH avec 5µM d’hydroxyurée (HU), un inhibiteur de l’enzyme de synthèse des dNTPs, induit un ralentissement des fourches de réplication parfaitement comparable à celui observé dans les cellules déficientes pour la RH. Après traitement à l’HU des cellules compétentes pour la RH, la fréquence de mitoses présentant des chromosomes surnuméraires augmente et devient similaire à la fréquence de mitoses avec des chromosomes surnuméraires pour les cellules déficientes pour la HR non traitées à l’HU. Nous avons mesuré l’impact de l’HU sur l’apparition des ponts anaphasiques et sur des défauts de ségrégation des chromosomes lors de la mitose. En l’absence de traitement, nous observons une fréquence plus élevée de ponts anaphasiques et de défauts de ségrégation dans des cellules déficientes pour la RH. Des traitements avec 5µM d’HU augmentent la fréquence des ponts anaphasiques et des erreurs de ségrégation dans les cellules compétentes pour la RH, pour atteindre un niveau comparable aux cellules déficientes pour la RH. Ainsi, une altération de la dynamique de réplication consécutive à une déficience de RH ou à un traitment avec de faibles doses d’HU induit des défauts au cours de la mitose. Un lien direct entre une dynamique de réplication anormale et l’apparition d’un nombre aberrant de centrosomes pourrait être la persistance en mitose d’ADN non répliqué ou endommagé.  Comme l’ADN non répliqué ou les fourches bloquées induisent la formation d’ADN simple brin couvert par RPA, nous avons compté le nombre de cellules en G2/M présentant des foyers de RPA, et observé que la fraction de cellules ayant plus de 5 foyers de RPA augmente dans des cellules déficientes pour Brca2. En conclusion, nous proposons un lien direct entre des altérations de la cinétique de réplication, l’apparition de centrosomes surnuméraires et des défauts de ségrégation des chromosomes en mitose dans les cellules déficientes pour la RH même non soumises à un stress exogène. L’utilisation de faibles doses d’HU dans des cellules compétentes pour la RH mime les phénotypes observés dans les cellules déficientes pour la RH et confirme notre modèle.Nous avons également cherché à comprendre les causes du ralentissement des fourches de réplication observé dans les cellules déficientes pour la RH. Il est ainsi possible que les arrêts de fourches spontanés soient une conséquence d’un stress oxydatif endogène. Dans les cellules compétentes pour la RH, le redémarrage des fourches bloquées est possible et assure une progression normale de la réplication de l’ADN. Ceci favorise une ségrégation équilibrée des chromosomes, le maintien de la diploïdie et la stabilité du génome. Dans des cellules déficientes pour la RH, les blocages de fourches devraient être délétères puisque les principaux mécanismes de redémarrage des fourches ne sont pas fonctionnels. De plus, l’arrêt prolongé des fourches ainsi que les cassures double brin générées par l’effondrement des fourches devraient activer des voies de signalisation. Nous avons néanmoins observé que les cellules ne sont pas bloquées dans le cycle cellulaire, ce qui suggère qu’un seuil supérieur de dommages doive être atteint pour induire l’arrêt du cycle. Les stress endogènes ne semblent donc pas suffisamment élevés pour atteindre ce seuil : même si l’ensemble des fourches parcourant le génome sont ralenties, l’activation d’origines cryptiques permet de compenser et ainsi de maintenir la progression dans le cycle. Mais puisque les cellules ne sont pas arrêtées dans le cycle, des fourches bloquées, de l’ADN endommagé ou non répliqué pourraient persister jusqu’à la transition G2/M et in fine perturber le déroulement de la mitose. Des centrosomes multipolaires provoquent la formation de fuseaux multipolaires et favorisent la ségrégation déséquilibrée des chromosomes, entraînant l’aneuploïdie, la déstabilisation du génome et le développement de cancers. / HR deficient cells show slow replication kinetics, aberrant centrosome number and aneuploidy even in the absence of any exogenous stress (Bertrand P 2003, Daboussi F 2005, 2008, Deng 1999, 2002, Griffin 2000, Kraakman-van der Zwet 2002). Frequency of mitosis with extra centrosomes is elevated and replication kinetics decreased in HR deficient compared to HR proficient cells, in the absence of exogenous stress. Thus the question arose, if replication slowing down in HR deficient cells has direct impact on the appearance of supernumerary centrosomes. Furthermore we wanted to know if this might directly impact chromosome segregation. The results we gained are brought together in the paper “Homologous recombination suppression causes spontaneous mitotic alterations through endogenous replication stress”. By treating our HR proficient cells with 5µM HU we found the perfect concentration to mimic replication dynamics of HR deficient cells in an HR proficient background. This concentration