Researchers from MIT and Analog Devices have made a 16-bit processor with 14,000 so-called carbon nanotube field-effect transistors. This is the largest chip based on carbon nanotubes to date.
The researchers named the chip RV16XNano and built it up based on the open source RISC-V architecture. Although it is a fairly basic processor, by current standards, it is capable of accurately executing a set of instructions. For example, the researchers managed to use a program to get the message “Hello, World! I am generating RV16XNano, made from CNTs’.
According to MIT, the science team is building on research that six years ago resulted in a 1-bit chip with 178 carbon nanotube field-effect transistors, or cnfets. Substantial progress was made in the subsequent period in reducing material and manufacturing defects, among other things.
Metallic contamination causes a small part of the carbon nanotubes to delay or stop the correct switching of the transistors. Metallic cnts are always conductive and it’s all about getting semiconductors. For a working chip of some size and complexity, the purity of the carbon nanotubes used must be 99.9999999 percent.
Thanks to a set of techniques that the researchers have named Dream, they can make functional chips, even if the purity is 99.99 percent. Dream stands for designing resiliency against metallic cnts. The basis of the method is to simulate different gate combinations to find out which ones are robust and which are not, due to defects. When designing the chip, they then used a program that automatically uses only the robust combinations.
In addition, the researchers have improved the manufacturing process. To make cnfets, carbon nanotubes are placed in a solution on a wafer with the transistor architecture on top. To prevent tube clumping, the wafer has been treated with an agent that promotes adhesion and the researchers also apply a specific polymer coating, after which the wafer is immersed in a solution. This last step causes the polymer, and with it the lumps of tubes, to be washed away. The researchers call this step Rinse, removal or incubated nanotubes through selective exfoliation .
Finally, they were able to determine the properties of transistors in a controlled way, which means that they are n or p-type transistors. That was one of the challenges with carbon nanotube transistors. The researchers succeeded in applying titanium or platinum, among other things. They call the technique Mixed, which stands for metal interface engineering crossed with electrostatic doping.
The minuscule tubes of carbon in a hexagonal grid of chicken wire are light, strong and in theory cheap to produce, and have good conductive properties. They have therefore been seen for years as a potential successor to silicon as a material for chips, which could become faster with lower heat production. Researchers encountered numerous obstacles in the actual production of chips, but according to MIT researchers, it is no longer a question whether, but when, chips based on carbon nanotubes appear. Max M. Shulaker from the research team even thinks that this can be the case within five years, partly because they use the same manufacturing techniques as the current chip industry.
The researcher publishes their work in the scientific journal Nature under the title Modern microprocessor built from complementary carbon nanotube transistors.