Chemistry Is Key to New Insulators, Barrier Films

Hafnium oxide, nickel silicide, bismuth strontium titanate: These are some of the exotic materials that will be finding their way into semiconductors in the years ahead. The challenge for chemical firms is finding the right way to apply these new materials.

Hafnium oxide, for example, is getting a lot of attention as a next-generation gate insulation material. In semiconductors, gate insulators separate the gate, which turns current flow on and off, from the channel through which the current flows.

Today’s gate insulators are made of silicon dioxide. As these insulators are thinned to support higher switching speeds, however, they tend to let electrons leak through, increasing power consumption. Hafnium oxide can form a thicker gate that blocks electron leakage while still allowing high switching speeds.

At the Semicon trade show in San Francisco last month, Praxair Electronics presented its capabilities in hafnium oxide precursors. According to Conrad T. Sorenson, program development manager at the firm’s Tonawanda, N.Y., R&D facility, one contender is tetrakis(dimethylamino)hafnium; others are the diethylamino and ethylmethylamino variants.

Sorenson explained that precursor molecules are applied to a silicon wafer via atomic layer deposition, a technique that lays down one—and only one—layer of molecules at a time. When the first layer hits the wafer, two of the organic ligands are knocked off, he said. Subsequent processing removes the other two, leaving an even hafnium oxide film.

Similar ligands and deposition techniques are starting to be used to deposit barrier films containing titanium, tantalum, and ruthenium. These films, Sorenson said, are needed to keep the copper-based circuitry in the most advanced computer chips from diffusing into the silicon oxide layer.