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.

Demonstration of Glass-based 3D Package Architectures with Embedded Dies for High Performance Computing

Abstract: This paper presents a technology demonstration of two novel 3D glass-based architectures for high performance computing applications. Current 3D technologies are limited by Through Silicon Vias (TSVs), and the proposed approached based on Glass Panel Embedding (GPE) eliminates TSVs resulting in a more robust 3D packaging platform that supports a variety of architectures. Two such architectures are designed and demonstrated in this paper. The first test vehicle shows multiple dies embedded and interconnected in a glass cavity, along with dies assembled on top using a microbump interface. The second test vehicle shows a 50x50 mm glass interposer package with 4 dies embedded in the core, 8 HBM emulators & 2 large SoCs assembled on top at 35 micron-bump pitch.

Reliability of Fine-Pitch μ m-Diameter Microvias for High-Density Interconnects

Abstract: Downscaling of package wiring has been the singular focus to achieve higher logic-memory interconnect density to meet next-generation needs for high-bandwidth computing. This article presents, for the first time, a systematic modeling and experimental study of sub-5- $\mu \text{m}$ -diameter microvia reliability. Geometry design considerations and build-up dielectric material properties in evaluating the microvia fatigue life are investigated. Finally, experimental thermal-cycling reliability results of sub-5- $\mu \text{m}$ -diameter microvias are correlated with the modeling results.

Delamination issues in integration of highly filled polymer-ceramic nanocomposite films on MCM-L compatible substrates

Abstract: The National Electronics Manufacturing Technology roadmap indicates that a capacitance density of 50 nF/cm/sup 2/ will be required in 2001 for successful implementation of integral passive technology. Polymer-ceramic composites are a favorable choice for thin-film capacitors in low-temperature MCM-L technology. Improvement in dielectric properties of the material, achievement of thin and defect-free films and integration on to large area substrates form the cornerstones for this technology. The Packaging Research Center at Georgia Tech. has been actively involved in achieving improved dielectric properties by developing high solids loading and well-dispersed suspensions based on colloidal techniques. The integration of thin-film composites into the subsequent fabrication process becomes increasingly challenging with higher filler content. For example, delamination of the composite from the bottom copper layer has been consistently observed during the modification of composite surface for increasing the adhesion between the electroless copper deposit (top electrode) and composite surface. The surface modification typically involves an etch-treatment with a powerful oxidizing agent such as permanganate. This delamination was not observed in fully cured neat epoxies. The role of fillers in preventing the curing of the epoxy, the reactions between permanganate and uncured epoxy and the lack of adhesion between ceramic and bottom electrode are some of the key issues involved in this delamination problem. This is further complicated by the classic copper-epoxy de-adhesion problem originating from the copper oxide film at the copper-epoxy interface. This work presents our investigation of the origin of this delamination problem by delineating these issues and identifying the key effects. In particular, the role of epoxy curing and filler content based on the interface and epoxy characterization is addressed here. A failure mechanism is proposed from the correlation between epoxy cure and the delamination during etch treatment.

RF inductors with sputtered nanomagnetic films on glass substrates

Abstract: Inductors utilize spiral or helical designs to enhance magnetic flux and inductance. Spiral inductors require large footprint while solenoid inductors require thick substrates to achieve adequate flux coupling. As such, both these designs impose trade offs in inductance density, Q, and frequency stability. Nanomagnetic films with stable and high permeability can enhance inductance density and correspondingly the Q factor. However, nanomagnetic films suffer from two disadvantages. They have high losses beyond 1.0 GHz. They are also sputter-deposited as thin films that are less than five microns. Therefore, the benefits of nanomagnetic films in RF inductors are not clear. Performance improvements and trade-offs in RF inductors with nanomagnetic films are analyzed through material and component-level modeling. In the first part of the paper, nanomagnetic films and properties are modeled. The inductance density and Q trade-offs with nanomagnetic films are simulated after imposing their constraints: frequency dependence and thickness of five microns. Both helical and spiral inductors are considered with and without the nanomagnetic films. The simulations show enhancement in inductance by 5× with mild degradation in Q for spiral inductors. However, the Q degraded more with solenoid inductors. Higher resistivity and ferromagnetic resonance frequency are critical to improve the performance of nanomagnetic film inductors.

Ultra-HDI with low cost photo-imageable polymers

Abstract: As IC devices continue to drive to finer and finer feature sizes, so must the packaging to support them. High-density-interconnects (HDI) fabricated on low cost organic laminates is widely regarded as a critical enabler for device and wafer level packaging. System-on-a-package (SOP) is rapidly being accepted as the future means by which ICs and other components will be packaged. However, if SOP is to be successful, low cost materials and processes must be optimized to reliably yield fine line structures. The PRC has been working to meet a number of challenges that confront the successful implementation of SOP. One of those challenges manifests itself in the area of fine line lithography. Here, low cost photoresists and photo-imageable polymer materials are being evaluated. In addition, processes to support these materials must be optimized for developing ultra fine line HDI substrates. Line widths and spaces of 10-15 /spl mu/m and microvia diameters of 25-50 /spl mu/m have been achieved on large area organic laminates. This paper discusses a number of critical process and material issues that may affect the integrity of fine line and microvia formation.

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.