A Review of Micro-Contact Physics for Microelectromechanical Systems (MEMS) Metal Contact Switches

Innovations in relevant micro-contact areas are highlighted, these include, design, contact resistance modeling, contact materials, performance and reliability. For each area the basic theory and relevant innovations are explored. A brief comparison of actuation methods is provided to show why electrostatic actuation is most commonly used by radio frequency microelectromechanical systems designers. An examination of the important characteristics of the contact interface such as modeling and material choice is discussed. Micro-contact resistance models based on plastic, elastic-plastic and elastic deformations are reviewed. Much of the modeling for metal contact micro-switches centers around contact area and surface roughness. Surface roughness and its effect on contact area is stressed when considering micro-contact resistance modeling. Finite element models and various approaches for describing surface roughness are compared. Different contact materials to include gold, gold alloys, carbon nanotubes, composite gold-carbon nanotubes, ruthenium, ruthenium oxide, as well as tungsten have been shown to enhance contact performance and reliability with distinct trade offs for each. Finally, a review of physical and electrical failure modes witnessed by researchers are detailed and examined

Physics-Based Electromigration Modeling and Analysis and Optimization

Long-term reliability is a major concern in modern VLSI design. Literature has shown that reliability gets worse as technology advances. It is expected that the future VLSI systems would have shorter reliability-induced lifetime comparing with previous generations. Being one of the most serious reliability effects, electromigration (EM) is a physical phenomenon of the migration of metal atoms due to the momentum exchange between atoms and the conducting electrons. It can cause wire resistance change or open circuit and result in functional failure of the circuit. Power-ground networks are the most vulnerable part to EM effect among all the interconnect wires since the current flow on this part is the largest on the chip. With new generation oftechnology node and aggressive design strategies, more accurate and efficient EM models are required. However, traditional EM approaches are very conservative and cannot meet current aggressive design strategies. Besides circuit level, EM also need to be thoroughly studied in system level due to limited power and temperature budgets among cores on chip. This research focuses on developing physical level EM model for VLSI circuits and system level EM optimization for multi-core systems in order to overcome the aforementioned problems. Specifically, for physical level, we develop two EM immortality check methods and a power grid EM check method. Firstly, a voltage based EM immortality analysis has been developed. Immortality condition in nucleation phase can be determined fast and accurately for multi-segment interconnect wires. Secondly, a saturation volume based incubation phase immortality check method has been proposed. This method can further reduce the redundancy in VLSI circuit design by immortality check in multiphase. Furthermore, both immortality check methods are integrated into a new power grid EM check methodology (EMspice) as filter for EM analysis. These filters can accelerate the simulation by filtering out immortal trees so that we only need to do simulation on fewer trees that are mortal. Coupled EM simulation considering both hydrostatic stress and electronic current/voltage in the power grid network will be applied to these mortal trees. This tool can work seamlessly with commercial synthesis flow. Besides physical level reliability models, system level reliability optimization is also discussed in this research. A deep reinforcement learning based EM optimization has been proposed for multi-core system. Both long term reliability effect (hard error) and transient soft error are considered. Energy can be optimized with all the reliability and other constraints fast and accurately compared to existing reliability management techniques. Last but not least, a scheduling based reliability optimization method for multi-core systems has been proposed. NBTI, HCI and EM are considered jointly. Lifetime of the system can be improved significantly compared to traditional methods which mainly focus on utilization

Constraint-Aware, Scalable, and Efficient Algorithms for Multi-Chip Power Module Layout Optimization

Moving towards an electrified world requires ultra high-density power converters. Electric vehicles, electrified aerospace, data centers, etc. are just a few fields among wide application areas of power electronic systems, where high-density power converters are essential. As a critical part of these power converters, power semiconductor modules and their layout optimization has been identified as a crucial step in achieving the maximum performance and density for wide bandgap technologies (i.e., GaN and SiC). New packaging technologies are also introduced to produce reliable and efficient multichip power module (MCPM) designs to push the current limits. The complexity of the emerging MCPM layouts is surpassing the capability of a manual, iterative design process to produce an optimum design with agile development requirements. An electronic design automation tool called PowerSynth has been introduced with ongoing research toward enhanced capabilities to speed up the optimized MCPM layout design process. This dissertation presents the PowerSynth progression timeline with the methodology updates and corresponding critical results compared to v1.1. The first released version (v1.1) of PowerSynth demonstrated the benefits of layout abstraction, and reduced-order modeling techniques to perform rapid optimization of the MCPM module compared to the traditional, manual, and iterative design approach. However, that version is limited by several key factors: layout representation technique, layout generation algorithms, iterative design-rule-checking (DRC), optimization algorithm candidates, etc. To address these limitations, and enhance PowerSynth’s capabilities, constraint-aware, scalable, and efficient algorithms have been developed and implemented. PowerSynth layout engine has evolved from v1.3 to v2.0 throughout the last five years to incorporate the algorithm updates and generate all 2D/2.5D/3D Manhattan layout solutions. These fundamental changes in the layout generation methodology have also called for updates in the performance modeling techniques and enabled exploring different optimization algorithms. The latest PowerSynth 2 architecture has been implemented to enable electro-thermo-mechanical and reliability optimization on 2D/2.5D/3D MCPM layouts, and set up a path toward cabinet-level optimization. PowerSynth v2.0 computer-aided design (CAD) flow has been hardware-validated through manufacturing and testing of an optimized novel 3D MCPM layout. The flow has shown significant speedup compared to the manual design flow with a comparable optimization result

