With conventional smooth heatsink surfaces “thermal conductivity is poor because of surface roughness”, Dr Jun Li of NASA’s Ames Research Center told Electronics Weekly. “Normally you have to put on grease or to solder with a eutectic to make it better.”
Li’s alternative uses a form of nanotube called carbon nanofibres in the interface.
After pointing out that figures are notoriously difficult to pin down because of their dependence on pressure and surface roughness, Li said: “With two [plain] pieces of silicon, the interface resistance is about 2cm²K/W which can be improved with grease or solder. People are shooting for less than 0.5cm²K/W; and 0.2 for the next generation. We can achieve 0.2 already and it is under-optimised.”
|A circular forest of carbon nanofibres cuts
thermal resistance between semiconductor
devices and heatsinks
The NASA interface relies on the incredible thermal conductivity of carbon nanotubes. “There is a lot of interest in this thermal conductivity. There is literature to say it is as high as diamond. But it is highly anisotropic,” said Li.
Along the length of the tube it has diamond-like thermal conductivity, but heat does not pass into the side of the tube easily. “If nanotubes are packed together like spaghetti, thermal conductivity is very poor. The heat doesn’t want to pass between the two [chip and heatsink] surfaces,” said Li.
However, the Ames team has a way to grow collections of orientated nanofibres “just like a brush”, said Li. “They can bend or buckle but they retain their vertical structure.”
The Ames team has deposited its thermal interface material on copper, where it has proved not only to have high thermal conductivity, but to be reusable as its fibres spring back. “You can’t do that with solder,” said Li
However, shear forces can flatten the fibres. To stop this the team has found a way to electro-chemically deposit a layer of copper around the bottoms of the fibres. “We leave a big portion exposed,” said Li. “The extra copper holds them straight in place, and second, improves the lateral heat spread.”
A firm called Nanoconduction has licensed the technology with a view to commercialising it.
NASA’s Dr Jun Li is working with a class of carbon nanotubes called carbon nanofibres. These are not the highly sought-after perfect single-wall nanotubes, or even multi-walled carbon nanotubes, but multi-walled nanotubes with some defects.
Nanofibres can be grown quickly. “We use plasma-enhanced chemical vapour deposition. The electric field helps to align the nanostucture vertically,” said Li. “The temperature is lower, 700°C, so we get more defects, but it is compatible with silicon.”