October 4, 2005

Letting Nature Be the Architect

Single-celled algae called diatoms could provide the structure for mass-produced nanodevices.

By Kevin Bullis

Inspired by the intricate beauty of the shapes formed by microorganisms -- and by those organisms' ability to reproduce rapidly -- a group of researchers centered in Georgia may have found an efficient way to create nanoscale parts for next-generation electronics.

Chemical engineer Kenneth Sandhage of the Georgia Institute of Technology and a team of biologists, geneticists, and electronic engineers have published details of a new process for converting the finely-detailed silica skeletons of diatoms, a type of single-celled algae, into synthetic replicas made of materials such as titanium dioxide, which conducts electricity and could be used in electronic devices.

The new techniques exploit the diatom's own ability to reproduce, and can be used to mass-produce intricate three-dimensional structures.

"Excellent work" is the description applied by Karl Berggren, head of the Quantum Nanostructures and Nanofabrication Group at MIT, who was not involved in the research. "It's a new concept for certain big problems in nanofabrication."
 
Sandhage says he got the idea after sitting next to a marine biologist on a bus trip. She showed him the elaborate, Christmas ornament-like structures made by diatoms. Sandhage decided to try growing the organisms as templates for potential nanodevices.

That part is easy, since diatoms reproduce through cell fission, creating two exact copies of their silica shells. After 40 generations, a single diatom will have multiplied itself into a trillion copies.

Sandhage then uses a handful of methods to either coat the diatom shells with metallic substances or completely replace them. He uses materials such as titanium dioxide (also known as titania) that are better conductors and can withstand thermal stress, two important features of materials to be used in electronics.

The resulting structures have features measured in tens of nanometers, comparable to the smallest features of chips produced today using conventional photolithographic techniques. The difference: complex three-dimensional shapes can be produced much more quickly using Sandhage's approach.

That's important because three-dimensional chip designs could help chipmakers keep delivering more powerful microprocessors at the pace set by Moore's Law, which says that the number of transistors that can fit on a chip doubles roughly every two years.

Conventional photolithography can be used to build three-dimensional structures by adding and etching one layer of silicon at a time, but it's a frustratingly slow process, says Berggren.

Pointing to an image published in Sandhage's article -- which appeared in the International Journal of Applied Ceramic Technology -- Berggren says, "There's no way I know of that we could make this structure without the technologies that they're developing."