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.

First Demonstration of Panel Glass Fan-Out (GFO) Packages for High I/O Density and High Frequency Multi-chip Integration

Abstract: Ultra-thin, panel-level glass fan-out packages (GFO) were demonstrated for next-generation fan-out packaging with high-density high-performance digital, analog, power, RF and mm-wave applications. The key advances with GFO include: 1) large area panel-scalable glass substrate processes with lower cost, 2) silicon-like RDL on large panels with 1-2 µm critical dimensions (CD), 3) lower interconnect loss and 4) improved board-level reliability enabled by the tailorability of the CTE of the glass panels and compliant interconnections. Daisy-chain test dies were used to emulate an embedded device with the size of 6.469 mm × 5.902 mm, thickness of 75 µm and pad pitch of 65 µm. Glass panels with 70 µm thickness and through-glass cavities were first fabricated, and then bonded onto a 50 µm thick glass panel carrier using adhesives. After glass-to-glass bonding, the test dies were assembled into the glass cavities using a high-speed placement tool. RDL polymers were then laminated onto both sides and cured to minimize the warpage of the ultra-thin package. A surface planar tool was then used to planarize the surface of the panel to expose the copper microbumps on the die, followed by a standard semi-additive process (SAP) for the fan-out RDL layer. The shift and warpage of the die were characterized during the multiple process steps. Initial modeling and measured results indicate the potential for less than 5 µm die shift and less than 10-15 µm warpage across a 300 mm × 300 mm panel size.

Nanostructured miniaturized artificial magnetic conductors (AMC) for high-performance antennas in 5G, IoT, and smart skin applications

Abstract: The emergence of fan-out packaging for 5G and IoT applications has brought escalating performance concerns that arise from the proximity of radiating components such as antennas to lossy materials such as metals and silicon. These concerns also arise in wearable (“smart skin”) electronics where the human tissues act as the lossy substrate. Artificial magnetic conductors (AMC) are widely explored for enhancing the performance of antennas that are in close proximity to metallic surfaces and other lossy substrates such as silicon and human tissue. High-permittivity or high-permeability materials can be used to significantly reduce the sizes of AMCs. Nanostructured materials provide unique opportunities to provide stable properties up to the GHz and mm-Wave frequency ranges. In this paper, novel miniaturized nanostructured AMCs utilizing Barium Strontium Titanate (BST) thinfilms and ceramic-polymer composites are demonstrated through full-wave modeling analysis. The size reduction rates are 50.6 % when using 3 μm of BST thinfilm and 77 % when using 400 μm of ceramic-polymer composites. This concept can be further extended with high-permeability magnetic films, and thicker nanocomposite films to achieve further size and performance improvement for a variety of frequency bands and applications.

Determination of propagation constants of transmission lines using 1-port TDR measurements

Abstract: The propagation constants of transmission lines were measured from 1-port TDR measurements. Since the TDR measurement is a 1-port measurement, error can be smaller than 2-port measurement techniques. Moreover, the available frequency is determined by the rise time of the TDR step pulse unlike TRL methods. The propagation constant of a lossy transmission line was extracted from DC to 10 GHz. Simulation of the lossy transmission line using the extracted propagation constant shows good agreement with TDR measurement, demonstrating the accuracy of the TDR measurement technique.

Electrical Modeling and Analysis of Tapered Through-Package via in Glass Interposers

Abstract: This paper proposes a wideband scalable circuit model for tapered through-package vias (TPVs) in glass interposers. By slicing TPVs horizontally into infinitesimally thin pieces and integrating along TPVs, an analytical solution was derived for the parasitic resistance ( $R$ ), while semianalytical expressions were derived for the parasitic inductance ( $L$ ), capacitance ( $C$ ), and conductance ( $G$ ) in the proposed model. Then, this model was verified against a 3-D electromagnetic solver in terms of S-parameters for various TPV dimensions, with the differences of $S_{21}$ magnitude and $S_{21}$ phases being less than 0.01 dB and 1°, respectively. In addition, two dual-via chains of different lengths were designed, fabricated, and measured to benchmark the proposed model up to 20 GHz. The excellent consistency between them further proves the validity of the proposed model. Finally, the effect of the TPV taper was comprehensively studied on the RLCG parameters. It was found that the TPV taper was beneficial to reduce the parasitic capacitance and conductance by as much as 40%. Despite these benefits, tapered TPVs are not preferred, because the parasitic resistance and inductance are the dominant factors for TPVs in glass, and the TPV taper increases the parasitic resistance drastically and also increases the parasitic inductance by as much as 80%.

Capillary Performance of Micropillar Arrays in Different Arrangements

Abstract: In this study, permeability and capillary pressure of copper micropillar structures (height: 50 µm, diameter: 50 µm) in different arrangements (hexagonal, rectangular, and square) and different por...

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.