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Heat transfer enhancement using flow-induced vibration of a microfin array
Sensors and Actuators A 90 (2001) 232±239
Heat transfer enhancement using ¯ow-induced vibration of a micro®n array Jeung Sang Goa,b,*,1, Sung Jin Kima, Geunbae Limb, Hayong Yunb, Junghyun Leeb, Inseob Songb, Y. Eugene Pakb a
Department of Mechanical Engineering, KAIST, 373-1 Kusong-dong, Yusong-ku, Taejon 305-701, South Korea b MEMS Laboratory, Samsung Advanced Institute of Technology (SAIT), San 14-1 Nongseo-ri Kiheung-up Yongin City Kyunggi-do, 449-712, South Korea Accepted 9 February 2001
Abstract Advanced computers are facing thermal engineering challenges from both high heat generation due to rapid performance improvement and the reduction of an available heat removal surface due to large packaging density. Ef®cient cooling technology is desired to provide reliable operation of microelectronic devices. This paper investigates the feasibility of heat transfer enhancement in laminar ¯ow using the ¯ow-induced vibration of a micro®n array. The micro®ns are initially bent due to the residual stress difference. In order to characterize the dynamics of the micro®n ¯ow-induced vibration, a micro®n sensor is fabricated. Increase in air velocity provides larger vibrating de¯ection, while the vibrating frequency of the micro®n is independent of the air velocity. The thermal resistances are measured to evaluate the thermal performance of the micro®n heat sink and compared with those of a plain-wall heat sink. For a ¯uid velocity of 4.4 m/s, the thermal resistance of the micro®n array heat sink is measured to be 4.458C/W and that of the plain-wall heat sink to be 4.698C/W, which indicates a 5.5% cooling enhancement. At a ¯ow velocity of 5.5 m/s, the thermal resistance of the micro®n array heat sink is decreased by 11.5%. From the experimental investigations, it is concluded that the vibrating de¯ection plays a key role in enhancing the heat transfer rate. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Micro®n array; Flow-induced vibration; Hydrodynamic mixing; Thermal resistance; Heat transfer enhancement
1. Introduction Advanced computers are facing thermal engineering challenges from both high heat generation and reduction of an available heat removal surface area. The consumer's demand for high performance computers leads to high clock speed and large circuit integration for multifunction. As a result, heat generation from electronic components has been remarkably increased [1]. In addition, compact size and large packaging density have reduced the overall size of a computing system, which results in the decrease of the available cooling surface area. The working temperature of the electronic components may exceed the desired temperature level in the absence of suf®cient heat removal. The elevated temperature also causes an increase in system *
Corresponding author. Present address: MEMS Laboratory, SAIT, Kyunggi-do 449-712, South Korea, Tel.: 82-31-280-6942; fax: 82-31-280-6955. E-mail addresses: [email protected], [email protected] (J.S. Go). 1 Tel.: 82-42-869-3083; fax: 82-42-869-3095.
failure rate. Therefore, the employment of a high performance computing system requires ef®cient and compact cooling technology to provide reliable system operation. Despite the high heat removal capability of liquids, air is commonly used in advanced computer cooling for reasons of cost-effective implementation and simple assembly. According to Newton's law of cooling, two conventional strategies can be employed to improve the forced convection heat transfer rate under the speci®ed temperature difference. One strategy involves extending the surface area of a heat sink. However, the high packaging density in advanced computers limits the surface area extension of the heat sink. An alternative strategy is the enhancement of the heat transfer coef®cient. Roughness elements and vortex generators may help to overcome the constraint of available heat removal surface area extension. The heat transfer mechanism of roughness elements is closely related with the ¯ow pattern around them. As the ¯ow reattaches and impinges on the roughness elements, the heat transfer rate can be improved [2,3]. In laminar ¯ow, however, the rough elements decrease the heat
0924-4247/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 ( 0 1 ) 0 0 5 2 2 - 2
J.S. Go et al. / Sensors and Actuators A 90 (2001) 232±239
transfer rate rather than augmenting it since they provide an obstruction to a main through-¯ow [4]. It is demonstrated that a signi®cant heat transfer enhancement is achieved only when the height of the roughness elements is larger than the thermal boundary layer thickness in laminar ¯ow [5]. A halfdelta-wing vortex generator also increases the heat transfer coef®cient by hydrodynamic mixing obtained from vortex generation. A counter rotating vortex generator with 1±3 cm height provided a 24% increase of the local heat transfer in turbulent ¯ow [6]. However, direct application of the conventional vortex generator to the heat sink is not appropriate due to its size. Based on the heat transfer enhancement that results from using the roughness elements and the vortex generator, it can be reasoned that the hydrodynamic mixing is effective in improving the heat transport. To this end, vibrating actuators have been introduced for active hydrodynamic mixing. The experimental measurements of the heat and mass transfer around a PZT-actuated vibrating ¯at plate of 2:0 cm 7:5 cm were performed in a channel for Reynolds numbers ranging from 130 to 350 [7]. It was shown that the vibrating plate increased the heat transfer rate signi®cantly and also controlled the boundary layer thickness effectively. Efforts to reduce frictional drag in turbulent ¯ow have been attempted, while heat transfer enhancement by using microactuators has not been reported. A magnetic microactuator for a delta-wing rolling motion control created vortices along the leading edge and controlled the ¯uid boundary layer [8]. Even though magnetic actuation provides a large stroke of vibration due to the strong force, it is not applicable to advanced computer cooling since the microelectronic devices are sensitive to the strong magnetic ®eld. Piezoelectrically driven microcantilevers were fabricated to generate counter-rotating vortices and to control the turbulent boundary layer [9]. However, the PZT driven actuators have disadvantages in the complex fabrication process and the small displacement. In this paper, we propose a novel heat transfer enhancement method in laminar ¯ow regime. We fabricate a bulkmicromachined micro®n array heat sink, which goes through the ¯ow-induced vibration of the micro®n array for hydrodynamic mixing. In addition, a micro®n sensor characterizes the dynamics of the micro®n vibration. The thermal performance of the micro®n array heat sink is experimentally evaluated by comparing its thermal resistances with those of a plain-wall heat sink fabricated with the same material. 2. Theoretical design Fig. 1 shows the schematic view of a heat sink with the ¯ow-induced vibrating micro®n array. The micro®n with a ¯ush-mounted cavity consists of two dissimilar thin ®lms and is initially bent due to the built-in residual stress difference. The initial de¯ection of the micro®n
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Fig. 1. Schematic view of a microfin array heat sink.
