Dynamic Finite Element Analysis on Underlay Microstructure of Cu/low-k Wafer during Wirebonding

The aim of present research is to investigate dynamic stress analysis for microstructure of Cu/Low-K wafer subjected to wirebonding predicted by finite element software ANSYS/LS-DYNA. Two major analyses are conducted in the present research. In the first, the characteristic of heat affected zone (HAZ) and free air ball (FAB) on ultra thin Au wire have been carefully experimental measured. Secondary, the dynamic response on Al pad/beneath the pad of Cu/low-K wafer during wirebonding process has been successfully predicted by finite element analysis (FEA). Tensile mechanical properties of ultra thin wire before/after electric flame-off (EFO) process have been investigated by self-design pull test fixture. The experimental obtained hardening value has significantly influence on localize stressed area on Al pad. This would result in Al pad squeezing around the smashed FAB during impact stage and the consequent thermosonic vibration stage. Microstructure of FAB and HAZ are also carefully measured by micro/nano indentation instruments. All the measured data serves as material inputs for the FEA explicit software ANSYS/LS-DYNA. Because the crack of low-k layer and delamination of copper via are observed, dynamic transient analysis is performed to inspect the overall stress/strain distributions on the microstructure of Cu/low-k wafer. Special emphasizes are focused on the copper via layout and optimal design of Cu/low-k microstructure. It is also shown that the Al pad can be replaced by Al-Cu alloy pad or Cu pad to avoid large deformation on pad and cracking beneath the surface. A series of comprehensive experimental works and FEA predictions have been performed to increase bondability and reliability in this study

Development of Novel Sensor Devices for Total Ionization Dose Detection

abstract: Total dose sensing systems (or radiation detection systems) have many applications, ranging from survey monitors used to supervise the generated radioactive waste at nuclear power plants to personal dosimeters which measure the radiation dose accumulated in individuals. This dissertation work will present two different types of novel devices developed at Arizona State University for total dose sensing applications. The first detector technology is a mechanically flexible metal-chalcogenide glass (ChG) based system which is fabricated on low cost substrates and are intended as disposable total dose sensors. Compared to existing commercial technologies, these thin film radiation sensors are simpler in form and function, and cheaper to produce and operate. The sensors measure dose through resistance change and are suitable for applications such as reactor dosimetry, radiation chemistry, and clinical dosimetry. They are ideal for wearable devices due to the lightweight construction, inherent robustness to resist breaking when mechanically stressed, and ability to attach to non-flat objects. Moreover, their performance can be easily controlled by tuning design variables and changing incorporated materials. The second detector technology is a wireless dosimeter intended for remote total dose sensing. They are based on a capacitively loaded folded patch antenna resonating in the range of 3 GHz to 8 GHz for which the load capacitance varies as a function of total dose. The dosimeter does not need power to operate thus enabling its use and implementation in the field without requiring a battery for its read-out. As a result, the dosimeter is suitable for applications such as unattended detection systems destined for covert monitoring of merchandise crossing borders, where nuclear material tracking is a concern. The sensitive element can be any device exhibiting a known variation of capacitance with total ionizing dose. The sensitivity of the dosimeter is related to the capacitance variation of the radiation sensitive device as well as the high frequency system used for reading. Both technologies come with the advantage that they are easy to manufacture with reasonably low cost and sensing can be readily read-out.Dissertation/ThesisDoctoral Dissertation Electrical Engineering 201

Experiments in Graphene and Plasmonics

Graphene nanoribbons, graphene based optical sensors, and grating based plasmonics are explored experimentally. Graphene nanoribbons exhibit highly insulating states that may allow for graphene based digital applications. We investigate the sensitivity of these states to local charged impurities in ultra high vacuum. We look into the possibility of isolating two-dimensional films of H-BN and BSCCO, and test for any interesting phenomena. We also assess graphene*s applicability for optical sensing by implementing a new style of spectral detector. Utilizing surface plasmon excitations nearby a graphene field-effect transistor we are able to produce a detector with wavelength sensitivity and selectivity in the visible range. Finally, we study another plasmonic phenomenon, and observe the resonant enhancement of diffraction into a symmetry-prohibited order in silver gratings

Development of a compact wireless SAW Pirani vacuum microsensor with extended range and sensitivity

