Experimental Investigation of Micro Heat Pipes Fabricated in Silicon Wafers

An experimental investigation was conducted to determine the thermal behavior of arrays of micro heat pipes fabricated in silicon wafers. Two types of micro heat pipe arrays were evaluated, one that utilized machined rectangular channels 45 μm wide and 80 μm deep and the other that used an anisotropic etching process to produce triangular channels 120 μm wide and 80 μm deep. Once fabricated, a clear pyrex cover plate was bonded to the top surface of each wafer using an ultraviolet bonding technique to form the micro heat pipe array. These micro heat pipe arrays were then evacuated and charged with a predetermined amount of methanol. Using an infrared thermal imaging unit, the temperature gradients and maximum localized temperatures were measured and an effective thermal conductivity was computed. The experimental results were compared with those obtained for a plain silicon wafer and indicated that incorporating an array of micro heat pipes as an integral part of semiconductor devices could significantly increase the effective thermal conductivity; decrease the temperature gradients occurring across the wafer; decrease the maximum wafer temperatures; and reduce the number and intensity of localized hot spots. At an input power of 4 W, reductions in the maximum chip temperature of 14.1°C and 24.9°C and increases in the effective thermal conductivity of 31 and 81 percent were measured for the machined rectangular and etched triangular heat pipe arrays, respectively. In addition to reducing the maximum wafer temperature and increasing the effective thermal conductivity, the incorporation of the micro heat pipe arrays was found to improve the transient thermal response of the silicon test wafers significantly.