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.

Glass-Based IC-Embedded Antenna-Integrated Packages for 28-GHz High-Speed Data Communications

Abstract: Chip-embedded mm-wave antenna-integrated modules are demonstrated, for the first time, on panel-scale ultra-thin glass substrates, for high-speed 5G communication standards in the n257 band (26.5 – 29.5 GHz) defined by 3GPP. Co-packaging of amplifiers, filters, and antennas with minimal package parasitics is the key to realize mm-wave package systems. Parasitics arise from on-package and chip-to-package interconnects. This paper focuses on reduced chip-to-package losses and implementation of filters and antennas with chip-embedding structures in glass substrates. To demonstrate the benefits of glass-panel embedding (GPE) for 5G communications, the interconnect losses are benchmarked with the C4-bump based flip-chip technique. The electrical performance shows that the chip-embedding structure with a glass substrate lead to 3X lower insertion loss from chip to antenna than the flip-chip assembly method with C4 bumps. This reduced chip-to-antenna insertion loss brings about the enhanced efficiency and gain of the patch antennas integrated on top of the glass substrates. The process development and electrical performance are benchmarked with emerging 5G substrate technologies such as fan-out wafer level packaging.

Nanoindentation of single crystal and polycrystalline copper nanowires

Abstract: Metal nanowires are attracting considerable interest because of their potential importance to the technology of miniaturization of electronic devices in need of metallic contacts. One of the important attributes of nanowires is their potentially high mechanical strength. Nanoindentation is the most realistic tool at the present time to determine the mechanical properties of nanowires. The load-displacement behavior during nanoindentation of electrodeposited single crystal and 500 nm diameter polycrystalline copper nanowires was performed and the results are reported in this paper. The behavior has also been compared with that of bulk nanocrystalline and annealed copper. The hardness values for 50 nm grain size polycrystalline nanowires and those of extruded bulk 50 nm grain size copper were comparable, 2.1 GPa, and that of a 50 nm single crystal copper nanowire was 1.8 GPa.

New paradigm in IC package interconnections by reworkable nano-interconnects

Abstract: We propose new IC packaging technologies that have the potential to bring about disruptive innovations in interconnect pitch, best electrical and mechanical properties, low-cost and chip size. Current approaches for chip to package interconnections are limited in terms of either pitch or electrical-mechanical trade-off properties. For example, lead free solder interconnects fail mechanically as the pitch is brought down from current 200 micron pitch to 20 micron. Compliant leads, on the other hand, solve mechanical reliability but at the expense of electrical performance. Solution-derived materials for reworkable nano-interconnects can be a viable technology to meet these two challenges. Nano-grained electroplated copper is chosen as the primary interconnect material. Compliancy was addressed by tuning the process to electroplate high-aspect-ratio structures. Reworkability was addressed by a thin, liquid lead-free solder interface between the interconnect and the package. Two approaches, sol-gel and electroless plating were used in this work to deposit these liquid interface films of lead free solders of the order of 50-300 nm. In the sol-gel process, metal-organic polymer solutions were heat-treated in a reducing atmosphere at 300/spl deg/C to form lead-free solders (Sn-Ag-Cu). In the other approach, lead-free alloy films were deposited from aqueous plating solutions consisting of suitable metal salts and reducing agents. This process was done at temperatures of 45/spl deg/C. The lead-free solder composition was controlled by altering the plating bath formulation. Lead-free solder films formed from both the above approaches were demonstrated to bond copper pads. Solution-derived nano-solder technology is an attractive low-cost method for bump-less nano-interconnects and other applications such as MEMS hermetic packaging and compliant interconnect bonding.

RF Characterization of Magnetodielectric Material Using Cavity Perturbation Technique

Abstract: Magnetodielectric (MD) materials find application in many areas of microwave engineering, and therefore, measurement of their dielectric and magnetic properties is very important. This paper presents a novel MD material characterization method using the cavity perturbation technique (CPT) with substrate-integrated waveguide (SIW) cavity resonators. Frequency dependent complex permittivity and permeability of MD material can be extracted with a single SIW cavity structure by inserting the sample material into different locations. The fundamental theory of CPT is explained and its analysis for SIW cavity is discussed. Cobalt nanoparticles are synthesized with a fluoropolymer matrix to realize the MD materials and their properties are measured in the frequency range 1–4 GHz. The effect of volume fraction and density of the synthesized MD materials on the dielectric and magnetic properties has been studied.

Ultrathin Antenna-Integrated Glass-Based Millimeter-Wave Package With Through-Glass Vias

Abstract: This article presents the design and demonstration of a high-bandwidth antenna-in-package (AiP) module focusing on low-loss interconnects and Yagi–Uda antenna performance fabricated on a 100- $\mu \text{m}$ low coefficient-of-thermal-expansion (CTE) glass for the 28-GHz band. It shows the modeling, design, and characterization of key technology building blocks along with the process development of advanced 3-D glass packages. The building blocks include impedance-matched antenna-to-die signal transitions, Yagi–Uda antenna, and 3-D active–passive integration with backside die assembly on 100- $\mu \text{m}$ glass substrates. The design and stack-up optimization of antenna-integrated millimeter-wave (mm-wave) modules is discussed. Process development to achieve high-density interconnects and precise dimensional control in multilayered thin glass-based packages is also described. The characterization results of the key technology building blocks show an insertion loss of 0.021 dB per through-package via (TPV), leading to the whole-chain loss of less than 1 dB and a return loss lower than 20 dB. The fabricated Yagi–Uda antenna features high repeatability of wide bandwidth due to the process control enabled by glass substrates. The antenna measurements show a bandwidth of 28.2%, which covers the entire 28-GHz fifth-generation (5G) frequency bands (n257, n258, and n261). The flip-chip assembled low-noise amplifier with 80- $\mu \text{m}$ solder balls shows a maximum gain of 20 dB as desired. The performance of the glass-based package integrated antennas is benchmarked to other 5G substrate technologies, such as organic laminates or co-fired ceramic-based substrates.

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.