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.

Chip-last fan-out package with embedded power ICs in ultra-thin laminates

Abstract: The demand for ultra-miniaturized mobile electronics systems has placed stringent requirements on the form-factor, especially thickness, of electronic modules. A novel technology to enable embedding in 1 or 2 metal layer substrates using chip-last technology was introduced previously [1]. This paper focuses on first ever demonstration of chip-last embedding of functional dies in 1–2 metal layer thin core substrates, to achieve form factor and performance comparable to wafer level fan-out with improved yield, cycle time reduction, cost, testability and thermal management. Ultra-slim modules were obtained as a result of embedding thin-chips within the core instead of the build-up layers reported previously [2]. Performance of chip-last embedded actives has been successfully demonstrated previously [3] using low power RF ICs. Embedding power management ICs (PMIC), however, not only challenges high power dissipation capabilities of chip-last fan-out package but also its high current carrying capability, with many I/Os drawing ∼1A current. Since the embedded PMIC is rated at 2.3W dissipation, finite element modeling (FEM) was carried out to simulate thermal performance of the package under steady state conditions. Assuming uniform distribution of the 2.3W over the entire PMIC, FEM simulations depicted a maximum temperature of ∼52°C on the top of the embedded IC, a rise of ∼27°C from the base temperature of 25°C, indicating the heat dissipation to be a non-issue. Module substrates were built using 100μm BT as core and ABF as the cavity layer dielectric. Thinned functional ICs were assembled in the laser-ablated cavities using previously demonstrated Cu-Cu thermo-compression bonding [4, 5]. The resulting module thickness with the embedded IC was ∼200μm, a reduction of more than 55% from the incumbent. The demonstrated ultra-thin laminate based fan-out package with embedded PMIC uses a simple fabrication process flow making it a manufacturing-friendly and cost effective solution. Therefore, the low layer count chip-last embedding technology has the potential to achieve ultra-miniaturization for future embedded sub-systems and systems.

Ultra-Thin Wireless Power Module with Integration of Wireless Inductive Link and Supercapacitors

Abstract: This paper describes advances in integrated ultra-thin wireless power module components for Internet of Things (IoT) and wireless sensor applications. A typical wireless module integrates both an inductive link for wireless power transfer, and supercapacitor as a storage element. The performance of the inductive link is enhanced with innovative high-permeability and low-loss magnetic films for improved inductive coupling between the receiver and the transmitter. Up to 4X enhancement in power transfer efficiency is demonstrated with the innovative magnetic films. Energy storage is achieved through a thinfilm micro fabricated supercapacitor layer with interdigitated graphene-based planar electrodes and solid-state electrolytes for easier integration. The component properties demonstrated through this work are projected to achieve high power levels with ultra-thin form-factors.

Ultra-High Density, Thin-Film Tantalum Capacitors with Improved Frequency Characteristics for MHz Switching Power Converters

Abstract: High-density passive components are needed for continued miniaturization of complex high-performance electronic systems. Tantalum (Ta) capacitors provide some of the highest volumetric densities achieved due to their combination of high-surface area and relatively high dielectric constant, but suffer from low frequency stability and large form-factors due to the electrode design. In this paper, a printed thin-film tantalum capacitor design is presented. Tantalum capacitor arrays of 1 μF/mm2 up to and beyond 1 MHz. The improved frequency stability comes from the ultra-thin structure of the capacitors, which reduces the path length of the charging and discharging current. The capacitors showed low equivalent series resistance and consistent electrical performance before and after thermal moisture testing at 65°C and 95% relative humidity for 500 h and 1000 h. Due to the ultra-low form-factor, the thin-film Ta capacitor technology can be extended to highly-miniaturized power converters with efficient substrate- or wafer-scale integration.

First Demonstration of Silicon-Like >250 I/O Per mm Per Layer Multilayer RDL on Glass Panel Interposers by Embedded Photo-Trench and Fly Cut Planarization

Abstract: This paper demonstrates for the first time >250 I/O per mm per layer on glass panels utilizing embedded photo-trench and fly cut planarization processes to meet the demands of next generation high bandwidth 2.5D Interposers. This paper combines the use of novel photo-imageable dielectrics developed by TOK and advances in Disco's Fly-Cut planarization technique to deliver unit processes capable of delivering >250 I/O per mm per layer.

Development of Stretch Solder Interconnections for Wafer Level Packaging

Abstract: A wafer level packaging technique has been developed with an inherent advantage of good solder joint co-planarity suitable for wafer level testing. A suitable weak metallization scheme has also been established for the detachment process. During the fabrication process, the compliancy of the solder joint is enhanced through stretching to achieve a small shape factor. Thermal cycling reliability of these hourglass-shaped, stretch solder interconnections has been found to be considerably better than that of the conventional spherical-shaped solder bumps.

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.