Adaptive Cooling of Integrated Circuits Using Digital Microfluidics: Adaptive Cooling of Integrated Circuits Using Digital Microfluidics.jpg

Contents
Preface ...........................................................................................11
Chapter 1
Thermal Management of Integrated Circuits ..................................15
1.1 Introduction....................................................................................................15
1.2 Low-Level Power Consumption in Integrated Circuits..................................16
1.2.1 Work, Power, and Energy ........................................................................16
1.2.2 Heat and Temperature..............................................................................17
1.2.3 Thermal Effects on Performance and Reliability.....................................17
1.2.4 Dynamic Power Consumption .................................................................18
1.2.5 Static Power Consumption.......................................................................20
1.3 Circuit-Level Power Consumption in Integrated Circuits..............................23
1.3.1 Transistor Reordering ..............................................................................23
1.3.2 Half-Frequency and Half-Swing Clocks..................................................24
1.3.3 Low-Power Flip-Flop Design ..................................................................24
1.3.4 Technology Mapping ...............................................................................25
1.4 Low-Power Interconnect Design....................................................................26
1.4.1 Bus Inversion...........................................................................................26
1.4.2 Crosstalk Reduction.................................................................................26
1.4.3 Low-Swing Buses....................................................................................26
1.4.4 Other Methods .........................................................................................27
1.5 Low-Power Memory Design..........................................................................27
1.5.1 Partitioning Memory................................................................................27
1.5.2 Specialized Power-Friendly Caches ........................................................28
1.6 Real-World Example—The Pentium M Processor.........................................28
1.7 Summary ........................................................................................................29
References............................................................................................................29
Chapter 2
Cooling Devices for Integrated Circuits .........................................33
2.1 Introduction....................................................................................................33
2.2 Design Considerations for IC Cooling ...........................................................34
6 Adaptive Cooling of Integrated Circuits Using Digital Microfluidics
2.2.1 Performance.............................................................................................34
2.2.2 Cost..........................................................................................................35
2.2.3 Space........................................................................................................35
2.2.4 Power.......................................................................................................35
2.3 Categorization of IC Cooling Techniques......................................................35
2.3.1 Passive Versus Active Cooling ................................................................35
2.3.2 Adaptive Versus Nonadaptive Cooling ....................................................36
2.4 Current Methods for IC Cooling ....................................................................37
2.4.1 Fan-Based Cooling ..................................................................................37
2.4.2 Macrofluidic-Based Cooling ...................................................................38
2.4.3 MEMS-Based Cooling.............................................................................38
2.4.4 Refrigeration-Based Cooling ...................................................................41
2.4.5 Microfluidics-Based Cooling...................................................................42
2.5 Summary and Conclusions.............................................................................46
References............................................................................................................47
Chapter 3
Adaptive Hot-Spot Cooling Principles and Design ........................49
3.1 Introduction....................................................................................................49
3.2 Requirements for Adaptive Hot-Spot Cooling ...............................................50
3.3 Digital Microfluidics as a Cooling Platform ..................................................51
3.3.1 Electrowetting-Based Actuation of Droplets ...........................................52
3.3.2 Fabrication of Digital Microfluidic Devices............................................55
3.3.3 Digital Microfluidic Operations...............................................................56
3.3.4 Digital Microfluidic Adaptive Cooling—Flow-Through Method ...........60
3.3.5 Digital Microfluidic Adaptive Cooling—Programmable
Thermal Switch Method ..........................................................................63
3.4 Feedback Control Mechanisms ......................................................................68
3.4.1 Thermal Sensor Feedback Control...........................................................69
3.4.2 Flow-Rate Feedback Control...................................................................69
3.4.3 Electro- and Thermocapillary Feedback Control.....................................69
3.5 IC Level Integration .......................................................................................70
3.6 Performance Comparisons for Flow-Through Methods.................................71
3.6.1 Implementation Parameters .....................................................................71
3.6.2 Heat-Transfer Parameters ........................................................................72
3.7 Summary and Conclusions.............................................................................74
References............................................................................................................75
Chapter 4
Technology Development ...............................................................77
4.1 Introduction....................................................................................................77
Contents 7
4.2 Temperature Measurement .............................................................................77
4.2.1 Infrared Imaging ......................................................................................78
4.2.2 On-Chip Resistive Temperature Devices (RTDs) ....................................84
4.3 Hot-Spot Source Design.................................................................................89
4.3.1 Surface Mount Resistors..........................................................................90
4.3.2 On-Chip Thin-Film Heaters.....................................................................90
4.4 Capacitance Detection....................................................................................92
4.5 Design, Fabrication, and Testing of Initial Flow-Through Prototypes ...........95
4.6 Digital Microfluidics on Printed Circuit Board (PCB)...................................97
4.7 Summary and Conclusions...........................................................................103