Vakuumsensoren haben nach wie vor einen begrenzten Messbereich und erfordern eine aufwendige Verkabelung sowie eine komplexe Integration in Vakuumkammern. Ein kompakter Sensor, der in der Lage ist, den Erfassungsbereich zwischen Hochvakuum und Atmosphärendruck zu erweitern und dabei drahtlos zu arbeiten, ist äußerst wünschenswert. Der Schwerpunkt dieser Arbeit liegt auf dem Entwurf, der Simulation, der Herstellung und der experimentellen Validierung eines drahtlosen kompakten Vakuum-Mikrosensors mit erweiterter Reichweite und Empfindlichkeit. Zunächst wurde ein neuer Sensor unter Verwendung vorhandener und neu entwickelter Komponenten entworfen. Zweitens wurden die Sensorkomponenten simuliert, um ihre Parameter zu optimieren. Drittens wurde ein Prototyp unter Verwendung der verfügbaren Mikrobearbeitungs- und Halbleitertechnologien hergestellt und montiert. Viertens wurde der Prototyp unter Umgebungs- und Vakuumbedingungen charakterisiert, um seine Leistungen zu validieren. Für das Wandlerprinzip wurden zwei Techniken kombiniert, nämlich Pirani-Sensorik und akustische Oberflächenwellen. Das Design der Sensorkomponenten bestand aus vier Einheiten: Sensoreinheit, Heizeinheit, Abfrageeinheit und Gehäuse. Alle Einheiten wurden in einen kompakten Würfel eingebaut. Einige Komponenten wurden neu entwickelt, während andere gekauft, modifiziert und dann miteinander verbunden wurden. Die Sensoreinheit besteht aus einem neuen Chip mit verbesserter Sensorleistung dank eines optimierten Verhältnisses von Oberfläche zu Volumen. Die Heizeinheit wurde aus zwei induktiv gekoppelten Spulen und der zugehörigen Konditionierungselektronik zusammengesetzt. Die Abfrageeinheit wurde mit einer Mikro-Patch-Antenne hergestellt. Ein würfelförmiges Polymergehäuse wurde entwickelt, um alle Komponenten in einer Vakuumkammer unterzubringen. Zweitens wurde die Simulation des Verhaltens der Sensorkomponenten behandelt. Die für die Druckmessung verantwortliche Wärmeübertragung des Sensorchips wurde vom Hochvakuum bis zum Atmosphärendruck untersucht, um seine Abmessungen zu optimieren. Die Verwendung eines hängenden Lithium-Niobat-Chips mit Y-Z-Schnitt und einem TCF von 94 ppm/K führte zu einer verbesserten Leistung in einem Messbereich zwischen \num{d-4}~Pa und \num{e5}~Pa. Die elektronische Kopplung der Heizspulen wurde ebenfalls simuliert, um die Leistungsübertragung und den Kopplungsabstand zu optimieren. Der dritte Teil betrifft die Herstellungs- und Montageschritte des Prototyps unter Verwendung der verfügbaren Halbleitertechnologien und -ausrüstung. Ein SAW Chip wurde mit einer 100~nm dicken Goldschicht an der Unterseite gesputtert, um den Heizwiderstand zu bilden, und mit Hilfe von Drahtbonding elektrisch mit dem Rest des Sensors verbunden. Es wurde eine Leiterplatte vorbereitet, die die Heiz- und Sensoreinheit enthält. Ein kubisches Gehäusewurde aus PTFE hergestellt. Viertens wurden die Sensorkomponenten zunächst separat charakterisiert, um ihre Leistungen zu überprüfen, und dann zusammen unter Umgebungsbedingungen. Später wurde der Sensor im Vakuum integriert, und es wurde ein druckabhängiges Verhalten des Sensorchips beobachtet. Die Relevanz eines drahtlosen Übertragungsverfahrens wurde den herkömmlichen drahtgebundenen Methoden gegenübergestellt. Die Ergebnisse der experimentellen Arbeiten außerhalb und innerhalb des Vakuums zeigten die Machbarkeit und Relevanz des neuen Konzepts

