Researchers at the UCLA engineering school are coaxing more efficiency out of hot silicone chips by spraying them with water.
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 per cent.
Brown 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 said to be 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 moving 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 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.
The UCLA researchers tested two types of chip: 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 said to be impressive, boosting performance by as much as 34 per cent. 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.
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 was found to be about equal.
The performance improvements were achieved by spray cooling directly on the top of the transistor die. The top surface of silicone 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 silicone 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.