plays a key role in structural vibration in the presence of ¯uid ¯ow. We use a bimorph micro®n with different residual stresses in order to generate the initial de¯ection of the micro®n. The bimorph micro®n is composed of an upper ®lm of Tb±Fe alloy and a lower ®lm of p-silicon. The measured micromechanical properties of p-silicon show a Young's modulus of 125 GPa and a tensile residual stress of 77 MPa [10]. The Tb±Fe alloy with a Young's modulus of 76 GPa and a compressive residual stress of 21.5 MPa [11] is deposited on the p-silicon layer. Hence, the upward initial de¯ection can be obtained due to the moment caused by the residual stress difference. By applying the elastic energy method, the initial de¯ection of the micro®n can be expressed as [10] Ms L2 dmax P 2 Ej Ij
(2.1)
where Ms denotes the moment caused by two different residual stresses, L the microfin length and EjIj the elastic stiffness of each thin film. To ensure a safe structural design, the maximum stress applied on the micro®n should be smaller than the material yield strength. The maximum stress occurs at the bottom of the p-silicon layer and is obtained as Ej y j M s smax P syield Ej I j
(2.2)
where yjis the distance from the neutral axis, Ej theYoung's modulus of the material and syield, the yield strength. Bulk-micromachining techniques were used to fabricate the micro®n array heat sink. The fabrication process limits the con®guration of the micro®n because the silicon is etched anisotropically. Under the fabrication constraints, a side gap around the micro®n is required to provide a vibration space in the trench of the silicon substrate. Here, we consider a 100 mm-wide side gap. For the micro®n actuation, we propose a ¯ow-induced actuation. In the presence of ¯uid ¯ow, any suf®ciently blunt structure sheds vortices. As vortices alternately shed, the distributed pressure acting on the blunt structure periodically oscillates [12]. Periodic oscillating pressure causes the elastically mounted blunt structure to vibrate. In the micro®n actuation, the periodic drag force caused by the periodic
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pressure acts on the micro®n and generates the ¯ow-induced vibration of the micro®n. The lift force on the micro®n is ignored since the thickness of the micro®n is small compared to the width. The ¯ow-induced vibrating frequency of the micro®n depends on the vortex shedding frequency ws S
U D
(2.3)
where S indicates the Strouhal number, D the characteristic length, and U the wind speed normal to the microfin. The vibrating frequency of the microfin can not be calculated directly from Eq. (2.3) since information on the Strouhal number S is still missing. Even though a heat transfer model for a micro®n array heat sink using ¯ow-induced vibration has not been reported, it is expected that an increase in the number of the micro®ns induces more active hydrodynamic mixing so that the heat transfer rate of the micro®n array heat sink can be enhanced. For this reason, the micro®ns are positioned as closely as possible in order to increase the number of the micro®ns under the structural limit and fabrication process constraints. The thermal performance of the micro®n array heat sink is experimentally evaluated by measuring the thermal resistance based on Newton's law of cooling Rth
Tw
T1 Q
1 hA
(2.4)
where Tw means the wall temperature, T1 the coolant temperature, Q the applied electrical power to a thermofoil heater, h the convection heat transfer coefficient, and A the area wetted by fluid. The slope obtained from the measured values of several combinations of the measured temperature difference and the applied electrical power represents the thermal resistance of the heat sink. In addition, the thermal performance of the microfin array heat sink is compared with that of the plain-wall heat sink fabricated from the same material.
Fig. 2. Microfabrication processes of an initially bent bimorph microfin.
micro®n by using corner compensation. The fabrication process is completed by the ®nal step shown in Fig. 2f, in which a 0.5 mm-thick Tb±Fe alloy is deposited on the psilicon micro®n and the initial de¯ection induced by the residual stress difference is obtained simultaneously. A SEM photograph of the initially de¯ected micro®n array is shown in Fig. 3. The fabricated micro®n was 2 mm long and 100 mm wide. We measured a total micro®n thickness of 3.7 mm from the SEM photograph. By using the optical microscope, the initial de¯ection of the micro®n was also measured to be about 250 mm. Based on the structural analysis of the fabricated micro®n, a maximum stress of 36.3 MPa was applied on the p-Si when considering the 100 mm-peak vibrating de¯ection. The natural frequencies can be estimated as 1.28, 8.05 and 22.55 kHz.
3. Microfabrication process Fig. 2 illustrates a single mask micromachining process for an initially de¯ected micro®n array heat sink of 5 cm 5 cm. In the bulk silicon micromachining process, EDP is used as an anisotropic silicon etching solution and p-silicon as an etch-stop layer for the micro®n de®nition. The fabrication starts with the thermal growth of a 1 mmthick silicon dioxide ®lm that acts as a barrier for the subsequent deep boron diffusion process. In step (b) of Fig. 2, the microcantilever beam is de®ned by patterning the silicon dioxide. Fig. 2c shows the 12 h deep boron diffusion process for the p-silicon micro®n. Step (d) of Fig. 2 includes the removal of the silicon dioxide and a boron rich glass in BOE solution. In Fig. 2e, anisotropic silicon etching is illustrated. This step is performed to de®ne the
Fig. 3. SEM photograph of the fabricated microfin array with initial deflection.
J.S. Go et al. / Sensors and Actuators A 90 (2001) 232±239
4. Experimental evaluation For the experimental investigation, we fabricated a heat sink with the initially de¯ected micro®n array. The ¯owinduced actuation of the micro®n array for hydrodynamic mixing was observed and measured. Finally, the thermal performance of the micro®n array heat sink was evaluated and compared with that of a plain heat sink. Fig. 4 illustrates the test apparatus of the micro®n array heat sink. The specimen for thermal performance measurement consists of the micro®n array heat sink, a 3 mm-thick copper heat spreader and a thermofoil heater with a diameter of 2.5 cm. Each component was attached with a 0.13 mmthick adhesive tape (3M, 9885) with a relatively high thermal conductivity of 0.5 W/m-K. Three K-type thermocouples (OMEGA, CHAL-010) with the same spacing along the ¯ow direction were instrumented on the upper surface of the copper heat spreader to measure the temperature distribution. A layer with low thermal conductivity (6 mmthick commercial styrofoam) was attached to the backside of the thermofoil heater to minimize heat loss. Electrical power was applied to the thermofoil heater through a DC power supply (HP, E3620A). The uncertainty analysis was estimated at a 95% con®dence level [13]. 4.1. Flow-induced vibration test of the microfin The micro®n vibrates due to the periodic pressure distribution acting on the elastic micro®n in the presence of the
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¯uid ¯ow. A high-speed motion analyzer observed and recorded the vibration of the micro®n array. Fig. 5 shows the photograph of the ¯ow-induced vibration of the micro®n in the ¯uid ¯ow. In the absence of air¯ow, the micro®n array did not oscillate. As soon as air was blown over the micro®n, the micro®n started to vibrate. A larger stroke of the vibrating de¯ection was obtained as the ¯uid velocity increased. Due to the aliasing effect resulting from the limitation of the camera performance, the dynamics of the micro®n vibration could not be read. In order to overcome this, a micro®n with a piezoresistive sense element (micro®n sensor hereafter) was fabricated in order to characterize the dynamics of the micro®n vibration. The micro®n sensor was bonded on a PCB (printed circuit board) for signal processing by using aluminum wire as shown in Fig. 6. The measured electrical resistances of the sensing resistors were 2.33 and 2.30 kO, respectively. The resistances of the reference resistors were also measured as 2.36 and 2.38 kO, respectively. The analytical natural frequencies of the fabricated micro®n sensor are estimated as 1.17, 7.34 and 20.54 kHz. A dynamic signal analyzer (HP, 35670A) and an oscilloscope (Tektronix, TDS 784A) read the outputs of the micro®n sensor vibration. Speci®cally, the dynamic signal analyzer determines the dominant vibrating frequency of the micro®n by measuring the power spectral density. The ¯owinduced vibration of the micro®n sensor for air velocities of 3, 4, 5 and 6 m/s was examined. An anemometer (KANOMAX, 6631) was also placed to measure the air velocity.
