nano-bio- electronic, photonic and mems packaging · nano-bio- electronic, photonic and mems...

Nano-Bio- Electronic, Photonic and MEMSPackaging

C.P. Wong · Kyoung-Sik (Jack) Moon · Yi LiEditors

Nano-Bio- Electronic,Photonic and MEMSPackaging

123

EditorsC.P. WongSchool of Materials Science &

EngineeringGeorgia Institute of Technology771 Ferst Drive NW.,Atlanta GA 30332-0245, [email protected]

Yi LiIntel CorporationCH5-1595000 W. Chandler Blvd.Chandler AZ 85226, [email protected]

Kyoung-Sik (Jack) MoonSchool of Materials Science &

EngineeringGeorgia Institute of Technology771 Ferst Drive NW.,Atlanta GA 30332-0245, [email protected]

ISBN 978-1-4419-0039-5 e-ISBN 978-1-4419-0040-1DOI 10.1007/978-1-4419-0040-1Springer New York Dordrecht Heidelberg London

Library of Congress Control Number: 2009941849

© Springer Science+Business Media, LLC 2010All rights reserved. This work may not be translated or copied in whole or in part without the writtenpermission of the publisher (Springer Science+Business Media, LLC, 233 Spring Street, New York,NY 10013, USA), except for brief excerpts in connection with reviews or scholarly analysis. Use inconnection with any form of information storage and retrieval, electronic adaptation, computer software,or by similar or dissimilar methodology now known or hereafter developed is forbidden.The use in this publication of trade names, trademarks, service marks, and similar terms, even if they arenot identified as such, is not to be taken as an expression of opinion as to whether or not they are subjectto proprietary rights.

Printed on acid-free paper

Springer is part of Springer Science+Business Media (www.springer.com)

Preface

For the past decade, we have witnessed tremendous advances in nano- and bio-related science, with enormous technical information and literature publishedworldwide. However, limited numbers of commercialized and industrialized itemshave appeared. Reproducibility, low yield, and difficulties in packaging, whichare critically needed to manufacture usable devices and systems, are issues thatresearchers strive to resolve.

This book provides comprehensive reviews and overviews on the latest devel-opments and cutting edges on nano- and bio-packaging technologies and theirscience, including nano- and biomaterials, devices and thermal issues for nanobio-packaging, and the molecular or atomistic scale modeling to predict those smallworld phenomena.

This book is composed of 20 Chapters written by well-recognized world expertsin this field. Chapters 1–4, 8, and 10 review various nanomaterials for nanopack-aging technologies and their most recent research including nanomaterials forelectrical and thermal interconnections and nanosurface manipulation for nanopack-aging. Chapter 5 addresses a novel combustion method for nanoparticle synthesisand nanosurface coating. Novel nanomaterials for renewable energy and energyconversion devices are reviewed in Chapters 6 and 7. Chapter 9 addresses pas-sive devices by using nanomaterials, and Chapter 11 reviews structural analysisof nanoelectronics and optical devices. Latest reviews on nano-thermal science arepresented in Chapters 12 and 13. Some of the hot issues in the health care arenaare the biomedical device and NEMS packaging, and the biosensor device andmaterials. Their recent development and research are reviewed in Chapters 14–17.Computational molecular and atomistic scale simulations could provide us witha profound insight on the understanding of nano- and bio-related systems whichare difficult to realize without high-cost equipment. These modeling efforts arereviewed in Chapters 18 to 20.

We are indebted to all the contributions and efforts of the authors who share theirexpertise in these vital areas in delivering this book to our readers.

Atlanta, Georgia, USA C.P. WongAtlanta, Georgia, USA Kyoung-Sik (Jack) Moon

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Contents

Nanomaterials for Microelectronic and Bio-packaging . . . . . . . . . . 1C.P. Wong and Kyoung-sik (Jack) Moon

Nano-conductive Adhesives for Nano-electronics Interconnection . . . . 19Yi Li, Kyoung-sik (Jack) Moon, and C.P. Wong

Biomimetic Lotus Effect Surfaces for Nanopackaging . . . . . . . . . . 47Yonghao Xiu and C.P. Wong

Applications of Carbon Nanomaterials as ElectricalInterconnects and Thermal Interface Materials . . . . . . . . . . . . . . 87Wei Lin and C.P. Wong

Nanomaterials via NanoSpray Combustion Chemical VaporCondensation, and Their Electronic Applications . . . . . . . . . . . . . 139Andrew Hunt, Yongdong Jiang, Zhiyong Zhao,and Ganesh Venugopal

1D Nanowire Electrode Materials for Power Sourcesof Microelectronics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167Jaephil Cho

Mechanical Energy Harvesting Using Wurtzite Nanowires . . . . . . . 185Xudong Wang and Zhong Lin Wang

Nanolead-Free Solder Pastes for Low Processing TemperatureInterconnect Applications in Microelectronic Packaging . . . . . . . . . 217Hongjin Jiang, Kyoung-sik (Jack) Moon, and C.P. Wong

Introduction to Nanoparticle-Based Integrated Passives . . . . . . . . . 247Ranjith John and Ajay P. Malshe

Thermally Conductive Nanocomposites . . . . . . . . . . . . . . . . . . 277Jan Felba

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viii Contents

Physical Properties and Mechanical Behavior of CarbonNano-tubes (CNTs) and Carbon Nano-fibers (CNFs) as ThermalInterface Materials (TIMs) for High-Power Integrated Circuit(IC) Packages: Review and Extension . . . . . . . . . . . . . . . . . . . 315Yi Zhang, Ephraim Suhir, and Claire Gu

On-Chip Thermal Management and Hot-Spot Remediation . . . . . . . 349Avram Bar-Cohen and Peng Wang

Some Aspects of Microchannel Heat Transfer . . . . . . . . . . . . . . . 431Y. Joshi, X. Wei, B. Dang, and K. Kota

Nanoprobes for Live-Cell Gene Detection . . . . . . . . . . . . . . . . . 479Gang Bao, Won Jong Rhee, and Andrew Tsourkas

