Placing Nanowires on Semiconductor Wafers for Future Nanoscale Electronics

A new system for manipulating and accurately positioning individual nanowires on semiconductor wafers has been developed by researchers at the National Institute of Standards and Technology (NIST).

Nanowires can be defined as structures that have a lateral size constrained to tens of nanometers or less and an unconstrained longitudinal size; they are not observed spontaneously in nature and must be produced in a laboratory.

Although they have been intensively studied for their possible applications in future nanoscale electronics, there are still some unsolved issues, like how to put wires only a handful of atoms in diameter where you want them.

The smallest-diameter nanowires today are built in a "bottom-up" fashion, assembled atom-by-atom through a chemical growth process such as chemical vapor deposition, which is essentially a bulk process that produces haystacks of jumbled nanowires of varying lengths and diameters.

"The normal research approach," explains NIST electronics engineer Curt Richter, "is to throw a whole bunch of these down on the test surface, hunt around with a microscope until you find a good-looking wire in about the right place, and use lithography to attach electrical contacts to it."

Searching for ways to control their disposition, the NIST engineers modified a standard probe station used to test individual components in microelectronic circuits. The station includes a high-resolution optical microscope and a system for precisely positioning work surfaces under a pair of customized titanium probes with tips less than 100 nanometers in diameter.

This involves a two-step process, in which silicon nanowires suspended in a drop of water are deposited on a special staging wafer patterned with a grid of tiny posts, and dried. Resting on the tops of the posts, selected nanowires can be picked up by the two probe tips, which they cling to by static electricity. The test structure wafer is positioned under the probes, the nanowire is oriented by moving either the probe tips or the wafer, and then placed on the wafer in the desired position.

However, mass production is still a few years away, as the new method of control allows NIST engineers to place single nanowires wherever they want to create elaborate structures for testing nanowire properties using only optical microscopy and conventional photolithographic processing instead of advanced and expensive tools such as focused ion or electron beams.