Near-junction microfluidic cooling for high power-density GaN-on-Si electronics: A wafer, device, packaging, and system-level investigation

Electrification of the energy section, from generation to end-use, plays an essential role in reducing global CO2 emission. Innovations in power electronics are required to increase conversion efficiency and power density. Gallium nitride (GaN) transistors have emerged over the last decade as a viable alternative to silicon-based devices to address these needs and pave the way for power integrated circuits that replace traditional discrete devices. However, integration of high power components causes thermal challenges that ultimately limits integration density. Microfluidic cooling is a promising candidate to overcome these limitations. By integrating microchannels directly in a chip, and passing coolant through the device, over 1 kW/cm2 of heat can be extracted. While this approach has been studied for silicon ICs, GaN RF devices, an in-depth study on the application to GaN-on-Si power devices is lacking. In this thesis, we explore the possibility of improving thermal performance of GaN-on-Si power devices, by turning the silicon substrate from a low-cost carrier into a high-performance heat sink. 4 distinct levels of integration are investigated, and at each level, we benchmark the cooling performance and efficiency, and provide a breakdown of which components form the bottleneck in heat transfer. On a system level, the usage of a hierarchical microfluidic cold-plate structure to locally extract heat from a GaN-based converter with 20 individual heat sources in an efficient manner. We show that well-designed microfluidic heat sinks can provide system-level advantages such as higher power density and low pumping power despite the small channel dimensions. At the packaging level, we integrating cooling directly inside the silicon substrate of a GaN-on-Si power IC to eliminate interface and packaging thermal resistances, combined with new packaging concepts based on additive manufacturing and coolant-delivery PCBs. Using in-chip liquid cooling, a 7.5-fold increase in power density was obtained compared to commercially available systems. To reduce convective thermal resistance, a new GaN device manufacturing method is presented where microfluidic cooling and electronics are co-designed, resulting in a monolithically-integrated 3-dimensional manifold microchannel heat sink within the silicon substrate. Heat fluxes up to 1.7 kW/cm2 could be extracted within a compact form factor at low pressure drops and limited pumping power. To facilitate the commercialization, we developed a GaN-on-Si substrates with epitaxially-integrated cooling. This substrate can be considered as a drop-in replacement for traditional GaN-on-Si wafers onto which devices can be fabricated, that offers the benefits of microfluidic cooling without requiring a change in foundry process. Finally, a comparison is made between single-phase water cooling and two-phase flow boiling using R1233ZD(E) as a refrigerant. A benchmark is presented that shows comparable cooling efficiencies for single-phase cooling using co-designed manifold microchannel heat sinks. The benchmark and breakdown of thermal resistance presented in each chapter function as a reference for deciding which level of microfluidic cooling is required for a given application. The high cooling performance demonstrated in this thesis confirms that microfluidic cooling for GaN ICs may become an interesting thermal management technology for certain demanding applications.