References..........................................................................................................103
Chapter 5
Thermal Effects of Digital Microfluidic Devices .........................105
5.1 Introduction..................................................................................................105
5.2 Experimental Methods .................................................................................105
5.2.1 Investigation of In-Oil Parameters.........................................................106
5.2.2 Investigation of In-Air Parameters ........................................................107
5.3 Experimental Results for a System in Oil ....................................................107
5.3.1 Global Temperature Effects on Vth in Oil ..............................................107
5.3.2 Interfacial Tension Effects on Vth ..........................................................108
5.3.3 Oil Viscosity Effects on Vth ................................................................... 110
5.4 Experimental Results for a System in Air .................................................... 111
5.4.1 System-Global Effects ........................................................................... 111
5.4.2 Contact-Angle Temperature Dependence .............................................. 112
5.5 Summary and Conclusions........................................................................... 114
References.......................................................................................................... 114
Chapter 6
Flow-Through–Based Adaptive Cooling ......................................117
6.1 Introduction.................................................................................................. 117
6.2 Experimental Methods ................................................................................. 117
6.2.1 Digital Microfluidic Platform Development.......................................... 117
6.2.2 Transport of Various Cooling Liquids....................................................120
6.2.3 Heat Transfer Characterization ..............................................................120
6.3 Digital Microfluidic PCB Platform Development........................................124
6.3.1 Fabrication Parameters ..........................................................................124
6.3.2 Design Parameters .................................................................................126
6.3.3 Closed-Loop Dispensing, Transport, and Recycling of Droplets...........128
6.4 Hot-Spot Cooling in an Open System ..........................................................129
6.4.1 Effects of Switching Frequency.............................................................131
8 Adaptive Cooling of Integrated Circuits Using Digital Microfluidics
6.4.2 Effects of Hot-Spot Heat Flux Density..................................................132
6.5 Hot-Spot Cooling in a Closed System..........................................................135
6.5.1 Effect of Switching Frequency ..............................................................135
6.5.2 Effect of Droplet Volume.......................................................................135
6.5.3 Effect of Background Heating ...............................................................137
6.6 Static Heat Transfer Characterization...........................................................141
6.6.1 Effect of Heat Flux Density ...................................................................142
6.6.2 Effect of Droplet Aspect Ratio...............................................................143
6.7 Summary and Conclusions...........................................................................143
Chapter 7
Programmable Thermal Switch–Based Adaptive Cooling ...........145
7.1 Introduction..................................................................................................145
7.2 Experimental Methods .................................................................................145
7.2.1 Mercury Droplet Transport ....................................................................145
7.2.2 Steady-State Heat Transfer of Mercury .................................................146
7.2.3 Transient Heat Transfer of Mercury.......................................................147
7.3 Mercury Droplet Transport ..........................................................................148
7.4 Steady-State Heat Transfer of Hot Spots Using Mercury ............................150
7.5 Transient Heat Transfer of Hot Spots Using Mercury .................................154
7.5.1 Effect of Copper Via on Heat Transfer ..................................................154
7.5.2 Effect of Heat Flux Density on Heat Transfer .......................................155
7.5.3 Effect of Cooling Area on Heat Transfer ...............................................156
7.6 Summary and Conclusions...........................................................................158
Reference ...........................................................................................................159
Chapter 8
Concluding Remarks.....................................................................161
8.1 Summary of Concepts ..................................................................................161
8.1.1 Digital Microfluidics on PCB................................................................162
8.1.2 Thermal Effects on Droplet Transport ...................................................162
8.1.3 Flow-Through–Based Cooling ..............................................................163
8.1.4 Programmable Thermal Switch–Based Cooling....................................163
8.2 Future Work..................................................................................................164
8.2.1 Flow-Through–Based Cooling ..............................................................164
8.2.2 Programmable Thermal Switch–Based Cooling....................................165
Appendix A
Image Analysis Software Using MATLAB ..................................167
Contents 9
A.1 Introduction .................................................................................................167
A.2 Video Frame Handling ................................................................................167
A.2.1 Loading a Video File.............................................................................168
A.2.2 Displaying the Video in False Color .....................................................169
A.3 Processing Video for Temperature Measurement ........................................172
A.3.1 Selecting a Region of Interest...............................................................172
A.3.2 Calculating Average Intensity ...............................................................173
A.3.3 Outputting Data to a File ......................................................................175
Appendix B
Microfluidic Chip Design Reference ............................................177
Appendix C
RTD/Heater Top Plate Design Reference......................................185
Appendix D
Material Safety Data Sheet: Mercury............................................189
About the Authors.......................................................................197
Index ...........................................................................................201

Preface
Thermal management is a critical issue in integrated circuit (IC) design. With each
new IC technology generation, feature sizes decrease, while operating speeds and
package densities increase. These factors contribute to elevated die temperatures
detrimental to circuit performance and reliability. Furthermore, hot-spots due to
spatially nonuniform heat flux in ICs can cause physical stress that further reduces
reliability. While a number of techniques to address these issues have been
proposed, most are still unable to handle the varying thermal profiles of an IC.
