| Pacific Rim Symposium on Surfaces, Coatings and Interfaces (PacSurf 2014) | |
| Thin Films | Tuesday Sessions |
| Session TF-TuM |
| Session: | Interfaces in Multilayers & Nanocomposites |
| Presenter: | Sekwon Na, Sungkyunkwan University, Korea, Republic of Korea |
| Authors: | S. Na, Sungkyunkwan University, Korea, Republic of Korea H. Lee, Sungkyunkwan University, Korea, Republic of Korea |
| Correspondent: | Click to Email |
As scale-down of metal-oxide-semiconductor field effect transistors (MOSFETs) continues, the portion of resistance rising from the contact of total resistance has become burdensome. This has fueled a drive to find a new contact material with a low contact resistance. In particular, rare-earth (RE) metal silicides (Er, Yb, etc.) are promising as source/drain contact materials, since they have a very low Schottky barrier height (SBH) on n-type silicon (0.2~0.4eV).
Among several RE silicides, ytterbium silicide deserves much attention due to its some favorable characteristics for low SBH applications. In this study, we embarked full investigation of Yb silicide by thoroughly analyzing how the formation of the epitaxial silicide layer influences electrical properties including sheet resistance and SBHs. Moreover, we alloyed Yb with Mo to improve oxidation resistance of the material at high temperatures.
Ytterbium and molybdenum were deposited on an n-type silicon (001) substrate with a resistivity of 1-10ohm·cm using a radio frequency (rf) magnetron sputtering system. For the Mo-alloyed sample, the composition of Mo was found around 20 at. % from EDS analysis. To remove the native oxide of the silicon substrate, the wafer was dipped in 1% HF solution and then rinsed with deionized water. The films were deposited at room temperature and at a working pressure of 9mTorr in Ar ambient. Subsequently, a tantalum nitride (TaN) capping layer with the thickness of 50nm was deposited. To measure SBH, we fabricated Schottky diodes by forming circular dots (diameter: 50µm) of Yb via lift-off. The samples were annealed using rapid thermal annealing (RTA) at various temperatures (300°C ~800°C) for 1 minute in N2 ambient. For characterization, we utilized transmission electron microscopy and X-ray diffraction. The electrical characteristics were examined using an HP semiconductor parameter analyzer.
Dynamic microstructural evolution of the Yb/Si contact, in which solid-state amorphization between Yb and Si, nucleation and growth of an epitaxial YbSi2-X layer, and disruption of the layer due to oxidation occurred successively with the temperature increasing. However, the microstructures of Mo-alloyed samples annealed at high temperatures show that the Mo-rich region is formed above the epitaxial layer, and it fended off oxygen diffusion. Thus, the epitaxial layer remained up to 800°C. While the epitaxial Yb silicide led to low SBHs, oxidation increased significantly SBHs at high temperatures in Yb/Si samples. Protection of epitaxial layer by Mo-segregated region helped to remain low SBHs over a wide temperature range (600~800°C).