was applied to HR proficient cells. After HU treatment the frequency of mitosis with extra centrosomes was elevated in HR proficient cells. Now they showed the same frequency of mitosis with extra centrosomes, than unchallenged HR deficient cells. We measured the impact of HU treatment on occurrence of anaphase bridges or aberrant mitotic segregation. In the absence of treatment higher frequency of anaphase bridges and aberrant mitotic segregation was detected for HR deficient cells. With 5µM HU the frequency of anaphase bridges and aberrant mitosis could be elevated in HR proficient cells. Now they showed aberrant mitotic features with the same frequency than unchallenged HR deficient cells. A direct link between abnormal replication kinetics and aberrant centrosomes might be unreplicated or damaged DNA, that enter mitosis. Unreplicated or blocked DNA might harbour ss DNA bound RPA. Thus we counted G2/M cells with RPA foci. Indeed the fraction of cells that harbour more than 5 RPA foci was elevated in Brca2 deficient in comparison to Brca2 proficient cells. In conclusion we propose a direct link between delayed replication, supernumerary centrosomes and aberrant chromosome segregation in unchallenged HR deficient cells. If we mimicked replication kinetics of HR deficient cells in an HR proficient background, we also mimicked frequency of mitosis with extra centrosome number and aberrant chromosome segregation. Furthermore we investigated the causes of replication slowing down in HR deficient cells. It can be hypothesized that endogenous oxidative stress is implicated in spontaneous fork arrest. In HR proficient cells, reactivation of stalled replication forks and therefore normal replication progression is assured. This favours balanced chromosome segregation, diploidy and genetic stability.In HR deficient cells, replication fork blockage might be detrimental as the main restart mechanism for blocked forks is absent. Prolonged fork blockage or DSB’s arising by fork collapse or resolution of blocked replication forks might activate signalling pathways. However cells are not arrested in cell cycle progression, suggesting that a threshold should be reached to activate cell cycle arrest. Endogenous stress is not sufficient high to reach this threshold. Replication is genome wide slowed down. In this context, the activation of cryptic origins compensates at least partly the slow replication velocity. However, because cells were not arrested in cell cycle progression, some blocked replication forks and damaged or unreplicated DNA regions might persist until G2/M phase and affect centrosome duplication and chromosome segregation. Multipolar centrosomes cause multipolar spindles and favour unbalanced chromosome segregation leading to aneuploidy, genetic instability and cancer development.

Composite materials are increasingly used in many industries due to the high strength and low weight properties that they exhibit. Since composites are becoming more popular, they are being used in applications such as aircraft, boats, wind turbine blades, and even sports equipment. Composite behavior is complicated since they are made up of two completely different materials such as strong thin fibers and a relatively weaker resin material that hold the fibers together. It is becoming more important to understand how composites behave in different situations so that equipment designers have reliable material information in order to design safe products that will not harm human life. Fabrication of composite material is not perfect and introduces defects such as the fibers being wavy and the matrix having voids. These defects decrease the strength of composites and if not accounted for in design, could be detrimental. To better understand the effects of these defects in composite materials, experimental tests can be performed to determine the material properties but it costs a lot of money and time. If the material properties of the composite do not match what is desired, different constituent materials are selected to create new composite specimens and the tests must be repeated which costs more time and more money. Computational approaches such as Finite Element Modeling (FEM) are gaining popularity as a way to predict composite behavior without the high cost of fabrication and equipment. Another advantage is the ability to test various materials and various defects by simply changing parameters in the computation. For this thesis, an FEM protocol is developed to model composites made from the material AS4/8552. First, the strength properties are extracted from a model without defects and then, defects such as waviness in the fiber and voids in the matrix are added to the model to see its effect. Knowing the effect of certain defects may help motivate composite fabricators to develop processes that eliminate detrimental defects.