Packaging for Bio-micro-electro-mechanical Systems(BioMEMS) and Microfluidic Chips . . . . . . . . . . . . . . . . . . . . 505Edward S. Park, Jan Krajniak, and Hang Lu

Packaging of Biomolecular and Chemical Microsensors . . . . . . . . . 565Peter J. Hesketh and Xiaohui Lin

Nanobiosensing Electronics and Nanochemistryfor Biosensor Packaging . . . . . . . . . . . . . . . . . . . . . . . . . . . 613Dasharatham G. Janagama and Rao R. Tummala

Molecular Dynamics Applications in Packaging . . . . . . . . . . . . . . 665Yao Li, Jeffrey A. Hinkley, and Karl I. Jacob

Nanoscale Deformation and Strain Analysis by AFM/DIC Technique . . 695Y.F. Sun and John H.L. Pang

Nano-Scale and Atomistic-Scale Modeling of Advanced Materials . . . 719Ruo Li Dai, Wei-Hsin Liao, Chun-Te Lin, Kuo-Ning Chiang,and Shi-Wei Ricky Lee

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 759

Contributors

Gang Bao Department of Biomedical Engineering, Georgia Instituteof Technology, Atlanta, GA, USA, [email protected]

Avram Bar-Cohen Department of Mechanical Engineering, Universityof Maryland, College Park, MD, USA, [email protected]

Kuo-Ning Chiang Department of Power Mechanical Engineering, National TsingHua University, Hsinchu, Taiwan, [email protected]

Jaephil Cho School of Energy Engineering, Ulsan National Institute of Science& Technology, Ulsan, Korea, [email protected]

Ruo Li Dai Department of Mechanical and Automation Engineering, The ChineseUniversity of Hong Kong, Shatin, NT, Hong Kong, [email protected]

B. Dang IBM Corporation, Armonk, NY, USA, [email protected]

Jan Felba Institute of Microsystem Technology, Wrocław Universityof Technology, Wrocław, Poland, [email protected]

Claire Gu Department of Electrical Engineering, University of California, SantaCruz, CA, USA, [email protected]

Peter J. Hesketh School of Mechanical Engineering, Georgia Instituteof Technology, Atlanta, GA, USA, [email protected]

Jeffrey A. Hinkley Langley Research Center, NASA, Hampton, VA, USA,[email protected]

Andrew Hunt nGimat Co., Atlanta, GA, USA, [email protected]

Karl I. Jacob Department of Polymer, Textile and Fiber Engineering, GeorgiaInstitute of Technology, Atlanta, GA, USA, [email protected]

Dasharatham G. Janagama Package Research Center (PRC), Georgia Instituteof Technology, Atlanta, GA, USA, [email protected]

Hongjin Jiang Intel Corporation, Chandler, AZ, USA, [email protected]

Yongdong Jiang nGimat Co., Atlanta, GA, USA, [email protected]

ix

x Contributors

Ranjit John College of Engineering, University of Arkansas, Fayetteville,AR, USA, [email protected]

Y. Joshi School of Mechanical Engineering, Georgia Institute of Technology,Atlanta, GA, USA, [email protected]

K. Kota School of Mechanical Engineering, Georgia Institute of Technology,Atlanta, GA, USA, [email protected]

Jan Krajniak School of Chemical and Biomolecular Engineering, GeorgiaInstitute of Technology, Atlanta, GA, USA, [email protected]

Shi-Wei Ricky Lee Department of Mechanical Engineering, Hong Kong Instituteof Science and Technology, Kowloon, Hong Kong, [email protected]

Yao Li Department of Polymer, Textile and Fiber Engineering, Georgia Instituteof Technology, Atlanta, GA, USA, [email protected]

Yi Li Intel Corporation, Chandler, AZ, USA, [email protected]

Wei-Hsin Liao Department of Mechanical and Automation Engineering,The Chinese University of Hong Kong, Shatin, NT, Hong Kong,[email protected]

Chun-Te Lin Department of Power Mechanical Engineering, National Tsing HuaUniversity, Hsinchu, Taiwan, [email protected]

Wei Lin School of Materials Science and Engineering, Georgia Institute ofTechnology, Atlanta, GA, USA, [email protected]

Xiaohui Lin School of Mechanical Engineering, Georgia Institute of Technology,Atlanta, GA, USA, [email protected]

Hang Lu School of Chemical and Biomolecular Engineering, Georgia Instituteof Technology, Atlanta, GA, USA, [email protected]

Ajay P. Malshe College of Engineering, University of Arkansas, Fayetteville,AR, USA, [email protected]

Kyoung-sik (Jack) Moon School of Materials Science and Engineering, GeorgiaInstitute of Technology, Atlanta, GA, USA, [email protected]

John H.L. Pang School of Mechanical & Aerospace Engineering, NanyangTechnological University, Singapore, Singapore, [email protected]

Edward S. Park School of Chemical and Biomolecular Engineering, GeorgiaInstitute of Technology, Atlanta, GA, USA, [email protected]

Won Jong Rhee Department of Biomedical Engineering, Georgia Instituteof Technology, Atlanta, GA, USA, [email protected]

Ephraim Suhir Department of Electrical Engineering, University of California,Santa Cruz, CA, USA, Bell Laboratories, Physical Sciences and Engineering

Contributors xi

Research Division, Murray Hill, NJ, USA (ret); Department of MechanicalEngineering, University of Maryland, College Park, MD, USA; and ERS Co.,Los Altos, CA, USA, [email protected]

Y.F. Sun School of Mechanical & Aerospace Engineering, NanyangTechnological University, Singapore, Singapore, [email protected]

Andrew Tsourkas Department of Bioengineering, University of Pennsylvania,Philadelphia, PA, USA, [email protected]

Rao R. Tummala Package Research Center (PRC), Georgia Institute ofTechnology, Atlanta, GA, USA, [email protected]

