To make the contacts, the researchers evaporated a 10 nm-thick layer of titanium onto silicon wafers and used optical lithography to remove some of the metal, leaving patterned electrodes behind. Then they formed titanium silicide by heat-treating the samples at 800 °C for 30 seconds and 900 °C for 5 seconds.
Examining the resulting TiSi2 contacts using atomic force microscopy revealed a narrow diffused shadow "some nanometres high" on the silicon next to the electrodes. On silicon (111) wafers, the shadow was about 140 nm wide, while on silicon (100) wafers the shadows were between 200 and 500 nm wide and more irregular. The scientists reckon that the shadows are caused by titanium atoms diffusing laterally through the electrode border.
"Our aim was to develop stable contacts on silicon that were able to withstand the strong treatments used to obtain highly ordered silicon surfaces," researcher Vincenzo Palermo of ISOF told nanotechweb.org. "At first we considered the lateral diffusion of the metal as a secondary, negative effect of the thermal treatment, but then we discovered that the interfaces we were obtaining were very ordered and uniform. Our results suggest that metal diffusion, until now considered a dangerous drawback, can be used instead to improve resolution beyond lithographic limits."
Palermo added that diffusion is a process well known by technologists, highly controllable and can be easily implemented on wafer-scale production lines. "As a potential application, we can imagine a production line where lateral metal diffusion, stimulated with a simple oven, is used to shrink transistor gate length," he added. "This implementation looks simple, cheap and fast."
Palermo says there are still several issues that need resolving, however, mainly regarding reproducibility and electrical resistance. The researchers reported their work in Applied Physics Letters.