Modeling and fabrication of silicon micro-grooved heat pipes

Abstract

Micro heat pipes (MHPs) are of current interest in the cooling of electronic components due to their high heat removal capacity as a result of the phase change mechanism. This thesis work focuses on finite element modeling and fabrication of a silicon micro-grooved heat pipe system. The computational model is developed to design an MHP system which consists of cooling and heating units and micro-grooves. The 3-D computational model is developed by using the phase change results of a detailed computational model on a unit cell as a boundary condition. Finite element modeling is also used for the design of the cooling channels and the heaters of the MHP system. The 3-D temperature distribution on an MHP system is obtained, and the effects of multiple channels, which cannot be captured by the unit cell analysis are reported. Two different main fabrication techniques, namely lithography-based and mechanical-based, have been assessed for the fabrication of micro-groove structures. For the lithography-based fabrication, deep reactive ion etching together with photo-lithography is used. Many process parameters are tested and optimized to achieve the desired micro-groove structure. According to the tested parameters, a final recipe is prepared and tested on a < 100 > Si wafer. Square micro-grooves with a width and a depth of 200 µm are obtained for 580 cycle dry etching with grassing formation which is below 5% (acceptable) of the micro-groove height. For the mechanical fabrication, cutting with an automated dicing saw, and high-precision machining with a diamond tool and a PCD tool have been assessed. Satisfactory results have been achieved by the dicing saw. A drawback of the dicing saw technique is the presence of a curve-shaped profile at the beginning and end of the grooves. This study showed the dicing saw to be a fast and cost effective alternative to other techniques. On the other hand, the results of high-precision machining are found to be unsatisfactory for the fabrication of micro-grooves. Moreover, the machining time and the cost of this technique turns out to unfeasible for the fabrication of a MHP system. The cooling channels are fabricated using PDMS molding, and the chromium heaters are fabricated using photolithography and sputtering. The bonding of the layers of the MHP system is accomplished by plasma treatment. The lithography-based fabrication and the dicing saw techniques are performed at the Bilkent University National Nanotechnology Research Center, and high-precision machining is performed at the Bilkent University Micro System and Design Center.