The most promising techniques have been based on continuous-flow microfluidic
architectures. However, their capacity to remove a large amount of heat is
undermined by their lack of reconfigurability of flows.
In this book, we introduce alternative cooling architectures based upon a
recently invented “digital microfluidic” platform. This novel digital fluid handling
platform uses a phenomenon known as electrowetting, and allows for a vast array
of discrete droplets of liquid, ranging from microliters to nanoliters to potentially
picoliters, to be independently moved along a substrate. While this technology
was originally developed for a biological and chemical lab-on-a-chip, we have
adapted it to be used as a fully reconfigurable, adaptive cooling platform.
Since this technology is relatively new to the research community, students,
researchers, and other users of this book require only a basic knowledge of fluid
dynamics, plus a general knowledge of integrated circuit and microprocessor
design. This book is intended to be used as a reference for those interested in
tackling the thermal management problem using digital microfluidics or similar
droplet-based architectures.
This textbook begins with a general overview of thermal management issues
that IC design engineers face today. In Chapter 1, the detrimental effects in digital
ICs caused by overheating are explained. While many of the same problems are
found in analog integrated circuits, they are typically not as extreme and are
therefore excluded from this textbook. This introductory chapter continues with
the study of the sources of this power dissipation, followed by descriptions of
practical solutions at the microarchitectural level.
Chapter 2 provides a number of examples of methods used for cooling
integrated circuits. The techniques described range from simple heat-sink-fan
implementations to complex MEMS structures used to enhance air flow and heat
dissipation. Several microfluidic devices are also discussed in this chapter.
12 Adaptive Cooling of Integrated Circuits Using Digital Microfluidics
The fundamental principles of how a droplet moves by electrowetting are
described in Chapter 3. A number of design requirements for an adaptive cooling
platform are outlined here, and examples are given of how electrowetting-based
motion of droplets can be exploited in such an adaptive cooling system. Many
examples are also given in the text to illustrate the basics of this microfluidic
technology.
Chapter 4 focuses on the development of the basic digital microfluidic
platform toward a cooling platform and deals with two main themes: how to
measure the temperature across the surface of the chip and identify hot spots, and
how to integrate a digital microfluidic chip with an existing device. Key issues
such as temperature measurement and calibration are first discussed before
moving onto simulation of thermal hot spots. Finally, a cost-effective method to
manufacture a chip using printed circuit board (PCB) processes is demonstrated.
The temperature effects on moving droplets in a digital microfluidic platform
are studied in Chapter 5. This study is crucial in determining the feasibility of the
proper operation of droplet handling subject to high temperatures. It is
experimentally shown in this chapter that droplet transport is actually enhanced at
elevated temperatures and is well-aligned with chip-cooling applications.
Chapters 6 and 7 investigate two methods of chip cooling using digital
microfluidic devices. In a flow-through approach, cooling droplets are actuated
independently via electrowetting through user-defined reprogrammable flow paths
and speeds. This high level of reconfigurability enables us to create an “adaptive”
hot-spot cooling module that can be affixed directly onto the IC itself, whereby
the system reconfigures itself on the fly in response to a changing thermal profile.
In a programmable thermal switch approach, an array of liquid-metal droplets can
be manipulated such that any area in the cooling device can be selectively
switched from a low-to-high or high-to-low thermal conductivity mode. In this
way, a higher heat flux can be drawn away from the hot spot, resulting in a
uniform thermal profile. The two cooling methods developed and characterized in
these chapters demonstrate the feasibility to adaptively perform thermal
management to dynamically cool hot spots. This kind of adaptive cooling system
will pave the way for new IC thermal management design strategies to perform
temperature-aware cooling, and we hope that the users of this book will help
contribute to this ongoing research.
Acknowledgment for this work is made first and foremost to my doctorate
advisor Dr. Krishnendu Chakrabarty of Duke University and to my colleague and
mentor, Dr. Vamsee K. Pamula, a cofounder of Advanced Liquid Logic, Inc. Both
Dr. Chakrabarty and Dr. Pamula, who are coauthors of this book, contributed
much of their expertise and time in making this book possible. Dr. Vijay
Srinivasan and Dr. Michael G. Pollack of Advanced Liquid Logic, Inc., also
contributed ideas and encouragement that helped in the completion of the research
used in this book. Financial support received by the National Science Foundation
(NSF) is gratefully acknowledged. I also thank NSF Program Director Dr. Sankar
Basu for his interest in this project.
Preface 13
We also want to thank Professor Richard B. Fair of Duke University’s
Department of Electrical and Computer Engineering, who currently directs the
Duke University Digital Microfluidics Laboratory. Many thanks also go to my
wife, Minae Kim, for all her encouragement. Others in my family who helped,
either directly or indirectly, are my mom, my wife’s parents, and my brothers,
Peter and Paul.
Philip Y. Paik
Durham, North Carolina
April 2007

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