Insertion of Hafnium Interlayer to Improve the Thermal Stability of Ultrathin TiSi_<i>x</i> in TiSi_<i>x</i>/n⁺-Si Ohmic Contacts

Serious agglomeration of ultrathin TiSixemployed 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 TiSixin TiSix/n+-Si ohmic contacts were systematically studied. As-prepared ultrathin TiSixand TiSix/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 TiSixsignificantly. With 1-nm Hf, amorphous HfO2, and Hf silicate are readily formed, which hinders the interdiffusion between Ti and Si atoms and resultant agglomeration of ultrathin TiSixfilms. This is thought to be responsible for such a remarkably enhanced thermal stability of ultrathin TiSixin TiSix/n+-Si ohmic contacts.