Corona-driven air propulsion for cooling of microelectronics

Corona-driven air propulsion for cooling of microelectronics By Fumin Yang A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Electrical Engineering University of Washington 2002 Program Authorized to Offer Degree: Electrical Engineering University of Washington Graduate School This is to certify that I have examined this copy of a master’s thesis by Fumin Yang and have found that it is complete and satisfactory in all respects, and that any and all revisions required by the final examining committee have been made Committee Members: _ Alexander Mamishev _ Jiri Homola _ Ann Mescher Date: In presenting this thesis in partial fulfillment of the requirements for a Master’s degree at the University of Washington, I agree that the Library shall make its copies freely available for inspection I further agree that extensive copying of this thesis is allowable only for scholarly purposes, consistent with "fair use" as prescribed in the U.S Copyright Law Any other reproduction for any purposes or by any means shall not be allowed without my written permission Signature Date University of Washington Abstract Corona-driven air propulsion for cooling of microelectronics by Fumin Yang Chair of the Supervisory Committee Assistant Professor Alexander Mamishev Department of Electrical Engineering Rapid development of microelectronics has led to high component density that has doubled every 12 months in the last decade Each semiconductor component emits heat associated with its electrical resistance With higher density of electronic components on a chip, heat sinks get denser and channels between them get narrower Existing cooling devices are not efficient because gases become viscous in narrow channels, which greatly hinders the air movement The problem of heat dissipation is one of the most profound obstacles in the electronics industry today The object of this thesis is to develop an electrostatic air pump that could be later incorporated into a chip structure for heat withdrawal from microelectronics and MEMS devices This thesis explores the possibility of building an electrostatic air pump used for cooling at chip level Numerical simulations are conducted for different device geometries and materials to achieve the optimal performance of air pumps Based on the results of simulations, several prototypes of the electrostatic air pump were built Measurements conducted to characterize this device included air velocity profile at the outlet, voltage-air speed relationship, current-voltage relationship, and air resistance Working efficiency of the device is calculated It is found that the efficiency of current air pump with single channel geometry has the same magnitude as that of traditional computer cooling fans At the same time, it has more efficient airflow profile and several other advantages compared to rotational computer fan A possibility of enhanced heat exchange through evaporation is explored Analytical model of forces involved in the dehumidification process of air pumps is being developed Comparison of columbic and dielectrophoretic forces is provided The latter is rarely discussed in framework of electrostatic devices, but may become a significant force component under certain conditions Future direction of this research project towards miniaturization of existing devices is proposed TABLE OF CONTENTS List of Figures List of Tables Acknowledgements Chapter Introduction 1.1 Background 1.2 Motivation 1.3 State of the art 1.3.1 Corona driven pump for air movement .6 1.3.2 Corona discharge 1.4 Thesis Outline .7 Chapter Basic principles of electrostatic air pump operation .9 2.1 Operation of the electrostatic air pump 2.2 Ion generation in gases 10 2.2.1 Properties of gas in corona discharge 11 2.2.2 Ionization processes 12 2.2.3 Mathematical description of corona discharge .12 2.3 Positive and negative corona discharges 13 2.3.1 Positive corona 14 2.3.2 Negative corona 15 2.4 Theoretical current-voltage relationship 15 2.5 Electric field distribution .17 2.6 Enhancement of heat exchange through water evaporation .19 2.6.1 Charging process .20 2.6.2 Electric drag 22 2.6.3 Stability of a charged liquid droplet 23 2.7 Advantages of corona technology in micro-cooling 24 Chapter Theoretical background .27 3.1 Comparison of forces acting on water droplets and particles in the air 27 3.2 Columbic force 27 3.3 Dielectrophoretic (polarization) forces 28 3.4 Biot-Savart force 35 i Chapter Device design and simulation 36 4.1 Simulation of a single pair electrodes air pump 36 