A well designed runner and feeding system should produce castings with minimal defects and low pour weight. This thesis investigates how the filling regime and solidification of the mould influences defects in the castings produced from that mould. Design guidelines to reduce such defects are proposed and tested. An existing shrinkage fault in a Grey Iron disc brake casting is simulated using a commercial finite-difference computer program. Three criteria are used to predict the defect and the effect of changes to the feeder geometry. Critical Fraction Solidification analysis is used to determine whether the feeder remains in liquid contact with the casting during solidification and this approach is shown to correctly predict the presence or absence of porosity* The feeder block is extended below the ingate of the casting to improve liquid contact between the casting and feeder without significantly increasing the feeder mass. Plant trials confirm the change to the feeder eliminates the porosity defect. The runner system and mould venting for a thin walled Ductile Iron casting are investigated. Trials show that by setting the total mould vent area to be greater than the net ingate area of the castings, the cold-shut frequency is halved. A method for runner system design based on peak linear flow velocity in the runner during mould filling is proposed. A new pressurised runner system produces castings with significantly fewer defects and reduced pour weight when runner areas are designed to maintain peak velocity below 1 m/s. Peak velocity and magnesium levels are demonstrated to be critical factors in the elimination of cold-shut defects. A pressurised runner system is also shown to isolate inclusion defects from castings more effectively than an unpressurised system. From this work, a technique is proposed which allows the yield of an existing runner and feeder system for iron castings to be improved with confidence in the results.

Wet thermal oxidations of polycrystalline Si₀.₅₄Ge₀.₄₆ films at 600°C for 30 and 50 min were carried out. A stable mixed oxide was obtained for films that were oxidized for 50 min. For film oxidized for 30 min, however, a mixed oxide with Ge nanocrystallites embedded in the oxide matrix was obtained. A trilayer gate stack structure that consisted of tunnel oxide/oxidized polycrystalline Si₀.₅₄Ge₀.₄₆/rf sputtered SiO₂ layers was fabricated. We found that with a 30 min oxidized middle layer, annealing the structure in N₂ ambient results in the formation of germanium nanocrystals and the annealed structure exhibits memory effect. For a trilayer structure with middle layer oxidized for 50 min, annealing in N₂ showed no nanocrystal formation and also no memory effect. Annealing the structures with 30 or 50 min oxidized middle layer in forming gas ambient resulted in nanocrystals embedded in the oxide matrix but no memory effect. This suggests that the charge storage mechanism for the trilayer structure is closely related to the interfacial traps of the nanocrystals. / Singapore-MIT Alliance (SMA)

Ph.D. / Materials Science and Engineering / Polytitanates in the BaO- TiO2 system with Ba:Ti ratios ranging from 1:2 to 1:5 were prepared using a low temperature technique developed by Pechini. The samples were heated at 600 to 1300°C in oxygen. Room temperature Raman spectroscopy was used to investigate the phase relations in this system. Results of this study indicate the following: except for BaTi4O9, the powders of these compounds were amorphous when heated at 600°C for 4hrs; the compound BaTi2O5 is a low temperature stable phase; Ba6Ti17O40 forms only at temperatures above 1100°C; Ba4Ti13O30 does not form below 1000°C; the single phase BaTi4O9 structure was observed at 1200°C'; the Ba2Ti9O20 phase is obtained only after long heat treatment at 1200°; BaTi5O11 was stable up to 1200°C, at which it decomposes into Ba2Ti4O20 and TiO2. After determination of stability relationships in this system, the electrical conductivities of these compounds were examined as a function of temperature and oxygen partial pressure. For all the temperatures (850-1150°C) studied, the conductivities of these compounds increased with decreasing oxygen partial pressure resulting in n-type properties throughout the whole P[subscript O2] range (10[superscript -19 - 1atm). The P[subscript O2] dependencies of the electrical conductivity were found to be linear for an extensive range of oxygen partial pressures. On the basis of structural considerations the conductivity data was described by a majority defect model consisting of both singly and doubly ionized oxygen vacancies. For lower oxygen partial pressure values a drastic change in the electrical conductivity was observed. This is believed to result from increasing defect interaction for larger departures from stoichiometry. A defect model based on this interaction is proposed to account for the observed sharp change in the electrical conductivity values.