Ganesh Venugopal nGimat Co., Atlanta, GA, USA, [email protected]

Peng Wang Department of Mechanical Engineering, University of Maryland,College Park, MD, USA, [email protected]

Xudong Wang School of Materials Science and Engineering, Georgia Instituteof Technology, Atlanta, GA, USA, [email protected]

Zhong Lin Wang School of Materials Science and Engineering, Georgia Instituteof Technology, Atlanta, GA, USA, [email protected]

X. Wei IBM Corporation, Armonk, NY, USA, [email protected]

C. P. Wong School of Materials Science and Engineering, Georgia Institute ofTechnology, Atlanta, GA, USA, [email protected]

Yonghao Xiu School of Materials Science and Engineering, Georgia Institute ofTechnology, Atlanta, GA, USA, [email protected]

Yi Zhang Department of Electrical Engineering, University of California, SantaCruz, CA, USA, [email protected]

Zhiyong Zhao nGimat Co., Atlanta, GA, USA, [email protected]

Nanomaterials for Microelectronicand Bio-packaging

C.P. Wong and Kyoung-sik (Jack) Moon

Abstract This chapter addresses the state-of-art nanoscience and technologiesrelated to the next generation high-density microelectronics and bio-packagingapplications, including carbon nanotubes (CNTs) for electrical/thermal devices,lead-free nanoalloys for lead-free interconnection, nano-conductive adhesives,molecular wires for electrical interconnects, low-stress and high thermal conduc-tive flip-chip underfills, high-k dielectric for embedded passives, bio-mimic Lotuseffect with both nano-micro surfaces for self-cleaning and molecular dynamic (MD)simulations for nanomaterial study and prediction, etc.

Keywords Nanomaterials · Carbon nanotubes interfaces · Electrical and thermalinterconnection · Solar cell packaging · Nanoparticle synthesis · Melting pointdepression · Nano-embedded capacitors · Nanocomposites · Molecular dynamicsimulation

1 Introduction

Microelectronics, photonics, and bio-packaging technology trends have been alwaysrelying on increasing the packaging density, resulting from meeting consumerdemands for light-weight, compact, highly reliable and multifunctional electronic,biomedical, or communication devices. To address such demanding ultra-high-density packaging technology, nanotechnology, nanoscience, and bioengineeringplay an important role in this arena. For over a decade, nano/biotechnologies suchas carbon nanotubes (CNTs), nanoparticles, molecular self-assembly, nanoimprints,and sensors have been explored and studied for mechanical, physical, chemical,photonic, electronic, and biological properties. The packaging scientists prospect

C.P. Wong (B)School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA, USAe-mail: [email protected]

1C.P. Wong et al. (eds.), Nano-Bio- Electronic, Photonic and MEMS Packaging,DOI 10.1007/978-1-4419-0040-1_1, C© Springer Science+Business Media, LLC 2010

2 C.P. Wong and K-s.(J.) Moon

beyond nanomaterials or devices, aiming at system-level components (chip tochip or chip to package interconnection, thin-film components, dielectrics, pas-sives, encapsulants, coatings, power sources, etc.) leading to nano/biomodules. Thisemphasis is expected to lead to a number of commercial applications during thenext decades. However, the implementation of nanotechnology in the system-levelapplications requires a thorough understanding of nano/bioscience and engineering.

In this chapter, several fundamental researches related to nanotechnologies formicroelectronics, photonics, and bio-packaging are discussed, including carbonnanotubes (CNTs) for electrical/thermal interconnects, nanosolder materials withmelting point depression, molecular wires for high-performance electrical inter-connects, high-k nanocomposites, high thermally conductive nanocomposites, andbio-mimic self-cleaning nanosurfaces.

1.1 Interfaces of Carbon Nanotubes (CNTs)/Substratesfor Electrical and Thermal Interconnects

Carbon nanotubes (CNTs) have attracted much interest due to their extraor-dinary electrical, thermal, and mechanical properties, and their wide range ofpotential applications [1–4]. The quality control of the CNTs grown, e.g., purity(catalyst residual and amorphous carbon), diameter, length, and defects, directlyaffect the final properties of CNTs. Unfortunately, the results reported so farvary much and reproducible growth of high-quality CNTs is still an issue.Aligned CNT (ACNT) structures make the best use of the 1-D transport prop-erties of individual CNTs and have been proposed as the promising candidatefor thermal interface materials (TIM) and electrical interconnect materials, gassensors and electrodes, etc. Chapter 4 will address the carbon materials inmore detail.

However, there are challenges in the chemical vapor deposition (CVD) forACNT fabrication, including its high growth temperature and few suitable sub-strate materials for the CVD. To address these key issues, three low-temperatureACNT transfer techniques to position ACNTs onto various surfaces are reportedby Georgia Tech team, as shown in Fig. 1, which use in-situ open-ended andsurface functionalized, aligned multiwall-CNTs via (1) solder, (2) polymer adhe-sives, and (3) self-assembled monolayer as transfer media [4, 5]. Introducinga trace amount of oxidants during the CVD process can eliminate amorphouscarbons on CNT surfaces, open the CNT tips, and controllably functionalizethe tips and walls. Consequently, the CNTs are of high purity and superiorelectrical conductivity. A number of research papers on CNTs (recently ongraphene – a monolayer of graphite, another emerging carbon material) havebeen published in the past decade resulting in tremendous amount of findingson their material properties, structures, fabrications, manipulation, and applica-tions. Nevertheless, due to a lack of knowledge on the defect control and theirinterface manipulation, there are still barriers in using CNTs and graphenes in

Nanomaterials for Microelectronic and Bio-packaging 3

Fig. 1 Low-temperature ACNT transfer technology: solder transfer, adhesive transfer, and chem-ical transfer (from left to right in sequence)

device-level applications. Therefore, more understanding on defects and interfacesshould be made in the near future in order to fully utilize their exotic materialproperties.

