However, the performance wasn’t even close to the maximum amount of cooling that the microrefrigerator is capable of when it’s not confined to the chip housing. “We’ve found good performance,” Venkatasubramanian says, “but there are still a lot of challenges.” When engineers put the cooler inside the package, there are a number of additional contact points where the cooler is connected to the copper plate and packaging electronics, he says. Prasher explains that the thermal characteristics of these contacts play a significant role in reducing the cooler’s efficiency: “By itself, [reducing resistance of thermal contacts] is a significant research area.” People are exploring different types of solder and even carbon nanotubes to reduce the resistance at the interface, he says, but the problem still has to be resolved.
Regardless, Ali Shakouri, a professor of electrical engineering at the University of California, Santa Cruz, is impressed by the work so far. “This is a good achievement,” he says. “The idea [that] there’s an uneven distribution of temperature in a microprocessor, and that by selectively cooling certain locations you can do a better job and save power, has been around for a while, but it hadn’t been demonstrated on a chip before.”
Shakouri notes that as the microprocessor industry moves toward using multiple cores or processing centers on a chip, the problem of hot spots will get worse, because workloads are shifted from core to core, creating more transient hot spots. Fans, used in many computers today, don’t respond quickly or effectively. “If you could selectively have microrefrigerators throughout a multicore chip,” he says, “you could lower power and increase performance.”
The researchers don’t have a timeline for commercialization. Right now, even though the cooler could be incorporated into traditional chip packaging, it would still be prohibitively expensive. After all, says Venkatasubramanian, adding a cooler is essentially adding a completely new layer of electronics to a chip. He says that if the cost and scalability of these coolers can be addressed, then he’s confident that they’ll find a market.