July 29, 2002 Borrowing from a method often used to cool down on a hot summer day, researchers at the UCLA engineering school are coaxing more efficiency out of hot silicon chips by spraying them with water.
The technology has numerous applications including improving the efficiency of the communications platform aboard unmanned aircraft and the performance of electric car and train motors.
Maintaining lower operating temperatures allows transistors to be driven harder, causing them to produce more power. It also allows chips to survive in harsh temperature environments that would otherwise cause them to fail.
Under the leadership of Elliott Brown, professor of electrical engineering and Vijay K. Dhir, interim dean of the Henry Samueli School of Engineering and Applied Science, researchers found that liquid spray-cooling could improve the performance of transistors as much as 34 percent. Their findings were presented in June in San Diego at the Eighth Intersociety Conference on Thermal and Thermomechanical Phenomena in Electronic Systems.
Researchers were able to achieve "significantly greater power than can be achieved with the same chips using conventional cooling," Brown said. He called it "a promising approach to improving thermal management at higher heat density or elevated temperature."
He and Dhir discovered that compared to other methods, such as liquid immersion or forced-convective (fan) cooling, spraying improves the transfer of heat away from the chip by combining the effects of convection with vaporization.
Although applying the concept to electronics isn't completely new — there are commercially available products that spray-cool the entire package of components, including the circuit boards — the UCLA team is the first to employ micro-spraying, which isolates the spray to each individual chip. For their research, the spray-cooling equipment was scaled down, with the size of the nozzles tailored to match the size of the chip.
Electrons speeding through transistors create heat. In devices such as cell phones, the heat is usually negligible. But when the circuitry must generate large amounts of power — to drive motors or operate radar equipment — their temperatures can exceed the boiling point of water (100 degrees Celsius). Above temperatures of 150 degrees Celsius, chips break down faster and the results they produce become unreliable. At 200 degrees Celsius, they cease to function.
In addition to increasing their power, creating a method of keeping chips below 150 degrees Celsius also allows power-amplifier chips to operate in harsh temperature environments such as those aboard unmanned aerial vehicles (UAVs).
In the cramped confines of a drone, where space and weight come at a high premium, cooling systems must be small and light. In addition to the operating temperatures of the chips, ambient temperatures aboard a drone flying over the desert vary by as much as 100 degrees Celsius between day and night, creating "conditions that lead to very high temperatures — too high for silicon to survive," Brown said. The cooling system Brown and Dhir have designed is small, lightweight and consumes only a small amount of power.
Two types of chips were tested: Insulated Gate Bipolar Transistors (IGBTs), used to drive electric motors in trains, electric cars and elevators, and LD-MOSFET transistors used in 500-MHz radio frequency power amplifiers. Such chips traditionally power radar base stations.
Results for the IGBTs were impressive, boosting performance by as much as 34 percent. Using the same technique on the LD-MOSFETs was an order of magnitude more effective at removing heat.
For example, Brown said, in a 60-watt radio frequency power amplifier, spray-cooling disburses about 20 watts of heat.
Power amplifier chips, which operate in the radio frequency range, produce very high temperatures. Inside the fuselage of a drone circling above the desert, operating temperatures of these chips are even further challenged.
Calculating the proper flow rates required Dhir's expertise in phase change heat transfer.
UCLA researchers found that in this temperature range, water is the best high-density liquid flux. Heat is disbursed by both thermal convection and evaporation. Heat dissipation by convection and evaporation were found to be about equal.
"Like spraying your face with an atomizer on a hot day, atomizing the water increases the surface area, disbursing every cubic centimeter into a zillion droplets. And each of those droplets removes heat as it evaporates." Brown said.
The performance improvements were achieved by spray-cooling directly on the top of the transistor die. The top surface of silicon die was coated with Parylene-C, a conformal polymer with excellent dielectric properties.
Measuring 4.86 mm x 1.53 mm, the nozzle matrix used for the LD-MOSFETs consisted of 28 holes horizontally and 18 holes vertically. Brown pointed out that the size and matrix of the nozzle array was constructed "to exactly match the layout of the active cells." He said they "tailored the design of the nozzle to the heat source distribution of the transistor."
Because silicon is so chemically robust with respect to acids and other harsh chemicals and inexpensive to mass-produce, it is an ideal material for the nozzle array, Brown said. Reactive-ion etching, the same process used to create the transistors themselves, was used to create the nozzles 35 microns in diameter. The process produces very smooth sidewalls compared to any known mechanical machining, so there is less of a tendency to trap contaminants and become clogged.
Researchers also found that at higher temperatures, an even larger amount of heat can be dissipated by spray-cooling.
Looking ahead, Brown plans to experiment with the use of spray-cooling on wide band gap semiconductors, which run even hotter than LD-MOSFETs.
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