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.

High-frequency characterization of through package vias formed by focused electrical-discharge in thin glass interposers

Abstract: This paper presents the modeling, design, fabrication and characterization, up to 30 GHz, of low loss and high aspect-ratio 55 μm diameter through package vias (TPVs) in 300 μm thick glass interposers. These TPVs were fabricated using a novel, high-throughput, focused electrical discharge method and low cost panel-based double-side metallization processes. Such a glass interposer is targeted at two emerging applications, (a) large 30 mm to 60 mm body size 2.5D interposers to achieve 28.8 Gbps logic-memory bandwidth and (b) 3D interposers for mm wave applications at 28 GHz local multipoint distribution service (LMDS) for future 5G networks. Accurate measurement of the electrical performance of fine pitch metallized through vias in glass up to 30 GHz and beyond is critical for both these high performance interposer applications. In this paper, two novel characterization methods are applied: 1) the short-circuit-and-open-circuit method and 2) the dual-via-chain method. The resistance and the inductance of a single via are extracted by using a short-circuit structure along with an open-circuit structure. At 10 GHz, the values for the series resistance and inductance have average values of 0.1 Ω and 160 pH respectively. Long dual-via chains were designed to evaluate their performance in insertion loss, delay and eye diagram. The insertion loss achieved with the longest dual-via chain was found to be less than 1 dB/cm up to 30 GHz with only a 6.2 ps delay in the TPVs, and the simulations indicate a wide open eye.

Thin-Film Packaging

Abstract: Thin film refers to a coating layer of thickness typically in the range of from a few (2–3) atomic layers to a few (1–5) microns. However, film thicknesses up to 20–50 μm are frequently regarded as thin-film in electronic packaging. Thin-film packaging is the technology for conductors and dielectrics deposition and patterning in package fabrication, which resembles the technology used for integrated-circuit (IC) chip fabrication. Thin-film packaging is distinguished from thick-film packaging, mainly ceramic and printed wiring board (PWB) packaging, in two aspects: (a) the typical dimensions of conductors and dielectrics are about 2–25 μm versus 100 μm and above in thick-film packaging; (b) the typical methods of thin-film deposition include sputtering, evaporation, chemical vapor deposition (CVD), more recently electro/electroless plating and polymeric solution coating, and other similar methods, which are all typical sequential processes, whereas in thick-film packaging parallel processes are more common, such as lamination and cofiring of ceramic greensheets. Nevertheless, the current trends in electronic packaging are the enhancement of thick-film technology to fabricate finer feature sizes which are traditionally considered only achievable by thin-film technology and the adoption of some of the parallel thick-film processes for thin-film fabrication. Due to these developments, the traditional division between thin- and thick-film packaging technologies is diminishing and the definition of thin-film packaging is becoming less obvious. Still, the mainstream of thin-film packaging can be well identified which allows a fairly extensive compilation on this technology. In this chapter, the need for thin-film packaging, electrical performance considerations, and typical structures in thin-film packages are first elucidated. Subsequently, the materials, processes, and cost analyses of thin-film packaging are discussed in detail. Repair is especially important in thin-film packaging and is dealt with in the context of cost and yield. Reliability of packages has been discussed in Chapter 5, “Package Reliability,” and reliability issues which are unique to thin-film packages are identified here. Some major commercial applications of thin-film packaging are reviewed, followed by a summary of the emerging technologies. Integration of various passive components is becoming imperative for thin-film packages to enhance electrical performance and packaging efficiency, which is also included in this chapter along with some applications and future predictions. Finally, the current development and future directions of thin-film packaging technology are summarized and projected.

Wideband Power/Ground Noise Suppression in Low-Loss Glass Interposers Using a Double-Sided Electromagnetic Bandgap Structure

Abstract: In this article, we propose a double-sided electromagnetic bandgap (DS-EBG) structure for glass interposers (GIs) with low substrate loss to suppress power/ground noise. For the first time, we validated wideband power/ground noise suppression in the GI using the proposed DS-EBG structure based on dispersion analysis and experimental verification. We experimentally verified that the proposed DS-EBG structure achieved the power/ground noise suppression (below −40 dB) between 2.5 and 8.9 GHz in the GI. Derived stopband edges, $f_{L}$ and $f_{U}$ based on the dispersion analysis, and 3-D electromagnetic (EM) simulation showed a good correlation with measurements. The effectiveness of the proposed DS-EBG structure on the power/ground noise suppression is verified by analyzing noise propagation in the power distribution network and coupling to the GI channel. Using the 3-D EM simulation, we verified that the proposed DS-EBG structure suppressed the power/ground noise coupling and improved the eye diagram of the GI channel. Finally, we propose a design methodology to broaden the isolation bandgap or miniaturize the dimensions based on the dispersion analysis.

Design and Demonstration of Ultra-Thin Glass 3D IPD Diplexers

Abstract: This paper demonstrates, for the first time, 3D integrated passive device (IPD) diplexers on ultra-thin glass substrates for wireless local area network (WLAN) application in mobile devices. The designed LC-based diplexer was composed of a low-band filter and a high-band filter, built on ultra-thin glass substrates. The two filters were designed on each side of the glass substrate and interconnected by through-package-vias (TPVs) to form a 3D IPD. Ultra-thin and low-loss dryfilm dielectrics were utilized for improved electrical performance as well as to achieve high-density of passives integration. The demonstrated 3D IPD diplexer is 3-4X thinner than current LTCC devices, with lateral dimensions of 1.1mm x 1.3mm in a thickness of 200µm resulting in a low insertion loss of less than 1dB for pass bands and more than 24dB stop-band rejection.

Adhesion and reliability of direct Cu metallization of through-package vias in glass interposers

Abstract: Direct metallization of bare glass with copper is required to reach the full potential low-cost benefit of glass interposers. However, this poses a fundamental materials challenge associated with copper-to-glass adhesion. Intermediate polymer liners on glass have been used by others, adding an extra material and processing step. In this paper, three approaches to direct metallization of copper to glass interposers are explored and reported. Electroless plating, sputtering followed by electrolytic plating, and sol-gel were investigated as Cu deposition methods with an emphasis on adhesion and reliability of copper to bare glass. The adhesion and reliability performance of films were characterized by tape-testing, peel-strength measurements, and thermal-shock testing. Based on these results, individual assessments are made for each approach and compared with others to assess future directions.

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.