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.

Design of Low-Profile Integrated Transformer and Inductor for Substrate-Embedding in 1-5kW Isolated GaN DC-DC Converters

Abstract: This paper presents the design analysis of low-profile transformers in 1-5kW DC-DC resonant converters. Recent advances in GaN devices are expected to improve the efficiency and power density of power converters, while the magnetics still remain the major bottleneck to miniaturization. A low-profile integrated magnetic component design that enables low-loss and high-power density is proposed for 400V/48V DC-DC resonant converters with 1kW output power. The winding loss, core loss, and power density of the novel magnetic component design is analyzed and compared with the conventional design approach using planar cores. Higher power density and lower loss are demonstrated with the novel design approach.

Insulation reliability of fine-pitch through-vias in glass fiber reinforced halogen-free epoxy substrates

Abstract: Insulation failures from electrochemical migration is a major reliability concern for achieving reliable small conductor spacings in glass fiber reinforced substrates in the presence of humidity and DC bias voltage. In this study, insulation reliability of fine-pitch copper plated-through-vias in two different halogen-free epoxy substrates was investigated using accelerated testing condition (85 °C, 85 % RH and 100 V DC). The test vehicles included two different conductor geometry: (1) through-via to through-via (spacing: 100 and 150 μm) and (2) through-via to surface-trace (spacing: 75 μm). In accelerated testing, the through-via to through-via test vehicles exhibited insulation failures (failure criterion: 1 MΩ) with a strong dependence on via spacing with through-via spacing of 100 μm showing significantly shorter time to failures compared to test vehicles with spacing of 150 μm. Failure analysis revealed cracking in resin-glass fiber interfaces and within the resin matrix between the failed through-vias. The through-via to surface-trace test vehicles, on the other hand, did not exhibit failures based on the 1 MΩ criterion. However, occurrence of electrochemical migration was visible after optical inspection of the test vehicles. Elemental characterization revealed the presence of copper and chlorine in the resin-glass fiber interface, similar to the previously reported chloride-containing conductive anodic filament compound in printed wiring boards. Accelerating testing and failure analysis in this study indicates a strong dependence of insulation reliability on conductor spacing, geometry and substrate material properties.

Sputtered Ti-Cu as a superior barrier and seed layer for panel-based high-density RDL wiring structures

Abstract: This paper demonstrates that sputtered Ti-Cu is a superior barrier and seed layer on glass and organic panel substrates, over traditional electroless seeding, for the fabrication of ultra-fine copper traces (2–5µm) on dry film polymer dielectrics for high-density 2.5D interposers. The current semi-additive processes using electroless Cu seed face several challenges in scaling the copper trace widths below 5µm due to two main reasons: high-roughness of dielectric and high-thickness of copper seed. In this paper, both the above limitations are addressed by an advanced Physical Vapor Deposition (PVD) process that can be scaled to large panels with high throughputs. The PVD process developed in this study is capable of depositing Ti-Cu barrier and seed layer on 500 mm size panels at a low enough temperature for dry film polymer dielectrics of glass transition temperatures (T g ) of 150–160°C. The superiority of sputtered Ti-Cu over the conventional electroless Cu seeding for achieving good and reliable adhesion between Cu and dry film polymer dielectrics was investigated by peel strength measurements after highly-accelerated stress tests (HAST). The results indicate that sputtered process results in higher peel strengths and without adhesive failures at the Ti-Cu-polymer interfaces. Adhesive failures, however, were observed with the traditional electroless seed processes. In addition, the PVD processes resulted in small 2–5µm Cu traces on smooth dielectric films like ZS-100, requiring no desmear treatment. Such a process promises to be scalable to large panels leading to low-cost fabrication of high-density 2.5D interposers.

Interposer with polymer-filled or polymer-lined optical through-vias in thin glass substrate

Abstract: An optical interposer that includes a glass substrate having one or more optical vias extending through the glass substrate. A first optical polymer may be bonded to the substrate and to interior surfaces of the one or more optical vias. Implementations include one or more optical via cores comprising a second optical polymer that has a greater refractive index than the first optical polymer. The one or more optical via cores may be at least partially surrounded by the first optical polymer. Embodiments include encapsulated optical waveguides in communication with the optical vias and/or via cores. Example implementations include layers of electrical insulation, electrical traces, and electrical vias. A method of manufacture includes forming the optical vias by laser ablation. Certain embodiments may include chemically etching the inside of the vias to improve surface roughness.

High-Voltage Capacitors for Next-Generation Power Modules in Electric Vehicles

Abstract: This paper describes an innovative approach to develop high density and ultra-thin solid aluminum capacitors for high voltage automotive applications that sustain high temperatures. Form factors of less than 100 µm thickness are achieved with densities of 16.85, 1.91, 1.13, and 0.705 µF/cm2, for capacitors anodized to 10, 100, 150, and 200 V respectively. The capacitors exhibit good frequency stability up to 100 kHz and demonstrate the first aluminum capacitors to come in such a thin format. These capacitors can be used in new era automotive electronics where high density and high voltage stability is critical.

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.