Fig. 4. Heat sink configuration for the thermal performance measurement.
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Fig. 7. Measured power spectral densities of the microfin vibration.
Fig. 5. Measured flow-induced vibration of a microfin. (a) opticallyfocused microfin before air blowing; (b) microfin vibration after air blowing.
By analyzing the power spectral density of the micro®n vibration (Fig. 7) for four different air velocities, it was determined that the vibrating frequency of the micro®n was 1.173 kHz and independent of the air velocity. Through a comparison of the measured vibrating frequency with the natural frequency of 1.17 kHz, which was obtained theoretically from the structural analysis, it was also determined that the micro®n sensor vibrated at the fundamental natural frequency. Fig. 8 shows that the second mode of the micro®n vibration appeared at 7.35 kHz at an air velocity of 6 m/s.
Fig. 6. Microfin sensor bonded on the PCB by using aluminum wire.
The voltage outputs during 10 ms were read for observing the long-term behavior of the micro®n sensor vibration. The magnitude of the voltage output indicates the vibrating de¯ection qualitatively. As shown in Fig. 9, the histogram of the micro®n vibration con®rms that the micro®n does not vibrate with a constant vibrating de¯ection. Fig. 10 illustrates the measured peak-to-peak voltage outputs of the micro®n sensor and the vibrating de¯ections analytically estimated at the measured vibrating frequency of 1.173 kHz. While the estimated vibrating de¯ection continued to increase, the measured voltages tended to saturate with the increasing air velocity. It is assumed that the saturation of the vibrating de¯ection resulted from the lock-in phenomenon of the micro®n vibration as was experimentally shown in the torsional vibrations of a plate [12].
Fig. 8. Measured power spectral density for an air velocity of 6 m/s.
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Fig. 11. Measured thermal performances at the fluid velocity of 4.4 m/s. Fig. 9. Histogram of the microfin sensor output for an air velocity of 3 m/s.
The thermal performance of the micro®n array heat sink can be evaluated by measuring the thermal resistance. The measurement of the thermal resistance in the micro®n array heat sink was performed at ¯ow velocities of 4.4 and 5.5 m/s and compared with that in a plain-wall heat sink. A micronanometer (Furnace, FCO-12) was used for velocity measurement. Figs. 11 and 12 show the measured thermal performances. Temperature differences measured from three thermocouples indicated that thermal spreading resistance in the copper heat spreader was small. At an air velocity of 4.4 m/s, the thermal resistance of the micro®n array heat sink was 4.45 (with uncertainty of 0.089)8C/W, which was averaged from the three values measured from the three thermocouples. The measured thermal resistance of the plain-wall heat sink was 4.69 (0.092)8C/W, indicating a 5.5% enhancement in cooling performance. At an air
velocity of 5.5 m/s, the micro®n array heat sink showed a thermal resistance of 4.05 (0.257)8C/W compared with 4.56 (0.052)8C/W of the plain-wall heat sink, which means an increase of 11.5% in the heat transfer rate. From the measured thermal performance results, increase in the ¯uid velocity from 4.4 to 5.5 m/s provided a larger vibrating de¯ection of the micro®n, which in turn improved the heat transfer rate. It is therefore determined that a larger vibrating de¯ection in the micro®n array heat sink plays a key role in enhancing the heat transfer rate. The improvement of heat transfer in the micro®n array heat sink may be achieved by the ¯ow-induced vibration of the micro®n array and partly by the increased surface area obtained by introducing the micro®n array. In order to determine the effect of the increased surface area on the heat transfer rate, we removed the micro®n array from the heat sink and compared the thermal performance with that of the plain heat sink. The comparison test was performed at an air velocity of 6 m/s, which was the maximum air velocity of the wind tunnel. The experimental results showed that the
Fig. 10. Peak-to-peak output voltages and estimated vibrating deflections for different air velocity.
Fig. 12. Measured thermal performances at the fluid velocity of 5.5 m/s.
4.2. Thermal performance measurement
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References
Fig. 13. Comparison of the thermal performance of the plain and finremoved heat sink.
increased surface area did not enhance the heat transport as shown in Fig. 13. The ratio of the Colburn factor to the friction factor was not considered since the pressure drop was too small to measure. Also, the robustness of the micro®n array heat sink against dust was tested by exposing it to air¯ow for 3 days. It was determined that the micro®n array heat sink is very resistant to dust due to the ¯ow-induced vibration of the micro®n array. 5. Conclusions A micro®n array heat sink using the ¯ow-induced vibration of a micro®n array was investigated to identify its effect on heat transfer enhancement in laminar ¯ow regime. Additionally, a micro®n sensor was also fabricated to determine the dynamics of the micro®n vibration. The micro®n array heat sink was fabricated by using bulkmicromachining technology. The initial de¯ection of the micro®n for the ¯ow-induced vibration was obtained from the residual stress difference in two dissimilar thin ®lms. By measuring the thermal resistance, the thermal performance of the micro®n array heat sink was compared with that of a plain-wall heat sink fabricated with the same material. Based on the comparison of the thermal performance, it was determined that an increase in the air velocity improves the heat transport in the micro®n array heat sink. From the dynamics of the fabricated micro®n sensor, it is shown that the micro®n vibrates at resonance and the vibrating frequency of the micro®n is independent of the air velocity. By increasing the air velocity, the vibrating de¯ection of the micro®n is increased. Based on the dynamics of the micro®n vibration and the thermal performance comparison, it is concluded that the vibrating de¯ection of the micro®n plays a key role in enhancing the heat transfer rate from the micro®n array heat sink.
