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Author

Rao Tummala

Other affiliations: Qualcomm, IBM, AVX Corporation  ...read more
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.


Papers
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Proceedings ArticleDOI
01 May 2018
TL;DR: In this paper, an advanced photosensitive dielectric material (PDM) was developed to realize a high resolution and low coefficient thermal expansion (CTE) for next-generation high-density re-distribution layer (RDL) for 2.5D interposer and high density fan-out package applications.
Abstract: In this paper, we introduce an advanced photosensitive dielectric material (PDM) we recently developed to realize a high resolution and low coefficient thermal expansion (CTE) for next-generation high-density re-distribution layer (RDL) for 2.5D interposer and high-density fan-out package applications. For high-density RDL, photosensitive materials need to have (1) a high resolution, (2) high insulation reliability and (3) semi-additive process (SAP) compatibility. We have developed an advanced photosensitive dielectric material witch meets these three requirements. We demonstrate ultra-small via (3 um) by i-line stepper. For SAP compatibility, we have fabricated copper traces of 2um lines and spaces on the PDM by using SAP with sputtered Ti/Cu seed layer. The insulation reliability test was performed at the condition of Bias-Highly Accelerated-Stress Test, 135 deg.C. 85% R.H. on test coupon with 5um thick PDM. As a result, over 300 hours insulation reliability was confirmed. These results mean that the newly-developed PDM is suitable for next generation 2.5D interposer and high-density fan-out package applications. In terms of a package reliability, a low CTE and low process temperature are desired. In order to reduce the CTE of the material and maintain high resolution, nano-size fillers were integrated into the material. As a result, CTE of 30-35 ppm / deg.C was achieved. Curing temperature of the PDM is designed at 180 deg.C which is lower than most of the advanced dielectric materials. These two features contribute to reduce warpage of high density substrates and interposers, so it is expected to be applied to multiple layer application. We also fabricated test coupon with daisy chain structure connected by ultra-small vias and very stable via resistance was confirmed. In conclusion, our newly-developed PDM is a promising dielectric material for highly reliable high-density redistribution layer (RDL) for 2.5D interposers and fan-out package applications.

6 citations

Proceedings ArticleDOI
24 Aug 2004
TL;DR: In this paper, the authors demonstrate how to reduce the "as received" surface roughness of a HDI board, which may additionally contain metal lines up to 18 /spl mu/m thick, to a roughness in general and to /spl plusmn/20 nm or less in special cases over distances of 500 /spl m/m.
Abstract: Polymer materials are becoming increasingly important for integrated optics, in part due to the easy and low cost manufacturability. Embedded polymer waveguides and other polymer devices such as gratings, micro lenses, switches, splitters, etc. are increasingly used in packages and printed wiring boards (PWBs) for optical interconnection and are becoming an emerging technology to enable high data rates over centimeter lengths on-board. A nano scale local surface roughness is desirable for the implementation of these devices in order to avoid unnecessary optical losses. Yet it is a great challenge to control surface roughness on PWB surfaces to the nanometer scale over distances of a millimeter. In addition, board flexibility, lack of planarity, and thermal expansion create great challenges for the implementation of integrated digital-optical technology on packages or boards. A buffer layer produced by a low cost process and which results in a nano scale local roughness is a critical solution that enables the integration of low loss waveguides and other optical passives on boards. It is one of the objectives of this article to demonstrate how to reduce the "as received" surface roughness of a HDI board, which may additionally contain metal lines up to 18 /spl mu/m thick, to a roughness of /spl plusmn/20 nm in general and to /spl plusmn/5 nm or less in special cases over distances of 500 /spl mu/m. We will also discuss fabrication of low loss, low multimode waveguides with 0.24 dB/cm at 1.32 /spl mu/m and 0.52 dB/cm at 1.55/spl mu/m, as well as other optical passives such as micro lenses having 50-250 /spl mu/m base diameter and 80 to 200 /spl mu/m focal length, surface relief gratings with 250 lines/mm and a 35-degree blaze angle, micro mirrors, as well embedded photo detectors, including I-MSM thin film detectors and off-the-shelf PiN bare die detectors. Finally, metallization of bonding pads structures on waveguide polymer material will also be discussed.

