One of the key challenges for future CMOS technology nodes is to form source/drain junctions with very small parasitic series resistance values. This requires fundamentally new junction and contact formation technologies to produce ultra-shallow junctions with super-abrupt doping profiles, above equilibrium dopant activation and contact resistivity values near 10-8 ohm-cm2. Recently, this laboratory demonstrated a new junction formation technology based on selective deposition of heavily doped Si1-xGex alloys in source/drain regions isotropically etched to the desired depth. The results to date indicate that the technology has the potential to meet all junction and contact requirements of future CMOS technology nodes. Of particular interest to this thesis is the smaller bandgap of Si1-xGex resulting in a smaller metal-semiconductor barrier height, which is a key advantage in reducing the contact resistivity of a metal-semiconductor contact. In this work, formation of germanosilicide contacts to heavily boron doped Si1-xGex alloys was studied with the intention to find a contact solution for future CMOS technology nodes beyond 100 nm.
During the early stages of the research project, germanosilicides of Ti, Co, Ni, Pt, W, Ta, Mo and Zr were studied to identify the most promising candidates as source/drain contacts. The first set of experiments showed that Zr, Ni and Pt may have advantages over other candidates. Of the three germanosilicides, zirconium di-germanosilicide, Zr(Si1-xGex)2 exhibited the best thermal stability but suffered from a high resistivity and excessive substrate consumption. Ni and Pt germanosilicides were considered attractive because they were both mono-germanosilicides and consumed much less Si1-xGex than Zr(Si1-xGex)2. Additionally, both had resistivity values lower than that of Zr germanosilicide which could be reached at temperatures as low as 300 °C.
Of the three germanosilicides, NiSi1-xGex was found to be the only one capable of yielding the desired contact resistivity of ~ 10-8 ohm-cm2 on both p+ and n+ Si1-xGex. Unfortunately, NiSi1-xGex was found to suffer from Ge out-diffusion, which had a direct negative impact on its thermal stability. NiSi1-xGex formed at temperatures above 450 °C exhibited high sheet resistance and a rough germanosilicide/Si1-xGex interface. Below this temperature, ultra-shallow p+-n juntions with self-aligned NiSi1-xGex contacts were formed with excellent reverse bias junction leakage characteristics. It was also observed that the thermal stability of NiSi1-xGex formed on heavily boron doped Si1-xGex was noticeably better.
A new approach was proposed to form ultra-thin Ni Si1-xGex layers with enhanced thermal stability. By inserting a thin Pt interlayer between Ni and Si1-xGex, the thermal stability of NiSi1-xGex was found to be significantly improved. On boron doped Si1-xGex, the material was found to be stable at least up to 700 °C with a total starting metal thickness of 10 nm. Pt incorporation was also found to result in better surface and interface roughness.
This work has shown that high quality boron doped Si1-xGex source/drain junctions with NiSi1-xGex contacts can be formed. The junctions exhibit contact resistivity values near 10-8 ohm-cm2, which satisfies the requirements of future CMOS technology nodes.