1.2 Tin/Silver Alloy Nanoparticle Pastes for Low-TemperatureLead-Free Interconnect Applications

Tin/silver (96.5Sn3.5Ag) is one of the promising lead-free alternatives for a Sn/Pbsolder that has been a de facto standard bonding material in microelectronicspackaging [6]. However, the melting point (Tm) of 96.5Sn3.5Ag alloy (221 C) isaround 40◦C higher than that of eutectic Sn/Pb solders. The higher Tm requiresthe higher reflow temperature in the electronics manufacturing process, which hasadverse effects not only on energy consumption, but also on the package reliabil-ity. Therefore, studies on lowering the melting point of lead-free metals have beendrawing much attention of electronic packaging engineers.

The Georgia Tech team has reported that different sized SnAg alloy nanoparti-cles were successfully synthesized by the chemical reduction method. An organic


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Fig. 2 Relationship betweenthe radius of theas-synthesized SnAg alloynanoparticles and theircorresponding melting pointsand heat of fusion,respectively

surfactant was used to not only prevent aggregation, but also protect the SnAgalloy nanoparticles from oxidation. Both the particle size-dependent melting pointdepression and latent heat of fusion have been observed (Fig. 2). It has alreadybeen found that surface melting of small particles occurs in a continuous mannerover a broad temperature range, whereas the homogeneous melting of the solid coreoccurs abruptly at the critical temperature Tm [7]. For smaller sized metal nanopar-ticles, the surface melting is strongly enhanced by curvature effects. Therefore withthe decreasing of particle size, both the melting point and latent heat of fusionwill decrease. Among all the synthesized particles, the 5 nm (radius) SnAg alloynanoparticles have a melting point at ∼194.3◦C, which will be a good candidatefor the low melting temperature lead-free solders and can solve the issues fromthe high-temperature reflow for the micron-sized lead-free particles. The wettingproperties of the (32 nm in radius) SnAg alloy nanoparticle pastes on the cleanedcopper surface were studied in a 230◦C oven with an air atmosphere. The cross-section of the sample after reflow is shown in Fig. 3. It was observed that the SnAg

Fig. 3 SEM image of thecross-section of the wettedSnAg alloy nanoparticles(32 nm in radius) on thecleaned copper foil


alloy nanoparticles completely melted and wetted on the cleaned copper foil sur-face. The energy-dispersive spectroscopy (EDS) results revealed the formation ofthe intermetallic compound (Cu6Sn5), which showed scallop-like morphologies inFig. 2.

This study has demonstrated the feasibility of the SnAg alloy nanoparticlepastes as a candidate for the low processing temperature lead-free interconnectapplications.

The melting point can be dramatically decreased when the size of substancesis reduced to nanometer size [8]. To date, the size-dependent melting behavior hasbeen proved both theoretically and experimentally [9–11]. The high ratio of the sur-face area to volume of nanoparticles has been known as one of the driving forces forthe size-dependent melting point depression, which is employed currently in vari-ous areas, in particular, where low-temperature bonding, high thermal or electricalconductivity is required. Chapter 8 addresses this as well.

1.3 Enhanced Electrical Properties of Anisotropically ConductiveAdhesive with π -Conjugated Molecular Wire Junctions forEnhanced Electrical Properties

As microelectronics requirements are driven toward smaller, higher density, andmore cost-effective solutions, anisotropically conductive adhesives (ACAs) arewidely used in electronics industry [12, 13]. The ACA joint is established by trap-ping conductive particles dispersed in the polymer matrix in between integratedcircuit and conductive pads when heat and pressure are applied. ACAs can provideelectrical conductivity in Z-direction as well a mechanical interconnect betweenintegrated circuit and conductive pads. The most significant advantage of ACAs isfine pitch capability. However, the ACA joints have lower electrical conductivity andpoor current-carrying capability due to the restricted contact area and poor interfacebetween conductive particles and IC/conductive pads, compared to the metallurgi-cal joint of the metal solders. In order to enhance the electrical performance of ACAjoints, the Georgia Tech team has introduced π-conjugated self-assembled molecu-lar wire junctions to improve the electrical properties of anisotropically conductiveadhesives (ACAs) [14] and nonconductive films (NCFs) [15] for the electronic inter-connects in previous work. Crucial for successful applications of self-assemblymonolayers (SAMs) in ACAs and NCFs are interfacial properties (chemisorptionor physisorption), the characteristics of the molecules (conjugated or unconju-gated, thermal stability, and so on), surface binding geometry, and packing density.Therefore, current work investigates the effects of those factors on the performanceof ACAs in detail by using a series of molecules with different functional groups.The electrical properties of an anisotropic conductive adhesive (ACA) joint wereinvestigated using a submicron-sized (∼500 nm in diameter) silver (Ag) particle asconductive filler with the effect of π-conjugated self-assembled molecular wires(a)). The ACAs with submicron-sized Ag particles have higher current-carrying


capability (∼3400 mA) than those with micro-sized Au-coated polymer particles(∼2000 mA) and Ag nanoparticles (∼2500 mA). More importantly, by constructionof π-conjugated self-assembled molecular wire junctions between conductive parti-cles and integrated circuit (IC)/substrate, the electrical conductivity has increased byone order of magnitude and the current-carrying capability of ACAs has improvedby 600 mA (Fig. 4).

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Fig. 4 (a) Cross-section of ACA joints with π-conjugated molecular wire junctions; (b) I–V curveof ACA filled with Ag particles (Ctrl), TPN-treated Ag particles, TPA-treated Ag particles, andCBA-treated particles


1.4 Low-Stress and High Thermal Conductive Underfillfor Cu/Low-k Application

The underfill materials have been used to improve the flip-chip device reliability byeffectively distributing thermal stress of the package evenly to all solder joints/ballsversus concentrating on the farthest to the distance neutral point (DNP), result-ing in dramatically enhancing solder joint fatigue life [16–18]. In addition to usinglow modulus and CTE underfills for reducing the stress, researches also show thethermally conductive underfills can enhance the reliability as well, because of itsthermal dissipative capability [18]. Therefore, the high thermal conductive under-fill material is expected to play an important role to improve the Cu/low-k devicereliability, as the thermal management is becoming the bottleneck of electronicpackaging technology due to the reduced device profile, increased package den-sity (e.g., 3D packaging), and application of porous materials in Cu/low-k device.However, there is no underfill material available, which can absorb such highthermal stress and dissipate heat to protect the next generation Cu/low or ultra-low-kand high-power devices. Consequently, a novel high-performance underfill/polymercomposite material is required for the highly reliable next generation fine pitchCu/low-k devices.

