Instead of remaining where they were made, the tiny holes actually move around a bit, sometimes lining up in the equivalent of cornrows, the researchers found.
“We think this is important for several reasons,” said Dr. John J. Boland, professor of chemistry at UNC. “First, it gives us a better understanding of the fundamental chemistry of etching. Previously, models of what was happening completely neglected the possibility of motion, and people thought etching was a kinetic process determined by how fast reactions occurred.”
Researchers found the process is really driven by heat, and the holes can move in a diffusion process into the most energy-efficient configurations.
“This knowledge also could eventually help lead to smaller silicon chips for computers and other devices,” Boland said. “The smaller a device is, the more efficient it has to be, and this information suggests we can achieve greater precision in the manufacturing process.” The smoother the walls of the hole are, the better electrons can flow from the source to the immediate target, which is called the “drain,” he said.
A report on the studies appears in the current (Sept. 10) issue of Physical Review Letters, a professional journal. Dr. Cari F. Herrmann, a former chemistry graduate student, worked with Boland and is first author.
By bombarding silicon surfaces with bromine or other halogen-containing chemicals, scientists and technicians create patterns in those surfaces, Boland said. Previously, the process was thought to be something like sand blasting a wall to wear away mortar or chiseling a hole in a piece of wood.
Herrmann and her mentor used a scanning tunneling microscope in real time to analyze and show what was really happening. The holes -- atoms missing at the bombardment sites -- migrate and become more stable patterns with increased heat, between 600 and 700 degrees centigrade.
“Prior to this, people thought there were two mechanisms for creating etch patterns, one for low temperatures and one for high temperatures,” Boland said. “In fact, there is only one. Holes we made rearranged and continued to rearrange as we changed the temperature and ultimately tended to form the most energy-efficient structure, which looks like a different mechanism but really isn’t.
“We are saying now that the roughness you get isn’t from the etching but from the rearrangement of these holes, or ‘vacancies,’” he said. “If we can control the interactions of atoms on the surfaces, we can control the roughness.”
Note: Herrmann can be reached at (919) 962-5466. Boland has been delayed in Germany because of the terrorist attacks.
By DAVID WILLIAMSON
UNC News Services