[1] R.C. Chu, Heat transfer in electronic systems, in: Proceedings of the 8th International Conference on Heat Transfer, New York, USA 1986, pp. 293±305. [2] D.A. Aliaga, J.P. Lamb, E.E. Klein, Convection heat transfer distributions over plates with square ribs from infrared thermography measurement, Int. J. Heat Mass Transfer 37 (3) (1994) 363±374. [3] J. Davalath, Y. Bayazitoglu, Forced convection cooling across rectangular blocks, J. Heat Transfer 109 (1987) 321±328. [4] F.J. Rowley, S.V. Patankar, Analysis of laminar flow and heat transfer in tubes with internal circumferential fins, Int. J. Heat Mass Transfer 27 (4) (1984) 553±560. [5] R.C. Henry, R.J. Hansman Jr., K.S. Breuer, Heat transfer variation on protuberances and surface roughness elements, J. Thermophys. Heat Transfer 9 (1) (1995) 175±180. [6] P.A. Eibeck, J.K. Eaton, Heat transfer effects of a longitudinal vortex embedded in a turbulent boundary layer, J. Heat Transfer 109 (1987) 16±24. [7] T. Tsutsui, M. Akiyama, H. Sugiyama, Active boundary layer control by elastically vibrating flat plate, JSME 61 (585) (1995) 216±222. [8] C. Liu, T. Tsao, Y.-C. Tai, Out-of-plane permalloy magnetic actuators for delta-wing control, in: Proceedings of the IEEE Micro Electro Mechanical Systems Workshop (MEMS'95), Amsterdan; Netherland 1995, pp. 7±12. [9] S.M. Kumar, W.C. Reynolds, T.W. Kenny, MEMS-based transducers for boundary layer control, in: Proceedings of the IEEE Micro Electro Mechanical Systems Workshop (MEMS'99), Orlando, Florida, USA, 1999, pp. 135±140. [10] J.S. Go, Y.-H. Cho, B.M. Kwak, K. Park, Snapping microswitches with adjustable acceleration threshold, Sens. Actuators A54 (1996) 579±583. [11] E. Quandt, B. Gerlach, T. Gerst, K. Seemann, Magnetostrictive thin film actuators, in: Proceedings of the 4th International Conference on New Actuators, Bremen, Germany 1994, pp. 229±231. [12] R.D. Blevins, Flow-Induced Vibration, Van Nostrand Reinhold, New York, 1977. [13] S.F. Richard, E.B. Donald, Theory and Design for Mechanical Measurements, 2nd Edition, Wiley, New York, 1995.
Biographies Jeung Sang Go received a BS degree from Pusan National University in 1993 and an MS degree from the Korea Advanced Institute of Science and Technology (KAIST) in 1995. His major was Mechanical Engineering. Beginning from May 2000, he has been working at the Samsung Advanced Institute of Technology (SAIT) in Korea, where he is conducting research on MicroCooling Systems using micro electro mechanical systems (MEMS). In addition, he is also currently a graduate student at KAIST working on a PhD in Mechanical Engineering. His research interests include microfluidics and micro heat transfer using MEMS technology and its applications. Dr. Sung Jin Kim is an Associate Professor in the Department of Mechanical Engineering at the Korea Advanced Institute of Science and Technology (KAIST). Until July 1997, he was a group leader in the Thermal Engineering Center at the IBM Tucson Laboratory where he had worked for almost 8 years. He was responsible for the investigation and development of advanced cooling techniques for application to electronic systems. He received a PhD degree in
J.S. Go et al. / Sensors and Actuators A 90 (2001) 232±239 Mechanical Engineering from Ohio State University in 1989. He has received two IBM Invention Achievement Awards and five Author Recognition Awards, holds four patents, and has published 30 papers in the area of convective heat transfer. Also, he recently edited a book entitled Air Cooling Technology for Electronic Equipment. Dr. Hayong Yun (M'99) received his BS degree from Seoul National University in 1991, and MS and PhD degrees from M.I.T in 1993 and 1995, respectively. His major was Mechanical Engineering. From September 1991 to September 1995, he was a research assistant in the Cryogenic Engineering Laboratory at M.I.T. In January 1996, he started work at Samsung Electronics Co. in Korea as a senior engineer. In January 1999, he moved to the Samsung Advanced Institute of Technology in Korea, where he is currently conducting research on micro electro mechanical systems (MEMS) at the MEMS Laboratory. Dr. Geunbae Lim (Non-member) received his BS and MS degrees in Electrical Engineering from Yeungnam University, Korea, in 1990 and 1992, respectively. He received his PhD degree in Precision Engineering from Tohoku University, Japan, in 1996 and then joined the Microsystems Laboratory, SAIT, Korea. His research interests are in the areas of BioMEMS and nanomachining. Dr. Junghyun Lee received his BS degree in Mechanical Engineering from Yonsei University, Seoul in 1986; and MS and PhD degrees in Aerospace Engineering from Case Western Reserve University, Cleveland, Ohio in 1990 and 1994, respectively. He was awarded the National Research Council Research Fellowship
239
from 1993±1996. He worked as NRC Resident Research Associate at the NASA Lewis Research Center in Cleveland, Ohio from 1993±1996. He is currently working as a member of the Research Staff at the Samsung Advanced Institute of Technology MEMS laboratory and as Micro Thermo Fluid Elementary Technical Leader. Dr. Inseob Song received his BS degree from Hanyang University, Korea in 1985, and MS and PhD degrees from the Korea Advanced Institute of Science and Technology (KAIST), Korea in 1988 and 1994, respectively. His major was Mechanical Engineering. From September 1993 to January 1996, he was employed as a senior engineer at Samsung Heavy Industries. Since February 1996, he has been working at the Samsung Advanced Institute of Technology (SAIT) in Korea as a member of the Research Staff. He is currently conducting research on micro cooling systems in the MEMS Laboratory at SAIT. Dr. Y. Eugene Pak received his BS degree in Mechanical Engineering from the State University of New York at Buffalo in 1980, and MS and PhD degrees in Mechanical Engineering from Stanford University in 1982 and 1985, respectively. He joined the Grumman Corporate Research Center in 1985 where he carried out research in micromechanics of smart material. In 1994, he took an academic position at the State University of New York at Stonybrook. In 1995, he joined the Samsung Advanced Institute of Technology (SAIT) where he is currently serving as Director of the MEMS Laboratory. Dr. Pak's research interests include Micro and Nanoscale Mechanics, MEMS reliability and Biotechnolgy.