Development of a nanogap fabrication method for applications in nanoelectromechanical systems and nanoelectronics

There is a great need for a well-controlled nanogap fabrication technique compatible with NEMS applications. Theoretically, a displacement sensor based on vacuum tunnel junction or a nanogap can be capable of performing quantum-limited measurements in NEMS applications. Additionally, in the context of nanoelectronics, nanogaps are widely demanded to characterize nanostructures and to incorporate them into nanoscale electronic devices. Here, we have proposed and implemented a fabrication technique based on the controlled shrinkage of a lithographically defined gap between two suspended structures by thermal evaporation. We have consistently produced rigid and stable metallic vacuum tunneling junctions at nanometer or subnanometer sizes. The fabricated nanogaps were characterized by I-V measurements and their gap sizes and potential barrier heights were interrogated using the Simmons' model. Throughout this work, high tensile stress silicon nitride thin films were preferred for the fabrication of suspended structures because they have high resonance frequencies with low dissipation, they are mechanically stable, and they are resilient to stiction problem. However, high-stress nitride structures experience a complex shape deformation once they are suspended. The shape deformation is undesired when the precise positioning of the structures is required as in nanogap fabrication. We developed a new method in which the built in stress gradient is utilized to tune the distance between two suspended structures. The technique was simulated by finite element analysis and experimentally implemented to demonstrate a gap tuning capability beyond the lithographic resolution limits

Electrical detection of spin state switching in electromigrated nanogap devices

Spin crossover is an effect shown in some transition metal complexes where the spin state of the molecule undergoes a transition from a low spin to a high spin state via the application of light, pressure or a change in temperature. This behaviour makes these complexes an attractive candidate to form electronic molecular-scale switches as the electrical resistance of the compound differs between the two spin states. Although the spin crossover effect is commonly studied in its bulk form, the integration of a single molecule into a solid-state device while maintaining the magnetic bi-stability is highly desirable, but remains challenging. This is not only due to difficulties in capturing a single molecule between electrodes and making electrical connections but it is also due to the strong coupling effects imparted on the molecule by the high-density metallic states of the electrodes that can prevent the spin transition from occurring.In recent years there have been many attempts at studying spin crossover complexes at a single molecule level. Many of these have used scanning tunneling microscopy or break junction techniques. While these studies have highlighted the unique and promising electronic properties of these compounds, these techniques are unsuitable for real world devices. This thesis demonstrates a means to make electrical contact to single or small numbers of molecules between gold electrodes fabricated using a bilayer nanoimprint lithography and a feedback controlled electromigration method. This method, enabling high throughput and low-cost fabrication is potentially suitable for scaling to large area planar devices and as such may be used for commercially producing molecular devices.To validate the quality of the nanogaps, devices containing self-assembled monolayers of benzenethiol were first studied. The shape and magnitude of I-V curves measured on nanogap devices containing the benzenethiol monolayers are in good agreement with previously published work using similar molecules in mechanically controlled break junctions. The resulting I-V characteristics were analyzed using the single level resonant tunneling model as well as transition voltage spectroscopy and are consistent with transport through molecular junctions in which the benzenethiol molecules are - stacked. These highly conducting molecular junctions may have potential uses for “soft” coupling to sensitive target molecules.Following validation of the molecular nanojunction fabrication and measurement process, the experimental work shifted to studying electronic transport through spin crossover complexes with a focus on Schiff-base compounds that are specifically tailored for surface deposition. In the case of measurements made on the bulk compound, a sharp spin transition centered at a temperature around 80 K was observed, while a shift to lower temperatures was found for thin films of the complex. In contrast, nanojunction devices containing single molecules displayed very different behaviour, with distinct and reproducible telegraphic-like switching between two resistance states when cooled below 160 K. These two states are attributed to the two different spin states of the complex. The presence of these two resistive states indicates that the spin crossover is preserved at the single molecule level and that a spin-state dependent tunneling process is taking place. Interestingly, in some cases a multi-level switching behaviour is detected with four possible conductance states. This behaviour is attributed to the presence of two spin crossover molecules in the nanogap

