Tip-Enhanced Raman Spectroscopy Images Buried Nanoscale Features

Hank Hogan

Thanks to researchers at the University of Rochester in New York, looking below the surface may be easier — at least on a small scale. Using tip-enhanced Raman spectroscopy, the group has mapped buried features with 30-nm resolution and has characterized their chemical makeup.

The technique could be useful in a number of areas; for example, probing a nanoscale device after it has been covered by multiple protective layers. Lukas Novotny, an optics and physics professor at the university, noted that it also could find a home in the semiconductor industry where chips are increasingly complex, being used for test and analysis applications such as the stress analysis of high-density integrated circuits.

Except when they are deployed as sensors, nanoscale devices must be shielded from the environment. This typically is achieved by capping a device with various layers, which remove it from view. Although various methods have been used to see below the surface, they do not reveal chemically specific information.

Raman spectroscopy provides this additional data. The use of an optical antenna, an irradiated metal tip, allows researchers to achieve sub-diffraction-limit resolution because it concentrates the localized electric field. What was not known until the present research was whether the enhancing electric field could be made to penetrate the surface far enough to be useful for the investigation of buried nanoscale structures.

Tip-enhanced Raman spectroscopy maps buried features and characterizes their chemical composition. At left is a surface image of a carbon nanotube covered, in part, by SiO₂. In the middle is the near-field image, taken at a wave number of 1592 cm^–1. At right are the vibrational spectra, showing that the signal from the buried nanotube has been captured. Courtesy of Lukas Novotny, University of Rochester.

In their experiment, the researchers used sharp gold tips 20 to 30 nm in diameter and positioned a few nanometers above the surface of the test sample to maximize the field penetration. They illuminated the tip with 633-nm light from a HeNe laser, polarized to align the electric field along the tip shaft. They directed the beam to the tip and collected the resulting Raman scattering with the same microscope apparatus. By raster scanning the tip across the sample, they produced 2-D images of the Raman response from the sample.

With this setup, they mapped the subsurface features of test structures: carbon nanotubes beneath a 5- to 10-nm-thick layer of SiO₂. From these results, they determined the resolution to be ~30 nm. Imaging several wave numbers this way would capture the chemical composition of the buried structures.

Novotny said that this technique might be combined with others to perform spectroscopic identification and three-dimensional reconstruction of buried features. Another area of improvement involves the development of more efficient antenna geometries, but that may prove difficult.

“The challenge is fabrication,” he said.

Nano Letters, online March 23, 2006, doi:10.1021/nl0600023.