Heat transfer enhancement using ¯ow-induced vibration of a micro®n array Jeung Sang Goa,b,*,1, Sung Jin Kima, Geunbae Limb, Hayong Yunb, Junghyun Leeb, Inseob Songb, Y. Eugene Pakb a
Department of Mechanical Engineering, KAIST, 373-1 Kusong-dong, Yusong-ku, Taejon 305-701, South Korea b MEMS Laboratory, Samsung Advanced Institute of Technology (SAIT), San 14-1 Nongseo-ri Kiheung-up Yongin City Kyunggi-do, 449-712, South Korea Accepted 9 February 2001
Abstract Advanced computers are facing thermal engineering challenges from both high heat generation due to rapid performance improvement and the reduction of an available heat removal surface due to large packaging density. Ef®cient cooling technology is desired to provide reliable operation of microelectronic devices. This paper investigates the feasibility of heat transfer enhancement in laminar ¯ow using the ¯ow-induced vibration of a micro®n array. The micro®ns are initially bent due to the residual stress difference. In order to characterize the dynamics of the micro®n ¯ow-induced vibration, a micro®n sensor is fabricated. Increase in air velocity provides larger vibrating de¯ection, while the vibrating frequency of the micro®n is independent of the air velocity. The thermal resistances are measured to evaluate the thermal performance of the micro®n heat sink and compared with those of a plain-wall heat sink. For a ¯uid velocity of 4.4 m/s, the thermal resistance of the micro®n array heat sink is measured to be 4.458C/W and that of the plain-wall heat sink to be 4.698C/W, which indicates a 5.5% cooling enhancement. At a ¯ow velocity of 5.5 m/s, the thermal resistance of the micro®n array heat sink is decreased by 11.5%. From the experimental investigations, it is concluded that the vibrating de¯ection plays a key role in enhancing the heat transfer rate. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Micro®n array; Flow-induced vibration; Hydrodynamic mixing; Thermal resistance; Heat transfer enhancement
1. Introduction Advanced computers are facing thermal engineering challenges from both high heat generation and reduction of an available heat removal surface area. The consumer's demand for high performance computers leads to high clock speed and large circuit integration for multifunction. As a result, heat generation from electronic components has been remarkably increased [1]. In addition, compact size and large packaging density have reduced the overall size of a computing system, which results in the decrease of the available cooling surface area. The working temperature of the electronic components may exceed the desired temperature level in the absence of suf®cient heat removal. The elevated temperature also causes an increase in system *
Corresponding author. Present address: MEMS Laboratory, SAIT, Kyunggi-do 449-712, South Korea, Tel.: 82-31-280-6942; fax: 82-31-280-6955. E-mail addresses: [email protected], [email protected] (J.S. Go). 1 Tel.: 82-42-869-3083; fax: 82-42-869-3095.
failure rate. Therefore, the employment of a high performance computing system requires ef®cient and compact cooling technology to provide reliable system operation. Despite the high heat removal capability of liquids, air is commonly used in advanced computer cooling for reasons of cost-effective implementation and simple assembly. According to Newton's law of cooling, two conventional strategies can be employed to improve the forced convection heat transfer rate under the speci®ed temperature difference. One strategy involves extending the surface area of a heat sink. However, the high packaging density in advanced computers limits the surface area extension of the heat sink. An alternative strategy is the enhancement of the heat transfer coef®cient. Roughness elements and vortex generators may help to overcome the constraint of available heat removal surface area extension. The heat transfer mechanism of roughness elements is closely related with the ¯ow pattern around them. As the ¯ow reattaches and impinges on the roughness elements, the heat transfer rate can be improved [2,3]. In laminar ¯ow, however, the rough elements decrease the heat
0924-4247/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 ( 0 1 ) 0 0 5 2 2 - 2
J.S. Go et al. / Sensors and Actuators A 90 (2001) 232±239
transfer rate rather than augmenting it since they provide an obstruction to a main through-¯ow [4]. It is demonstrated that a signi®cant heat transfer enhancement is achieved only when the height of the roughness elements is larger than the thermal boundary layer thickness in laminar ¯ow [5]. A halfdelta-wing vortex generator also increases the heat transfer coef®cient by hydrodynamic mixing obtained from vortex generation. A counter rotating vortex generator with 1±3 cm height provided a 24% increase of the local heat transfer in turbulent ¯ow [6]. However, direct application of the conventional vortex generator to the heat sink is not appropriate due to its size. Based on the heat transfer enhancement that results from using the roughness elements and the vortex generator, it can be reasoned that the hydrodynamic mixing is effective in improving the heat transport. To this end, vibrating actuators have been introduced for active hydrodynamic mixing. The experimental measurements of the heat and mass transfer around a PZT-actuated vibrating ¯at plate of 2:0 cm 7:5 cm were performed in a channel for Reynolds numbers ranging from 130 to 350 [7]. It was shown that the vibrating plate increased the heat transfer rate signi®cantly and also controlled the boundary layer thickness effectively. Efforts to reduce frictional drag in turbulent ¯ow have been attempted, while heat transfer enhancement by using microactuators has not been reported. A magnetic microactuator for a delta-wing rolling motion control created vortices along the leading edge and controlled the ¯uid boundary layer [8]. Even though magnetic actuation provides a large stroke of vibration due to the strong force, it is not applicable to advanced computer cooling since the microelectronic devices are sensitive to the strong magnetic ®eld. Piezoelectrically driven microcantilevers were fabricated to generate counter-rotating vortices and to control the turbulent boundary layer [9]. However, the PZT driven actuators have disadvantages in the complex fabrication process and the small displacement. In this paper, we propose a novel heat transfer enhancement method in laminar ¯ow regime. We fabricate a bulkmicromachined micro®n array heat sink, which goes through the ¯ow-induced vibration of the micro®n array for hydrodynamic mixing. In addition, a micro®n sensor characterizes the dynamics of the micro®n vibration. The thermal performance of the micro®n array heat sink is experimentally evaluated by comparing its thermal resistances with those of a plain-wall heat sink fabricated with the same material. 2. Theoretical design Fig. 1 shows the schematic view of a heat sink with the ¯ow-induced vibrating micro®n array. The micro®n with a ¯ush-mounted cavity consists of two dissimilar thin ®lms and is initially bent due to the built-in residual stress difference. The initial de¯ection of the micro®n
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Fig. 1. Schematic view of a microfin array heat sink.
