Rao Tummala

Bio: Rao Tummala is an academic researcher from Georgia Institute of Technology. The author has contributed to research in topics: Interposer & Capacitor. The author has an hindex of 43, co-authored 623 publications receiving 11663 citations. Previous affiliations of Rao Tummala include Qualcomm & IBM.

Ultra fine-pitch wafer level packaging with reworkable composite nano-interconnects

Abstract: The decrease in feature sizes of micro-electronic devices has underlined the need for higher number of I/O's in order to increase its functionality This has spurred a great interest in developing electronic packages with fine and ultra fine pitches (20-100 microns) Most of the compliant interconnects that are currently being developed have inductance and resistance higher than desirable This work presents a novel low-temperature fabrication process that combines polymer structures with electroless copper plating to create low stress MEMS structures for extremely fine pitch wafer level packages Finite element analysis of these structures shows tremendous reduction in the stresses at the interfaces and superior reliability as IC-package nano interconnects Low CTE polyimide structures with ultra-low stress, high toughness and strength were fabricated using plasma etching This dry etching process was tuned to yield a wall angle above 80 degrees The etching process also leads to roughened sidewalls for selective electroless copper plating on the sidewalls of polymer structures Metal-coated polymer structures from MEMS fabrication techniques can provide low-cost high-performance solutions for wafer-level-packaging This work also describes a material solution synthesis route to develop reworkable nano-dimensional interfaces for IC-package bonding Reworkability is addressed by a thin (200 nm) interface of lead-free high-strength solders using selective electroless plating Lead-free alloy films were deposited from aqueous plating solutions consisting of suitable metal salts and reducing agents at 45/spl deg/C The lead-free solder composition was controlled by altering the plating bath formulation and was characterized using SEM, XRD and XPS Solder film formed from the above approach was demonstrated to bond the metal-coated polymer interconnects with the copper pads on the substrate

Evaluation of alternative materials for system-on-package (SOP) substrates

Abstract: This work deals with the selection and evaluation of new board materials that can enable microminiaturized, multilayered multifunctional system-level packaging such as system-on-package (SOP) pursued by the Packaging Research Center, Georgia Institute of Technology. The effect of board properties such as coefficient of thermal expansion (CTE) and high elastic modulus upon the increase in flip chip reliability performance was investigated and guidelines for optimum board properties for realizing SOP were suggested based on the reliability results. Three different types of carbon fiber reinforced composites boards and two different kinds of inorganic boards were tested to evaluate the improvement in flip chip reliability compared to conventional glass fabric-epoxy boards (FR-4). Short carbon fiber, unidirectional carbon fiber, and carbon fiber-cloth reinforced polymer composites with a CTE close to 3 ppm//spl deg/C and three to nine times higher modulus than conventional FR-4 were selected as polymer-based composite boards. Metal matrix composites with moderate CTE ( Al/SiC; 8 ppm//spl deg/C) and ceramics with lower CTE (AlN, 4 ppm//spl deg/C) were selected as inorganic boards with high stiffness. The thermomechanical reliability of the electrical interconnections was evaluated with flip-chips assembled on five different boards by subjecting them to thermal shock treatments. Except AlN, all test vehicles fabricated with low CTE boards did not show any improvement in reliability even with underfill though they have matched CTE with Si. This is attributed to the higher dielectric stresses caused by the increased CTE mismatch between the boards and the build-up dielectric. The stress in dielectric leads to severe cyclic warpage of boards during heat cycle, resulting in dielectric fatigue cracking. With this mechanism, even underfill cannot improve the reliability. On the contrary, AlN board with highest modulus and matched CTE with Si significantly enhanced the reliability even without underfill because of its high modulus preventing cyclic warpage during thermal cycling. A new type of failure mechanism was proposed based on the optical microscopic failure mode analysis as well as analytical modeling of stress induced in the solder joints and dielectric layer. In-situ warpage of test vehicles during thermal cycling was measured in order to confirm the proposed failure mechanism. It can be concluded that to enhance the flip chip reliability without underfill, it is necessary to have high elastic modulus along with Si-matched CTE. Ultra-high stiffness is an important requirement for developing new board materials that can realize SOP concept.

