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Study on Droplet-based Liquid Cooling of an Hotspot using Digital Microfluidics
- 1. DIGITAL MICROFLUIDIC DEVICEFOR HOTSPOT COOLING 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. 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. 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. IC Hotspot Coolingusing 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. Objective of EWODDMF 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. Objective of EWODDMF 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. Temperature Measurement withITO 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 1410 Resistance 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. Experimental Setup EWOD AC Camera system Voltage supply Control Panel Heater/RTD DAQ Computer control DMF EWOD cooling device IMECE 2012 8
- 9. DMF hotspot coolingdevice Cr EWOD connections Liquid reservoir Droplet motion path ITO heater busbar EWOD device used in the experiment. IMECE 2012 9
- 10. Experimental Procedure • Simultaneousoperation 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. Experimental Procedure Cross sectionof device. Top view of device. IMECE 2012 11
- 12. Summary of Results 1490 No 105 drop Drop 95 1480 85 1470 Temperature ( 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. Summary of Results 1490 105 95 1480 85 1470 Temperature ( 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. Summary of Results 1490 105 95 1480 85 1470 Temperature ( 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. Observations • Distinct kinksobserved 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. 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. Conclusions • Demonstrated proofof concept • Temperature measurement using ITO RTD performed • Fundamentals of EWOD DMF cooling phenomena observed IMECE 2012 17
- 18. Future Work • Dropletgeneration – 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. Acknowledgements • This studywas 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
Editor's Notes
- #2 Good Morning everyone! I am Shreyas and today I am talking about a device for hotspot cooling in integrated circuits using digital microfluidics
- #3 Thermal management in integrated circuits is commonly understood by the heat transfer community. As we enter a new era of computing where performance of the IC increases as the real estate decreases, there arises a great demand for a compact cooling system which can take over the role of conventional cooling systems like heat sinks, fans and pipes
- #4 Explain highlighted words. An approach which we took at the IMNfL is by using Digital microfluidics. DMF is the process of micromanipulation of discrete droplets in small quantities and high automation…EWOD used as a DMF technique to transport, merge or create drops. Heat transfer point of view, the main advantages are pump less operation – no need of external pumps to drive coolants – and adaptive cooling – coolant can be sent anywhere on the chip to cool hotspots. Some of the applications include L-o-c devices for drug delivery etc.
- #5 Emphasize on near junction hotspot cooling….minimum contact resistance compared to ufluidics methods….thin form factor….easy integration….no pumps…pressure sources…..long term vision is targeting near junction IC cooling in 3D IC packagesIn 3D packages, the heat can be removed only from the top of the entirepackage….EWOD DMF device can be integrated in-between the individual layers of the 3D IC packageDo research on literature based on thru hole conducting vias….heat removal in 3D IC’s is made possible using THV’s but EWOD DMF can be also comparable
- #6 Using this technique, we list out objectives which includes illustrating the proof of concept of the combination of EWOD and the need for IC cooling by emulating hotspots. Coolant drops are moved by digitally actuating a series of electrodes in the desired direction of transport. In this case, drops are transported across hotspot to cool them…Why liquid cooling better than air cooling? Higher ‘k’; Higher ‘h’; q’’ evap higher
- #7 The second objective is to devise a microscale temperature measurement technique and the third is to study the EWOD DMF cooling phenomena at the microscale level to understand single-phase and phase change cooling
- #8 This plot shows the TCR curve of the ITO RTD for the temperature of interest. The reason we calibrated ITO was because its not a conventional material used for RTD – need for transparency
- #9 The experimental setup involves the voltage supply source for EWOD motion, the heater/RTD DAQ for rtd data acquisition, the camera system for the image capture and the control panel for setting up the experimental parameters. The ewod device under test is the focus of attention in the upcoming slides
- #15 Since measurement is not well controlled, the power we provided to the hotspot is dissipated elsewhere through the glass substrate. Hence, the actual power provided to the hotspot is not entirely available for cooling. We are in the process of improving the experimental design by lowering spreading or by coming up with a better RTD busbar design. Our main intention here was to demonstrate the drop in temperature with droplet cooling.
- #19 Past success/failures with Si substrate…..Don’t spend too much time on this slide
- #20 Just thank “other members for their continued recommendations and support throughout my research work”