Heterogeneous ferromagnetic-superconducting systems such as a regular array of ferromagnetic nano dots deposited on the top of a superconducting thin film have attracted many research teams both experimental and theoretical. The interest in these systems does not only stem from being good candidates for technological applications, but also because they represent a new class of physical systems where two competing order parameters can coexist. This work focuses on the theoretica laspects of these systems by studying the static and dynamics of few model systems. In the first part, the static properties of a superconducting thin film interacting with a ferromagnetic texture are considered within the London approximation. In particular, the ferromagnetic textures considered here are a circular dot of submicrometer size with in-plane magnetization, an elliptical dot magnetized in the direction perpendicular to the superconductor, and a ferromagnetic dot magnetized in the direction normal to the superconducting film and containing non magnetic cavities. I also consider the interaction of vortices in the superconductor with a ferromagnetic columnar defect which penetrates the supercondcting film. In each case the vector potential and magnetic field of the ferromagnet in the presence of the superconductor are calculated. Afterward the presence of vortices in the superconductor is assumed and the energy of vortex-texture system is found. The pinning potential and force supplied by the texture are then derived from the energy of interaction between the ferromagnet and superconductor. I show that if the magnetization of the ferromagnet exceeds a critical value then vortices are spontaneously created in the ground state of the system. Such spontaneous creation of vortices is possible mostly in a close vicinity of the superconducting transition temperature Ts. For every case, the threshold value of the magnetization at which vortices start to be spontaneously created in the SC is calculated as a function of the parameters of the texture geometry. The phase diagrams for transitions from vortexless regime to regimes with one or more vortices are determined for all cases. In the second problem, the transport properties of a ferromagnetic superconducting bilayer with alternating magnetization and vortex density are studied within a phenomenological model. I show that pinning forces do not appear for continuous distribution of vortices, so a discrete model for the bilayer system is constructed. Afterward, I calculate the pinning forces acting on vortices and antivortices resulting from highly inhomogeneous distribution of flux lines and prove that this system has strong transport anisotropy. In the absence of random pinning, the system displays a finite resistance for the current in the direction perpendicular to the domains while its resistance vanishes for the parallel current. The transport anisotropy strongly depends on temperature. I study this dependence and show that the ratio of parallel to perpendicular critical current is largest close to the superconducting transition temperature Ts and the vortex disappearance temperature Tv while it has a minimum in between them.

Software defect detection research typically focuses on individual inspection and testing techniques. However, to be effective in applying defect detection techniques, it is important to recognize when to use inspection techniques and when to use testing techniques. In addition, it is important to know when to deliver a product and use maintenance activities, such as trouble shooting and bug fixing, to address the remaining defects in the software.<p>To be more effective detecting software defects, not only should defect detection techniques be studied and compared, but the entire software defect detection process should be studied to give us a better idea of how it can be conducted, controlled, evaluated and improved.<p>This thesis presents a self-evolving software defect detection process (SEDD) that provides a systematic approach to software defect detection and guides us as to when inspection, testing or maintenance activities are best performed. The approach is self-evolving in that it is continuously improved by assessing the outcome of the defect detection techniques in comparison with historical data.<p>A software architecture and prototype implementation of the approach is also presented along with a case study that was conducted to validate the approach. Initial results of using the self-evolving defect detection approach are promising.

Les dimensions dels transistors disminueixen per a cada nova tecnologia CMOS. Aquest alt nivell d'integració complica el procés de fabricació dels circuits integrats, apareixent nous mecanismes de fallada. En aquest sentit, els mètodes de diagnosi actuals no són capaços d'assumir els nous reptes que sorgeixen per a les tecnologies nanomètriques. A més, la inspecció física de fallades (Failure Analysis) no es pot aplicar des d'un bon començament, ja que els costos de la seva utilització són massa alts. Per aquesta raó, conèixer el comportament dels defectes i dels seus mecanismes de fallada és imprescindible per al desenvolupament de noves metodologies de diagnosi que puguin superar aquests nous reptes. En aquest context, aquesta tesi presenta l'anàlisi dels mecanismes de fallada i proposa noves metodologies de diagnosi per millorar la localització de ponts (bridge) i oberts (open). Per a la diagnosi de ponts, alguns treballs s'han beneficiat de la informació obtinguda durant el test de corrent (IDDQ). No obstant no han tingut en compte l'impacte del corrent de dowsntream. Per aquesta raó, en aquesta tesi s'analitza l'impacte d'aquest corrent degut als ponts i la seva dependència amb la tensió d'alimentació (VDD). A més, es presenta una nova metodologia de diagnosi basada en els múltiples nivells de corrent. Aquesta tècnica considera els corrents generats per les diferents xarxes connectades pel pont. Aquesta metodologia s'ha aplicat amb èxit a un conjunt de xips defectuosos de tecnologies de 0.18 µm i 90 nm.Com alternativa a les tècniques basades en corrent, els shmoo plots també poden ser útils per a la diagnosi. Tradicionalment