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Silicon Dopants

3-D maps reveal clumping of desirable impurities

by Mitch Jacoby
September 10, 2007 | A version of this story appeared in Volume 85, Issue 37

An atom-probe map shows that arsenic dopant atoms (purple dots) in silicon cluster into nanosized loops (O is blue; Si, gray).
An atom-probe map shows that arsenic dopant atoms (purple dots) in silicon cluster into nanosized loops (O is blue; Si, gray).

FOR ALL OF SILICON'S GLORY as the cornerstone of modern microelectronics, its exploitable electronic properties are almost entirely due to impurities—dopant atoms deliberately introduced into the semiconductor during processing. A new study shows that, despite manufacturers' considerable efforts to distribute dopants uniformly, the atoms sometimes end up in dense nanosized clusters (Science 2007, 317, 1371).

The investigation, which yielded three-dimensional angstrom-resolution maps of the atomic components of silicon samples, highlights a shortcoming in silicon device fabrication methods that could become more problematic in future generations of devices. Nonuniform dopant distributions may adversely affect the performance of future microelectronics devices, which are soon expected to feature circuit components in the low-double-digit nanometer range.

Ion implantation is a common method for embedding charge-carrying dopants in silicon. The process alters the semiconductor's electronic properties in a desirable way, but it also wreaks havoc on the material's lattice structure. To undo the crystal defects and repair the damage, manufacturers subject the semiconductor to various heat treatments. Previous studies have probed the evolution of silicon crystal defects during thermal annealing, but the fate of dopant atoms in the vicinity of defects has remained unclear.

Now, Keith Thompson and Thomas F. Kelly of Imago Scientific Instruments, Madison, Wis., Philip L. Flaitz of IBM, Hopewell Junction, N.Y., and their coworkers have used atom-probe tomography and other methods to pinpoint arsenic dopant atoms in silicon.

The group found that ion implantation leads to spherical silicon crystal defects that trap arsenic atoms. Furthermore, as the crystals are subjected to high-temperature treatments, the defects rearrange, causing the arsenic dopants to coalesce into dense loops.

Lincoln J. Lauhon, an assistant professor of materials science at Northwestern University, notes that as the dimensions of circuit components continue to shrink to the tens-of-nanometer range, even the smallest defects may significantly impair electronic performance. The Imago-IBM study is a "major step in characterizing potentially killer defects with unprecedented spatial resolution," he says.



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