4.1.1 Methodology 37 4.1.2 Results .37 4.2 Simulation on optimum air movement vs collection efficiency .44 4.3 Design and simulation of the air pump with channel geometry .46 4.3.1 Design of the air pump with channel geometry 46 4.3.2 Maxwell simulation of an air pump with single channel geometry 48 Chapter Experimental setup, measurements, and results 53 5.1 Experimental setup 53 5.2 Air speed profile on the outlet of the air pump 56 5.3 Voltage-air speed relationship 58 5.4 Current-voltage relationship and air resistance 59 5.5 Energy efficiency .60 Chapter Future research 62 6.1 Current problem 62 6.2 Future plans .63 Chapter Conclusions 65 References 66 ii LIST OF FIGURES Chapter Figure 1.1 Structural levels of a computer [1] Chapter Figure 1.2 Microchannels on silicon chip [1] Chapter Figure 1.3 Air-cooled multi-chip module used in IBM 4381 Processor [1] Chapter Figure 1.4 Heat generation trend for Pentium microprocessors .5 Chapter Figure 2.5 Principle of operation of corona air pump High voltage power supply (HVPS) provides required potential difference 10 Chapter Figure 2.6 Corona current-voltage relationship .11 Chapter Figure 2.7 Visual difference between positive corona and negative corona [28] 14 Chapter Figure 2.8 Electrostatic dehumidification technology 20 Chapter Figure 2.9 Corona air pump can be used for cooling of computer chips 25 Chapter Figure 2.10 Contrast of air movement profile difference between a traditional fan and corona-driven pump 26 Chapter Figure 2.11 Dynamic airflow pattern can be controlled through varying voltage distribution .26 Chapter Figure 3.12 Columbic force distribution of an air pump .28 Chapter Figure 3.13 Dielectrophoretic force in an electric field of corona air pump 29 Chapter Figure 3.14 Columbic force and dielectrophoretic force along the radial position for a single water molecule with the 1e- net charge 31 Chapter Figure 3.15 Large water conglomerates in a strong electric field became polarized and elongated 31 Chapter Figure 3.16 Relationship between electric field gradient, dipole value, and the corresponding dielectrophoretic force produced .32 Chapter Figure 3.17 Calculated electric field intensity displayed as a function of dimensionless radial distance from corona electrode without space charge 33 Chapter Figure 3.18 Calculated electric field intensity displayed as a function of dimensionless radial distance from corona electrode with space charge 33 Chapter Figure 3.19 Calculated columbic force displayed as a function of dimensionless radial distance from corona electrode without space charge 33 Chapter Figure 3.20 Calculated columbic force displayed as a function of dimensionless radial distance from corona electrode with space charge 34 iii Chapter Figure 3.21 Calculated dielectrophoretic force displayed as a function of dimensionless radial distance from corona electrode without space charge 34 Chapter Figure 3.22 Calculated dielectrophoretic force displayed as a function of dimensionless radial distance from corona electrode with space charge 34 Chapter Figure 4.23 Basic design concept of a corona air pump pair 36 Chapter Figure 4.24 Design I of ionic pump 38 Chapter Figure 4.25 Electric field and equipotential line plot of Design I 38 Chapter Figure 4.26 Force distribution between two electrodes in Design I .39 Chapter Figure 4.27 Design II of ionic pump 40 Chapter Figure 4.28 Electric field and equipotential line plot of Design II .40 Chapter Figure 4.29 Force distribution between two electrodes in Design II 41 Chapter Figure 4.30 Design III of ionic pump 42 Chapter Figure 4.31 Electric field and equipotential line plot of Design III .42 Chapter Figure 4.32 Force distribution between two electrodes in Design III 43 Chapter Figure 4.33 Geometry of a single pair of electrodes with possible non-linear voltage distribution at sidewalls 45 Chapter Figure 4.34 Field strength and voltage distribution of the electrode geometry for optimum air movement 45 Chapter Figure 4.35 Field strength and voltage distribution of the electrode geometry for optimum collecting efficiency .46 Chapter Figure 4.36 Corona electrodes are shielded with walls separating them .47 Chapter Figure 4.37 Channel geometry with film collector electrodes attached on sidewalls 48 Chapter Figure 4.38 Electric field and equipotential line distribution of Geometry I without space charge 50 Chapter Figure 4.39 Dielectrophoretic force distribution of Geometry I without space charge 50 Chapter Figure 4.40 Electric field and equipotential line distribution of Geometry I with constant space charge distribution 