Portland, Ore. - As major semiconductor fabs tackle the submicron nanoscale patterning of wafers, researchers at Pennsylvania State University have already moved to the angstrom scale. Their organic monolayers with 5-angstrom features promise to enable the self-assembly of patterns too small for lithography by serving as templates for chip atoms.
"We use molecules that are deliberately designed to be less stable in their substrate attachment than other related molecules, so they would be unlikely to be used directly," professor Paul Weiss said. "Rather, they will be used to shore up patterns and to stabilize the precision of the patterns at this subnanometer scale."
The subnanometer scale is measured in angstroms-one-tenth of a nanometer. Weiss, a recent visiting professor of electronic science and engineering at Kyoto University in Japan, heads a semiconductor research group associated with Penn State University's Materials Research Institute. Two Penn State doctoral candidates, Arre-laine Dameron and Lyndon Charles, helped Weiss perform his experiments.
Self-assembled monolayers (SAMs) offer a way to create intricate angstrom-scale patterns that can be tuned by adjusting their chemical makeup and thereby precisely adjusting their resulting physical properties. Using these patterns, which serve as placeholders, single-molecule devices can potentially be arrayed across wafers. The SAMs consist of adamantanethiol, a commonly used organic molecule for this kind of work. Weiss' group is developing a catalog of useful chemical formulas that can create a variety of self-assembled monolayers that serve as patterns for single-molecule semiconductor devices.
The demonstration used organic alkanethiolate as self-assembled monolayers on gold substrates. Using chemisorption of the alkanethiolate's sulfur head group, the self-assembled monolayer achieved 5-angstrom resolution but was highly stable because of the strong sulfur-gold bond and the relatively weak van der Waals forces between adjacent alkyl chains.
Points of attachment
The resulting patterns formed large, consistent domains of molecules with identical rotations and tilts. In addition, the boundaries separating each domain were discrete and often contained molecules protruding out of the monolayer; those could potentially serve as attachment points for semiconductors.
The researchers are extending the current demonstration on a gold substrate to traditional silicon. "These same strategies can be applied on more-conventional semiconductor substrates, but the chemistry is substantially less-developed," said Weiss. "We are just now embarking on a research program to advance self-assembly on technological surfaces."
When they studied the monolayer with a scanning tunneling microscope (STM), Weiss and his fellow researchers found that a cage structure had formed that could contain future semiconductors for molecular-scale devices. Each molecular cage comprised 10 carbon atoms with four fused cyclohexane rings in chair conformations. Each carbon cage was tetrahedrally hybridized and exhibited only limited strain, because each carbon-carbon bond was staggered. By adding a sulfur group, Weiss' group was able to create adamantanethiol with the carbon cage strongly tethered to the gold surface with a highly symmetrical round topology.
The STM revealed that the adamantanethiol monolayers formed flat, highly ordered hexagonal close-packed thin films that were precisely one molecule thick. In addition, the adamantanethiolate SAM was unstable with respect to displacement, so it was potentially useful as a temporary protective layer during the fabrication of single-molecule semiconducting devices. When used as a placeholder, the self-assembled monolayer could keep useful molecular devices in place while maintaining their full functionality as transistors or other semiconducting devices.
"The most important aspect of this work is that we are using what we know about supramolecular [intermolecular] interactions to make nanostructures that are in the difficult area between chemical synthesis (1 nm and less) and nanolithography (100 nm and up)," Weiss said. These strategies may well come to the rescue of advancing semiconductor technologies as they move down to smaller and smaller scales. Our advance here was to extend the range at the low end of useful intermolecular interactions and to show how those can be applied to make nanostructures with greater precision."
Semiconductor Research Corp. recently sponsored a workshop on self-assembly. The results will be forwarded as recommendations to the National Nanotechnology Initiative and National Science Foundation, where Weiss' techniques and those of others will be examined with an eye toward determining how they can help plumb the nanoscale and angstrom scale for future semiconducting devices.