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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.