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.

Development of high-k embedded capacitors on printed wiring board using sol-gel and foil-transfer processes

Abstract: Sol-gel ceramic films were fabricated for organic system-on-package compatible integral capacitor applications. The films were synthesized on Ti and Ni foils which were then transferred onto organic boards using a lamination step. SrTiO/sub 3/ and BaTiO/sub 3/ films were synthesized with capacitance as high as 700 nF/cm/sup 2/ and loss as low as 0.005. It should be noted that the high permeability of Ni (approximately 100 in bulk form) and lower conductivity compared to copper decreases the skin depth and increases the resistivity of copper. This can have a deleterious effect on Q. More studies are underway to investigate this effect.

Label-Free Protein Detection by ZnO Nanowire Based Bio-Sensors

Abstract: There is an increasing demand for portable, reliable, and cost effective bioelectronic systems for applications ranging from clinical diagnosis to homeland security. Conventional detection systems involve labeling the probe molecules, large amount of target molecules to enable detection, and elaborate signal transduction methods. Most of them also have to couple with optical detection equipments that are bulky and expensive. One dimensional (1-D) and two dimensional (2-D) structures such as nanowire, nanobelts and films are capable of detecting the molecular interactions in terms of significant change in their electrical properties leading to ultrahigh sensitivity and easy integration. In this paper, we demonstrate ZnO nanowires based bio-sesnors to detect IgG antibodies. Current-voltage (I-V) and Scanning Electron Microscopy (SEM) characterization were used to monitor the change in the conductivity as well as morphology. By comparing with the reference sample, the specific binding event between anti-IgG and IgG antibodies was detected. The data indicated a conductivity change by more than 12% after the protein hybridization. SEM images confirm the morphological change from reference samples to reacted samples. In addition, same experiment protocols are carried out for ZnO thin film devices. Similar change in I-V characteristics and morphologies are observed. Through this work, we have demonstrated to use ZnO nanowires as building blocks to fabricate bio-sensors which can potentially detect any protein. Conductimetric sensing results in a label-free detection system as it detects the protein hybridization events electrically. It is a cost effective process, which can be exploited further by expanding into arrays and integrating with microfluidics. When integrated on the SOP platform, this technology can lead to portable, reliable and cost effective biosensors with applications in many areas.

Low-temperature, organics-free sintering of nanoporous copper for reliable, high-temperature and high-power die-attach interconnections

Abstract: A novel die-attach joining technique based on low-temperature film sintering of nanoporous Cu is demonstrated Nanoporous Cu films are proposed as a low-cost replacement of nano-sintering pastes with the following benefits: (i) synthesis by electrochemical dealloying, compatible with standard lithography processes; (ii) no organic content to minimize risks of voiding and corrosion; and (iii) controllable physical properties post sintering through tailorable initial nanostructure and morphology As a first proof-of-concept, thin films of nanoporous Cu with 25–50nm feature size and ∼60% relative density were synthesized by dealloying of Cu-Si films The nanoporous Cu films were then sintered on bulk Cu metallizations at temperatures of 200–250°C for 5–15min with an applied pressure of 6–9MPa, in reducing atmosphere A maximum shear strength of 42kgf was achieved and analysis of the fracture profiles showed failure through the sintered joints, confirming strong metallurgical bonding to bulk Cu Cross-sections of joints formed at 200°C and 250°C −15min observed by SEM showed relative density as high as 85%, achieved for the first time with sintered copper

Multichip Embedding Technology Using Fine-Pitch Cu–Cu Interconnections

Abstract: Increasing performance and functional density while maintaining low cost is a catalyst for technological progress in the field of packaging. From flip-chip with solder to a hybrid approach of copper and solder, many methods have been created to reach this objective. The 3-D Packaging Research Center at Georgia Tech has been revolutionizing interconnection technology with the multichip embedding chip-last approach, which utilizes ultrathin adhesive-bonded copper bumps to enable ultrafine-pitch chip-to-package interconnections. This technology has been proven to be highly reliable using a low-cost low-temperature direct copper-to-copper bonding approach at 30-μm pitch and ~20-μm standoff height copper-to-copper interconnections. This interconnection method provides a platform for integration with flip-chip packages through its proven ability to work well with different die sizes and thicknesses bonded to the surface of ultrathin organic substrates. The next step in advancing the chip-last approach is to investigate chip embedding at the single-chip and multichip levels. Consequently, this paper focuses on: 1) the design and fabrication of the test vehicle to examine the reliability of the previously demonstrated copper-to-copper interconnections after embedding a thin die in an organic substrate, and 2) assembly process development and reliability data for the interconnections. Specifically, advances in the assembly process include: 1) a novel method to perform chip-last assembly at the panel level leading to a 10-15 times reduction in assembly time per die, and 2) an improved two-step assembly process to achieve simultaneous die embedding and cavity planarization. This embedding technology and its advancements not only allow actives to be embedded in organic substrates but also enables higher functional integration at high-throughput, making chip-last adhesive bonding with low-profile copper-to-copper interconnections a robust chip embedding solution for the next generation of highly integrated heterogeneous subsystems.

Large low-CTE glass package-to-PCB interconnections with solder strain-relief using polymer collars

Abstract: This paper reports the use of circumferential polymer collars as a strain-relief mechanism to improve the fatigue life of low-CTE package-to-PCB solder interconnections, while preserving SMT-compatibility and reworkability. Acting as a partial underfill, the polymer-collar serves to block shear deformation at the solder-package interface, and redistributes the load to reduce the overall plastic strain concentration in the solders. It also suppresses failure initiation from defective surface sites and, thus further enhances reliability. Ultra-thin glass 100μm interposers were fabricated in 18.4 mm × 18.4 mm size to model, design and demonstrate the reliability enhancement with the polymer-collar approach. The detailed interposer design and fabrication process with laminated dielectric and metallization layers on both sides is presented. A new class of epoxies with low modulus, without the incorporation of silica fillers, was used to act as the polymer collars. The polymer collars are formed by spin-coating with an optimized thickness to provide the best compromise between the effective strain relief and reworkability. Board-level assembly was performed using standard SMT processes for glass interposers with and without polymer collars. Thermal cycling reliability testing (-40°C to 125°C) of interposers, assembled on PCBs with and without polymer collars for various thicknesses of the collar was performed.

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.