Study of hotspot cooling for integrated circuits using electrowetting on dielectric digital microfluidic system.

摘要

Thermal management in electronics is an ever-growing challenge which needs constant innovations to regulate hotspots in integrated circuits for safe operation within its Thermal Design Power (TDP) envelope. As the number of transistors increase due to advancements in photolithography to enable die shrink, portable electronics are on the rise and so is the demand for microscale cooling systems because conventional cooling components like heat sinks, heat pipes and fans are not capable of meeting design requirements for cooling solutions. In addition to the benefits enjoyed by microscale cooling systems like compact and lightweight design along with simple operation, electrowetting on dielectric (EWOD) based digital microfluidic (DMF) hotspot cooling promises an innovative and novel cooling based on valveless and pumpless motion of coolant droplets directly on hotspots. Along with reduction in thermal resistance, this technology is also useful for site specific cooling by moving droplets to multiple hotspots on chip.;In this research, Indium Tin Oxide (ITO), a transparent conducting material, was patterned on glass substrates to emulate hotspots and also provide the electric field for EWOD pumping of droplets. Two cooling systems were designed, fabricated and tested for demonstrating temperature drop, namely Ionic Liquids (ILs) and De-Ionized (DI) water, using Liquid Crystals and ITO Resistance Temperature Detectors (RTDs) as the temperature sensing techniques. ILs were initially used as coolants to suppress evaporation but turned out to have poor cooling capability than DI water from LCT results. Apart from conduction and convection heat transfer from the hotspot to the droplet, phase-change was also responsible in achieving cooling for DI water.;To facilitate detailed study of hotspot cooling, an innovative approach to regulate hotspot temperature was demonstrated by creating a hydrophilic spot (H-spot) on the heater which retains a small droplet while the main coolant droplet passes over the hotspot. High-speed video was taken and synchronized with RTD data to identify different phases of droplet motion and observations were made to explain the heat transfer modes in each phase of motion. Transient heat transfer simulations in COMSOL Multiphysics including emulation of phase-change effects were performed for varying thermal diffusivity values of the droplet to understand its effect on the heater temperature. Finally, an analytical model was proposed based on literature which related droplet evaporation on the H-spot to the overall heat transfer coefficient. The evaporation rate was also calculated experimentally and compared with the ideal values used in the COMSOL simulations which resulted in the evaporation rates being in good agreement with each other.