Optimizing the thermal conductivity and mechanical properties of these high per-formance underfill materials by surface modification of high thermal conductiveinorganic filler has been focused. SiC particles are high thermally conductive buthave relatively low amounts of the hydroxyl group on the particle surfaces, hencethe difficulty to be involved in the surface reaction and poor compatibility with theepoxy resin. The SiC particle surfaces were thermally coated with a nanolayer ofSiO2 by oxidizing Si atoms on the surface at high temperature, as shown in Fig. 5a.Further surface modification of SiC by γ-glycidoxypropyl-trimethoxysilane (GOP)(Fig. 5b) was applied to graft epoxide groups onto SiC surface. The surface modifi-cation improves the chemical bonding density between filler and polymer matrixand reduces the surface phonon scattering, resulting in the thermal conductivityimprovement in epoxy/SiC composites. Moreover, results showed that the addedsilane improved the adhesion strength between the underfill and the silicon wafer.Also, mechanical property measurements indicated the relatively low moduli andlow coefficient of thermal expansion, which will help to reduce the global thermalstress of the underfilled flip-chip structure.

1.5 High-Dielectric Constant (k) Polymer Nanocomposites forEmbedded Capacitor Applications

Embedded passives technology is one of the key emerging techniques for realizingthe system integration such as system-in-package (SiP), system-on-package (SOP),which offers various advantages over traditional discrete components, includinghigher component density, increased functionality, improved electrical performance,


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increased design flexibility, improved reliability, and reduced unit cost. Novel mate-rials for embedded capacitor applications are in great demand, for which highdielectric constant (k), low dielectric loss, and process compatibility with printed cir-cuit boards (PCBs) are the most important material prerequisites. Chapter 9 coversthe embedded component packaging in more detail.

Dielectric nanocomposites based on nanoparticles with controlled parameterswere designed and developed to fulfill the balance between sufficiently high-k andlow dielectric loss and satisfy the requirements to be a feasible option for embeddedcapacitor applications [19]. To this end, people have explored the use of materialand process innovations to reduce the dielectric loss sufficiently while maintain thehigh-k of the nanocomposites [20, 21]. They take advantage of the high-k polymermatrix and the inter-particle barrier layer formed intrinsically by an insulating shell


around the conducting metal core to achieve a simultaneously high-k and low dielec-tric loss conductive filler/polymer composite. First, a silver (Ag)-epoxy mixture wasprepared via an in situ photochemical method, in which Ag nanoparticles were gen-erated by chemical reduction of a metallic precursor upon UV irradiation. Figure 6displays TEM micrograph and histogram of Ag nanoparticles synthesized via an insitu photochemical reduction in epoxy resin. The particle size analysis results showthat well-dispersed Ag nanoparticles with the size mostly smaller than 15 nm inpolymer matrix were obtained. The average size was calculated as 5.2 nm [22].

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The as-prepared Ag-epoxy mixture was then utilized as a high-k polymer matrixto host self-passivated aluminum (Al) fillers for preparation of Al/Ag-epoxy com-posites as embedded capacitor candidate materials. Figure 7 shows the dielectricproperties of Al/epoxy composites and Al/Ag-epoxy composites as a function of Alfiller loading at a frequency of 10 kHz. The Al/Ag-epoxy composites exhibit morethan 50% increase in k values compared with Al/epoxy composites with the samefiller loading of Al, which demonstrated that the incorporation of well-dispersed Ag


Fig. 7 Dielectric constant (k)values of Al/epoxy andAl/Ag-epoxy composites as afunction of Al filler loading(at 10 kHz). Dielectric losstangent (tan δ) values ofAl/epoxy and Al/Ag-epoxycomposites are displayed inthe inset

nanoparticles in the polymer matrix delivers a huge impact on the dielectric con-stant enhancement. It is also notable that the dielectric loss tangent values (see theinset of Fig. 7) for Al/Ag-epoxy composites with different Al filler loadings are allbelow 0.1, which is tolerable for some applications such as decoupling capacitors.The low dielectric loss can be attributed to the good dispersion of Ag nanoparti-cles in the polymer matrix together with the thin self-passivated aluminum oxidelayer forming an insulating boundary outside of the Al particles. The results suggestthat the metal–polymer nanocomposites via an in situ photochemical approach canbe employed as a high-k polymer matrix to enhance the k value of the compositeswhile maintaining the relatively low dielectric loss tangent.

The trends of the dielectric properties with the variation of filler loadingsbehaved differently for Al/epoxy composites and Al/Ag-epoxy composites. Thisphenomenon can be explained in terms of the origins and mechanisms of the dielec-tric loss of the heterogeneous conductive filler/polymer composite material. Theconcentration of silver nanoparticles in the polymer matrix plays a significant rolein determining the dielectric properties of the high-k nanocomposites. At low Agconcentration, the dielectric behaviors of Al/Ag-epoxy composites are mainly deter-mined by interfacial polarization, while conduction and electron transport of Agdominate the Al/Ag-epoxy composites at higher Ag concentration [22].

