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Study on Droplet-based Liquid Cooling of an Hotspot using Digital Microfluidics

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Thermal management in present day integrated circuits (ICs) has become extremely challenging to deal with, as more number of transistors is packed into smaller die sizes. Conventional macro-scale and bulky cooling mechanisms like heat sinks, fans and heat pipes are unsuitable to handle the non-uniform spatial power distributions (hotspots) found on these small, yet, powerful ICs. To tackle this thermal management issue, we present a digital microfluidics (DMF) microscale liquid cooling system working on the principle of electrowetting on dielectric (EWOD). EWOD is an efficient and low power consuming actuation technique to pump liquids at microscale. In EWOD DMF, fluids are handled in droplet-wise by external electric field, thus, mechanical pumps and valves are not necessary to control the liquid motion.

In this demonstration, the EWOD system comprises a parallel arrangement of thin film Indium Tin Oxide (ITO) coated glass devices separated by spacer gap of 150µm. The bottom device is patterned with a 3D arrangement of ITO heaters/RTDs (Resistance temperature detectors) with EWOD electrodes separated by a passivation layer. By using the heaters and RTDs in a 600µm x 600µm area on the bottom device, we emulate hotspots found on ICs by controlling and sensing the temperature. A reservoir holds a pool of de-ionized water from which a small liquid drop of 800nL is dispensed and delivered to the hotspot at high velocities. When multiple drops are passed over the hotspot, considerable cooling will occur.

With the help of the ITO thin film RTDs and a pre-calibrated temperature coefficient of resistance data, the temperature of the hotspot before and after cooling is recorded for different dwell times of water droplets on the hotspot and heat fluxes. A plot between the temperature and the droplet traveling time for various speeds and heat flux is established. By using a high speed camera and synchronizing it with the RTD measurement, the meniscus of the droplet on the hotspot is examined for phase change at various heat fluxes to identify and study its effects on the hotspot temperature. This study is crucial to distinguish single phase and phase change of the coolant in estimating the performance of the hotspot cooling. This demonstration provides a foundation to a novel microfluidic hotspot cooling system in current generation ICs and can be extended to 3D ICs.

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Study on Droplet-based Liquid Cooling of an Hotspot using Digital Microfluidics

