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.

Vapor phase infiltration of aluminum oxide into benzocyclobutene-based polymer dielectrics to increase adhesion strength to thin film metal interconnects

Abstract: Interfacial adhesion between metallic thin films and polymers is a critical performance metric for a number of microelectronics and packaging applications. Delamination of metal-polymer interfaces is a frequent failure mode for many multilayer structures, like those used for electronics packaging. Such a failure is even more likely when electronic packages are operated under extreme conditions like high-power, high-temperature, and/or high-humidity operation. Roughening or direct chemical modification of the few layers of atoms that make up the interface is often used to promote adhesion at these interfaces. Here, the authors investigate a new process, vapor phase infiltration, that infiltrates inorganic constituents into the bulk of the polymer, creating an interpenetrating network within the subsurface of the polymer that further enhances interfacial adhesion. For the authors’ model system of copper films on a benzocyclobutene polymer, they are able to increase the interfacial adhesion strength by as much as 3×, resulting in cohesive rather than adhesive failure. The authors attribute this increased interfacial adhesion to physicochemical interlocking of the organic and inorganic phases within the subsurface of the polymer, generating a “root system” that impedes interfacial delamination.

System-on-a-package (SOP) substrate and module with digital, RF and optical integration

Abstract: The Packaging Research Center has been developing next generation system-on-a-package (SOP) technology with digital, RF, and optical system integration on a single package. SOP aims to utilize the best of on-chip SOC integration and package integration to achieve the highest system performance at the lowest cost. The micro-miniaturized multi-functional SOP package is highly integrated and fabricated on large area substrates similar to the wafer-to-IC concept. In addition to novel mixed signal design methodologies, SOP research at PRC is targeted at developing enabling technologies for package level integration including ultra-high density wiring, embedded passive components, embedded optical interconnects, wafer level packaging and fine pitch assembly. Several of these enabling technologies have been recently integrated into the first successful system level demonstration of SOP technology using the intelligent network communicator (INC) testbed. This paper reports on the latest INC and SOP testbed results at the PRC and provides an insight into the future SOP integration strategy for convergent microsystems. The focus of this paper is on integration of materials, processes and structures in a single package substrate for system-on-a-package (SOP) implementation.

Thermomechanical Reliability of Nickel Pillar Interconnections Replacing Flip-Chip Solder Without Underfill

Abstract: Interconnect technologies between ICs and packages or boards have a significant impact on the IC performance and packaging density. Today, the interconnections are typically accomplished with either wire bonding or flip-chip solders. While both of these technologies are incremental, they also run into either electrical or mechanical barriers as they are extended to higher density of interconnections. Downscaling traditional solder bump interconnect might not satisfy the thermomechanical reliability requirements at very fine-pitches. Alternate interconnection approaches such as compliant interconnects typically require lengthy connections and are therefore limited in terms of electrical properties, although expected to meet the mechanical requirements. This paper reports fine-pitch interconnection technologies using nano-structured nickel as primary interconnection material. The nano-grained nickels are produced by electroplating process. The primary nano-structured interconnects are assembled with different bonding methods to provide organic compatible low-temperature fabrication. Au-Sn and Sn-Cu are used for solder-based assembly of nano-nickel interconnections. Low modulus anisotropic conductive films (ACFs) are also used as an alternate bonding route of the solders. No underfilling is used in all the interconnect structures evaluated in this paper. Assembly are accomplished on different coefficient of thermal expansion (CTE) substrates including FR-4 with 18 ppm/degC, advanced organic substrates with 10 ppm/degC, novel low CTE (3 ppm/degC) substrates based on carbon-silicon carbide (C-SiC). The thermomechanical reliability of all the nano-interconnects assembled on different CTE substrates with different bonding approaches is evaluated by thermal shock testing and finite-element analysis. Nano-nickel interconnects bonded with the ACF showed the highest reliability withstanding 1500 cycles. In all cases, no apparent failure was observed in the primary nano-nickel metal interconnects. This technology is expected to be easily downscaled to submicrometer and nano-scale unlike the current solder technologies leading to true nano-interconnections.

Ultra-fine photoresist image formation for next generation high-density PWB substrate

Abstract: System-on-a-Package (SOP) is fast becoming a primary key towards the drive for integration of mixed technologies such as Rf, digital, analog, MEMS and optical at the electronic package level. A key enabler of this technology is a fully integrated substrate, called Single Level Integrated Module (SLIM), with very high wiring density and integrated passive and optoelectronic components. This paper presents an overview of the SLIM testbed integration process and test vehicles, with particular emphasis on ultra-fine resist lithography and microvia processing. This paper will focus on the issues, challenges and results for achieving very fine line and ultra fine line image formation on fiberglass reinforced epoxy substrates for next generation electronic packages. Four commercial photo-resist materials were evaluated for their imaging resolution. The exposure and development processes have been optimized and several related effects, which limit the fine line imaging, have been investigated. Line widths down to 7.5 μm have been achieved at the Packaging Research Center (PRC) on PWB substrates using low cost liquid photoresist and associated processes over a large area.

Ultra-thin and ultra-small 3D double-side glass power modules with advanced inductors and capacitors

Abstract: This paper demonstrates 3D functional modules that are ultra-miniaturized, high-performance and low-cost, based on an innovative 3D Integrated Passive and Active Component (3D IPAC) concept [1]. The 3D IPAC concept utilizes an ultra-thin (30–100 microns) and ultra-low-loss glass substrate, low-cost through-package-vias (TPVs) and double-side redistribution layers (RDL) for assembly of both active and passive components. In this concept, both active and passive components are integrated on both sides of the glass substrate, either as thinfilms or as discretely fabricated and assembled components, separated by only about 50–100 microns in interconnection length. This paper specifically addresses the power functional modules with passive components by integrating ultra-thin high-density capacitors on one side and power-supply inductors on the other side. The first part of the paper describes the electrical modeling and design of power inductors and capacitors in 3D IPAC structure. The second section describes the fabrication for both the building block L and C components and the assembly of integrated modules. The last section presents the electrical characterization. The paper, thus, provides a first demonstration of a novel power module platform for double-side thin active and passive component integration for power module applications.

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.