Algebraic level sets for CAD/CAE integration and moving boundary problems

Boundary representation (B-rep) of CAD models obtained from solid modeling kernels are commonly used in design, and analysis applications outside the CAD systems. Boolean operations between interacting B-rep CAD models as well as analysis of such multi-body systems are fundamental operations on B-rep geometries in CAD/CAE applications. However, the boundary representation of B-rep solids is, in general, not a suitable representation for analysis operations which lead to CAD/CAE integration challenges due to the need for conversion from B-rep to volumetric approximations. The major challenges include intermediate mesh generation step, capturing CAD features and associated behavior exactly and recurring point containment queries for point classification as inside/outside the solid. Thus, an ideal analysis technique for CAD/CAE integration that can enable direct analysis operations on B-rep CAD models while overcoming the associated challenges is desirable. ^ Further, numerical surface intersection operations are typically necessary for boolean operations on B-rep geometries during the CAD and CAE phases. However, for non-linear geometries, surface intersection operations are non-trivial and face the challenge of simultaneously satisfying the three goals of accuracy, efficiency and robustness. In the class of problems involving multi-body interactions, often an implicit knowledge of the boolean operation is sufficient and explicit intersection computation may not be needed. Such implicit boolean operations can be performed by point containment queries on B-rep CAD models. However, for complex non-linear B-rep geometries, the point containment queries may involve numerical iterative point projection operations which are expensive. Thus, there is a need for inexpensive, non-iterative techniques to enable such implicit boolean operations on B-rep geometries. ^ Moreover, in analysis problems with evolving boundaries (ormoving boundary problems), interfaces or cracks, blending functions are used to enrich the underlying domain with the known behavior on the enriching entity. The blending functions are typically dependent on the distance from the evolving boundaries. For boundaries defined by free form curves or surfaces, the distance fields have to be constructed numerically. This may require either a polytope approximation to the boundary and/or an iterative solution to determine the exact distance to the boundary. ^ In this work a purely algebraic, and computationally efficient technique is described for constructing signed distance measures from Non-Uniform Rational B-Splines (NURBS) boundaries that retain the geometric exactness of the boundaries while eliminating the need for iterative and non-robust distance calculation. The proposed technique exploits the NURBS geometry and algebraic tools of implicitization. Such a signed distance measure, also referred to as the Algebraic Level Sets, gives a volumetric representation of the B-rep geometry constructed by purely non-iterative algebraic operations on the geometry. This in turn enables both the implicit boolean operations and analysis operations on B-rep geometries in CAD/CAE applications. Algebraic level sets ensure exactness of geometry while eliminating iterative numerical computations. Further, a geometry-based analysis technique that relies on hierarchical partition of unity field compositions (HPFC) theory and its extension to enriched field modeling is presented. The proposed technique enables direct analysis of complex physical problems without meshing, thus, integrating CAD and CAE. The developed techniques are demonstrated by constructing algebraic level sets for complex geometries, geometry-based analysis of B-rep CAD models and a variety of fracture examples culminating in the analysis of steady state heat conduction in a solid with arbitrary shaped three-dimensional cracks. ^ The proposed techniques are lastly applied to investigate the risk of fracture in the ultra low-k (ULK) dies due to copper (Cu) wirebonding process. Maximum damage induced in the interlayer dielectric (ILD) stack during the process steps is proposed as an indicator of the reliability risk. Numerical techniques based on enriched isogeometric approximations are adopted to model damage in the ULK stacks using a cohesive damage description. A damage analysis procedure is proposed to conduct damage accumulation studies during Cu wirebonding process. Analysis is carried out to identify weak interfaces and potential sites for crack nucleation as well as damage nucleation patterns. Further, the critical process condition is identified by analyzing the damage induced during the impact and ultrasonic excitation stages. Also, representative ILD stack designs with varying Cu percentage are compared for risk of fracture

Untersuchungen zu den Mechanismen des Ultraschall-Drahtbondens

Ultrasonic (US) wire bonding is a predominating interconnection technique in the microelectronic packaging industry. Despite its long-term usage and wide applications, the mechanisms, especially those of the friction and softening phases, are still unclear more than half a century after its invention. Targeting on reducing the big gap to a good understanding of the mechanisms, this dissertation focuses on the relative motions at the wire/substrate and wire/tool interfaces, and the oxide removal process. In addition, an energy flow model from the electrical input energy to the different energies involved in the mechanisms is developed and quantified. The relative motions at the two interfaces were investigated by a real-time observation system with which the micrometer-motions of the tool and the wire were captured. The motions were then tracked and quantified. In addition, the influences of the process parameters including the normal force, US power and process time were analyzed and the combined effect of the normal force and US power was emphasized. By a further investigation on the changes of the surface topography and elements distribution, it was proved that the relative displacement amplitudes at different locations of the wire/tool interface differ. With the substitution of the metal substrate by a transparent glass, the bonding process was visualized and different areas including the contact, friction, stick, microwelds and oxides areas were detected. The oxide removal process was studied with artificial coatings on either the wire or the substrate. A complete removal process including cracks, detachment, milling and transportation was studied. The transportation further includes penetration, oxide flow, pushing and metal splash. The quantification of energy flows shows that most US energy flows to the vibration induced friction at the two interfaces and the vibration induced formation, deformation and breakage of microwelds. Based on the energy flow to the wire/substrate interface and to the formation of microwelds, the optimal combination of the normal force and the ultrasonic power is determined

