Mechanics of Non Planar Interfaces in Flip-Chip Interconnects

With the continued proliferation of low cost, portable consumer electronic products with greater functionality, there is increasing demand for electronic packaging that is smaller, lighter and less expensive. Flip chip is an essential enabling technology for these products. The electrical connection between the chip I/O and substrate is achieved using conductive materials, such as solder, conductive epoxy, metallurgy bump (e.g., gold) and anisotropic conductive adhesives. The interconnect regions of flip-chip packages consists of highly dissimilar materials to meet their functional requirements. The mismatches in properties, contact morphology and crystal orientation at those material interfaces make them vulnerable to failure through delamination and crack growth under various loading patterns. This study encompasses contact between deformable bodies, bonding at the asperities and fracture properties at interfaces formed by the interconnects of flip-chip packages. This is achieved through experimentation and modeling at different length scales, to be able to capture the detailed microstructural features and contact mechanics at interfaces typically found in electronic systems

Formation mechanisms of low-dimensional semiconductor nanostructures grown by OMCVD on nonplanar substrates

Semiconductor quantum wires (QWRs) are promising structures for optoelectronics applications, since they can provide quantum confinement for charge carriers in two dimensions. The advantage that they offer over conventional quantum wells (QWs) is due to the sharper density of states characteristic of these structures, yielding narrower spectral lines and higher optical gain. However, to exhibit clear confinement characteristics, QWRs must meet stringent requirements in terms of size, uniformity and interfacial quality. Different methods have been explored for QWR fabrication. Techniques based on etching and regrowth suffer from defect incorporation into the lateral interfaces, since they are not formed in situ, and are limited in size by the lithographic features. On the other hand, growth of fractions of monolayers on vicinal substrates, although overcoming the above limitations, gives rise to size nonuniformities and graded interfaces. In this project, (In)GaAs/AlxGa1-xAs QWRs are obtained by organometallic chemical vapor deposition (OMCVD) growth of quantum wells on patterned, V-grooved substrates. In this way, the lithographically defined pattern serves as a seed for QWR formation. The self-ordering properties of OMCVD on nonplanar surfaces ensure the creation of a self-limiting profile at the bottom of the grooves, on which the wires are grown. This method overcomes the size limitations imposed by lithography, allows the in situ formation of interfaces and, thanks to the self-ordering mechanism, yields structures with high uniformity, whose characteristics are determined solely by the growth conditions. Although nonplanar growth has been employed for more than ten years for QWR fabrication, the understanding of the self-ordering mechanisms originating the profiles at the bottom of the grooves has been until now only phenomenological. The attainment of self-limiting profiles takes place via transients of the growth rates at the bottom of the groove. Current models of nonplanar growth can predict the formation, evolution or disappearance of facets at the 100nm-μm size. However, they cannot describe the transient behaviors at the nm scale that lead to self-limiting growth. This thesis project has been aimed at elucidating the physical mechanisms of this self-organized growth. A fundamental part of the project has been the creation of a wide experimental database to understand the dependence of the self-limiting profiles on the materials and growth conditions. The profiles at the bottom of the groove exhibit a faceted structure, consisting of a central (100) plane, surrounded by two {311}A ones. Cross-sectional transmission electron microscopy (TEM) shows that the bottom facets become wider as the growth temperature increases and as the Al mole fraction x of AlxGa1-xAs layers decreases. It appears therefore that surface diffusion is a key element in determining self-limiting growth. TEM cross sections show also that the establishment of self-limiting profiles takes place via self-adjusting growth rates on these facets. In addition to this geometrical self-ordering, AlxGa1-xAs alloys exhibit also a compositional self-ordering at the bottom of the groove. Due to the higher mobility of Ga species, with respect to the Al ones, the facets at the bottom of the groove are Ga rich, with respect to the sidewall planes, giving rise to so-called vertical quantum wells (VQWs). To determine the composition of the VQWs, we have developed a technique employing cross-sectional atomic force microscopy (AFM) in air. This method is based on the dependence of the AlxGa1-xAs oxidation rates on the Al content x. Through a calibration on a reference sample, we were able to measure compositions with an accuracy of ±0.02. The enhanced Ga content of the VQWs follows classical models of segregation, and reaches a maximum of Δx ≅ 0.15 for x ≅ 0.55 at a growth temperature of 700°C. We also studied the three dimensional structure of the self-limiting surface profiles by top-surface AFM in air of the nonplanar samples after cool-down and removal from the OMCVD reactor. Each of the planes composing the groove presents a monolayer step structure that reflects directly the morphology of surfaces of the same orientation found in planar epitaxy. However, on the facets forming on corrugated substrates the step structure exhibits a higher degree of ordering, with respect to planar epitaxy. This is due to a modification of surface diffusion, when the trench width becomes comparable to or lower than the adatom surface diffusion length. In the last part of the project, we have developed a model ascribing the self-ordering phenomena observed above to local variations of the surface chemical potential μ. Since μ becomes lower as the concavity of the surface increases, it induces a curvature-dependent capillarity flux towards the bottom of the groove. In the absence of capillarity, if the growth rate is higher on the sidewall planes than on the bottom facets, the capillarity-enhanced growth rate at the bottom can balance exactly the sidewall growth rate, thus leading to self-limiting growth. The different behavior of nonplanar OMCVD (where self-ordering is usually observed at the bottom of the grooves) and molecular beam epitaxy (where self-ordering rather takes place at the top of the corrugations) can be explained by the different growth rate anisotropies of the two techniques. In a ternary alloy, the composition is locally different at the bottom of the groove, due to the different diffusion lengths of Ga and Al. The resulting entropy of mixing, which is lower than the one for a uniform composition, tends however to oppose this segregation, thus affecting the alloy self-limiting profiles. The predictions of the model have been successfully verified on our OMCVD-grown profiles. They can be used to design and optimize a variety of nanostructures, including VQWs, QWRs and QWR superlattices in the GaAs/A1GaAs system, and can be further extended to the strained InGaAs/AlGaAs system

