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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