Dec 17, 2003
IBM exploits self-assembly to make nanocrystal memory
Researchers at IBM, US, have used self-assembly of polymer molecules to make a FLASH memory device. The scientists combined the technique with conventional semiconductor processing methods to create an array of silicon nanocrystals each with a diameter of around 20 nm.
“One of the challenges in circuit fabrication is shrinking the size of circuit features,” IBM scientist Kathryn Guarini told nanotechweb.org. “Conventional lithography is limited by the wavelength of light. We were interested in new opportunities for making features smaller and more dense - self-assembly is suitable for that.”
To make the devices, Guarini and colleagues deposited a thin film of diblock copolymer - a mixture of polystyrene and poly(methyl methacrylate) (PMMA) molecules - onto a thermal oxide coating on a silicon substrate. Then they annealed the polymer so that it separated into phases consisting of nanoscale PMMA cylinders in a polystyrene matrix. Removing the PMMA with an organic solvent left a porous polystyrene film. This film acted as a sacrificial layer to define nanocrystals.
The scientists created the nanocrystals by etching the polystyrene pattern into the oxide layer, growing a 2-3 nm thick layer of program oxide, conformally depositing amorphous silicon and then etching away the top surface of the silicon. To complete the devices, the team added a 7-12 nm layer of control oxide on top of the nanocrystal array and then added a polysilicon gate. The program and control oxide layers acted to electrically isolate the silicon nanocrystals. The nanocrystals had a diameter of 20 nm and a centre-to-centre spacing of 40 nm, giving a nanocrystal density of 6.5 x 1010/sq. cm. According to the researchers, using a polymer with a lower molecular weight could provide a route for obtaining smaller nanocrystals.
“The emphasis was to demonstrate that we can combine nanoscale and standard semiconductor processing technologies,” added Guarini. “It’s the same concept as FLASH memory in digital cameras, cellphones, etc. It is used for data storage - each device is either a zero or a one. Industry is trying to make the devices smaller to pack more in, and it’s becoming difficult to do that with traditional structures.”
Now the scientists are developing silicon-based self-assembly devices for microelectronics, biosensing, biomedical, memory and logic circuit applications. They’re also looking to improve the self-assembly technique. “[The nanocrystals] are 20 nm today and we’d like to make them even smaller,” said Guarini.
The researchers reported their work at the International Electron Devices Meeting in the US.
About the author
Liz Kalaugher is editor of nanotechweb.org.