Laser printing a bonding layer onto microchips is expected to keep them cooler

A printed bonding layer up to 120 microns thick on the back of microchips could enable a heat sink that keeps processors 10 degrees Celsius cooler than comparable systems today.

Heat is believed to be behind most of the failures of electronics that lead to the expense of having to replace data servers. Keeping circuit boards cooler will extend their lives and researchers at Binghamton University’s mechanical engineering department have developed a manufacturing technique to keep electronics cooler by 10 degrees Celsius (18 degrees Fahrenheit). The technique is a laser deposition process which prints a bonding layer onto the back of a semiconductor wafer to allow a broad range of heat sink materials to be printed on top.

“A lot of servers will break after five years and you have to replace the hardware. I understand that a lot of the failures are thermally induced,” says Scott Schiffres, an assistant professor in Binghamton’s mechanical engineering department. “We use the backside of the silicon wafers that are not polished to atomic smoothness then we print several layers of this titanium alloy and that is enough for a lot of cooling devices.”

Binghamton University logo printed on silicon. Credit: Binghamton University

The team’s testing of this alloy printing has put it through high and low temperatures. For a week it was exposed to temperatures from 130 degrees Celsius to -40 degrees Celsius, and no defects were detected. It has printed three layers, each 40 microns thick. The Binghamton team plans to print microchannels on a chip in a spiral or maze pattern that allows coolant to flow around chip’s actual surface for cooling. “It will mean big changes for high-end electronics, data centers and computationally intense programs such as video editing tools and video games,” Schiffres adds.

Traditionally, a heat sink is attached to the central processing unit (CPU) or the graphics processor via a thermal interface material which includes a thermal paste. This interface attaches the actual heat sink, typically copper, to the microchip, and also facilitates heat transfer itself. However, the interface has two layers, the first which attaches to the microchip and the second which attaches to the heat sink. This interface, according to Schiffres and his colleagues, impedes good heat flow for cooling.

The interface layers exist because of the difficulty in bonding some materials to silicon. The Binghamton researchers have used a laser to selectively melt and bond an alloy, which will attach to silicon, directly onto the wafer. The alloy is tin, titanium silver alloy and it is applied in the form of titanium-silicide. A silicide is a compound of silicon. By laser processing, the time to create this silicide bond was reduced to microseconds. This is fast enough for the additive manufacturing of the metal heat sink directly onto the silicon’s surface.

“It’s not the cheapest mix,” Schiffres says of the alloy’s component elements. “We were just trying to get the technique working. We could tune the mix of elements to make it more economically feasible.” He emphasised that titanium is only up to 4% of the alloy’s content. His team is considering, “a few elements,” for alternative silicides, including Chromium.

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