Equilibration of nonstoichiometric Ta-Si deposits on polycrystalline silicon at high temperatures

Abstract

The formation of tantalum disilicide from either a metal- or silicon-rich amorphous film was studied to provide information on the kinetics of phase formation and the corresponding details of atom diffusion. A thin-film structure consisting of ∼3000 Å of cosputtered Ta-Si on ∼4000 Å of annealed polycrystalline silicon (which will be referred to as ‘‘silicon’’) on ∼1250 Å of silicon dioxide was deposited over monocrystalline Si(100) wafers. The deposition variables probed were (1) arsenic doped versus undoped silicon, (2) Si/Ta atom ratios of 3.2 and 1.3, and (3) capping silicon dioxide layers versus no capping present during the anneals. One hour anneals were performed in vacuum between 850 and 1000 °C at 50 °C intervals. Rutherford backscattering spectrometry and transmission electron microscopy measurements show that the metal-rich film consumes an appropriate amount of silicon to form the stable tantalum disilicide phase even at the lowest anneal temperature. On the other hand, the rate of formation of TaSi2 from the Si-rich film is affected by both the presence of capping oxide and dopant in the underlying silicon. The main mechanism for Si ejection in the uncapped specimens is evaporation from the surface. In the capped specimens, the main Si loss mechanism is growth of a Si layer between the silicon and tantalum disilicide. In this case, formation of TaSi2 and simultaneous growth of the Si layer is enhanced by the presence of arsenic dopant.