plays a key role in structural vibration in the presence of ¯uid ¯ow. We use a bimorph micro®n with different residual stresses in order to generate the initial de¯ection of the micro®n. The bimorph micro®n is composed of an upper ®lm of Tb±Fe alloy and a lower ®lm of p-silicon. The measured micromechanical properties of p-silicon show a Young's modulus of 125 GPa and a tensile residual stress of 77 MPa [10]. The Tb±Fe alloy with a Young's modulus of 76 GPa and a compressive residual stress of 21.5 MPa [11] is deposited on the p-silicon layer. Hence, the upward initial de¯ection can be obtained due to the moment caused by the residual stress difference. By applying the elastic energy method, the initial de¯ection of the micro®n can be expressed as [10] Ms L2 dmax P 2 Ej Ij
(2.1)
where Ms denotes the moment caused by two different residual stresses, L the microfin length and EjIj the elastic stiffness of each thin film. To ensure a safe structural design, the maximum stress applied on the micro®n should be smaller than the material yield strength. The maximum stress occurs at the bottom of the p-silicon layer and is obtained as Ej y j M s smax P syield Ej I j
(2.2)
where yjis the distance from the neutral axis, Ej theYoung's modulus of the material and syield, the yield strength. Bulk-micromachining techniques were used to fabricate the micro®n array heat sink. The fabrication process limits the con®guration of the micro®n because the silicon is etched anisotropically. Under the fabrication constraints, a side gap around the micro®n is required to provide a vibration space in the trench of the silicon substrate. Here, we consider a 100 mm-wide side gap. For the micro®n actuation, we propose a ¯ow-induced actuation. In the presence of ¯uid ¯ow, any suf®ciently blunt structure sheds vortices. As vortices alternately shed, the distributed pressure acting on the blunt structure periodically oscillates [12]. Periodic oscillating pressure causes the elastically mounted blunt structure to vibrate. In the micro®n actuation, the periodic drag force caused by the periodic
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J.S. Go et al. / Sensors and Actuators A 90 (2001) 232±239
pressure acts on the micro®n and generates the ¯ow-induced vibration of the micro®n. The lift force on the micro®n is ignored since the thickness of the micro®n is small compared to the width. The ¯ow-induced vibrating frequency of the micro®n depends on the vortex shedding frequency ws S
U D
(2.3)
where S indicates the Strouhal number, D the characteristic length, and U the wind speed normal to the microfin. The vibrating frequency of the microfin can not be calculated directly from Eq. (2.3) since information on the Strouhal number S is still missing. Even though a heat transfer model for a micro®n array heat sink using ¯ow-induced vibration has not been reported, it is expected that an increase in the number of the micro®ns induces more active hydrodynamic mixing so that the heat transfer rate of the micro®n array heat sink can be enhanced. For this reason, the micro®ns are positioned as closely as possible in order to increase the number of the micro®ns under the structural limit and fabrication process constraints. The thermal performance of the micro®n array heat sink is experimentally evaluated by measuring the thermal resistance based on Newton's law of cooling Rth
Tw
T1 Q
1 hA
(2.4)
where Tw means the wall temperature, T1 the coolant temperature, Q the applied electrical power to a thermofoil heater, h the convection heat transfer coefficient, and A the area wetted by fluid. The slope obtained from the measured values of several combinations of the measured temperature difference and the applied electrical power represents the thermal resistance of the heat sink. In addition, the thermal performance of the microfin array heat sink is compared with that of the plain-wall heat sink fabricated from the same material.
Fig. 2. Microfabrication processes of an initially bent bimorph microfin.
micro®n by using corner compensation. The fabrication process is completed by the ®nal step shown in Fig. 2f, in which a 0.5 mm-thick Tb±Fe alloy is deposited on the psilicon micro®n and the initial de¯ection induced by the residual stress difference is obtained simultaneously. A SEM photograph of the initially de¯ected micro®n array is shown in Fig. 3. The fabricated micro®n was 2 mm long and 100 mm wide. We measured a total micro®n thickness of 3.7 mm from the SEM photograph. By using the optical microscope, the initial de¯ection of the micro®n was also measured to be about 250 mm. Based on the structural analysis of the fabricated micro®n, a maximum stress of 36.3 MPa was applied on the p-Si when considering the 100 mm-peak vibrating de¯ection. The natural frequencies can be estimated as 1.28, 8.05 and 22.55 kHz.
3. Microfabrication process Fig. 2 illustrates a single mask micromachining process for an initially de¯ected micro®n array heat sink of 5 cm 5 cm. In the bulk silicon micromachining process, EDP is used as an anisotropic silicon etching solution and p-silicon as an etch-stop layer for the micro®n de®nition. The fabrication starts with the thermal growth of a 1 mmthick silicon dioxide ®lm that acts as a barrier for the subsequent deep boron diffusion process. In step (b) of Fig. 2, the microcantilever beam is de®ned by patterning the silicon dioxide. Fig. 2c shows the 12 h deep boron diffusion process for the p-silicon micro®n. Step (d) of Fig. 2 includes the removal of the silicon dioxide and a boron rich glass in BOE solution. In Fig. 2e, anisotropic silicon etching is illustrated. This step is performed to de®ne the
Fig. 3. SEM photograph of the fabricated microfin array with initial deflection.
J.S. Go et al. / Sensors and Actuators A 90 (2001) 232±239
4. Experimental evaluation For the experimental investigation, we fabricated a heat sink with the initially de¯ected micro®n array. The ¯owinduced actuation of the micro®n array for hydrodynamic mixing was observed and measured. Finally, the thermal performance of the micro®n array heat sink was evaluated and compared with that of a plain heat sink. Fig. 4 illustrates the test apparatus of the micro®n array heat sink. The specimen for thermal performance measurement consists of the micro®n array heat sink, a 3 mm-thick copper heat spreader and a thermofoil heater with a diameter of 2.5 cm. Each component was attached with a 0.13 mmthick adhesive tape (3M, 9885) with a relatively high thermal conductivity of 0.5 W/m-K. Three K-type thermocouples (OMEGA, CHAL-010) with the same spacing along the ¯ow direction were instrumented on the upper surface of the copper heat spreader to measure the temperature distribution. A layer with low thermal conductivity (6 mmthick commercial styrofoam) was attached to the backside of the thermofoil heater to minimize heat loss. Electrical power was applied to the thermofoil heater through a DC power supply (HP, E3620A). The uncertainty analysis was estimated at a 95% con®dence level [13]. 4.1. Flow-induced vibration test of the microfin The micro®n vibrates due to the periodic pressure distribution acting on the elastic micro®n in the presence of the
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¯uid ¯ow. A high-speed motion analyzer observed and recorded the vibration of the micro®n array. Fig. 5 shows the photograph of the ¯ow-induced vibration of the micro®n in the ¯uid ¯ow. In the absence of air¯ow, the micro®n array did not oscillate. As soon as air was blown over the micro®n, the micro®n started to vibrate. A larger stroke of the vibrating de¯ection was obtained as the ¯uid velocity increased. Due to the aliasing effect resulting from the limitation of the camera performance, the dynamics of the micro®n vibration could not be read. In order to overcome this, a micro®n with a piezoresistive sense element (micro®n sensor hereafter) was fabricated in order to characterize the dynamics of the micro®n vibration. The micro®n sensor was bonded on a PCB (printed circuit board) for signal processing by using aluminum wire as shown in Fig. 6. The measured electrical resistances of the sensing resistors were 2.33 and 2.30 kO, respectively. The resistances of the reference resistors were also measured as 2.36 and 2.38 kO, respectively. The analytical natural frequencies of the fabricated micro®n sensor are estimated as 1.17, 7.34 and 20.54 kHz. A dynamic signal analyzer (HP, 35670A) and an oscilloscope (Tektronix, TDS 784A) read the outputs of the micro®n sensor vibration. Speci®cally, the dynamic signal analyzer determines the dominant vibrating frequency of the micro®n by measuring the power spectral density. The ¯owinduced vibration of the micro®n sensor for air velocities of 3, 4, 5 and 6 m/s was examined. An anemometer (KANOMAX, 6631) was also placed to measure the air velocity.