Update - Body of Knowledge (BOK) for Copper Wire Bonds

Copper wire bond technology developments continue to be a subject of technical interest to the NASA (National Aeronautics and Space Administration) NEPP (NASA Electronic Parts and Packaging Program) which funded this update. Based on this new research, additional copper bond wire vulnerabilities were found in the literature - Crevice corrosion, intrinsic degradation of palladium coated copper wire, congregation of palladium near ball bond interface leading to failure, residual aluminum pad metallization impact on device lifetimes, stitch cracking phenomena, package delamination's that have resulted in wire bond failures and device failure due to elemental sulfur. A search of the U.S.A. patent web site found 3 noteworthy patents on the following developments: claim of a certain IMC (Intermetallic Compound) thickness as a mitigation solution to chlorine corrosion; claim of using materials with different pHs to neutralize contaminants in a package containing copper wire bonds; and a discussion on ball shear test threshold values for different applications. In addition, an aerospace contractor of military hardware had a presentation on copper bond wires where it was reported that there was a parametric shift and noise susceptibility of devices with copper bond wires which affected legacy design performance. A review of silver bond wire (another emerging technology) technical papers found that an electromigration failure mechanism was evident in device applications that operate under high current conditions. More studies may need to be performed on a comprehensive basis. Research areas for consideration are suggested, however, these research and or qualification/standard test areas are not all inclusive and should not be construed as the element (s) that delivers any potential copper wire bond solution. A false sense of security may occur, whenever there is a reliance on passing any particular qualification, standard, or test protocol

Experimental Characterization Of Cu Free-Air Ball And Simulations Of Dielectric Fracture During Wire Bonding

Wire bonding is the process of forming electrical connection between the integrated circuit (IC) and its structural package. ICs made of material with low dielectric constant (low-k) and ultra low-k are porous in nature, and are prone to fracture induced failure during packaging process. In recent years, there is increasing interest in copper wire bond technology as an alternative to gold wire bond in microelectronic devices due to its superior electrical performance and low cost. Copper wires are also approximately 25% more conductive than Au wires aiding in better heat dissipation. At present, validated constitutive models for the strain rate and temperature dependent behavior of Cu free-air ball (FAB) appear to be largely missing in the literature. The lack of reliable constitutive models for the Cu FAB has hampered the modeling of the wire bonding process and the ability to assess risk of fracture in ultra low-k dielectric stacks. The challenge to FAB characterization is primarily due to the difficulty in performing mechanical tests on spherical FAB of micrometers in size. To address this challenge, compression tests are performed on FAB using custom-built microscale tester in the current study. Specifically, the tester has three closed-loop controlled linear stages with submicron resolution, a manual tilt stage, a six-axis load cell with sub-Newton load resolution for eliminating misalignment, a milliNewton resolution load cell for compression load measurement, a capacitance sensor to estimate sample deformation and to control the vertical stage in closed loop, a high working depth camera for viewing the sample deformation, and controllers for the stages implemented in the LabVIEW environment. FAB is compressed between tungsten carbide punches and a constitutive model is developed for Cu FAB through an inverse modeling procedure. In the inverse procedure, appropriate constitutive model parameter values are iterated through an automated optimization workflow, until the load-displacement response matches the experimentally observed response. Using the material properties obtained from the experiment, a macroscale finite element model for the impact and ulatrasonic vibration stages of wire bonding process is constructed to simulate (a) Plastic deformation of the Cu FAB at different time steps (b) Evolution of contact pressure (c) Phenomenon such as pad splash and lift-off. The deformations from the macroscale model are provided as input to a microscale model of the dielectric with copper vias as well as line-type heterogeneities. The microscale model is used to identify potential crack nucleation sites as well as the crack path within the ILD stack during wire bonding. The modeling provides insight into the relative amounts of damage accumulated during the impact and the ultrasonic excitation stages. In general, Bonding over Active Circuit (BOAC) has made wire bonding a considerable challenge due to the brittleness of the dielectric. Identifying and locating microscale fractures beneath the bond pads during wire bonding require extensive sample preparation and investigation for microscopic characterization. While simulations of fracture are an attractive alternative to trial and error microscopic characterization, the length scale of components involved in wire bonding varies from millimeters to nanometers. Therefore, constructing a finite element mesh across the model is computationally costly. Also, a multi-scale simulation framework is necessary. Such a modeling framework is also developed in this work to predict crack nucleation and propagation in wire bond induced failure

