All-copper chip-to-substrate interconnects for high performance integrated circuit devices

In this work, all-copper connections between silicon microchips and substrates are developed. The semiconductor industry advances the transistor density on a microchip based on the roadmap set by Moore's Law. Communicating with a microprocessor which has nearly one billion transistors is a daunting challenge. Interconnects from the chip to the system (i.e. memory, graphics, drives, power supply) are rapidly growing in number and becoming a serious concern. Specifically, the solder ball connections that are formed between the chip itself and the package are challenging to make and still have acceptable electrical and mechanical performance. These connections are being required to increase in number, increase in power current density, and increase in off-chip operating frequency. Many of the challenges with using solder connections are limiting these areas. In order to advance beyond the limitations of solder for electrical and mechanical performance, a novel approach to creating all-copper connections from the chip-to-substrate has been developed. The development included characterizing the electroless plating and annealing process used to create the connections, designing these connections to be compatible with the stress requirements for fragile low-k devices, and finally by improving the plating/annealing process to become process time competitive with solder. It was found that using a commercially available electroless copper bath for the plating, followed by annealing at 180 C for 1 hour, the shear strength of the copper-copper bond was approximately 165 MPa. This work resulted in many significant conclusions about the mechanism for bonding in the all-copper process and the significance of materials and geometry on the mechanical design for these connections.Ph.D.Committee Chair: Kohl, Paul; Committee Member: Bidstrup Allen, Sue Ann; Committee Member: Fuller, Thomas; Committee Member: Hesketh, Peter; Committee Member: Hess, Dennis; Committee Member: Meindl, Jame

Ultra thin ultrafine-pitch chip-package interconnections for embedded chip last approach

Ever growing demands for portability and functionality have always governed the electronic technology innovations. IC downscaling with Moore s law and system miniaturization with System-On-Package (SOP) paradigm has resulted and will continue to result in ultraminiaturized systems with unprecedented functionality at reduced cost. The trend towards 3D silicon system integration is expected to downscale IC I/O pad pitches from 40µm to 1- 5 µm in future. Device- to- system board interconnections are typically accomplished today with either wire bonding or solders. Both of these are incremental and run into either electrical or mechanical barriers as they are extended to higher density of interconnections. Alternate interconnection approaches such as compliant interconnects typically require lengthy connections and are therefore limited in terms of electrical properties, although expected to meet the mechanical requirements. As supply currents will increase upto 220 A by 2012, the current density will exceed the maximum allowable current density of solders. The intrinsic delay and electromigration in solders are other daunting issues that become critical at nanometer size technology nodes. In addition, formation of intermetallics is also a bottleneck that poses significant mechanical issues. Recently, many research groups have investigated various techniques for copper-copper direct bonding. Typically, bonding is carried out at 400oC for 30 min followed by annealing for 30 min. High thermal budget in such process makes it less attractive for integrated systems because of the associated process incompatibilities. In the present study, copper-copper bonding at ultra fine-pitch using advanced nano-conductive and non-conductive adhesives is evaluated. The proposed copper-copper based interconnects using advanced conductive and non-conductive adhesives will be a new fundamental and comprehensive paradigm to solve all the four barriers: 1) I/O pitch 2) Electrical performance 3) Reliability and 4) Cost. This thesis investigates the mechanical integrity and reliability of copper-copper bonding using advanced adhesives through test vehicle fabrication and reliability testing. Test vehicles were fabricated using low cost electro-deposition techniques and assembled onto glass carrier. Experimental results show that proposed copper-copper bonding using advanced adhesives could potentially meet all the system performance requirements for the emerging micro/nano-systems.M.S.Committee Chair: Prof. Rao R Tummala; Committee Member: Dr. Jack Moon; Committee Member: Dr. P M Ra

