Metal-dopant-compound formation in TiSi2 and TaSi2: Impact on dopant diffusion and contact resistance

The refractory metal disilicides TiSi2 and TaSi2 were investigated for their usefulness as dopant diffusion sources. During furnace annealing and rapid thermal processing, strong decomposition reactions occur between the dopants D (B or As) and the respective silicide (MSi2) to form MxDy compounds. With the help of special sample preparation methods and various analytical techniques, the compound phases TiB2, TiAs, TaB2, and TaAs were unambiguously detected. The fraction of freely diffusing B in TaSi2 is determined to be below 5% of the total dose; by far, the major part of the dopant is bound within the TaB2 phase detected. Careful sample preparation and analysis of secondary-ion-mass spectrometry profiles is necessary to avoid artifacts caused by these compound particles. The MxDy-compound formation has detrimental consequences: The solubility of arsenic and even more of boron in TiSi2 and TaSi2 is limited to rather low-concentration levels (e.g., B in TaSi2: 4 × 1018 B/cm3 < CB(900 °C) < 1.6 × 1019 B/cm3) and the outdiffusion into poly- or monocrystalline silicon is strongly retarded. Also, the low interface dopant concentrations achievable result in unacceptably high values of contact resistance. The observations on metal-dopant- (M-D-) compound formation are demonstrated to agree well with the predictions from thermodynamic calculations on the respective M-Si-D system. The effects on junction formation are compared to the case of WSi2 and CoSi2, which, from a parallel study, are known not to form compounds. In all cases these comparisons support our statements on the tremendous impact of M-D-compound formation, because much improved data on diffusion and junction formation were obtained for CoSi2 and WSi2. The same holds for a comparison on contact resistances for silicide diffused junctions, which was performed for TiSi2 and CoSi2.