6 citations

Proceedings ArticleDOI
27 May 2008
TL;DR: In this article, a low temperature bonding process using high surface energy metallic nanoparticles such as copper and gold was proposed to assist fine pitch bonding, which is based on selective wetting or selective deposition of nanoparticles.
Abstract: High speed digital and mixed signal applications are driving short and more reliable fine pitch interconnection with higher I/O count in 3D architectures. Thin film die to wafer and wafer to wafer bonding with copper-based interconnections have several benefits in terms of low cost, process compatibility with semiconductor infrastructure, and the shortest interconnection with the best electrical performance. However, the bonding is accomplished at around 400 C, with pressures exceeding 30 N/cm2 which may not be compatible with thinned dies, and in ultrahigh vacuum and cleanroom environments with careful copper oxide cleaning procedures. The bonding time is typically 1 hour, which also limits the throughput. The process windows are relatively narrow with several temperature compatibility issues. This paper deals with low temperature bonding process using high surface energy metallic nanoparticles such as copper and gold. Bonding is enhanced by accelerated diffusion kinetics. Self patterning technique has also been developed to assist fine pitch bonding. This is based on selective wetting or selective deposition of nanoparticles.

6 citations

Journal ArticleDOI
TL;DR: In this article, the design, fabrication and characterization of ultra-thin inductors using a new class of multilayered ferromagnetic-polymer composite structures as magnetic cores was presented.

6 citations

Proceedings ArticleDOI
01 May 2018
TL;DR: In this paper, the authors present the latest lithographic advances to enable scaling of packaging redistribution layer (RDL) feature size towards 1-2 µm line and space on panel-based glass substrates.
Abstract: This paper presents the latest lithographic advances to enable scaling of packaging redistribution layer (RDL) feature size towards 1-2 µm line and space on panel-based glass substrates. Critical dimension (CD) of 1 µm line and space with aspect ratio (AR) of 5 was demonstrated in a newly developed chemically amplified plating photoresist by using i-line contact mask aligners and low numerical aperture (NA) 1x projection steppers. The minimum line and space demonstrated was 0.9 µm with AR of 5.5. Such high AR package RDL has much lower trace resistance compared to silicon back-end-of-line (BEOL) RDL, enabling power-efficient higher bandwidth. The relationship between resolution (R) and depth-of-focus (DOF) for low NA projection stepper is discussed. Low cost, low NA, large field steppers are better suited to large panel and large size package fabrication. A cost-effective mechanical fly-cut process was implemented for planarization of the entire wafer or panel with < 1 µm flatness that ensures the success of 1 µm with AR of 5 lithography by using low cost lithographic tools.

6 citations


Cited by
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Journal ArticleDOI

[...]

08 Dec 2001-BMJ
TL;DR: There is, I think, something ethereal about i —the square root of minus one, which seems an odd beast at that time—an intruder hovering on the edge of reality.
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 …

33,785 citations

Journal ArticleDOI
TL;DR: Shape-memory polymers as discussed by the authors are an emerging class of active polymers that can change their shape in a predefined way from shape A to shape B when exposed to an appropriate stimulus.

1,575 citations

Journal ArticleDOI
14 Feb 2008-Nature
TL;DR: This work establishes a methodology for scavenging light-wind energy and body-movement energy using fabrics and presents a simple, low-cost approach that converts low-frequency vibration/friction energy into electricity using piezoelectric zinc oxide nanowires grown radially around textile fibres.
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.

1,473 citations

Journal ArticleDOI
TL;DR: This work demonstrates the vertical and lateral integration of ZnO nanowires into arrays that are capable of producing sufficient power to operate real devices and uses the vertically integrated nanogenerator to power a nanowire pH sensor and a Nanowire UV sensor, thus demonstrating a self-powered system composed entirely of nanowiring.
Abstract: The lateral and vertical integration of ZnO piezoelectric nanowires allows for voltage and power outputs sufficient to power nanowire-based sensors.

1,465 citations

Journal ArticleDOI
TL;DR: In this paper, the authors focus on the important role and challenges of high-k polymer-matrix composites (PMC) in new technologies and discuss potential applications of highk PMC.

1,412 citations