Conductive Anodic Filament Failures in Fine-Pitch Through-Via Interconnections in Organic Package Substrates

Abstract: Failures due to conductive anodic filament (CAF) formation in copper-plated through-vias have been a concern in printed wiring boards since the 1970s. With the continuous reduction in through-via pitch to meet high circuit density demands in organic packages, the magnitude of CAF failures is expected to be significantly higher. In this study, an accelerated test condition [130°C, 85% relative humidity (RH), and 100 V direct current (DC)] was used to investigate CAF in two organic package substrates: (1) cyclo-olefin polymer–glass fiber composite (XR3) and (2) epoxy–glass fiber composite (FR4). Test coupons with through-via spacing of 100 μm and 200 μm were investigated in this study. CAF failures were not observed in either substrate type with spacing of 200 μm. With spacing of 100 μm, insulation failures were observed in FR4, while XR3 exhibited stable insulation resistance during the test. The substrates were characterized using gravimetric measurement, and XR3 was found to exhibit significantly lower moisture absorption compared with FR4. The CAF failures in FR4 were characterized using scanning electron microscopy and energy-dispersive x-ray spectroscopy. The results suggest a strong effect of moisture sorption of organic resins on CAF failure at smaller through-via spacing in package substrates.

Systems and methods for fabricating high-density capacitors

Abstract: The present invention describes systems and methods for fabricating high-density capacitors. An exemplary embodiment of the present invention provides a method for fabricating a high-density capacitor system including the steps of providing a substrate and depositing a nanoelectrode particulate paste layer onto the substrate. The method for fabricating a high-density capacitor system further includes sintering the nanoelectrode particulate paste layer to form a bottom electrode. Additionally, the method for fabricating a high-density capacitor system includes depositing a dielectric material onto the bottom electrode with an atomic layer deposition process. Furthermore, the method for fabricating a high-density capacitor system includes depositing a conductive material on the dielectric material to form a top electrode.

I and i

Abstract: There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality. Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

Microfibre–nanowire hybrid structure for energy scavenging

Abstract: Nanodevices don't use much energy, and if the little they do need can be scavenged from vibrations associated with foot steps, heart beats, noises and air flow, a whole range of applications in personal electronics, sensing and defence technologies opens up. Energy gathering of that type requires a technology that works at low frequency range (below 10 Hz), ideally based on soft, flexible materials. A group working at Georgia Institute of Technology has now come up with a system that converts low-frequency vibration/friction energy into electricity using piezoelectric zinc oxide nanowires grown radially around textile fibres. By entangling two fibres and brushing their associated nanowires together, mechanical energy is converted into electricity via a coupled piezoelectric-semiconductor process. This work shows a potential method for creating fabrics which scavenge energy from light winds and body movement. A self-powering nanosystem that harvests its operating energy from the environment is an attractive proposition for sensing, personal electronics and defence technologies1. This is in principle feasible for nanodevices owing to their extremely low power consumption2,3,4,5. Solar, thermal and mechanical (wind, friction, body movement) energies are common and may be scavenged from the environment, but the type of energy source to be chosen has to be decided on the basis of specific applications. Military sensing/surveillance node placement, for example, may involve difficult-to-reach locations, may need to be hidden, and may be in environments that are dusty, rainy, dark and/or in deep forest. In a moving vehicle or aeroplane, harvesting energy from a rotating tyre or wind blowing on the body is a possible choice to power wireless devices implanted in the surface of the vehicle. Nanowire nanogenerators built on hard substrates were demonstrated for harvesting local mechanical energy produced by high-frequency ultrasonic waves6,7. To harvest the energy from vibration or disturbance originating from footsteps, heartbeats, ambient noise and air flow, it is important to explore innovative technologies that work at low frequencies (such as <10 Hz) and that are based on flexible soft materials. Here we present a simple, low-cost approach that converts low-frequency vibration/friction energy into electricity using piezoelectric zinc oxide nanowires grown radially around textile fibres. By entangling two fibres and brushing the nanowires rooted on them with respect to each other, mechanical energy is converted into electricity owing to a coupled piezoelectric–semiconductor process8,9. This work establishes a methodology for scavenging light-wind energy and body-movement energy using fabrics.

Self-powered nanowire devices.

Abstract: The lateral and vertical integration of ZnO piezoelectric nanowires allows for voltage and power outputs sufficient to power nanowire-based sensors.