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.

1. DIGITAL MICROFLUIDIC DEVICE FOR 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
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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
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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
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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.
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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)
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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
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.
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8. Experimental Setup
EWOD AC Camera system
Voltage
supply
Control Panel
Heater/RTD
DAQ
Computer control
DMF EWOD
cooling device
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9. DMF hotspot cooling device
Cr EWOD connections
Liquid reservoir
Droplet motion path
ITO heater
busbar
EWOD device used in the experiment.
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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.
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11. Experimental Procedure
Cross section of device.
Top view of device.
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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
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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
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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
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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
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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
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17. Conclusions
• Demonstrated proof of concept
• Temperature measurement using ITO RTD performed
• Fundamentals of EWOD DMF cooling phenomena observed
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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
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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!
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