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.

Fabrication and reliability demonstration of 5μm redistribution layer using low-stress dielectric dry film

Abstract: Low-stress and low-warpage dielectrics are gaining importance as we move towards large-body Multi-chip Modules (MCMs). This paper demonstrates fabrication of redistribution layer (RDL) with 5 μm linewidth/spacing using a novel low-stress dielectric dry film- PLSF (Panasonic low stress film) as a build-up layer. Microvias formation down to 7 μm diameter has also been demonstrated in this paper. The main feature of PLSF is its low- stress and low-warpage. In this paper, we have studied residual stress characteristics of PLSF and its comparison with the industry-standard dielectric (ISD). The lower tensile modulus of PLSF as compared to ISD results in significantly lower residual stress and warpage on the substrate. Electrical and thermomechanical reliability of RDL with PLSF as build-up layer has also been studied and the results are presented in this paper.

Ultra-thin, self-healing decoupling capacitors on thin glass interposers using high surface area electrodes

Abstract: This paper presents a novel set of low-cost materials and processes to fabricate thinfilm decoupling capacitors on silicon and glass substrates, using printed valve-metal electrodes. The valve-metals such as Al and Ta allow for easy formation of conformal, robust and high insulation-strength dielectrics. By utilizing self-healing counter electrodes, high yield with low leakage currents were demonstrated. Such a thinfilm capacitor processing is compatible with large-area, high through-put glass substrates with through-vias. The process, therefore, allows double-side passive or active component integration in ultrathin substrates, leading to a unique and novel passive and active component integration technology referred to as 3D IPAC (3-Dimensional Integrated Actives and Passive components) for miniaturized and complete power or RF modules.

A ceramic substrate having a protective coating and method of protection

Abstract: Disclosed is a ceramic substrate having a protective coating on at least one surface thereof which includes: a ceramic substrate (52) having at least one electrically conductive via (54) extending to a surface of the substrate; an electrically conductive I/O pad (56) electrically connected to at least one of the vias; an I/O pin (58) brazed to the I/O pad (56), the brazed pin having a braze fillet (60); and a protective layer of polymeric material (62) fully encapsulating the I/O pad, wherein the layer of polymeric material protects the I/O pad, from corrosion. Also disclosed is a method of protecting a ceramic substrate from corrosion, the ceramic substrate of the type having a plurality of electrically conductive vias extending to a surface of the substrate, a multilayer metallic I/O pad electrically connected to at least one of the vias, and an I/O pin brazed to the I/O pad, the brazed pin having a braze fillet, the method comprising the step of: encapsulating fully the I/O pad with a protective layer of polymeric material wherein the layer of polymeric material protects the I/O pad from corrsoion. In a preferred embodiment, the I/O pin is selectively exposed to plasma ashing to remove any errant polymeric material from the pin shank, thereby assuring electrical contact to the pin shank.

Modeling, design, and demonstration of low-temperature Cu interconnections to ultra-thin glass interposers at 20 μm pitch

Abstract: This paper reports the first design and demonstration of a manufacturable 20 µm pitch Cu interconnection technology to ultra-thin glass interposers. Bonding is accomplished at temperatures below 200 o C without the need for solders. Manufacturability challenges such as substrate warpage, bump noncoplanarity and assembly throughput with low bonding times are addressed with this technology. The modeling and experimental results indicate that the ultra-fine pitch Cu interconnection offsets more than 3 µm non-coplanarity. Bonding interfaces were characterized to show that metallurgical bonding microstructure is formed even with a bonding time of 5 seconds, with superior electrical properties. A mechanism for low-temperature metallurgical bonding is proposed based on the characterization results.

Synthesis of electromagnetic band gap structures using stepped impedance resonators

Abstract: An approach to synthesize electromagnetic band gap (EBG) structures which offer isolation in the desired frequency band is proposed. The methodology treats EBGs as stepped impedance resonators (SIRs) and predicts the fundamental band gap of EBGs from the resonance condition of SIRs. The methodology is validated through simulations and measurements. © 2011 Wiley Periodicals, Inc. Microwave Opt Technol Lett 53:2378–2382, 2011; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.26249

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.