s'ha considerat que valors baixos de VDD són més apropiats per a la detecció de ponts. Tanmateix es demostra en aquesta tesi que en presència de ponts connectant xarxes equilibrades, valors alts de VDD són fins i tot més apropiats que tensions baixes, amb la conseqüent implicació que això té per a la diagnosi.En relació als oberts, s'ha dissenyat i fabricat un xip amb la inclusió intencionada d'oberts complets (full opens) i oberts resistius. Experiments fets amb els xips demostren l'impacte de les capacitats d'acoblament de les línies veïnes. A més, pels oberts resistius s'ha comprovat la influència de l'efecte història i de la localització de l'obert en el retard. Tradicionalment s'ha considerat que el retard màxim s'obté quan un obert resistiu es troba al principi de la línia. No obstant això no es pot generalitzar a oberts poc resistius, ja que en aquests casos es demostra que el màxim retard s'obté per a una localització intermèdia. A partir dels resultats experimentals obtinguts amb el xip, s'ha desenvolupat una nova metodologia per a la diagnosi d'oberts complets a les línies d'interconnexió. Aquest mètode divideix la línia en diferents segments segons la informació de layout de la pròpia línia. Aleshores coneixent els valors de les línies veïnes, es prediu la tensió del node flotant, la qual es compara amb el resultat experimental obtingut a la màquina de test. Aquest mètode s'ha aplicat amb èxit a un seguit de xips defectuosos pertanyents a una tecnologia de 0.18 µm.Finalment, s'ha analitzat l'impacte que tenen els corrents de túnel a través del terminal de porta en presència d'un obert complet. Com les dimensions disminueixen per a cada nova tecnologia, l'òxid de porta és suficientment prim com per generar corrents de túnel que influencien el node flotant. Aquests corrents generen una evolució temporal al node flotant fins fer-lo arribar a un estat quiescent, el qual depèn de la tecnologia. Es comprova que aquestes evolucions temporals són de l'ordre de segons per a una tecnologia de 0.18 µm. Tanmateix les simulacions demostren que aquests temps disminueixen fins a uns quants µs per a tecnologies futures. Degut a l'impacte dels corrents de túnel, un seguit d'oberts complets s'han diagnosticat en xips de 0.18 µm. / Transistor dimensions are scaled down for every new CMOS technology. Such high level of integration has increased the complexity of the Integrated Circuits (ICs) manufacturing process, arising new complex failure mechanisms. However, present diagnosis methodologies cannot afford the challenges arisen for future technologies. Furthermore, physical failure analysis, although indispensable, is not feasible on its own, since it requires high cost equipment, tools and qualified personnel. For this reason, a detailed understanding and knowledge of defect behaviours is a key factor for the development of improved diagnosed methodologies to overcome the challenges of nanometer technologies. In this context, this thesis presents the analysis of existing and new failure mechanisms and proposed new diagnosis methodologies to improve the diagnosis of faults, focused on bridging and open faults.IDDQ is a well known technique for the diagnosis of bridging faults. However, previous works have not considered the impact of the downstream current for the diagnosis of such faults. In this thesis, the impact and the dependence of the downstream current with the power supply voltage (VDD) is analyzed and experimentally measured. Furthermore, a multiple level IDDQ based diagnosis technique is presented. This method takes benefit from the currents generated by the different network excitations. This technique is successfully applied to real defective devices from 0.18 µm and 90 nm technologies.As an alternative to current based techniques, shmoo plots can be also useful for diagnosis purposes. Low voltage has been traditionally considered as an advantageous condition for the detection of bridging faults. However, it is demonstrated that in presence of bridges connecting balanced n- and p-networks, high VDD values are also advantageous for the detection of bridges, which has its direct translation into diagnosis application. Experimental evidence of this fact is presented.Related to open faults, an experimental chip has been designed and fabricated in a 0.35 µm technology, where full and resistive open defects have been intentionally added. Different experiments have been carried out so that the impact of the neighbouring coupling capacitances has been quantified. Furthermore, for resistive opens, experiments have demonstrated the influence of the history effect and the location of the defect on the delay. Traditionally, it has been reported that the highest delay is obtained when the resistive open is located at the beginning of the net. Nevertheless, this thesis demonstrates that this is not true for low resistive open, since the highest delay is obtained for an intermediate location. Experimental measurements prove this behaviour.Derived from the results obtained with the fabricated chip, a new methodology for the diagnosis of interconnect full open defects is developed. The FOS (Full Open Segment) method divides the interconnect line into different segments based on the topology of the faulty line. Knowing the logic state of the neighbouring lines, the floating net voltage is predicted and compared with the experimental results obtained on the tester. This method has been successfully applied to a set of 0.18 µm defective devices. Finally, the impact of the gate tunnelling leakage currents on the behaviour of full open defects has also been analyzed. As technology dimensions are scaled down, the oxide thickness is thin enough so that the gate tunnelling leakage currents influence the behaviour of floating lines. They cause transient evolutions on the floating node until reaching the steady state, which is technology dependent. It is experimentally demonstrated that these evolutions are in the order of seconds for a 0.18µm technology. However, for future technologies, simulations show that the evolutions decrease down to a few µs. Based on this factor, some full open faults present in 0.18 µm technology devices are diagnosed.