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.

A numerical analysis of crack growth and morphology evolution in chip-to-packages nano-interconnections

Abstract: The International Technology Roadmap for semiconductors (ITRS) has predicted that by the year 2007, integrated chip (IC) packages will contain feature sizes of 65 nm and an I/O pitch for the die-to-package interconnects approaching 80 mum. These will reduce even further in the next five years. The current approach of using surface mount technology and flip chip are mainly solder based and the lead and lead-free solder interconnects are known to fail mechanically as the pitch is reduced from 200 mum down to lower levels due to the thermal mismatch between the substrate and the chip. Although compliant interconnection could solve some of the mechanical issues, it is done at the expense of the electric performance. The PRC at Georgia Institute of Technology is proposing re-workable copper based nano-interconnections as a new interconnection paradigm as the next step beyond lead-free solders for future low-cost, high performance and high reliability packages. However, very limited data is published about the fatigue life of nano-crystalline materials and specifically those of nano-crystalline copper. It is important to predict crack growth as it can aid the understanding of the useful life of the IC-packages' interconnections. Multiple mechanisms may be responsible for crack initiation, but eventually most dominant fatigue cracks form a surface crack, which often have a semi-elliptical shape. Hence, the fatigue crack growth life predictions in this study are based on the assumption of elliptical and semi-elliptical cracks being initiated in the nano-interconnections. In this study, numerical analysis using the J-integral stress intensity parameter, in conjunction with experimental fatigue crack growth data, has been employed to study semi-elliptical crack growth and morphology evolution in nano-interconnection subject to uniaxial fatigue loading in linear-elastic conditions. The results indicate that a J-integral finite element analysis, using the loading portion of the fatigue cycle, in conjunction with known rates of fatigue crack growth can approximate surface crack morphology evolution. This study also predicts that the long crack growth is a relatively small portion of the total fatigue life of the material for the experimental LCF conditions. Hence, initiation of the cracks in the interconnection is the main criterion used to predict its fatigue life.

A hands-on multi-disciplinary product development course for micro systems packaging education at Georgia Tech

Abstract: A cross-disciplinary two-part Design-Build-Operate (DBO) program has recently been developed by the Packaging Research Center (PRC) at Georgia Institute of Technology. This program enables undergraduate and graduate students to gain hands-on experience in design, fabrication processes, material systems, electrical testing, flip-chip assembly, thermal management, and reliability assessment. The entire program is offered in two semesters. The first course, DBO1, covers the fundamentals of substrate fabrication processes and is augmented by an interactive multimedia education presentation that makes the course material remotely accessible through the Internet. The second course, DBO2, encompasses electronics manufacturing processes, material systems, testing, thermal management and reliability assessment.

First Demonstration of Photoresist Cleaning for Fine-Line RDL Yield Enhancement by an Innovative Ozone Treatment Process for Panel Fan-Out and Interposers

Abstract: This paper describes for the first time an innovative approach to improve re-distribution layer (RDL) yields in advanced semi-additive processes (SAP). An atmospheric pressure ozone based treatment is proposed as an alternative to oxygen plasma treatment. The ozone treatment process is scalable, being appropriate for process wafers up to large panels, and is suited for small feature sizes down to 1 micron that are required for interposers and future fan-out packages. The ozone process provides an environmentally friendly solution that can also replace wet cleaning processes, eliminating the need for hazardous chemicals that require abatement. The paper demonstrates the potential for two opportunities for integration of ozone treatment steps in the SAP flow for RDL fabrication. These particular steps are identified as 1) ozone treatment after dry film resist (DFR) patterning to improve the electrolytic copper plating yield and 2) removal of DFR residue prior to seed layer etch cleaning. Both these steps resulted in significantly improved RDL yields and demonstrate the feasibility of integrating ozone based cleans in high-yield, high volume cost effective manufacturing of RDL in Advanced Packaging. The paper also proposes ozone treatment as a higher throughput alternative to the plasma treatment process for electrolytic copper plating. Since the ozone gas is generated from oxygen, and reduced to oxygen upon process completion, no hazardous gas is required, or discharged into the atmosphere. To exhibit the scalability of the ozone treatment to both wafer-scale and panel-scale processing, two different types of copper seed layers, physical vapor deposition (PVD) Ti-Cu, and electroless plated copper, were evaluated. Samples were treated with either ozone or oxygen plasma, and the results were compared to a control sample with no treatment. After the photolithography step, in which 7 micron thick DFR laminated on the copper seed layers was resolved to 3 micron feature size, the substrates were subjected to ozone or plasma treatments. The subsequent water contact angle measurements show significant wettability improvement on the surfaces of substrates with copper seed layer, DFR, and DFR mesh patterned on a copper seed layer. Samples were then compared for the quality of the copper plating. Excellent copper metallization quality was achieved in the samples that had been treated with ozone and plasma due to the creation of a hydrophilic surface. An additional benefit emerged in that the ozone treatment was effective at 50 deg C, which minimized any impact on DFR stripping. The ozone treatment was also applied to clean the DFR residue after resist stripping and the results confirmed that the ozone process removed any remaining photoresist residue from the copper surface.

Method of making a matrix print head

Abstract: @ The method for fabricating a matrix print head for use in nonimpact electrolytic printers involves stacking and laminating green ceramic sheets (11) having holes (12) filled with conductive paste to form electrodes (13) then embossing the top ceramic sheet (11) with insulators (14) and screening the same with a layer (15,16) of ruthenium dioxide. The assembly is then sintered at less than 1000°C and the resulting structure smoothed by lapping and finished in a convention fashion.

3D Integrated High-Precision Passives on Thin Glass Substrates for Miniaturized and High-Performance RF Components

Abstract: Double-side or 3-D integration of high-precision and high-performance bandpass and lowpass filters that are interconnected with through-vias were designed and demonstrated on 100-micron th...

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.