Laser Deposits High-k Silicate Films
Daniel S. Burgess
Researchers at Université du Quebec in Varennes, Canada, hope to enable semiconductor manufacturers to continue reducing the size of integrated circuits. The laser-deposited films of titanium silicate created by the team suggest that an answer to the gate electrode problem may not be too far off.
Dilip K. Sarkar, a member of the team who is now at the University of Waterloo in Ontario, explained that the current International Technology Roadmap for Semiconductors projects that the dielectric layer at the gate electrode will be only 1 nm thick in 2012. The silicon dioxide used today displays a high leakage current at this scale, however, making it unsuitable for use in metal-oxide semiconductor field-effect transistor devices.
Thus, the industry is searching for alternate dielectric gate materials to avoid this materials limit -- specifically, amorphous materials with a dielectric constant (k) of between 10 and 20, Sarkar said. Potential candidates include other oxides, such as those based on tantalum, titanium, zirconium, hafnium, yttrium and aluminum, but these can form a layer of SiO2 at the interface with crystalline silicon, lowering the capacitance. On the other hand, their high-k silicates offer an abrupt interface.
Films without oxides
The researchers investigated the properties of titanium silicate thin films grown by pulsed laser deposition on crystalline silicon and on platinum-coated silicon. They produced titanium/silicon targets by laying 0.5 x 4-cm pieces of titanium foil on a 3-in.-diameter silicon substrate. A GSI Lumonics PulseMaster KrF laser ablated the target at 9.3 Pa and at room temperature.
The resulting films displayed an average dielectric constant of 11 for frequencies between 100 kHz and 13 MHz. Employing Fourier transform infrared transmission spectroscopy, x-ray photoelectron spectroscopy and x-ray diffraction techniques, the researchers determined that the process yielded crystalline films without forming oxides on the substrates.
Sarkar noted that laser ablation can produce micron-size particles on the substrate, leading to nonuniform layers. In the study, however, the average grain size of the particles was 3.5 nm. The researchers plan to examine issues of thermal stability and phase separation in the thin films, with the goal of producing only amorphous titanium silicate.
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