Adaptive hot-spot cooling of integrated circuits using digital microfluidics.

摘要

Thermal management is a critical issue in integrated circuit (IC) design. With each new IC technology generation, feature sizes decrease, while operating speeds and package densities increase. These factors contribute to elevated die temperatures detrimental to circuit performance and reliability. Furthermore, hot-spots due to spatially non-uniform heat flux in ICs can cause physical stress that further reduces reliability. A number of techniques to address these issues have been proposed to date, yet most are unable to handle the varying thermal profile of an IC due to their inherent lack of reconfigurability.; In this thesis, we introduce two alternative cooling architectures based upon a "digital microfluidic" platform. In a flow-through approach, cooling droplets are actuated independently via electrowetting through user-defined reprogrammable flow paths and speeds. This high level of reconfigurability enables us to create an "adaptive" hot-spot cooling module that can be affixed directly onto the IC itself, whereby the system reconfigures itself on-the-fly in response to a changing thermal profile. In a programmable thermal switch approach, an array of liquid-metal droplets can be manipulated such that any area in the cooling device can be selectively switched from a low-to-high or high-to-low thermal conductivity mode. In this way, a higher heat flux can be drawn away from the hot-spot, resulting in a uniform thermal profile.; This thesis first studies the temperature-dependent behavior of digital microfluidic systems. It is shown that the transport of droplets immersed in oil is facilitated at elevated temperatures. Various hot-spot cooling prototypes were designed, based on a coplanar electrowetting scheme developed to simplify fabrication and assembly steps. For flow-through based systems, cooling prototypes were used to demonstrate the closed-loop dispensing, transport, and recycling of droplets necessary for adaptive cooling. Heat transfer using moving droplets is shown to improve significantly for higher mass flow rates and heat flux densities. However, while the proof-of-concept for this flow-through technique was successfully demonstrated, the maximum flow-rates using current prototypes appear to be insufficient in view of the high heat-flux densities in today's IC devices. Future studies of alternative dielectric materials will help to overcome these limits.; For a programmable thermal switch-based architecture, heat transfer parameters of mercury droplets were studied in both steady-state and transient conditions, in which the cooling of hot-spots was shown to be dependent on parameters such as effective cooling areas and the number of conductive vias. These parameters are manipulated on-the-fly to create a non-uniform thermal conductance layer corresponding to the thermal profile of the IC substrate. The ability to manipulate a dense array of these liquid-metal droplets in response to changing heat-flux densities allows for a completely self-automated adaptive cooling system.; The work presented in this thesis addresses the fundamental design challenges in developing an adaptive cooling architecture based on the digital microfluidic platform. The two cooling methods developed and characterized here demonstrate the feasibility to adaptively perform thermal management to dynamically cool hot-spots. Such an adaptive hot-spot cooling system will pave the way for new IC thermal management design strategies to perform temperature-aware cooling.