Modeling of Thermally Aware Carbon Nanotube and Graphene Based Post CMOS VLSI Interconnect

This work studies various emerging reduced dimensional materials for very large-scale integration (VLSI) interconnects. The prime motivation of this work is to find an alternative to the existing Cu-based interconnect for post-CMOS technology nodes with an emphasis on thermal stability. Starting from the material modeling, this work includes material characterization, exploration of electronic properties, vibrational properties and to analyze performance as a VLSI interconnect. Using state of the art density functional theories (DFT) one-dimensional and two-dimensional materials were designed for exploring their electronic structures, transport properties and their circuit behaviors. Primarily carbon nanotube (CNT), graphene and graphene/copper based interconnects were studied in this work. Being reduced dimensional materials the charge carriers in CNT(1-D) and in graphene (2-D) are quantum mechanically confined as a result of this free electron approximation fails to explain their electronic properties. For same reason Drude theory of metals fails to explain electronic transport phenomena. In this work Landauer transport theories using non-equilibrium Green function (NEGF) formalism was used for carrier transport calculation. For phonon transport studies, phenomenological Fourier’s heat diffusion equation was used for longer interconnects. Semi-classical BTE and Landauer transport for phonons were used in cases of ballistic phonon transport regime. After obtaining self-consistent electronic and thermal transport coefficients, an equivalent circuit model is proposed to analyze interconnects’ electrical performances. For material studies, CNTs of different variants were analyzed and compared with existing copper based interconnects and were found to be auspicious contenders with integrational challenges. Although, Cu based interconnect is still outperforming other emerging materials in terms of the energy-delay product (1.72 fJ-ps), considering the electromigration resistance graphene Cu hybrid interconnect proposed in this dissertation performs better. Ten times more electromigration resistance is achievable with the cost of only 30% increase in energy-delay product. This unique property of this proposed interconnect also outperforms other studied alternative materials such as multiwalled CNT, single walled CNT and their bundles

Thermal-AFM under aqueous environment

The aim of this thesis is to describe the work developing and demonstrating the use of Scanning Thermal Microscopy (SThM) in an aqueous electrically conductive environment for the first time. This has been achieved by using new instrumentation to allow conventional SThM probes to measure and manipulate the temperature of non-biological and biological samples. For the latter, the aqueous environment is crucial to allow in-vitro experimentation, which is important for the future use of SThM in the life sciences. SThM is known to be a powerful technique able to acquire simultaneous topographic and thermal images of samples. It is able to measure the microscopic thermal properties of a surface with nanoscale spatial resolution. However, SThM has traditionally been limited to use in vacuum, air and electrically inert liquids. The aqueous Scanning Thermal Microscopy (a-SThM) described in this thesis is an entirely novel technique that opens up a new field for thermal-AFM. The first challenge addressed in this work was the adaptation of a commercial Multimode Nanoscope IIIa AFM to permit electrical access to a SThM probe completely immersed in aqueous solutions. By employing a newly designed probe holder and electronic instrumentation, the probe could then be electrically biased without inducing electrochemical reactions. This approach permitted conventional microfabricated thermal probes to be operated whilst fully immersed in water. This innovation allowed SThM measurements under deionized (DI) water to be performed on a simple solid sample (Pt on Si3N4) and the results compared with in-air scans and accurate 3D Finite Element (FE) simulations. Once the validity of the technique was proven, its performance was investigated, including crucially the limit of its thermal-spatial resolution; this was investigated using nanofabricated solid samples (Au on Si3N4) with well-defined features. These results were compared to the FE model, allowing an understanding of the mechanisms limiting resolution to be developed. In order to demonstrate the advantages granted by the water’s superior thermal conductivity compared to air or other liquids, non-contact thermal images were also acquired using the same samples. The final part of this thesis was focused on extending SThM into the biological area; a completely new field for this technique. New results are presented for soft 4 samples: I-collagen gel and collagen fibrils, which were thermally manipulated using a self-heated SThM probe. This successfully demonstrated the possibility of using heat to alter a biological sample within a very well localised area while being operated for long time in an aqueous environment. The difference in force response originated from the AFM scans with different levels of self-heating further proved the robustness of the technique. Finally, the technique was employed to study MG-63 living cells: The SThM probe was left in contact with each cell for a pre-determined period of time, with and without self heating. The results demonstrated that only the heated cells, directly beneath the probe tip died, tallying with the highly localised temperature gradient predicted by FE analysis

Computer-Integrated Design and Manufacture of Integrated Circuits

Contains research goals and objectives, reports on sixteen research projects and a list of publications.Defense Advanced Research Projects Agency/U.S. Navy Contract N00174-93-K-0035Defense Advanced Research Projects Agency/U.S. Army Contract DABT 63-95-C-0088Multisponsored Projects Industrial/MIT Leaders for Manufacturing Progra