| Abstract: | Vapor deposited micro heat pipe (VDMHP) arrays were fabricated as an integral part of silicon wafers, using state-of-art and advanced micro-machining, lithography, vapor deposition and charging procedures. A transient numerical model was developed capable of predicting the 3-D temperature profile, localized heat flux and the temperature gradients of silicon wafers with and without the VDMHP arrays, based on the physical parameters of the wafer, location of the heat sinks and the heat sources. An analytical model was developed to predict the performance of a VDMHP with triangular cross-section. The capillary limit was the controlling factor in determining the maximum heat transport capacity of the VDMHP. The critical radius of curvature was 9.978 μm, with a critical fill of 16.35%. Fabrication of the micro heat pipes was accomplished by first establishing a series of grooves in a silicon wafer. Orientation dependent etching using a KOH-1 propanol solution on a (110) wafer with a (111) flat covered with an oxide mask, resulted in grooves 25 μm wide and 25 μm deep with sharp perpendicular edges. The wafers were predeposited with a layer of chromium 400-500 A thick followed by a layer of gold 1250 A thick to improve the adhesion characteristics. Dual electron beam vapor deposition, followed by planetary deposition process using molybdenum crucibles were used to deposit a layer of copper 31.5 μm to 33.0 μm thick, and provide complete closure of the grooves. A glass cover slip was bonded on the top of the deposited layer. The grooves were finally charged and sealed. The steady-state experimental data indicated a reduction in the surface temperature and temperature gradients of 29.0% and 41.7%, respectively, coupled with an improvement in the effective thermal conductivity of 47.1%, for wafers containing 66 VDMHP over plain silicon wafers. In addition, the transient response of wafers with micro heat pipe array demonstrated a 30% to 45% reduction in the time constant over plain silicon wafers. The numerical model was successfully used to predict steady-state and transient response of wafers both with and without the heat pipe array. |
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