SCIENCE & ENGINEERING NEWS
San Diego, CA — By prompting organic molecules to spontaneously line up along natural pathways on a silicon crystal, scientists at the National Research Council of Canada (NRC) have taken a key step toward the next revolution in microchip technology: the fabrication of tiny devices that sense, analyze, and respond to information in their environment.
“We are approaching the limits of standard microchip technology. In the push to break through that barrier and develop yet more powerful devices, scientists around the world are racing to develop nanoscale structures,” says Bob Wolkow of NRC’s Steacie Institute of Molecular Sciences (SIMS). “We are talking about devices on a scale about one thousand times smaller than a single bacterium. To get there, we need to find completely new ways to make things and to make them work.”
Nanotechnology received a tremendous boost in 1990 when researchers at IBM labs demonstrated that individual atoms could be placed on a surface using a scanning tunneling microscope (STM). While advances have been made on many fronts, several obstacles remain. Atom-by-atom crafting of structures with STM is too slow for any practical fabrication process and standard lithography techniques are too coarse for devices of this size. In addition, without some sort of nanoscale wiring system, the functional capacity of these nanoscale units simply can not be harnessed.
Wolkow, together with colleagues Danial Wayner and Gregory Lopinski have found a way to address these obstacles and to enhance semiconductor technology by adding the capabilities of organic matter to the realm of silicon devices. The SIMS team started with a silicon surface rendered unreactive by a coating of hydrogen atoms. Then, using STM, they plucked off one hydrogen atom, leaving a single, atom-sized reactive spot. Once that dangling bond had been created, they exposed the entire crystal to a gas of styrene molecules.
“The dangling bond is highly reactive, so it quickly grabbed a styrene molecule,” says Wolkow. “That made the silicon happy, but left the molecule unstable. So, the styrene’s natural reaction was to steal an adjacent hydrogen atom from the silicon surface. That created another reactive site and set off a chain reaction that resulted in the growth of a molecular line.”
In effect, the SIMS team has found a way to use STM to initiate a self-assembling nanoscale wiring system. Their calculations show electronic coupling between adjacent molecules, which suggests some molecular lines could be capable of transmitting signals down the pathway. Alternately, the lines could serve as templates to direct the growth of very small conductors applied with conventional lithographic methods.
The SIMS self-assembly method has two significant advantages over previous STM schemes for nanostructure fabrication in that it addresses problems of defect intolerance and mass production. Because they are so small, molecular devices will allow little or no margin for error. The SIMS method will make it possible to build in a degree of redundancy that ensures at least one transmission line will get the message from point A to point B. Equally important, because STM is used only to initiate the process, this method offers the first practical step toward mass production of molecular devices.
“Right now, it takes literally days to individually place atoms on a crystal using STM,” says Wolkow. “With this method, you could use STM to prepare a batch of devices. Once exposed to the desired molecule, multiple lines would grow simultaneously. You could grow a virtually unlimited array of identical structures, all in a matter of seconds.”
On another level, the SIMS work enables further exploration of the development of hybrid devices. Incorporating organic molecules is one way to expand the palette of materials available to semiconductor device designers, and could lead to completely new capabilities. Silicon technology is limited to electrical signal processing, but organic molecules have the capacity to sense changes in their environments and react to those changes. If science finds a way to marry these abilities-and to do it on the atomic scale-it may become possible to custom-make tiny devices that perform very specific functions.
“This discovery is an excellent example of the power of multidisciplinary research being conducted at the NRC,” says NRC President Dr. Arthur Carty. “By combining their knowledge of physics and chemistry, the SIMS team has opened the door to new technologies with enormous potential in many fields-from communications to medicine. Their achievement also demonstrates how important it is for NRC to support fundamental research in Canada.”
Prompted by the realization that the field had reached a major roadblock, Wolkow and Wayner first began work on this concept approximately four years ago. While much work remains to be done, they are optimistic about the prospects of nanotechnology and the potential of hybrid devices. The SIMS team believes their technique for self-directed growth of molecular lines will spur advances on a number of fronts.
“The appeal of the technique is that we’ve limited the need for arduous atom-by-atom crafting of structures with an STM, and unleashed a spontaneous process to automatically drive nanostructure growth,” says Wolkow. “On top of that, we’ve opened the door to further exploration of the possibilities inherent in marrying the power of electronics with the sensitivity of organic molecules.”