A Significantly Improved Method for Evaluating Ultrathin Films


Delving into the Nanoscopic

Source: Weizmann Institute of Science
Rehovot, Israel

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.

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