Carbon nanotubes prove excellent heat transfer vehicles for silicon
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Carbon nanotubes prove excellent heat transfer vehicles for silicon

West Lafayette (IN) - Researchers at Purdue University have discovered that carbon nanotubes can move the heat generated by silicon chips more efficiently and less costly than other methods.  By taking the heat from a chip and moving it to the metal casing more efficiently, our silicon based products (CPUs) can be pushed further.

CPUs generate heat from electrons in motion and the resistance they encounter within the semiconductors.  This generated heat is one of the biggest stumbling blocks toward achieving better performance through higher clock speeds.  Transistors, the base components operating at the very heart of a CPU, can only switch on and off so fast within a given thermal envelope.  Beyond that the generated heat begins to grow quickly and, without sufficient mechanisms to remove that heat, the hotter CPUs will just eventually stop working.

Scientists at Purude have discovered a way to grow carbon nanotubes which affix themselves directly to a processor die, allowing for very efficient heat removal.  According to the research, the carbon nanotubes are grown vertically on the silicon die between the die and metal casing.  The surface is so full of nanotubes that under a microscope it looks like a thick, dense carpet.  When the metal casing is mounted, the tubes bend slightly as they come in contact with the metal.  This would be like placing a thick sheet of plexiglass on carpet where you could see the fibers bending under the weight.  This has the effect of creating a great deal of contact surface area with the metal.  And, since the carbon nanotubes are physically affixed to the silicon substrate material, the heat flow becomes significant toward the cooler metal.

Carbon nanotubes prove excellent heat transfer vehicles for silicon

Scientists physically create the nanotubes using a process called "microwave plasma chemical vapor deposition."  It involves taking a silicon substrate (like the back side of a CPU die), and introducing branching molecules called dendrimers which anchor to the silicon.  Metal catalyst particles are then applied in a solution.  The solution is removed by adding heat, leaving only the particles behind.  From there the surface is exposed to methane gas in a microwave field.  The microwave signal breaks down the methane gas and deposits the carbon onto the dendrimer/catalyst combos in the form of self-growing nanotubes.

While somewhat technical, the process being used by the researchers is not expensive or difficult to employ.  This is especially true when compared with some of the processes involved in manufacturing semiconductors today.  In addition it has been shown that existing alternative solutions, such as indium foil, grease and waxes, are all less efficient than the carbon nanotube solution.

Researchers believe this process could be introduced to large scale semiconductor manufacturing.  This would allow consumers to buy products which require smaller heatsinks while making our processors run faster.  The smaller heatsinks would be required because with a more efficient transfer rate of heat away from the silicon die, the temperature delta between the heatsink and CPU could be less while still achieving the same effective transfer rate.

Research for this project was funded by NASA, through the Institute for Nanoelectronics and Computing in Purdue's Discovery Park.  One of the researchers also received undisclosed forms of support from Intel and Purdue.