51 Chapter Figure 4.41 Dielectrophoretic force distribution of Geometry I with constant space charge distribution 51 Chapter Figure 4.42 Electric field and equip-potential line distribution of Geometry I with radially decreasing space charge distribution 52 Chapter Figure 4.43 Dielectrophoretic force distribution of Geometry I with radially decreasing space charge distribution 52 iv Chapter Figure 5.44 Experimental setup of a single channel air pump 53 Chapter Figure 5.45 The x-y-z translation stage to position the corona electrode 54 Chapter Figure 5.46 The corona electrode standing between collector electrodes .55 Chapter Figure 5.47 Semiconductive Kapton film attached to Teflon sheet forms the collector electrode .55 Chapter Figure 5.48 Zebra electrode: voltage gradient applied on insulating Kapton film through copper foil .56 Chapter Figure 5.49 Experimental setup with Zebra collector electrode 56 Chapter Figure 5.50 Air speed profile along the sidewall from the outlet 57 Chapter Figure 5.51 Air speed profile across the sidewall from the outlet 58 Chapter Figure 5.52 Measured corona voltage (Vc) vs air speed (lfm) on the outlet exhibits linear relationship 58 Chapter Figure 5.53 Measured corona voltage () vs current through collector electrode ( ) exhibits exponential dependence 59 Chapter Figure 5.54 Measured air resistance () as a function of corona voltage () 59 Chapter Figure 5.55 Energy efficiency as a function of input voltage 60 Chapter Figure 5.56 Fan efficiency in CFM/W as a function of input voltage 61 Chapter Figure 6.57 Contrast between the surface region without erosion and the region with erosion on a corona wire using SEM (Scanning Electron Microscopy) 63 v 54 Figure 5.45 The x-y-z translation stage to position the corona electrode Figure 46 shows a closer view of the single channel air pump A stainless steel razor corona electrode is positioned between two collector electrodes Insulating Kapton films attached around the razor are used to prevent breakdown of the high intensity corona field Two pieces of white Teflon sheets are used as the channel sidewalls Thin films made from different materials are attached on the sidewalls as collector electrodes Semi-conductive Kapton films are used as collector electrodes in Figure 47 In Figure 48, the collector electrode is made of copper foil paralleled on top of an insulating Kapton film We call it the “Zebra” electrode Each copper foil is wired to a specific node along a circuit, which generates a voltage gradient to optimize air movement The setup of it is shown in Figure 49 The wires stretching out of “Zebra” are connected to different nodes on a series circuit on a breadboard; the probe in front of DC voltage supply is a high voltage probe to measure the voltage applied on the corona electrode; the metal pole on the top of Teflon sidewalls is connected to an airflow sensor (VELOCICALC PLUS 8386, a multi-parameter ventilation meter) to measure outcoming air speed 55 Figure 5.46 The corona electrode standing between collector electrodes Figure 5.47 Semiconductive Kapton film attached to Teflon sheet forms the collector electrode 56 Figure 5.48 Zebra electrode: voltage gradient applied on insulating Kapton film through copper foil Figure 5.49 Experimental setup with Zebra collector electrode 5.2 Air speed profile on the outlet of the air pump Due to the symmetrical channel geometry of the corona air pump and the higher air movement resistance along the channel, the air speed has a non-uniform profile at the outlet along and across the channel sidewall The air speed along the channel sidewall can be seen in Figure 50 From the figure, it can be seen that two peaks of air speed are 57 present along the outlet This is not surprising since the airflow has the longest accelerating pass along those two directions The air speed across the channel sidewall of the outlet is shown in Figure 51 The peak is reached at the middle across the channel outlet The highest airflow speed of the air pump reached is 1115 lfm The unit of air speed here is lfm (linear feet per minute) It is much faster compared to conventional computer fan’s air speed of several hundred lfm 0 A ir s p e e d v a , ( lfm ) 0 0 0 0 0 0 0 0 0 M e a s u r in g p o s itio n a lo n g th e c h a n n e l s id e w a ll o n th e o u tle t , X ( m m ) Figure 5.50 Air speed profile along the sidewall from the outlet 58 0 A i r s p e e d v a , ( l fm ) 0 0 0 0 0 0 0 0 0 M e a s u r in g p o s it io n a c r o s s t h e c h a n n e l s id e w a ll o n t h e o u t le t , Y ( m m ) Figure 5.51 Air speed profile