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.

Microelectronic package and semiconductor package circuits

Abstract: PROBLEM TO BE SOLVED: To provide a stress relief barrier that provides thermal expansion and contraction stress relief and better metallization capabilities, which covers glass based interposer surfaces and/or through via walls.SOLUTION: In a method, a stress relief barrier 112 can be used to reduce effects of stress caused by different coefficients of thermal expansion, and acts as an adhesion promoter between a metallization and an interposer. Further, the stress relief barrier acts to absorb some of the stress caused by the different coefficients of thermal expansion and promotes adhesion to a conductive metal layer.SELECTED DRAWING: Figure 1

Next Generation High Dk, Low Df Organic Laminate for RF Modules and High Frequency Applications

Abstract: In this paper, we present a novel high density high performance ultra-thin organic laminate, X-R-1, with low cost standard PCB fabrication processes for RF and high frequency applications. The X-R-1 substrate, developed at Zeon Corporation is a new generation halogen-free high dielectric constant (Dk) and low loss tangent (Df) dielectric laminate material. Its dielectric constant is 6.5–6.7 in the range of 1–20GHz, similar to typical LTCC substrates but larger than most organic materials such as LCP, PTFE and Epoxy based materials. Its dielectric loss tangent is 0.003, similar to that of LTCC, LCP and PTFE but much lower than epoxy based materials. The thicknesses of the core used in this study are 50um. The clad copper on both sides is a 12um thick profile-free copper foil which provides extremely smooth surface. Microminiaturization of RF devices can be achieved by the combination of high Dk and ultra-thin substrate. The combination of low Df and smooth surface leads to RF and high frequency signals hav...

Advances in Panel Scalable Planarization and High Throughput Differential Seed Layer Etching Processes for Multilayer RDL at 20 Micron I/O Pitch for 2.5D Glass Interposers

Abstract: This paper describes the improvement of advanced semi-additive processes (SAP) to demonstrate 1.5-5 µm lines and spaces with 4-5 µm diameter photo-vias for multiple re-distribution layers (RDL) at 20 µm bump pitch on glass interposers. High performance computing systems for networking and graphics are driving ultra-high bandwidth interconnections between logic and memory devices. This signal bandwidth need with lowest power consumption has enabled the application of 2.5D interposers for high density chip-to-chip interconnections. Silicon interposers with through-silicon-vias (TSVs) are capable of ultra-high density wiring between logic and memory chips, but use back end of line (BEOL) dual damascene processes, requiring chemical mechanical polishing (CMP), leading to high process cost, which limits their expansion into lower cost and higher volume applications. On the other hand, organic substrates processed on large panels have large capture pads for via landing due to their poor dimensional stability, limiting the bump pitch scaling at chip level. Glass interposers have been proposed to address the limitations of both silicon interposers and organic substrates in recent years. This paper reports on research to extend low cost and large panel semi-additive processes (SAP) to below 5um lines and vias. To achieve this, high resolution lithography processes combined with photosensitive dry film polymer dielectrics were optimized to form fine patterns and ultra-small micro-vias. A major challenge for multilayer RDL is the non co-planarity of copper electroplating, and a new cost-effective copper surface planarization process was proposed and evaluated for surface co-planarity improvement, leading to better yields for multi-layer RDL fabrication.

Suppression of Vertical Electromagnetic Coupling in Multilayer Packages

Abstract: This paper proposes an effective approach to suppress vertical electromagnetic (EM) coupling in multilayer packages operating at GHz frequencies. In the case of packages with embedded actives where there are large apertures (die sized) in the metal planes and cavities in dielectric layers to accommodate the chips, the effect of EM field coupling is significant. The method involves EM band-gap structures for suppressing vertical coupling and the isolation band can be tuned over different frequency ranges. In addition, this paper puts forth a methodology for predicting the frequency range of the isolation band achieved by the coupling suppression technique. The proposed methodology is demonstrated through simulations and measurements.

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.