Investigation into stable failure to short circuit in IGBT power modules

This doctoral thesis investigates modes of failure of the IGBT power module and how these modes can be coerced from an open circuit failure mode (OCFM) to a stable short circuit failure mode (SCFM) by using different interconnect technologies and material systems. SCFM is of great importance for a number of applications where IGBT power modules are connected in series string e.g. high voltage modular multi-level converters (M2LC) where one module failing to an OCFM can shut down the whole converter. The failure modes of IGBT samples based on wirebond, flexible PCB, sandwich and press pack structured interconnect technologies have been investigated. Destructive Type-II failure test were performed which concluded that the SCFM is dependent on the energy level dissipating in the power module and the interconnect technology. The higher thermal mass and stronger mechanical constraint of the interconnect enables module to withstand higher energy dissipation. The cross-sections of the tested samples have been characterised with the scanning electron microscope and three dimensional X-ray computed tomography imaging. It was observed that the networked conductive phases within the solidification structure and the Sn-3.5Ag filled in cracks of the residual Si IGBT are responsible for low resistance conduction paths. The best networked conductive phase with lowest electrical resistance and high stability was offered by Ag if used as an intermediate interconnect material on emitter side of an IGBT. To offer a stable SCFM, a module has to be custom designed for a particular application. Hence for the applications which demand a stable SCFM, the IGBT module design becomes an integrated part of the complete power electronics system design

Manufacturing Methods for Magnetic Resonance Microscopy Tools with Application to Neuroscience

Magnetresonanztomographie (MR) ist ein unverzichtbares nicht-invases und hochselektives bildgebendes Verfahren in der Medizin. MR Tomographie wird kommerziell in der klinischen Diagnostik und der Forschung für Gehirnkrankheit, z.B. Epilepsie, Alzheimer und Parkinson, angewandt. In den Neurowissenschaften haben sich Kleintiere als biologische Modelle für die grundlegenden Studien zur diesen Gehirnkrankheiten etabliert. MR Methoden sind ein wertvolles Werkzeug um die Morphologie und den Metabolismus von Kleintieren zu untersuchen. Die Modelle für die Untersuchung von Gehirnkrankheiten schließen Zellen/Zellkulturen und organotypische hippocampale Schnittkulturen (OHSC) mit ein. Obwohl die MR Mikroskopie für die Untersuchung von OHSC schon angewandt wurde fehlt eine effektive Plattform für umfangreiche longitudinale Studien an OHSC wie sie in den Neurowissenschaften üblich sind. Zwei Detektorkonzepte für die MR Mikroskopie inklusive ihrer Auslegung, der Herstellung und der Charakterisierung, werden in dieser Arbeit beschrieben. Beide Konzepte basieren auf Herstellungsmethoden welche hohe Fertigungsgenauigkeiten zulassen und in ihrem Herstellungsvolumen skalierbar sind. Hohle solenoide Mikrospulen welche für hochauflösende Untersuchung von Zell und Zellanhäufungen geeignet sind werden eingeführt. Die Herstellung basiert auf dem automatisierten wickeln von Mikrospulen, eine skalierbare und hochpräzise Fertigungsmethode der Mikrotechnologie. Zudem werde induktiv gekoppelte Ober ächenspulen eingeführt. Diese Oberflächenspulen fokussieren den magnetischen Fluss und werden deshalb Lenz Linsen genannt. Die Lenz Linsen werden mit kabelgebundenen und induktiv gekoppelten Spulen verglichen. Ihre Breitband-Fähigkeit machen sie zu einem idealen Kandidaten für die Nutzung in verschiedensten MR Tomographie Systemen. Die Lenz Linsen wurden für den Einsatz in einer MR kompatiblen Inkubationsplattform ausgelegt, welche in dieser Arbeit entwickelt wurde. Der MR Inkubator erweitert die Funktionalität eines MR Tomographen um neurologische Gewebe (z.B. OHSC) über mehrere Stunden andauernde MR Messungen am Leben zu erhalten. Der MR Inkubator erlaubt longitudinale Studien an OHSC und bietet damit eine Plattform für umfangreiche Studien in den Neurowissenschaften. Die Lenz Linsen wurden zusammen mit dem MR Inkubator für MR Mikroskopie Mes- sung von akuten/ xierten hippocampalen Schnitten und OHSC genutzt. Die Resultate dieser MR Mikoskopie Messungen zeigen dass in OHSC die grobe Zytoarchitektur sicht- bar ist, ohne dass die OHSC während der Messungen sterben. Somit ist das eingeführte System bereit für longitudinale Studien an OHSC, welche bereits für die Aufklärung der Epilepsieprogression begonnen wurden