Modeling, optimization and thermal characterization of micropillar evaporator based high performance silicon vapor chamber
Date of Issue2017-12-27
School of Electrical and Electronic Engineering
To overcome the high heat generation challenge and ensure sustainable development in the semiconductor industries, thermal management for high density electronic devices has drawn much attention in recent years. Being a passive thermal management device with high heat removal capability, vapor chamber that spreads concentrated heat via liquid-to-vapor phase change phenomena was proven to be a promising cooling technique. The evaporator is the determinant section that governs the performance of a vapor chamber. Silicon vapor chamber with micropillar evaporator possess large thin film evaporation area, mature manufacturing technique and can be integrated with electronic devices homogeneously to eliminate excess thermal interface layer as well as avoid thermal expansion mismatch. However, a comprehensive and systematic study on silicon vapor chamber and micropillar evaporators is lacking. Predictive models that evaluate the performance limits of vapor chamber and evaporators are limited, selection of micropillar geometries is trial-and-error based on previous work. Investigation on wettability performance of micropillar structures was not systematically extended to a high temperature regime. In this thesis, we developed three semi-analytical models in predicting the capillary-limited dryout heat flux of uniform evaporators, biporous evaporators and sealed silicon vapor chambers. We performed optimization to determine the best geometric combinations of the evaporators and vapor chambers by taking the temperature rise into consideration. Accordingly, we fabricated the evaporators and vapor chambers by micro-electro-mechanical-systems (MEMS) process. We defined the micropillar patterns with deep-reactive-ion-etching (DRIE) and measured the temperature with embedded resistance-temperature-detectors (RTDs). We also conducted systematic experiments to determine the dryout heat flux and heat transfer coefficient of samples with various geometries. The models were validated against experiment results with less than 20% over-prediction. A 1 cm × 1 cm uniform and biporous evaporators could dissipate a maximum dryout heat flux of 25.7 and 55.9 W/cm2, respectively. A thin Si vapor chamber with a thickness of 1.25 mm can handle 98.1 W/cm2 heat flux before dryout. Heat transfer coefficient was found to increase with larger micropillar diameter/pitch ratio, smaller micropillar height, wider micropillar islands and narrower microchannel width. Besides the characterization of heat dissipation capabilities, we also examined the droplet behavior on superheated micropillar surfaces. Non-wetting droplets were observed to reside on top of micropillar structures at a temperature much lower than the Leidenfrost point of flat Si. Two peaks on droplet lifetime was observed, the nucleate boiling to non-wetting droplet transition temperature and Leidenfrost point were found to increase with l and h. This thesis provides deep insights, with experimental verification, into the design and optimization of the vapor chamber with micropillar evaporators.