  1. 1. DIGITAL MICROFLUIDIC DEVICE FOR HOTSPOTCOOLING IN ICS USING ELECTROWETTING ON DIELECTRIC ASME 2012 International Mechanical Engineering Congress & Exposition Shreyas Bindiganavale1, Hyejin Moon1*, Seung M You2 1Integrated Micro and NanoFluidics Lab (IMNFLab) 2Micro-scale Heat Transfer Laboratory University of Texas at Arlington hyejin.moon@uta.edu
  2. 2. Integrated Circuit Cooling – Introduction• Heat generation in IC? – Heat generated from the IC due to conversion of electric power to thermal energy – Dissipated through wire bonds and leads• Need for electronics cooling? – Improve operating efficiency & increase reliability of IC• Conventional cooling systems – Heat sinks, fans, heat pipes & vents – Not suitable for present day electronics IMECE 2012 2
  3. 3. Digital Microfluidics (DMF) – Introduction• Manipulation of discrete droplets in micro scale with high automation• Electrowetting on Dielectric (EWOD) – Digital microfluidic technique to transport, merge or create drops – Electric field induces relocation of charges at the solid liquid interface line – Wettability of droplet on hydrophobic surface changed Reversible• Main advantages – Pump less operation Change in wetting of drop when electric field is applied on a sessile drop – Adaptive cooling of hotspots Moon, et al. J. App. Phys. 2002, 92, 4080-4087 IMECE 2012 3
  4. 4. IC Hotspot Cooling using EWOD DMF• Long term application target – Near junction hotspot cooling in 3-D ICs – Small & thin form factor – Capability of integrating device in constraint 2D spaces – Minimum contact resistance compared to other cooling methods – Ease of fabrication – In-line with conventional cleanroom semiconductor fabrication – No mechanical parts like pumps, pressure sources, valves etc. – Easy integration techniques for 3D ICs EWOD cooling device *Image Source: IBM, 3M *Image Source: Rensselaer Polytechnic Institute IMECE 2012 4
  5. 5. Objective of EWOD DMF Cooling• To demonstrate proof of concept – Integrate fluidic components, EWOD device operation, temperature measurement and data collection – Motion of drops across hotspot by EWOD – miniature heaters emulates hotspots Top glass chip Cool Hot Cool Hot~ Droplet ~ Droplet ~ Droplet Droplet Electrical insulator ITO Heater/RTD Bottom glass chip Cross section of device and its operation. IMECE 2012 5
  6. 6. Objective of EWOD DMF Cooling• Direct microscale temperature measurement – To measure/control temperature at hotspot by thin film ITO (Indium Tin Oxide) RTD• Study EWOD DMF cooling phenomena – Cooling studies to help focus on untapped potential of EWOD cooling – Single-phase with traces of phase-change cooling observed during low heat flux application (< 10 W/cm2) – To build a strong practical and theoretical foundation for phase-change dominant studies (> 103 W/cm2) IMECE 2012 6
  7. 7. Temperature Measurement with ITO RTD • Motivation behind material selection – ITO thin film (150 nm) was standard material of choice for EWOD patterning – Provides optical transparency without sacrificing function – Cheap fabrication process when compared to other materials 1440 510 µm 1430 510 µm 1420 1410Resistance Ω 1400 - Curve indicates ITO is a PTC 1390 Top view of ITO RTD. - Linear within temperature range 1380 1370 - TCR value is 1.0797 1360 1350 1340 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 Temperature C Calibration of ITO thin film RTD. IMECE 2012 7
  8. 8. Experimental SetupEWOD AC Camera system Voltage supply Control PanelHeater/RTD DAQ Computer control DMF EWOD cooling device IMECE 2012 8
  9. 9. DMF hotspot cooling device Cr EWOD connections Liquid reservoir Droplet motion path ITO heater busbar EWOD device used in the experiment. IMECE 2012 9
  10. 10. Experimental Procedure• Simultaneous operation of the following – Droplet dispensing  Reservoir filling and droplet generation – Start EWOD motion  Automated control of droplet motion – Power ON heater  Supply required power to heater to emulate hotspot – Collect RTD data  Acquire RTD resistance and time data simultaneously Liquid inlet Liquid exit View of assembled device with tubing for liquid dispensing. IMECE 2012 10
  11. 11. Experimental ProcedureCross section of device. Top view of device. IMECE 2012 11
  12. 12. Summary of Results 1490 No 105 drop Drop 95 1480 85 1470Temperature ( C) 75 1460 2 No drop 36.6 W/cm2 Uncooled 65 1450 2 36.6 W/cm2 55 1440 2 20 W/cm2 uncooled No drop 45 1430 2 20 W/cm2 35 1420 2 8.7 W/cm2 uncooled No drop 25 1410 2 15 1400 8.7 W/cm2 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Time (s) Heating No. of drops Calib. ΔR/ΔT Switching EWOD element area time voltage 0.002601 cm2 6 1.085 Ω/K 1.5 sec 100 VAC, 1KHz IMECE 2012 12
  13. 13. Summary of Results 1490 105 95 1480 85 1470Temperature ( C) 2 36.6 W/cm2 Uncooled 75 1460 No drop 2 65 1450 36.6 W/cm2 55 1440 2 20 W/cm2 Uncooled No drop 45 1430 2 20 W/cm2 35 1420 2 8.7 W/cm2 uncooled No drop 25 1410 2 8.7 W/cm2 15 1400 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Time (s) Heating No. of drops Calib. ΔR/ΔT Switching EWOD element area time voltage 0.002601 cm2 7 1.085 Ω/K 1.25 sec 100 VAC, 1KHz IMECE 2012 13
  14. 14. Summary of Results 1490 105 95 1480 85 1470Temperature ( C) 2 36.6 W/cm2 Uncooled 75 1460 No drop 2 65 1450 36.6 W/cm2 55 1440 2 20 W/cm2 Uncooled No drop 45 1430 2 20 W/cm2 35 1420 2 No drop 8.7 W/cm2 uncooled 25 1410 2 15 1400 8.7 W/cm2 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Time (s) Heating No. of drops Calib. ΔR/ΔT Switching EWOD element area time (t) voltage 0.002601 cm2 9 1.085 Ω/K 1 sec 100 VAC, 1KHz IMECE 2012 14
  15. 15. Observations• Distinct kinks observed at high q’’ – At entry due to advancing meniscus formation over heater – At exit due to receding meniscus formation over heater – Visuals show evaporation and condensation  Lower temperature drop at kinks  Indication of phase-change heat transfer Advancing Receding Meniscus Meniscus Droplet Droplet IMECE 2012 15
  16. 16. Observations T’nd T’max• For fixed t, varying q’’ (Figure A) ΔTthigh T’d – ΔT’ (T’max – T’min) > ΔT (Tmax – Tmin)  Due to higher phase-change heat transfer ΔT’ q’’high thigh = 1.5s at q’’high than at q’’low T’min Tnd ΔTtlow• For varying t, high q’’ (Figure B) Td – ΔTthigh (T’nd – T’d) < ΔTtlow (Tnd – Td) Tmax  Indicates room for temperature stabilization ΔT tlow = 1s by further minimizing t q’’low Tmin Figure A Figure B thigh = 1.5s• For varying t , low q’’ (Figure C) – ΔTthigh = ΔTtlow = 0  Poor clarity in data as RTD noise higher at q’’low due to lower current tlow = 1s Figure C 16 IMECE 2012
  17. 17. Conclusions• Demonstrated proof of concept• Temperature measurement using ITO RTD performed• Fundamentals of EWOD DMF cooling phenomena observed IMECE 2012 17
  18. 18. Future Work• Droplet generation – High frequency – Long time duration• Looping of liquid supply to hotspot – To incorporate internal supply of liquid using return EWOD electrode paths• Silicon substrate to be chosen for future demonstration – Towards embedded IC cooling• Better synchronization between RTD data and high speed imagery – High speed camera imagery will be matched with RTD data for pinpoint data relation• Better RTD noise reduction techniques for data clarity IMECE 2012 18
  19. 19. Acknowledgements• This study was supported by: Defense Advanced Research Projects Agency/Microsystems Technology Office (DARPA/MTO) Dr. Avram Bar-Cohen, Program Manager Program grant number: W31P4Q-11-1-0012• Travel support was provided by the University of Texas at Arlington through the College of Engineering Thank you! IMECE 2012 19

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