Delving into the Nanoscopic
Source: Weizmann Institute of Science
September 26, 2000
Weizmann Institute scientists are reporting that they have
developed a significantly improved method for evaluating ultrathin films.
Potential benefits of this new method include diverse microelectronic
applications and a better understanding of chemical and biological systems.
A recent press release by the institute asks the question, "Ever
tried determining what's inside a layered chocolate cake without slicing it?
Now, how about tackling a similar task, yet on a nanometer-scale."
For decades, thinking big has often meant pursuing smaller and
smaller goals. Take the case of ultrathin films for instance. On average less
than 10-15 nanometers in width, ultrathin films are used in diverse
applications, from optoelectronics to biological sensors. (A nanometer is
roughly one 100,000th the width of a human hair.)
A major requirement for performing these nanoscale feats is
accurate composition and structural analysis. Yet, "looking inside" these films
- often multi-layered - calls for highly sensitive probes. Most available
techniques simply do not provide the depth information essential for evaluating
layered structures. Similarly, X-rays offer a spectacular glimpse into the human
body; however, determining the relative depth of individual structures is highly
difficult. The techniques that have been previously devised to solve this
problem are complicated and can damage the sample, which in turn distorts the
results.
Now, Dr. Hagai Cohen of the Weizmann Institute Chemical Services
and Prof. Israel Rubinstein of the Materials and Interfaces Department have
developed a novel method for evaluating ultrathin films, specifically,
non-conducting films on conducting substrates. Their study, recently appearing
in Nature, builds upon X-ray Photoelectron Spectroscopy (XPS), a common surface
analysis technique.
In XPS, the sample is irradiated with X-rays. This causes
photoelectrons to be ejected. By measuring the energy of the photoelectrons, it
is possible to determine the atoms from which they originated. Researchers have
routinely used an electron flood gun to neutralize the positive surface charge
formed in non-conducting samples as a natural consequence of the photoelectron
ejection, since the charging affects the photoelectrons' energy, distorting the
measurements.
However, proving that one person's stumbling block may be
another's stepping stone, Cohen and Rubinstein realized that the charging effect
actually provides structural information - the magnitude of the photoelectron
energy change correlates directly with the atoms' depth within the film (the
deeper the atom the smaller the change). They decided to turn things around,
using the electron gun to flood the sample with low energy electrons, thus
negatively charging the surface and causing controlled, easily detectable
changes in the energy of the ejected photoelectrons. By measuring these changes,
the researchers were able to determine both the atom type and its depth within
the film.
To test and evaluate their new approach, the scientists used one
of their previous research accomplishments - a highly organized ultrathin film,
which they laced with marker atoms at different depths. When tested on this
system, the new method provided depth information with a superior resolution of
about one nanometer while causing minimal damage to the sample. It also offered
a unique side-benefit, yielding information regarding the film's electrical
properties.
The Weizmann innovation should prove highly beneficial in
developing a wide range of microelectronic applications as well as in studying
various chemical and biological systems.
This research was conducted together with Prof. Abraham Shanzer of
the Organic Chemistry Department, Dr. Alexander Vaskevich of the Materials and
Interfaces Department, and doctoral students Ilanit Doron-Mor, Anat Hatzor and
Tamar van der Boom-Moav.
Rehovot,
Israel
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