1.6 Bio-mimetic Lotus Surface

The effect of surface structures on superhydrophobicity (SH) is of much interestdue to the dependence of a structure on the attainment of a high water contactangle (>150◦) and reduced contact angle hysteresis (<10◦). Superhydrophobicitywas first observed on lotus leaves where high water contact angle and low hystere-sis cause water droplet that falls on the surface to bead up and roll off the surface,


thereby leading to water repellency and self-cleaning characteristics [23–25]. Suchsurface properties are also critical for antistiction in microelectro-mechanical sys-tem (MEMS) [26], friction reduction [27], and anticorrosion [28] applications.In addition, superhydrophobic surfaces offer much promise for the formation ofhigh-performance micro/nanostructured surfaces with multifunctionality that can beused in optoelectronic [29, 30], photoelectric [31], microelectronic, catalytic, andbiomedical applications. In particular, its antireflection application in photovoltaic(PV) solar cell devices for enhancing the efficiency has been considered very impor-tant since a PV cell is a critical component for renewable energy developments.Figure 8 shows the typical PV module package structures where incident sunlighttransmits through a cover glass and a polymer layer and arrives at the top surface ofa silicon cell. Light reflection at the silicon top surface can significantly reduce thecell efficiency.

(a) (b)Glass

EVA (Silicone)

Backing Material(Tedlar)

Solar Cells

InterconnectionTabs

Fig. 8 (a) General structure of a PV module; (b) structure of passivated emitter, real locallydiffused (PERL) solar cell with near theoretical efficiency

The Georgia Tech team used a metal-assisted etching technique to formnanoscale roughness and thereby form hierarchical structures by metal-assistedetching of micron-size pyramid-textured surfaces (Fig. 9). These structures yieldsuperhydrophobic surfaces, which may have important application in light harvest-ing, antireflection, and light-trapping properties of solar cells.

A SiN coating on the pyramid-textured silicon surface has been employed toreduce the reflection. Recently, a nanosurface with two-tier roughness was foundto provide not only superhydrophobicity with self-cleaning capability but also verylow light reflectivity on a silicon cell even without SiN coating [32]. Because of thepresence of surface nanostructures, the surface absorbs most of the incident light,thus reducing reflection, especially in the 300–1000 nm wavelength regime. Thenanotextured surface may also increase the path length of light as it travels throughthe cell, which allows thinner solar cells with reduced cost; furthermore, the sur-face may trap the weakly absorbed light reflected from the back surface by totalinternal reflection at the front surface/air interface. On the micro/nanostructuredtwo-scale surfaces (Fig. 10), the weighted light reflectance is further reduced to3.8%, as shown in Table 1. More studies on nanostructure surfaces are introducedin Chapter 3.


(a)

(b)

Fig. 9 Schematics of two-tier surface roughness fabrication: (a) only micron-sized roughness and(b) additional nano-sized surface roughness

Fig. 10 (a) Cross section and (b) top view of Si pyramid surfaces etched made by GeorgiaTech team

Table 1 Weighted reflectance on different textured surfaces

Sample Weighted reflectance %

Flat Si surface 37.3Pyramid-textured surface 12.3Nanotextured surface 6.4Two-scale textured surface 3.8

1.7 Molecular Dynamic (MD) Simulations in Nanomaterial Study

Although a variety of nanomaterials have been studied and considered in manyapplications, a number of fundamental questions on their individual behavior havebeen asked, such as thermal behavior during cooling or heating, how they reactwith different organic molecules in specific solvents, and thermal transport at com-plicated interfaces, because the deep understanding of their individual behavior


enables scientists and engineers to manipulate the nanomaterials appropriately andutilize their properties fully. To answer those questions many researchers havebeen reporting their experimental results, which have been obtained and analyzedby sophisticated equipments. However, those experimental approaches still havetechnical limitations such as characterization resolutions, noise, and reproducibility.

In particular, for thermal behavior, various equipments such as transmissionelectron microscopy and electron diffraction [33, 34], x-ray diffraction (XRD)[35], and differential scanning calorimetry (DSC) [36, 37] have been applied tomonitor the melting behavior of the nano-sized materials. At the melting point,there are changes in the diffraction pattern associated with the disordering ofthe structure, thus XRD and electron diffraction have been used to investigatethe melting of the nanomaterials. Since melting is associated with the heat of asystem, DSC is also used to directly measure the thermal properties of the nano-materials. However, experimental techniques are normally incapable of resolvingthe melting temperature of a single nanoparticle, and the distribution in size ofnanoparticles often obscures the physics behind melting. Also, experimental mea-surements have another limitation, i.e., the melting of particles is influenced bythe type of the substrate on which the nanoparticles are placed. Moreover, exper-iments cannot give detailed information on the atomic-level behavior at elevatedtemperatures since direct observations are extremely difficult. As such, experimen-tal methods have some technical limitations when studying the melting behavior ofnanoparticles.

Theoretical models based on the kinetics, thermodynamics, and mechanicalproperties of solids that are typically bulk crystals have been employed. Thoseapproaches also have limitations since they do not have the information about theindividual atoms comprising the nanoparticle [38]. For the nano-scale substances, amore atomistic approach is necessary. In particular, molecular dynamic (MD) simu-lation is an effective way of investigating the coalescence or melting behavior at anatomic level. It is generally more difficult and more expensive to study the propertiesof a microstructure experimentally than to perform the corresponding simulation.As such, the MD simulation technique has been applied by several researchers toexplore the melting behavior of nanoparticles [39–41], including the melting pointdepression phenomenon, and the role of extrinsic defects (grain boundaries and freesurfaces) in the melting process. It was found that disorder and the melting transi-tion were initiated at the interface in the simulations. More details on MD simulationwill be shown in Chapter 18.

The thermal behavior and interaction of single elements or alloy nanoparticles oncertain substrates [42–48] are very interesting topics because various applicationshave already been employing nanomaterials in their systems to improve electricaland thermal properties. For instance, how the individual nanoparticles behave orinteract with adjacent particles during heating and cooling as a function of time canbe beautifully visualized as shown in Fig. 11.

