Fabrication of microchannels for use in micro-boiling experiments
Cummins, Gerard Pio
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Increased power densities in VLSI chips have led to a need to develop cooling methods that can cope with the increased heat produced by such chips. Currently one of the more attractive methods to meet this goal is through the use of two phase flow of a fluid as changing phase of the material allows high heat transfer rates for a low temperature change. To bring this technology to commercialisation a greater understanding of the underlying physics involved at the microscale is required as there is much debate within literature as to what occurs during two phase flow heat transfer at these scales. The work conducted as part of this thesis is a step towards improving the understanding of the mechanisms involved with this process. This thesis describes the fabrication of a novel microchannel structure, which can be used to experimentally characterise two phase heat transfer as it occurs. The final process reported for these microchannels structures provides the basis of a technology for the fabrication of microchannels with increased sensor densities. Two types of microchannel devices have been fabricated for this project. The first device of these was an array of parallel microchannels formed by the reactive ion etching (RIE) of silicon, which was then bonded with Pyrex glass. These microchannels were simple in that sensors were not integrated for local measurement. However the production of these devices incorporated fabrication techniques such as anodic bonding and inductively coupled plasma RIE that were essential to the fabrication of more complex devices. The second device built was a single microchannel that contained an integrated heater and several temperature sensors. The use of wafer bonding enabled the device to take full advantage of both bulk and surface micromachining technology as the placement of the temperature sensors on the channel floor would not be possible with conventional bulk micromachining. The initial microchannel structures demonstrated that wafer bonding could be used to fabricate novel devices, but they highlighted the difficulty of achieving strong anodic bonds due to the presence of dielectric films throughout the fusion bonded wafer stack used in the channel fabrication. To improve the performance of the device the process was optimised through the use of insitu, non-destructive test structures. These structures enabled the uniformity and strength of the bonds to be optimised through visualisation over the whole wafer surface. The integrated sensors enabled temperature measurements to be taken along the channel with a sensitivity 3.60 ΩK-1 while the integrated heater has delivered a controllable and uniform heat flux of 264 kWm-2.