Fig. 4. Heat sink configuration for the thermal performance measurement.
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Fig. 7. Measured power spectral densities of the microfin vibration.
Fig. 5. Measured flow-induced vibration of a microfin. (a) opticallyfocused microfin before air blowing; (b) microfin vibration after air blowing.
By analyzing the power spectral density of the micro®n vibration (Fig. 7) for four different air velocities, it was determined that the vibrating frequency of the micro®n was 1.173 kHz and independent of the air velocity. Through a comparison of the measured vibrating frequency with the natural frequency of 1.17 kHz, which was obtained theoretically from the structural analysis, it was also determined that the micro®n sensor vibrated at the fundamental natural frequency. Fig. 8 shows that the second mode of the micro®n vibration appeared at 7.35 kHz at an air velocity of 6 m/s.
Fig. 6. Microfin sensor bonded on the PCB by using aluminum wire.
The voltage outputs during 10 ms were read for observing the long-term behavior of the micro®n sensor vibration. The magnitude of the voltage output indicates the vibrating de¯ection qualitatively. As shown in Fig. 9, the histogram of the micro®n vibration con®rms that the micro®n does not vibrate with a constant vibrating de¯ection. Fig. 10 illustrates the measured peak-to-peak voltage outputs of the micro®n sensor and the vibrating de¯ections analytically estimated at the measured vibrating frequency of 1.173 kHz. While the estimated vibrating de¯ection continued to increase, the measured voltages tended to saturate with the increasing air velocity. It is assumed that the saturation of the vibrating de¯ection resulted from the lock-in phenomenon of the micro®n vibration as was experimentally shown in the torsional vibrations of a plate [12].
Fig. 8. Measured power spectral density for an air velocity of 6 m/s.
J.S. Go et al. / Sensors and Actuators A 90 (2001) 232±239
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Fig. 11. Measured thermal performances at the fluid velocity of 4.4 m/s. Fig. 9. Histogram of the microfin sensor output for an air velocity of 3 m/s.
The thermal performance of the micro®n array heat sink can be evaluated by measuring the thermal resistance. The measurement of the thermal resistance in the micro®n array heat sink was performed at ¯ow velocities of 4.4 and 5.5 m/s and compared with that in a plain-wall heat sink. A micronanometer (Furnace, FCO-12) was used for velocity measurement. Figs. 11 and 12 show the measured thermal performances. Temperature differences measured from three thermocouples indicated that thermal spreading resistance in the copper heat spreader was small. At an air velocity of 4.4 m/s, the thermal resistance of the micro®n array heat sink was 4.45 (with uncertainty of 0.089)8C/W, which was averaged from the three values measured from the three thermocouples. The measured thermal resistance of the plain-wall heat sink was 4.69 (0.092)8C/W, indicating a 5.5% enhancement in cooling performance. At an air
velocity of 5.5 m/s, the micro®n array heat sink showed a thermal resistance of 4.05 (0.257)8C/W compared with 4.56 (0.052)8C/W of the plain-wall heat sink, which means an increase of 11.5% in the heat transfer rate. From the measured thermal performance results, increase in the ¯uid velocity from 4.4 to 5.5 m/s provided a larger vibrating de¯ection of the micro®n, which in turn improved the heat transfer rate. It is therefore determined that a larger vibrating de¯ection in the micro®n array heat sink plays a key role in enhancing the heat transfer rate. The improvement of heat transfer in the micro®n array heat sink may be achieved by the ¯ow-induced vibration of the micro®n array and partly by the increased surface area obtained by introducing the micro®n array. In order to determine the effect of the increased surface area on the heat transfer rate, we removed the micro®n array from the heat sink and compared the thermal performance with that of the plain heat sink. The comparison test was performed at an air velocity of 6 m/s, which was the maximum air velocity of the wind tunnel. The experimental results showed that the
Fig. 10. Peak-to-peak output voltages and estimated vibrating deflections for different air velocity.
Fig. 12. Measured thermal performances at the fluid velocity of 5.5 m/s.
4.2. Thermal performance measurement
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References
Fig. 13. Comparison of the thermal performance of the plain and finremoved heat sink.
increased surface area did not enhance the heat transport as shown in Fig. 13. The ratio of the Colburn factor to the friction factor was not considered since the pressure drop was too small to measure. Also, the robustness of the micro®n array heat sink against dust was tested by exposing it to air¯ow for 3 days. It was determined that the micro®n array heat sink is very resistant to dust due to the ¯ow-induced vibration of the micro®n array. 5. Conclusions A micro®n array heat sink using the ¯ow-induced vibration of a micro®n array was investigated to identify its effect on heat transfer enhancement in laminar ¯ow regime. Additionally, a micro®n sensor was also fabricated to determine the dynamics of the micro®n vibration. The micro®n array heat sink was fabricated by using bulkmicromachining technology. The initial de¯ection of the micro®n for the ¯ow-induced vibration was obtained from the residual stress difference in two dissimilar thin ®lms. By measuring the thermal resistance, the thermal performance of the micro®n array heat sink was compared with that of a plain-wall heat sink fabricated with the same material. Based on the comparison of the thermal performance, it was determined that an increase in the air velocity improves the heat transport in the micro®n array heat sink. From the dynamics of the fabricated micro®n sensor, it is shown that the micro®n vibrates at resonance and the vibrating frequency of the micro®n is independent of the air velocity. By increasing the air velocity, the vibrating de¯ection of the micro®n is increased. Based on the dynamics of the micro®n vibration and the thermal performance comparison, it is concluded that the vibrating de¯ection of the micro®n plays a key role in enhancing the heat transfer rate from the micro®n array heat sink.