SILICON ON INSULATOR BIPOLAR JUNCTION TRANSISTORS FOR FLEXIBLE MICROWAVE APPLICATIONS

Microwave frequency flexible electronic devices require a high quality semiconducting material and a set of fabrication techniques that are compatible with device integration onto flexible polymer substrates. Over the past ten years, monocrystalline silicon nanomembranes (SiNMs) have been studied as a flexible semiconducting material that is compatible with industrial Si processing. Fabricated from commercial silicon on insulator (SOI) wafers, SiNMs can be transferred to flexible substrates using a variety of techniques. Due to their high carrier mobilities, SiNMs are a promising candidate for flexible microwave frequency devices. This dissertation presents fabrication techniques for flexible SiNM devices in general, as well as the progress made towards the development of a microwave frequency SiNM bipolar junction transistor (BJT). In order to overcome previous limitations associated with adhesion, novel methods for transfer printing of metal films and SiNMs are presented. These techniques enable transfer printing of a range of metal films and improve the alignment of small transfer printed SiNM devices. Work towards the development of a microwave frequency BJT on SOI for SiNM devices is also described. Utilizing a self-aligned polysilicon sidewall spacer technique, a BJT with an ultra-narrow base region is fabricated and tested. Two regimes of operation are identified and characterized under DC conditions. At low base currents, devices exhibited forward current gain as high as βF = 900. At higher base current values, a transconductance of 59 mS was observed. Microwave scattering parameters were obtained for the BJTs under both biasing conditions and compared to unbiased measurements. Microwave frequency gain was not observed. Instead, bias-dependent non-reciprocal behavior was observed and examined. Limitations associated with the microwave impedance-matched electrode configuration are presented. High current densities in the narrow electrodes cause localized heating, which leads to electrode material damage and ultimately dopant diffusion in the BJT. Finally, device design improvements are proposed to address the problem of localized heating and increase device lifetime under testing conditions. High values for DC current gain suggest that future modifications should improve microwave frequency performance and measurement reproducibility

Multiscale characterization of ferroelastic deformation in ceramic materials

Ceramic materials offer a variety of useful properties that make them desirable for a wide range of engineering applications, however, ceramics are limited in their utility by low toughness. Ferroelastic deformation provides a mechanism through which ceramics are intrinsically toughened, but the effect of microstructure on the deformation behavior has yet to be fully understood. In this present examination, the behavior of ferroelastic deformation was evaluated on a range of length scales, specifically highlighting the influence of several variables on the domain nucleation behavior. Ferroelastic domain nucleation was first evaluated in micro-scale single crystals. The stress required for domain nucleation was measured while crystal orientation was tracked. Domain nucleation was observed to not follow a critical resolved shear stress criterion, suggesting that orientation alone cannot be used to predict the deformation behavior. Furthermore, multiple types of deformation were observed to act in concert with ferroelastic deformation. This suggests that domain nucleation is a complex process that may involve multiple potential mechanisms of deformation. Domain nucleation in bulk polycrystals was also examined. Statistics collected on grain sizes that more frequently contain mechanically nucleated domains show that larger grains in close proximity to finer grains more frequently deform. The deformation behavior in polycrystals was contrasted against the domain nucleation behavior in single crystal nanopillars. The nanopillars exhibited high deformation stress, while prolific domain nucleation without fracture was observed in polycrystals. These results suggest that local constraints imposed by microstructure play a key role in locally increasing shear stresses responsible for domain nucleation. To design microstructures with specific characteristics, ceramic processing routes must also be developed to control microstructural development during fabrication. To this end, spark plasma sintering (SPS) offers a promising processing route for fabricating dense nanostructured ceramics. The densification mechanisms associated with ceramic processing using SPS have also been investigated in the present work. Results collected on many samples that were processed under identical processing control conditions convey significant variability in the resulting material properties between and within individually produced samples. Furthermore, the results indicate that electric current plays an important role in densifying ionic conducting ceramics during sintering using SPS. Overall, the research presented in this dissertation shows that ferroelastic domain nucleation is a complex process involving several competing and cooperating mechanisms, and that domain nucleation is affected by different microstructural variables. Domain nucleation cannot be predicted based solely on crystal orientation, however, other microstructural variables including grain size do significantly impact the ferroelastic deformation behavior. Microstructures with large ferroelastic grains embedded in a more finely grained matrix promote ferroelastic deformation even without fracture, and the deformation is sensitive to the stress state being applied. Several processing routes presented here result in these favorable bimodal grain size distributions and may be tested more thoroughly in the future to explore the effect that such microstructures have on the intrinsic toughness

Electron beam induced deposition (EBID) of carbon interface between carbon nanotube interconnect and metal electrode

