Carbon Nanotube Transistors May Soon Give Waning Moore’s Law a Boost


Carbon nanotubes (CNTs) have long been touted as a potential material to take us beyond the limits of faltering silicon chips, but they’ve proven tricky to manufacture. Now scientists have demonstrated a way to build CNT transistors in a commercial silicon fabrication plant.
Essentially CNTs are rolled-up sheets of the one-atom-thick wonder material graphene . They are excellent semiconductors and transport charge carriers very quickly , which has made them a promising option for replacing silicon, which is starting to hit fundamental physical limits.
Last year, a team led by engineers from MIT broke new ground with a 16-bit microprocessor made up of 15,000 CNT transistors that could execute a basic computer program. The project was part of a $61 million project funded by DARPA, but despite using industry-standard design processes the chip was still fabricated in a research lab rather than using industrial processes.
Now though, a team made up of many of the same researchers has developed a new process that makes it possible to produce CNT transistors in commercial chip manufacturing facilities using standard equipment. They successfully tested the new approach in two separate plants: a commercial silicon manufacturing facility run by Analog Devices and a high-volume semiconductor foundry run by SkyWater Technology.
The team achieved this by tweaking one of the most popular approaches to creating CNT transistors, in which silicon wafers are dipped into a solution of CNTs. The problem with previous implementations of this approach is that it can take on the order of days for enough CNTs to get deposited on the wafer and requires highly-concentrated solutions, which are expensive and degrade quickly as the CNTs in the solution stick together faster.
So the researchers carried out a rigorous analysis of the process by which the CNTs get deposited on the wafer, detailed in a paper in Nature Electronics . They found that it involves a careful balancing act between the rates at which the CNTs stick to the wafer and the rate at which they come loose. Early on there are very few CNTs on the wafer, so the rate of deposition is very high, but as they build up the rate at which they escape again rises.
This results in the rate of deposition gradually plateauing until the two processes balance out and the number of CNTs stabilizes. If the wafer is then removed from the solution and dried, the deposited CNTs stay in place. This prompted the team to devise two new techniques that would slow the rate at which CNTs come loose and speed up the rate at which they get deposited.
While previous approaches relied on long soaks in the solution to ensure enough CNTs g o t deposited, the researchers instead carried out a series of 10-second soaks interspersed with drying. At each cycle the deposited CNTs are fixed in place, and the next dip has the same...

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