across the sidewall from the outlet 5.3 Voltage-air speed relationship Measurement of air speed (at the spot with the highest air speed) when increasing the corona electrode voltage shows a linear relationship as seen in Figure 52 The noticeable airflow is detected after kV After that, the air speed increases almost linearly with the voltage When the applied voltage exceeds 11 kV, sparks occur between the corona electrode and collector electrodes with the current device 0 A ir s p e e d , v a ( lf m ) 0 0 0 0 0 0 10 C o r o n a e le c t r o d e v o lt a g e , V 11 C 12 (k V ) Figure 5.52 Measured corona voltage (Vc) vs air speed (lfm) on the outlet exhibits linear relationship 59 5.4 Current-voltage relationship and air resistance Current grows exponentially as applied voltage is higher than corona onset voltage in the measurement shown in Figure 53, which is in good agreement with classic theory It can be seen that the corona onset voltage is 5.2 kV Figure 54 shows the dramatic decrease of air resistance in log scale as the voltage on the corona electrode increases 120 90 60 30 5 6 7 Figure 5.53 Measured corona voltage ( VC ) vs current through collector electrode ( I C ) exhibits exponential dependence Figure 5.54 Measured air resistance ( RA ) as a function of corona voltage ( VC ) 60 5.5 Energy efficiency The prototype is tested for its energy efficiency One traditional definition of energy efficiency is the percentage of the kinetic energy of generated airflow over the input electric power For the air pump prototype, the kinetic energy is calculated from the air speed measured at the outlet; the input electric power is multiplication of the voltage applied on the corona electrode and the current flowing through the collector electrode As it is shown from Figure 55, the efficiency of air pump according this definition is very low, below 0.5% When the input voltage increases, the energy efficiency decreases further E ffic ie n c y ( % ) 0 0 0 0 0 10 11 12 I n p u t v o lta g e V , ( k V ) Figure 5.55 Energy efficiency as a function of input voltage V Another common measure of fan efficiency in industry is to divide the airflow rating in cfm (cubit feet per minute) by the power consumption in watts Its physical meaning is very self-explanatory: for a given amount of input power, the more and faster airflow is generated, the more efficient the fan is The efficiency of the air pump according this definition is shown in Figure 56 It can be seen that at lower input voltage, the fan efficiency is much higher 61 F a n e ffic ie n c y , ( C F M / W ) 6 11 In p u t v o lta g e V , ( k V ) Figure 5.56 Fan efficiency in CFM/W as a function of input voltage V For traditional rotational fan, the fan efficiency in CFM/W is normally provided in the manufacturer’s literature that accompanies the fan Traditional computer cooling fan’s efficiency in this unit normally ranges from to 15 cfm per watt Also, a rotational fan with a small diameter has lower efficiency than a fan with a bigger diameter [38] Basically, the corona air pump we built has the efficiency in the same magnitude with conventional rotational computer fans, with even smaller size However, as mentioned before, the airflow profile of corona air pump along the channel makes it much more efficient for cooling than parabolic airflow profile of conventional fans Also, corona air pumps for cooling purpose have several other advantages: no moving parts, low noise, customizable airflow profiles and compatibility with chip structure Therefore, from the perspective of energy efficiency and other advantages, it is worthwhile to minimize the corona air pump, and eventually build a micro air pump to test for computer chip cooling 62 C h a p t e r Future research 6.1 Current problem In this thesis, a macro single channel air pump with different geometric parameter, and materials has been simulated, built, and studied in detail However, translation of the research results to micro-scale, needs extensive research efforts Such efforts include numerous system integration issues; the choice of materials for corona and collector electrodes; a method to apply the ionization voltage between the fins of computer chips Eliminating the generation of ozone or removal of ozone production needs to be addressed Ozone generation should be less than required by Occupational Safety & Health Administration (OSHA) Moreover, in the current macro-scale design, the working voltage is in the range of kilovolts, which does not fall in the common electronics voltage working range Of course, as the size of the