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The End of the Line for Silicon Dioxide?

December 23, 2003

By means of computer simulations, scientists at the Technical Universities in Clausthal and Vienna are investigating new materials for even smaller and more efficient transistor generations.

The smaller the transistors, the faster they can operate. As a result, faster and faster processors can also be designed. The function of a transistor requires the presence of a thin insulating layer, the gate oxide. In only a few years, the thickness of this layer will be only one fifty-thousandth of that of a human hair. With continuing use of silicon dioxide as gate oxide, however, further miniaturisation of transistors - and thus the manufacture of even faster chips - will no longer be possible in a few years. Scientists all over the world have been racking their brains for years over the problem of further miniaturising transistors. Although the solution sounds simple, its realisation is quite formidable: a new material must be found.

If silicon dioxide - generally known as window glass - has a thickness of only a few atomic layers, it loses its insulating property. A kind of short circuit thus occurs in the transistor. The required material must therefore allow the application of a layer which is thicker and thus acts as an insulator, but which otherwise behaves as though it were an ultra-thin layer of silicon dioxide. After all, the objective is to design and manufacture transistors which are even smaller and more efficient. Strontium titanate has hitherto proved to be the most promising candidate for the purpose. However, only the "recipe" was previously known, not the combined effects of the ingredients. This knowledge deficit was a barrier to continuing development to achieve the set objective. The team of researchers from Vienna and Clausthal has now succeeded for the first time in determining precisely these combined effects. By means of computer simulations, they can explain the process of forming the oxide layer and thus indicate how their electrical properties can be controlled.

The scientific results achieved by Clemens J. Först, Karlheinz Schwarz - both at TU Vienna - as well as Christopher R. Ashman and Peter E. Blöchl at TU Clausthal have been published in the current issue of "Nature" (Nature 427, 53 (2004)). The article is entitled "The interface between silicon and a high -k oxide".

"Computer simulations shed some light into atomic dimensions, where one would otherwise be almost blind," explains Prof. Blöchl from TU Clausthal. By means of computer simulations, the team of researchers has succeeded in explaining, atom for atom, how a new gate oxide - that is, strontium titanate - can be applied to a silicon wafer. "One can imagine the composite of silicon and strontium titanate as two Lego building blocks positioned one over the other", says Clemens Först from TU Vienna in explaining the essential result. The surfaces of solids exhibit a characteristic atomic and electronic pattern which is governed by the arrangement of the atoms. The charge pattern of the oxide layer, which is comparable with the plug-in pattern of a Lego building block, matches the pattern of the silicon surface saturated with strontium.

For the researchers in Vienna and Clausthal, the conclusions concerning the electrical properties are also promising for the future. The oxide acts as a barrier to electrons and can thus be compared with a dam which holds back water. The higher the barrier is, the better the insulating properties are. For the first time, the scientists have demonstrated that the height of the barrier can be decisively increased by chemical processes at the interface. The properties of the gate oxide can thus be adapted to satisfy technological requirements.

The research work has been performed within the scope of the International Research Consortium - Integration of very high-k dielectrics with silicon CMOS technology (INVEST). The project is supported by the 5th General Program for Technology of the Information Society (IST) of the European Commission.

Clausthal, Technische Universitaet