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.

Embedded Trench Redistribution Layers at 2– $5~\mu \text{m}$ Width and Space by Excimer Laser Ablation and Surface Planer Processes for 20– $40~\mu \text{m}$ I/O Pitch Interposers

Abstract: This paper reports on one of the first demonstrations of the formation and metallization of 2–5- $\mu \text{m}$ lines and spaces by an embedded trench method in two dry-film polymer dielectrics, Ajinomoto build-up film and preimidized polyimide, without using chemical mechanical planarization. The trenches and vias in 8–15- $\mu \text{m}$ -thick dry-film dielectrics were formed by 308-nm excimer laser ablation, followed by the metallization of the trenches and vias by copper electrodeposition. A low-cost planarization process was used to remove the copper overburden with a surface planer tool. Using an optimized set of materials and processes, multilayer redistribution layers with 2– $5~\mu \text{m}$ trenches and vias were successfully demonstrated. Although thin film processes on silicon wafers have been able to achieve 40- $\mu \text{m}$ I/O pitch for interposers, the materials and processes integrated in this paper are scalable to large panel fabrication at much higher throughput, for interposers and high-density fan-out packaging at lower cost and higher performance than silicon interposers.

Ultra-Wideband, Glass Package-Integrated Power Dividers for 5G and mm-Wave Applications

Abstract: This paper presents an ultra-wideband, glass package-integrated, equal-split power divider with footprint smaller than the unit free-space wavelength corresponding to the operating frequency of 28 GHz 5G band. The utilization of precision low-loss redistribution layers (RDL) on ultra-thin glass substrates with stable electrical properties at mm-wave frequencies enable the ultra-wideband power dividing networks to have a small footprint in x-y-z dimensions along with excellent performance. This approach aggregates the benefits of ceramic, low-loss polymers and silicon: electrical performance of the ceramic, processability of polymers and the dimensional stability of silicon, simulated in glass substrate to realize fine features for the power dividers. The power divider exhibits low added insertion loss, minimal phase shift between its output ports and has a height less than 150-µm.

Material characterization for nano wafer level packaging application

Abstract: As the feature size of integrated circuit (IC) packages needs to be decreased significantly, computational methods in conjunction with experimental data have been employed to study the mechanical issues, which have become a concern for the components reliability. In this paper, the issues associated with and experimental methods necessary to perform material characterization for nano-wafer level packaging application will be investigated. Firstly, the need for nano-indentation to accurately characterize the modulus and hardness of copper thin film will be presented. Furthermore, some of the problems such as strain rate and thickness effects associated with extracting the modulus using nano-indentation will be addressed. Lastly, results from a fatigue experiment on a 200 /spl mu/m pitch solder column will also be given and factors affecting the failure criterion of these solder columns in fatigue conditions will be investigated.

Demonstration of Next-Generation Au-Pd Surface Finish with Solder-Capped Cu Pillars for Ultra-Fine Pitch Applications

Abstract: High-performance computing is driving sub-10µm substrate interconnect pitches to support high-density logic-to-memory interconnections in advanced 2.5D packaging. A new ultra-thin surface finish, electroless Pd autocatalytic Au (EPAG), was recently developed by Atotech GmBH to address extraneous plating encountered at these fine pitches with conventional finishes, such as electroless Ni immersion Au (ENIG) or electroless Ni electroless Pd immersion Au (ENEPIG). In this paper, the EPAG composition was optimized to demonstrate, for the first time, ultra-short copper pillar interconnections with superior pitch scalability, bonding strength and thermomechanical reliability as compared to standard ENEPIG. Variations in surface finish composition were considered, with EPAG finishes composed of 50 -- 100nm Pd and 50nm Au, electroless palladium (EP) finishes of 50 and 100nm Pd, and standard ENEPIG, used as reference. Solder wettability was first evaluated through contact angle measurements on solder sessile drop test samples. Copper pillar assemblies with limited solder volume were then formed on all surface finishes. They were subjected to high-temperature storage at 150°C for up to 500h, to study interfacial reactions, and subsequent intermetallic formation. While gold embrittlement was observed with both EP and ENEPIG finishes, it could be prevented with EPAG surface finish with 50nm Pd and 50nm Au, this specific ratio leading to formation of the single (Cu, Au, Pd) 6Sn5 intermetallic. Die shear and thermal shock tests were then carried out to determine the effect of the joints' composition on their strength and fatigue life. A shear strength of 40MPa was achieved with all EPAG compositions, exceeding the 6-11MPa and 5MPa achieved with EP and ENEPIG, respectively. Highest thermomechanical reliability was also achieved with EPAG finishes, surviving 300 cycles at -55/125°C even in silicon-to-FR-4 assemblies with high CTE mismatch, while 70% of the ENEPIG daisy chains failed after only 100 thermal cycles. The optimized EPAG composition was therefore demonstrated as a promising low-cost surface finish alternative to form ultra-short but highly-reliable, fine-pitch, Cu pillar interconnections.

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.