Electron Beam Induced Deposition (EBID) is an emerging additive nanomanufacturing tool which enables growth of complex 3-D parts from a variety of materials with nanoscale resolution. Fundamentals of EBID and its application to making a robust, low-contact-resistance electromechanical junction between a Multiwall Carbon Nanotube (MWNT) and a metal electrode are investigated in this thesis research. MWNTs are promising candidates for next generation electrical and electronic devices, and one of the main challenges in MWNT utilization is a high intrinsic contact resistance of the MWNT-metal electrode junction interface. EBID of an amorphous carbon interface has previously been demonstrated to simultaneously lower the electrical contact resistance and to improve mechanical characteristics of the MWNT-electrode junction. In this work, factors contributing to the EBID formation of the carbon joint between a MWNT and an electrode are systematically explored via complimentary experimental and theoretical investigations. A comprehensive dynamic model of EBID using residual hydrocarbons as a precursor molecule is developed by coupling the precursor mass transport, electron transport and scattering, and surface deposition reaction. The model is validated by comparison with experiments and is used to identify different EBID growth regimes and the growth rates and shapes of EBID deposits for each regime. In addition, the impact of MWNT properties, the electron beam impingement location and energy on the EBID-made carbon joint between the MWNT and the metal electrode is critically evaluated. Lastly, the dominant factors contributing to the overall electrical resistance of the MWNT-based electrical interconnect and relative importance of the mechanical contact area of the EBID-made carbon joint to MWNT vs. that to the metal electrode are determined using carefully designed experiments.Ph.D.Committee Chair: Dr. Andrei G. Fedorov; Committee Member: Dr. Azad Naeemi; Committee Member: Dr. Suresh Sitaraman; Committee Member: Dr. Vladimir V. Tsukruk; Committee Member: Dr. Yogendra Josh

Large Hybrid High Precision MEMS Mirrors

This thesis presents the design and microfabrication technology of a tip-tilt, 2 degree of freedom "mirror system" made out of Silicon On Insulator (SOI) wafers with two movable mirrors that have diameter of 10 mm. The system was intended for precise beam steering applications and tracking for inter-satellite telecommunication. Implementation of two mirrors allowed one mirror to have large static mechanical scan angles (±3.5°) and the other to have fast fine pointing capabilities within ±0.2° static mechanical scan angle. The first mirror was magnetically actuated and has a resonance frequency of 200 Hz and the large rotation angle. The second mirror was designed to use either electrostatic actuation or electromagnetic actuation. The resonance frequency is 1 KHz. The main objective of this thesis was the microfabrication of large mirrors (10 mm) and actuators. The study of upper limits of achievable resonant frequencies in combination with static deflection was another major aim of the project, as well as optimization of the flatness of the mirrors. The undertaken research presents a study of different non-contact actuation schemes for use in MOEMS devices, namely electrostatic and magnetic actuation schemes. Determination of the best magnetic actuation scheme for use in this thesis was carried out according to the design constraints and size limitations. It was concluded that the best magnetic actuation scheme consists of a moving magnet and a microfabricated stationary coil. This scheme created high force and the stationary microfabricated coils could easily dissipate heat through conduction into a heat sink. This configuration was of great advantage in space where heat convection does not exist. The considered electrostatic schemes included parallel plate actuators and vertical comb actuators. The vertical comb actuator was chosen to be fabricated due to its scalability that allows the microfabrication process to be easily adapted to smaller or larger devices. The microfabrication and implementation of the magnetically actuated mirrors was proven to be very straightforward resulting in high yield and uniformity of both the high and low frequency devices. The microfabrication of the electrostatic actuator was also achieved successfully. The operation performance of the electrostatically actuated mirror was not as satisfactory as the magnetically actuated mirrors. It was concluded that electrostatic actuation might be more suited to smaller devices, rather than the devices designed in the present study. The fabrication and actuation performance of the components of a "mirror system" that combines fast and large angle actuation was successfully demonstrated in this thesis. Very good mirror flatnesses were also obtained with RMS flatness of λ/10 for near infrared wavelengths. This project shows the general advantage of magnetic actuation for large MOEMS devices with large actuation ranges and proves the feasibility of their fabrication. Potential applications of these devices are robotic 3D vision, imaging LIDARs (Light Detection And Ranging), docking sensors and inter-satellite laser communications

METAL MOLD FABRICATION BY PROTON BEAM WRITING AND ITS APPLICATIONS

Ph.DDOCTOR OF PHILOSOPH

Electroplating of Nanostructures

The electroplating was widely used to electrodeposit the nanostructures because of its relatively low deposition temperature, low cost and controlling the thickness of the coatings. With advances in electronics and microprocessor, the amount and form of the electrodeposition current applied can be controlled. The pulse electrodeposition has the interesting advantages such as higher current density application, higher efficiency and more variable parameters compared to direct current density. This book collects new developments about electroplating and its use in nanotechnology