Understanding Smart Sensors - Nomads.usp

212 Understanding Smart Sensorsperformance, and increase reliability. A proposed approach is shown inFigure 9.9 [14]. After the channels are etched, multiple metallization layers arevacuum deposited to line the walls of the channel and seal the top. Heating thechip in a fluid bath fills approximately 20% of the cavity volume with fluid.The ends are sealed after this step to contain the fluid. The microheat pipe’soperation causes fluid to evaporate in high-temperature regions and condensein low-temperature areas, resulting in a more uniform temperature distributionacross the IC. Increasingly higher operating frequencies for higher performanceMPUs and MCUs may require this type of cooling to avoid increasing thepackaging size or the amount of external heatsinking.A patent has been issued in the United States for the cooling of highpowersemiconductor devices using microchannels formed by micromachiningor laser cutting techniques in silicon or silicon carbide structures [15]. Themicrochannels remove heat by forced convection or the use of fluid coolantlocated as close as possible to the heat source. The microchannels maximizeheatsink surface area and provide improved heat transfer coefficients for higherpower density of semiconductor devices without increasing junction temperatureor decreasing reliability.Gaseous flow in micron-sized channels is also being investigated. Theexchange of energy and momentum between MEMS and their gaseous environmentsoften governs the response characteristics of these systems. To studythe momentum exchange between a gaseous medium and microdevices, microchannelswere fabricated (52.25 mm wide, 1.33 mm deep, and 7,500 mm long)for conducting nitrogen flow experiments [16].Vapor deposited layersLayer 3Layer 2Layer 1SubstrateEtched channelMicroheat pipeFigure 9.9 A microheat pipe for cooling integrated circuits.