[1] R.C. Chu, Heat transfer in electronic systems, in: Proceedings of the 8th International Conference on Heat Transfer, New York, USA 1986, pp. 293±305. [2] D.A. Aliaga, J.P. Lamb, E.E. Klein, Convection heat transfer distributions over plates with square ribs from infrared thermography measurement, Int. J. Heat Mass Transfer 37 (3) (1994) 363±374. [3] J. Davalath, Y. Bayazitoglu, Forced convection cooling across rectangular blocks, J. Heat Transfer 109 (1987) 321±328. [4] F.J. Rowley, S.V. Patankar, Analysis of laminar flow and heat transfer in tubes with internal circumferential fins, Int. J. Heat Mass Transfer 27 (4) (1984) 553±560. [5] R.C. Henry, R.J. Hansman Jr., K.S. Breuer, Heat transfer variation on protuberances and surface roughness elements, J. Thermophys. Heat Transfer 9 (1) (1995) 175±180. [6] P.A. Eibeck, J.K. Eaton, Heat transfer effects of a longitudinal vortex embedded in a turbulent boundary layer, J. Heat Transfer 109 (1987) 16±24. [7] T. Tsutsui, M. Akiyama, H. Sugiyama, Active boundary layer control by elastically vibrating flat plate, JSME 61 (585) (1995) 216±222. [8] C. Liu, T. Tsao, Y.-C. Tai, Out-of-plane permalloy magnetic actuators for delta-wing control, in: Proceedings of the IEEE Micro Electro Mechanical Systems Workshop (MEMS'95), Amsterdan; Netherland 1995, pp. 7±12. [9] S.M. Kumar, W.C. Reynolds, T.W. Kenny, MEMS-based transducers for boundary layer control, in: Proceedings of the IEEE Micro Electro Mechanical Systems Workshop (MEMS'99), Orlando, Florida, USA, 1999, pp. 135±140. [10] J.S. Go, Y.-H. Cho, B.M. Kwak, K. Park, Snapping microswitches with adjustable acceleration threshold, Sens. Actuators A54 (1996) 579±583. [11] E. Quandt, B. Gerlach, T. Gerst, K. Seemann, Magnetostrictive thin film actuators, in: Proceedings of the 4th International Conference on New Actuators, Bremen, Germany 1994, pp. 229±231. [12] R.D. Blevins, Flow-Induced Vibration, Van Nostrand Reinhold, New York, 1977. [13] S.F. Richard, E.B. Donald, Theory and Design for Mechanical Measurements, 2nd Edition, Wiley, New York, 1995.
Biographies Jeung Sang Go received a BS degree from Pusan National University in 1993 and an MS degree from the Korea Advanced Institute of Science and Technology (KAIST) in 1995. His major was Mechanical Engineering. Beginning from May 2000, he has been working at the Samsung Advanced Institute of Technology (SAIT) in Korea, where he is conducting research on MicroCooling Systems using micro electro mechanical systems (MEMS). In addition, he is also currently a graduate student at KAIST working on a PhD in Mechanical Engineering. His research interests include microfluidics and micro heat transfer using MEMS technology and its applications. Dr. Sung Jin Kim is an Associate Professor in the Department of Mechanical Engineering at the Korea Advanced Institute of Science and Technology (KAIST). Until July 1997, he was a group leader in the Thermal Engineering Center at the IBM Tucson Laboratory where he had worked for almost 8 years. He was responsible for the investigation and development of advanced cooling techniques for application to electronic systems. He received a PhD degree in
J.S. Go et al. / Sensors and Actuators A 90 (2001) 232±239 Mechanical Engineering from Ohio State University in 1989. He has received two IBM Invention Achievement Awards and five Author Recognition Awards, holds four patents, and has published 30 papers in the area of convective heat transfer. Also, he recently edited a book entitled Air Cooling Technology for Electronic Equipment. Dr. Hayong Yun (M'99) received his BS degree from Seoul National University in 1991, and MS and PhD degrees from M.I.T in 1993 and 1995, respectively. His major was Mechanical Engineering. From September 1991 to September 1995, he was a research assistant in the Cryogenic Engineering Laboratory at M.I.T. In January 1996, he started work at Samsung Electronics Co. in Korea as a senior engineer. In January 1999, he moved to the Samsung Advanced Institute of Technology in Korea, where he is currently conducting research on micro electro mechanical systems (MEMS) at the MEMS Laboratory. Dr. Geunbae Lim (Non-member) received his BS and MS degrees in Electrical Engineering from Yeungnam University, Korea, in 1990 and 1992, respectively. He received his PhD degree in Precision Engineering from Tohoku University, Japan, in 1996 and then joined the Microsystems Laboratory, SAIT, Korea. His research interests are in the areas of BioMEMS and nanomachining. Dr. Junghyun Lee received his BS degree in Mechanical Engineering from Yonsei University, Seoul in 1986; and MS and PhD degrees in Aerospace Engineering from Case Western Reserve University, Cleveland, Ohio in 1990 and 1994, respectively. He was awarded the National Research Council Research Fellowship
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from 1993±1996. He worked as NRC Resident Research Associate at the NASA Lewis Research Center in Cleveland, Ohio from 1993±1996. He is currently working as a member of the Research Staff at the Samsung Advanced Institute of Technology MEMS laboratory and as Micro Thermo Fluid Elementary Technical Leader. Dr. Inseob Song received his BS degree from Hanyang University, Korea in 1985, and MS and PhD degrees from the Korea Advanced Institute of Science and Technology (KAIST), Korea in 1988 and 1994, respectively. His major was Mechanical Engineering. From September 1993 to January 1996, he was employed as a senior engineer at Samsung Heavy Industries. Since February 1996, he has been working at the Samsung Advanced Institute of Technology (SAIT) in Korea as a member of the Research Staff. He is currently conducting research on micro cooling systems in the MEMS Laboratory at SAIT. Dr. Y. Eugene Pak received his BS degree in Mechanical Engineering from the State University of New York at Buffalo in 1980, and MS and PhD degrees in Mechanical Engineering from Stanford University in 1982 and 1985, respectively. He joined the Grumman Corporate Research Center in 1985 where he carried out research in micromechanics of smart material. In 1994, he took an academic position at the State University of New York at Stonybrook. In 1995, he joined the Samsung Advanced Institute of Technology (SAIT) where he is currently serving as Director of the MEMS Laboratory. Dr. Pak's research interests include Micro and Nanoscale Mechanics, MEMS reliability and Biotechnolgy.