New Technique Allows Real-Time Monitoring of Atomic Changes Occurring at Material’s Surface

Researchers from Aix Marseille Université, based in Marseille, France, have devised a method that enables real-time and in-situ monitoring of physical processes that take place at materials’ surfaces at the atomic level.

Built on the principles of electron microscopy, this innovative method allows the scientists to investigate the kinetics of thermal decomposition of a thin silicon dioxide layer deposited on silicon. Silicon dioxide is a building block of micro-electronics and represents an important part of these devices. The thermal stability of silicon dioxide is equally important for device performance. The study was headed by Dr. Frédéric Leroy.

Over the past four decades, the decomposition of a thin silicon dioxide layer on silicon had attracted a great deal of scientific interest. Studies performed in the past demonstrated that such decompositions take place unevenly at the surface of a material, through holes that are formed locally in the oxide layer and expand in a lateral fashion.

In order to enhance the performance of silicon oxide, a better understanding of the basic atomic processes contributing to the holes’ opening velocity is required. Sophisticated characterization tools are required to gain a better insight into the nanomaterials properties.

We needed to be able to characterize the structural (crystallography, size, shape) and the chemical properties at the same time and to be able to follow in situ and in real time the changes during a given process for a rapid feedback on the experimental parameters. Our approach based on low energy electron microscopy is the cornerstone of our achievements.

Dr Frédéric Leroy, Aix Marseille Université

The researchers faced a number of issues, even with the use of the new instrument. It was difficult to acquire real-time measurements of the decomposition of silicon dioxide during a thermal treatment, because the entire process takes place within a span of several minutes under a narrow temperature threshold.

It was impossible to adjust all control parameters of the electron microscope before the decomposition process started since silicon dioxide is amorphous, so we had to adjust finely the settings within a few seconds as soon as the oxide decomposes in order to characterize the whole process.

Dr Frédéric Leroy, Aix Marseille Université

Despite these challenges, the detailed measurement produced some surprising results. The French research team uncovered experimental proof that showed that the process of decomposition does not initially remain in a stable state regime as projected by earlier studies.

Our results imply that the conventional view of a steady state regime for the silicon dioxide decomposition related to a simplified reaction Si+SiO2-> 2SiO(g) occurring at the hole edge is not generally true.

Dr Frédéric Leroy, Aix Marseille Université

The new results indicate that the decomposition of silicon dioxide takes place through hole nucleation and opening featuring a spherical shape. The opening velocity of the holes is closely associated with the decomposition speed of silicon dioxide at the holes’ edge. A chemical reaction is catalyzed by Si hydroxyls that exists within the hole. As a result of this reaction, large holes open quickly at first.

The scientists believe that Si hydroxyls begin to agglomerate during prolong thermal annealing, and are subsequently released within the holes during the decomposition process of silicon dioxide. The study offers important applications in micro-electronics, especially during all stages of thermal treatments.

We have shown that the silicon dioxide formed by a wet chemical treatment is highly defective after a long thermal annealing. The next step in our research is to study the interplay between chemical reactions and the enhancement of the mobility of nanostructures.

Dr Frédéric Leroy, Aix Marseille Université


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