The modified embedded atom method (MEAM) is popularly used in conjunctionwith MD simulations to investigate the effect of different particle sizes and tempera-ture ramp-up rates on melting behavior of nanoparticles. The MEAM is an empirical


Fig. 11 The x–z plane projection of the system with simulation temperatures of (a) 400 K and (b)1000 K, and simulation time of 5, 15, 30, and 60 ps, respectively [44]

extension of the embedded atom method (EAM), which can be applied to bothmetallic and covalent bonding by including angular forces [49, 50]. In those mod-eling studies, it is not simple to define what temperature will be the melting pointof particles and a heating rate is another of this complication. Thus, the evolutionof potential energy with respect to temperature is used to obtain the melting tem-perature of nanoparticles and the two-phase equilibrium method is used to identifythe melting points of nanoparticles with different sizes [46]. Also the pair correla-tion function (PCF) is used to monitor the crystal regularity of particles and studythe structure differences of shells and cores of a particle [51]. Comparisons of theseresearches with thermodynamic models [52] showed that the thermodynamic mod-els were not able to correctly predict the melting points of nanoparticles with verysmall sizes.

Figure 12 demonstrated that atoms located in the outer shell of the particle losttheir structural order before the atoms in core region did. This result confirmed thesurface melting behavior of nanoparticles of this size. The MD simulation has beenproved as a very powerful tool for exploring nanomaterial systems at atomistic scale,


0

1

2

3

4

5

2 4 6 8 10Interatomic Distance (Angstrom)

PC

F

Core, 470K

Core, 490K

Shell, 470K

Fig. 12 PCF of a Sn particle; heating rate 0.1 K/ps, diameter 8 nm, shell width is 1 nm, core radiusis 3 nm [46]

by itself as well as combined with other simulation methods and has been demon-strating its strong potential in this arena. We can expect that a numerous currentand future questions on such a small-scale world could be answered through thepowerful computational studies together with sophisticated experimental methods.

2 Conclusive Remarks

While a variety of researches on nanomaterials for nano-bio devices and systemshave been carried out, this field is still at its infancy. However, there are manypotential applications in electronic, photonic, and bio-medical applications, whichwill benefit our society. Therefore, much research and development work need tobe studied. This is an area where scientists and engineers need to work together tomake this goal a reality.

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22. Lu J., Moon K., and Wong C.P., Silver/polymer nanocomposite as a high-k polymer matrixfor dielectric composites with improved dielectric performance. J. Mater. Chem. 2008; DOI:10.1039/B807566B.

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Nano-conductive Adhesives for Nano-electronicsInterconnection

Yi Li, Kyoung-sik (Jack) Moon, and C.P. Wong

Abstract With the phasing out of lead-bearing solders, electrically conductiveadhesives (ECAs) have been identified as one of the environmentally friendlyalternatives to tin/lead (Sn/Pb) solders in electronics packaging applications. In par-ticular, with the requirements for fine-pitch and high-performance interconnectsin advanced packaging, nanoconductive adhesives are becoming more and moreimportant due to the special electrical, mechanical, optical, magnetic, and chemicalproperties that nano-sized materials can possess. There has been extensive researchfor the last few years on materials and process improvement of ECAs, as well asthe advances of nanoconductive adhesives that contain nano-filler such as nanoparti-cles, nanowires, or carbon nanotubes and nanomonolayer graphenes. In this chapter,recent research trends on electrically conductive adhesives (ECAs) and their relatednanotechnologies are discussed, with the particular emphasis on the emerging nan-otechnology, including materials development and characterizations, processingoptimization, reliability improvement, and future challenges/opportunities identi-fication. The state of the art on nanoisotropic/anisotropic conductive adhesivesincorporated with nanosilver, carbon nanotubes, and nanonickel, and their recentstudies on those for flexible nano/bioelectronics, transparent electrodes, and jettableprocesses are addressed in this chapter. Future studies on nanointerconnect materialsare discussed as well.

Keywords Electrically conductive adhesives (ECAs) · Isotropically conduc-tive adhesives (ICA) · Percolation threshold · Nanotechnology · Carbon nan-otubes (CNT) · Self-assembled monolayer (SAM) · Thermo-compression bonding(TCB) · Sintering · Current-carrying capability · Anisotropic conductive adhe-sives (ACAs) · Fine-pitch interconnection · Nanoconductive fillers · Transparentelectrodes

Y. Li (B)Intel Corporation, Chandler, AZ, USAe-mail: [email protected]

19C.P. Wong et al. (eds.), Nano-Bio- Electronic, Photonic and MEMS Packaging,DOI 10.1007/978-1-4419-0040-1_2, C© Springer Science+Business Media, LLC 2010

20 Y. Li et al.

1 Introduction

Miniaturization of electronic and photonic devices is a demanding trend in cur-rent and future technologies including consumer products, military, biomedical, andspace applications. In order to achieve this miniaturization, new packaging tech-nologies have emerged such as embedded active and passive components and 3Dpackaging, where new packaging designs and materials are required.

Fine-pitch interconnection is one of the most important technologies for thedevice miniaturizations. Metal solders including eutectic tin/lead solder and lead-free alloys are the most common interconnect materials in the electronic/photonicpackaging areas.

Conventional eutectic tin/lead solder or lead-free solders cannot meet the require-ments for fine-pitch assembly due to their stencil printing resolution limit andbridging issues of solders in fine-pitch bonding, which is an intrinsic characteristicof metal solders.

Recently, polymer-based nanomaterials have been intensively studied to replacethe metal-based interconnect materials, and many research efforts to enhancematerial properties of the electrically conductive adhesives (ECAs) are on going.Furthermore, incorporating nanotechnology into the ECA systems not only enablesthe ultra-fine-pitch capability but also enhances the electrical properties such asreducing the percolation threshold and improving the electrical conductivity.

In this chapter, recent research and study trends on ECAs and their relatednanotechnologies are discussed, and the future of the nanomaterial-based polymercomposites for fine-pitch interconnection is reviewed.

