Insertion of Hafnium Interlayer to Improve the Thermal Stability of Ultrathin TiSix in TiSix/n+-Si Ohmic Contacts

Serious agglomeration of ultrathin TiSi
_x
employed in source–drain (S–D) ohmic contacts after high-temperature annealing was manifested in previous work. This severely restricts its application in state-of-the-art DRAM peripheral 3-D FinFET transistors. In this work, the effects of hafnium (Hf) interlayer on the thermal stability of ultrathin TiSi
_x
in TiSi
_x
/n
⁺
-Si ohmic contacts were systematically studied. As-prepared ultrathin TiSi
_x
and TiSi
_x
/n
⁺
-Si ohmic contacts with 0-, 1-, 2-, and 3-nm Hf interlayers were characterized by means of specific contact resistivity (
$\rho _{\text {c}}$
), transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). It is found that compared to the counterparts with 0-, 2-, and 3-nm Hf, the presence of 1-nm Hf interlayer is proven to be effective in enhancing the thermal stability of ultrathin TiSi
_x
significantly. With 1-nm Hf, amorphous HfO
₂
, and Hf silicate are readily formed, which hinders the interdiffusion between Ti and Si atoms and resultant agglomeration of ultrathin TiSi
_x
films. This is thought to be responsible for such a remarkably enhanced thermal stability of ultrathin TiSi
_x
in TiSi
_x
/n
⁺
-Si ohmic contacts.