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.

Thermo-mechanical behavior of through silicon vias in a 3D integrated package with inter-chip microbumps

Abstract: Through-silicon via (TSV), being one of the key enabling technologies for 3D system integration, is being used to interconnect 3D vertically stacked devices, such as logic, memory, sensors, and actuators that are fabricated on separate wafers and then interconnected by either wafer-to-wafer or chip-to-wafer methods. However, thermo-mechanical analyses on TSVs are limited, and most of the existing studies focus on the thermo-mechanical analysis of TSVs in a freestanding wafer, rather than in an integrated package. In this paper, three-dimensional thermo-mechanical finite-element models have been built to analyze the stress/strain distribution in a 3D integrated package which contains stacked dice with TSVs, inter-chip microbumps, overmold, and underfilled solder bumps, and an organic substrate. Models show that the stresses in the TSV under packaging configuration could be generally lower than the stresses in the TSV in a free-standing wafer. Also, the models show that the high-strain region switches from TSV corners to microbumps.

2.5D Glass Panel Embedded (GPE) Packages with Better I/O Density, Performance, Cost and Reliability than Current Silicon Interposers and High-Density Fan-Out Packages

Abstract: This paper demonstrates for the first time a next generation high-bandwidth 2.5D glass panel embedding (GPE) architecture with better I/O density, performance, cost and reliability than silicon interposers and high density fan-out packages for heterogeneous integration. Silicon interposers were the first 2.5D technology to enter volume manufacturing, first with TSVs as CoWoS by TSMC and later as embedded bridge EMIB by Intel. High density fan-out packages by chip-first, such as InFO by TSMC, and RDL-first were recently developed. However, all current approaches face challenges in meeting future 2.5D I/O, performance, cost and reliability needs. This paper presents the first demonstration of a revolutionary new concept in scaling power-efficient bandwidth, cost, large package size and board-level reliability, called 2.5D glass panel embedding (GPE). High temperature and low CTE glass reduces die shifts from tens of microns in current molded fan-out to less than 2 microns in GPE. RDL connecting to embedded ICs with 1-2 micron lines and vias overcomes the solder thermo-compression bonding limitations. RDL with much lower resistance and capacitance than BEOL RDL continues power-efficient bandwidth scaling. Ultra-thin glass is readily available in large panels for lower cost. The tailorable CTE of glass allows for reliable direct SMT attach to board of large GPE packages. With a total package thickness of less than 200 microns, this paper describes the fabrication process for an ultra-thin 2.5D GPE, and a systematic parametric process optimization to reduce die shifts to less than 2 microns, leading to the first known demonstration of side-by-side embedding of HBM test chips with all-Cu interconnections at 40 micron I/O pitch.

Numerical and experimental investigations of large IC flip chip attach

Abstract: Because flip chips can achieve high electrical interconnect speed, high density, and low profiles, a team from the Fraunhofer Institute for Reliability and Microintegration in Berlin and from Georgia Tech undertake a study examining the extreme limits of flip chip input/output (I/O) capabilities and physical dimensions. Their starting point is a SIA estimate of memory requirements, based on Moore's Law, for the year 2012. In order to study the limitations of flip chip technology the groups are working on both, advanced thermomechanical simulation and hands-on interconnection technology resulting in the design of four flip chips. They have the dimensions of 10/spl times/10 mm/sup 2/, 20/spl times/20 mm/sup 2/, 30/spl times/30 mm/sup 2/, and 40/spl times/40 mm/sup 2/. With these designs both, the simulation and the interconnection technology departments of Fraunhofer IZM start to evaluate the feasibility of flip chips beyond 20/spl times/20 mm/sup 2/.

Novel nanomagnetic materials for high-frequency RF applications

Abstract: This paper describes leading-edge research to explore and demonstrate new and unique nanoscale magnetic composites for high-frequency RF applications Passivated cobalt nanoparticles were chemically synthesized and dispersed in epoxy to fabricate nanocomposite thick films The high permeability comes from enhanced coupling between the metal nanoparticles while the insulating polymer matrix prevents eddy current loss and improves stability with frequency Test vehicles were fabricated to demonstrate integration of these composites in organic substrates and to characterize the high-frequency properties The frequency-dependent magnetic properties in 100–500 MHz range were extracted by impedance spectroscopy Magnetic toroids were mechanically pressed with the metal-insulator powder By refining the processing, permeability of 27 was demonstrated at VHF frequencies The loss tangent was less than 004 at these frequencies The GHz frequency-dependent material characteristics of the magneto-dielectric films were extracted from corner-probing of parallel-plate resonators and strip inductors By engineering the composite structures at nanoscale, a combination of stable permeability of ∼2 at 1–5 GHz and permittivity of 7, not previously reported, was achieved with polymer composites for antenna miniaturization The magnetic nanomaterials with low loss, described in this paper, can benefit several other RF and power components, leading to their miniaturization and performance enhancement in emerging RF sub-systems The metal composite structures also lead to high permittivity in the GHz frequencies which can benefit such RF components as antennas, by allowing closer impedance matching with air

High performance spiral inductors embedded on organic substrates for SOP applications

Abstract: This paper presents the design, measured data, and systematic analysis of spiral embedded inductors fabricated on standard organic substrates using low-cost, large-area MCM-L technology. Several configurations for inductors were investigated to optimize the inductor layout dimensions such as conductor width, number of turns, inner diameter, spacing between inductor and ground, and inductor area. A maximum Q of 100 was measured for a 3.6 nH inductor at 1.8 GHz on an organic substrate with a self resonance frequency of 10.6 GHz within an inductor core area of 0.72 mm/sup 2/. The effects of configurational variables on inductor characteristics such as quality factor, self-resonance frequency, and inductance are discussed. High-Q inductors embedded on organic substrates can find numerous RF and microwave system-on-package (SOP) applications, such as VCOs, IF/RF bandpass filters, LNAs, etc., in which IC chips are flip-chip mounted on the package substrate.

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.