Electrically conductive adhesives (ECAs) are composites of polymeric matri-ces and electrically conductive fillers. The polymeric resin, such as an epoxy, asilicone, or a polyimide, provides physical and mechanical properties such as adhe-sion, mechanical strength, impact strength, and the metal filler (such as, silver, gold,nickel, or copper) conducts electricity. Metal-filled thermoset polymers were firstpatented as electrically conductive adhesives in the 1950s [1–3]. Recently, ECAmaterials have been identified as one of the major alternatives for lead-containingsolders for microelectronics packaging applications. ECAs offer numerous advan-tages over conventional solder technology, such as environmental friendliness, mildprocessing conditions (enabling the use of heat-sensitive and low-cost componentsand substrates), fewer processing steps (reducing processing cost), low stress onthe substrates, and fine-pitch interconnect capability (enabling the miniaturizationof electronic devices) [4–9]. Therefore, conductive adhesives have been used in flatpanel displays such as LCD (liquid crystal display), and smart card applications asan interconnect material and in flip-chip assembly, CSP (chip-scale package) andBGA (ball grid array) applications in replacement of the solder. However, no cur-rently commercialized ECAs can replace tin-lead metal solders in all applicationsdue to some challenging issues such as lower electrical conductivity, conductivityfatigue (decreased conductivity at elevated temperature and humidity aging or nor-mal use condition) in reliability testing, limited current-carrying capability, and poorimpact strength. Table 1 gives a general comparison between tin-lead solder and ageneric commercial ECA [6].

Nano-conductive Adhesives for Nano-electronics Interconnection 21

Table 1 Conductive adhesives compared with solder

Characteristic Sn/Pb solder ECA (ICA)

Volume resistivity 0.000015� cm 0.00035 � cmTypical junction R 10–15 mW <25 mWThermal conductivity 30 W/m-deg.K 3.5 W/m-deg.KShear strength 2200 psi 2000 psiFinest pitch 300 μm <150–200 μmMinimum processing temperature 215◦C <150–170◦CEnvironmental impact Negative Very minorThermal fatigue Yes Minimal

Depending on the conductive filler loading level, ECAs are divided into isotrop-ically conductive adhesives (ICAs), anisotropically conductive adhesives (ACAs)and nonconductive adhesives (NCAs). For ICAs, the electrical conductivity in allx-, y-, and z-directions is provided due to high filler content exceeding the percola-tion threshold. For ACAs or NCAs, the electrical conductivity is provided only inz-direction between the electrodes of the assembly. Figure 1 shows the schematicsof the interconnect structures and typical cross-sectional images of flip-chip jointsby ICA, ACA, and NCA materials illustrating the bonding mechanism for all threeadhesives.

Fig. 1 Schematic illustrations and cross-sectional views of (a, b) ICA, (c, d) ACA, and (e, f) NCAflip-chip bondings

Isotropic conductive adhesives, also called as “polymer solder,” are compos-ites of polymer resin and conductive fillers. The adhesive matrix is used to form amechanical bond for the interconnects. Both thermosetting and thermoplastic mate-rials are used as the polymer matrix. Epoxy, cyanate ester, silicone, polyurethane,etc., are widely used thermosets, and phenolic epoxy, maleimide acrylic preimidized

22 Y. Li et al.

polyimide, etc., are the commonly used thermoplastics. An attractive advantageof thermoplastic ICAs is that they are reworkable, i.e., can easily be repaired. Amajor drawback of thermoplastic ICAs, however, is the degradation of adhesionat high temperature. A drawback of polyimide-based ICAs is that they generallycontain solvents. During heating, voids are formed when the solvent evaporates.Most of commercial ICAs are based on thermosetting resins. Thermoset epox-ies are by far the most common binders due to the superior balanced properties,such as excellent adhesive strength, good chemical and corrosion resistances, andlow cost, while thermoplastics are usually added to allow toughening and reworkunder moderate heat. The conductive fillers provide the composite with electricalconductivity through physical contact between the conductive particles. The pos-sible conductive fillers include silver (Ag), gold (Au), nickel (Ni), copper (Cu),and carbon in various forms (graphites, carbon nanotubes, etc.), sizes, and shapes.Among different metal particles, silver flakes are the most commonly used con-ductive fillers for current commercial ICAs because of the high conductivity andthe maximum contact between flakes. In addition, silver is unique among all thecost-effective metals by nature of its conductive oxide. Oxides of most commonmetals are good electrical insulators. Copper powder, for example, becomes a poorconductor after oxidation/aging. Nickel and copper-based conductive adhesives gen-erally do not have good resistance stability. As such, electrical reliability issues ofECA joints, in particular on non-noble metal finishes at elevated temperature andhigh relative humidity are of critical concern for implementing ECAs into the high-performance device interconnects. Intensive studies on this ECA joint failure ledto understanding of the failure mechanisms, that corrosion at the ECA joints wasthe underlying mechanism rather than a simple oxidation process of the non-noblemetal used [10, 11] (Fig. 2). Under the elevated temperature/humidity (for exam-ple, 85◦C/85%RH), the non-noble metal acts as an anode, and is oxidized by losingelectrons, and then turns into metal ion (M–ne– = Mn+). The noble metal acts as acathode, and its reaction generally is 2H2O + O2 + 4e– = 4OH–. Then Mn+ com-bines with OH– to form a metal hydroxide, which further oxidizes to metal oxide.After corrosion, a layer of metal hydroxide or metal oxide is formed at the inter-face. Because this layer is electrically insulating, the contact resistance increasesdramatically.

Since this underlying mechanism was reported, many research activities for reli-able ECAs have been intensively conducted via various approaches, such as usinglow moisture absorption resins, incorporating corrosion inhibitors or oxygen scav-engers, using sacrificial anode materials and oxide-penetrating sharp particles [10,12–15]. In addition, many approaches to improve the electrical conductivity ofECAs have been reported, such as increasing the cure shrinkage of the matrix resin,replacing a lubricant of Ag flake with a shorter chain length carboxylic acid, addingan in-situ reducing agent for preventing the Ag flake-surface oxidation and addinglow melting point alloys (LMA) for enhancing conductivity [16–21]. With the helpof those intensive researches on enhancing the ECA performance, recently, ICAs

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