DEVELOPMENT OF MICROCHANNELS BY MICRO-USM ON SILICON WAFER FOR HEAT TRANSFER APPLICATIONS

Abstract

Miniaturization has brought about many striking changes in the fields of electronics, biotechnology, chemical processing, etc. Microchannels are common elements used in the miniaturized technologies for extracting heat from electronic devices, mixing of fluids, examining new cells, transferring drugs, etc. Smooth and high aspect ratio microchannels on silicon wafer substrate are used in heat removal application in various microelectronic components. Generally, etching techniques are used to fabricate silicon microchannels; however, the maximum achievable limit for the channel depth is a major concern. Micro-ultrasonic machining (micro-USM) is capable of machining high aspect ratio microchannels on hard and brittle material such as silicon, glass, ceramics, etc. In the present study, microchannels were fabricated on silicon wafer by the micro-ultrasonic machining technique. The effect of vital process parameters of the micro-USM was investigated to achieve the targeted dimensional accuracy (width and depth) of the microchannels. Response surface methodology was used to design the experiments and analyse the output responses. It was found that the fine abrasive grit size with low abrasive concentration and high workpiece feed rate were favorable to achieve the targeted aspect ratio of the microchannels. Overcut and edge damage (stray cut) are undesirable in precision machining while surface roughness of the microchannels can be set at an optimized value to attain maximum heat transfer. In order to improve the surface quality and accuracy of the machined channels, different viscous fluids (palm oil, transformer oil and water) were used as the abrasive slurry medium and their effect was investigated considering abrasive concentration and workpiece feed rate as other machining parameters. The experimental investigation revealed that use of low viscous fluids yields better surface roughness compared to high viscous fluid; however, overcut and stray cut were minimized while using high viscous fluids. Machining at higher feed rates could minimize the surface roughness, over cut and stray cut iv irrespective of the percentage abrasive concentration; possible interactions between the tool, abrasive and workpiece in the machining zone were analyzed. Further, three different serpentine microchannels (Rectangular, U and V) were fabricated on silicon wafer with the optimized process parameters of micro-USM. Fluid flow and heat transfer experiments were performed to investigate the heat transfer capability of the microchannels; a setup was developed for the heat transfer experiments. Thermal performance of the microchannels was experimentally investigated by making the working fluid (water) to flow through them at different Reynolds numbers (100 to 400) and at different input heat fluxes (10 kW/m2, 20 kW/m2 and 30 kW/m2). Results indicate that the overall thermal performance of the microchannel heat sink mostly depends on the type of pattern, as the fluid-substrate interaction area, defined by the Sink Area Factor (SAF), changes appreciably with the type of patterns. It was found that the U-serpentine microchannel exhibited the best thermal performance while compared to the other two serpentine microchannels. The sharp bends of the microchannel patterns and the surface roughness of the microchannel walls adversely affect the overall thermal performance of the microchannels. Two different cases were numerically analysed to evaluate the flow behavior inside the microchannels. In case – 1, the channel length was kept constant and the base area was varied among the three; in the case – 2, the base area was kept constant and the channel length was varied. The performances of the microchannels obtained with the experimental as well as numerical results were compared in terms of pressure drop, substrate and fluid temperatures and overall thermal performance. It was found that, higher Reynolds number flow conditions do not help improving thermal performance of the microchannels.