air pump shrinks, the voltage should be able to decrease to an acceptable range, as discussed in Section 4.3.1.1 Another problem in this technology is the lifetime of corona electrodes They erode due to the bombardment or attachment of charged dust particles in the air Figure 57 shows the contrast between the surface region without erosion on a corona wire and the region with erosion The black spots in the second picture correspond to defects on the surface after erosion Therefore, corona electrodes need to be cleaned or replaced after working for a period of time The current component of stainless steel razors as corona electrodes are more robust However, an unevenly eroded razor tip generates unevenly distributed airflow or even sparks in the current device This problem needs to be investigated further 63 S u r fa c e w ith o u t e r o s io n S u r fa c e w ith e r o s io n Figure 6.57 Contrast between the surface region without erosion and the region with erosion on a corona wire using SEM (Scanning Electron Microscopy) 6.2 Future plans The next step in this project is to continue optimizing macro air pump cooling efficiency with different geometries, materials, and working conditions to reach higher energy efficiency One of the most significant aspects in choosing fabrication materials is the desire to reduce electrode erosion and eliminate ozone generation The relatively low voltage used in the device would still result in high electric field intensity near the electrode tip while ozone concentration is expected to be lower Currently, stainless steel is used as the corona electrode material Alternative materials that might perform better include silicon, tungsten, lanthanum hexaboride, and alumina Another possible technique to eliminate ozone generation and reduce electrode erosion is to use liquid electrodes The main focus of future research is to shrink the size of the corona pump and eventually build a prototype on the scale of a computer chip for integration The technological and physical limitations of electrostatic air pump technology at micron distances will be investigated System integration issues mentioned above should be addressed MEMS modeling software can be utilized to simulate a micro air pump and its performance with different design parameters After all these, the design should be mature enough to be used to build the first prototype 64 Prototype testing and evaluation require special procedures that will be developed separately and require additional manufacturing steps The direct gas flow measurement at micron scale is difficult and, therefore, requires indirect methods Several test procedures should be developed during the time of prototype design and manufacturing, including measurement of ozone and evaluation of cooling efficiency C h a p t e r Conclusions This thesis explores the possibility of building an electrostatic air pump for enhancement of heat withdrawal from microelectronics and MEMS at chip level Corona-driven air pumps may serve as a catalyst for the next generation of high-density microelectronics Its main advantages include elimination of moving parts, low noise, dynamic airflow profiles, versatile shape and sizes, and compatibility with chip structure In the device modeling conducted in this thesis, different geometry variations of the air pump are simulated with finite-element software The channel geometry with the razor corona electrode is found to be one of the most promising designs for heat removal A prototype of air pump of this kind has been built on macro-scale for investigation The air speed measurement at the outlet confirmed laminar nature of airflow and uniform speed distribution in the cross-section of the air pump It has also been found that the measured air speed increases linearly as the applied voltage increases Measured current flowing through the device grows exponentially with increasing voltage after the corona onset voltage as expected The airflow speed of this air pump is about 1115 lfm (linear feet per minute), which is much higher than the speed of a conventional fan (several hundred lfm) Future research directions include optimization of geometry and driving electronics; system integration; numerical simulation of the electrodynamics of moving media in micro-scale; and a micro-scale fabrication REFERENCES [1] A E Bergles, Heat Transfer in Electronic and Microelectronic Equipment, Troy, New York, USA, Hemisphere Publishing Corporation, 1990 [2] Xie, J., Ali, A., and Bhatia, R., The Use of Heat Pipes 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