Heat transfer in integrated thermal microsystems.

摘要

The motivation of this thesis work is to construct fundamental study of the heat transfer in micro-domain.; In the first part of the thesis work, a systematic framework for the application of semiconducting microthermoresistors has been established. Polycrystalline thin films based on two semiconducting materials, Si and Ge, have been utilized to fabricate microthermistors. The thermistors are designed in a heavy-light-heavy doping concentration arrangement. The design, fabrication, analysis and characterization of a variety of thermistors under different doping schemes are described. The operation of the thermistors in a self-heating mode is also discussed.; Next, The design and fabrication of an integrated microsystem consisting of microchannels with temperature microsensors are described for the study of heat-transfer properties of fluid flow in microdomains. Surface micromachining technology is used to construct the microchannels about 1.4μm in height. Polysilicon thermistors, 4μm x 4μm x 0.4μm in size are suspended across the channels and directly exposed to the fluid for local temperature measurements. The integrated microsystem performance is theoretically analyzed, and the various heat-transfer mechanisms involved are subsequently discussed. It has been clearly demonstrated that conduction is, by far, the dominant heat transfer mechanism in such micro systems.; A unique technique of mask-less and self-aligned silicon etch between bonded wafers is developed for the fabrication of a microchannel heat sink device. The integrated microsystem is consisting of heater and an array of temperature microsensors. The microsystem allows direct temperature measurements for different levels of power dissipation under forced convection using either nitrogen or water as the working fluid. The measured temperature field is used to characterize the micro heat sink performance under forced convection boiling conditions. The onset of critical heat flux condition is investigated for different channel sizes and liquid flow rates. The results suggest that the bubble dynamic mechanism in microchannel is different from conventional channels.; Furthermore, the device has also been used to study the transient behavior of a thermal microsystem. Both heating-up and cooling-down times due to a pulsed-current input are determined for natural and forced convection conditions. The device transient temperature response to a periodic input power is characterized for different flow rate and input power levels. The device response under natural convection is successfully modeled as a first-order system, while characteristics of a second-order system are observed in the device response under forced convection with liquid. Under certain operating conditions of temperature cycling, a large peak-peak temperature can be achieved without the device damage.; Finally, a transparent microchannel heat sink system was fabricated and characterized by bonding a glass to a silicon wafer. No boiling plateau has been observed in the boiling curves of microchannel heat sinks. Three boiling modes, depending on the input power level, have been distinguished during the flow visualizations. Local nucleation boiling within the microchannels was observed at low power level, while a stable annular flow mode was observed at high power level.