Modeling the effect of elastic strain on ballistic transport and photonic properties of semiconductor quantum structures

The recent progress in microelectronic processing techniques has made it possible to fabricate artificial materials, dedicated and tailored directly for nanoelectronics and nanophotonics. The materials are designed to achieve a confinement of electrons to nanometer size foils or grains, often called quantum structures because of the quantization of the electron energies. In this work I have developed computationalmodels for the electronic structure, photonic recombination and carrier dynamics of quantum confined charge carriers of artificial materials. In this thesis I have studied in particular the effect of elastic strain on the ballistic transport of electrons, in silicon electron wave guides; and on the electronic structure and photonic properties of III-V compound semiconductor heterostructures. I have simulated two types of elastic strain. The strain in the silicon wave guides is induced by the thermal oxidation of the silicon processing and the strain of the III-V compound semiconductor structures is a result of a pseudomorphic integration of lattice mismatched materials. As one of the main results of this work, we have shown that the oxidation-induced strain can lead to current channeling effects in electron wave guides and a doubling of the conductance steps of the wave guide. In the case of the III-V compound semiconductor heterostructures, it was shown that piezoelectric potential (which is due to the elastic strain) complicates considerably the electron-hole confinement potential of strain-induced quantum dots. This has several consequences on the optical properties of these systems. Our results are well in agreement with experimental observations and do explain a set of experiments, which have so far lacked any explanation. This work does, thereby, imply a much better understanding of both silicon electron wave guides and strain-induced quantum dots. This could have implications for both further detailed experiments and future technological applications of the studied devices.reviewe

Aeronautical engineering: A continuing bibliography with indexes

This bibliography lists 425 reports, articles and other documents introduced into the NASA scientific and technical information system in January 1985

Aeronautical engineering: A continuing bibliography with indexes (supplement 237)

This bibliography lists 572 reports, articles, and other documents introduced into the NASA scientific and technical information system in February, 1989. Subject coverage includes: design, construction and testing of aircraft and aircraft engines; aircraft components, equipment and systems; ground support systems; and theoretical and applied aspects of aerodynamics and general fluid dynamics

Aeronautical engineering: A continuing bibliography with indexes (supplement 272)

This bibliography lists 719 reports, articles, and other documents introduced into the NASA scientific and technical information system in November, 1991. Subject coverage includes: design, construction and testing of aircraft and aircraft engines; aircraft components, equipment, and systems; ground support systems; and theoretical and applied aspects of aerodynamics and general fluid dynamics

Micro-Mechanical Voltage Tunable Fabry-Perot Filters Formed in (111) Silicon

The MEMS (Micro-Electro-Mechanical-Systems) technology is quickly evolving as a viable means to combine micro-mechanical and micro-optical elements on the same chip. One MEMS technology that has recently gained attention by the research community is the micro-mechanical Fabry-Perot optical filter. A MEMS based Fabry-Perot consists of a vertically integrated structure composed of two mirrors separated by an air gap. Wavelength tuning is achieved by applying a bias between the two mirrors resulting in an attractive electrostatic force which pulls the mirrors closer. In this work, we present a new micro-mechanical Fabry-Perot structure which is simple to fabricate and is integratable with low cost silicon photodetectors and transistors. The structure consists of a movable gold coated oxide cantilever for the top mirror and a stationary Au/Ni plated silicon bottom mirror. The fabrication process is single mask level, self aligned, and requires only one grown or deposited layer. Undercutting of the oxide cantilever is carried out by a combination of RIE and anisotropic KOH etching of the (111) silicon substrate. Metallization of the mirrors is provided by thermal evaporation and electroplating. The optical and electrical characteristics of the fabricated devices were studied and show promissing results. A wavelength shift of 120nm with 53V applied bias was demonstrated by one device geometry using 6.27 micrometer air gap. The finesse of the structure was 2.4. Modulation bandwidths ranging from 91KHz to greater than 920KHz were also observed. Theoretical calculations show that if mirror reflectivity, smoothness, and parallelism are improved, a finesse of 30 is attainable. The predictions also suggest that a reduction of the air gap to 1 micrometer results in an increased wavelength tuning range of 175 nm with a CMOS compatible 4.75V

Index to 1983 NASA Tech Briefs, volume 8, numbers 1-4

Short announcements of new technology derived from the R&D activities of NASA are presented. These briefs emphasize information considered likely to be transferrable across industrial, regional, or disciplinary lines and are issued to encourage commercial application. This index for 1983 Tech Briefs contains abstracts and four indexes: subject, personal author, originating center, and Tech Brief Number. The following areas are covered: electronic components and circuits, electronic systems, physical sciences, materials, life sciences, mechanics, machinery, fabrication technology, and mathematics and information sciences

1996 Laboratory directed research and development annual report

This report summarizes progress from the Laboratory Directed Research and Development (LDRD) program during fiscal year 1996. In addition to a programmatic and financial overview, the report includes progress reports from 259 individual R&D projects in seventeen categories. The general areas of research include: engineered processes and materials; computational and information sciences; microelectronics and photonics; engineering sciences; pulsed power; advanced manufacturing technologies; biomedical engineering; energy and environmental science and technology; advanced information technologies; counterproliferation; advanced transportation; national security technology; electronics technologies; idea exploration and exploitation; production; and science at the interfaces - engineering with atoms