Utilization of Silicides for VLSI-Contacts with Aluminum and Thermal Oxidation

Abstract

The potential role of silicides in VLSI (very large scale integration) Si technology is described. A survey of trends and requirements exposes two difficult technological issues as the device dimensions shrink: contacts to the individual devices and interconnections between devices or functional blocks on the Si chip. For both, silicides play an important role. The contact between the Si that contains the device and the top Al layer (Al is the preferred metal for metallization) has to be thermally stable, reproducible, reliable, and have low contact resistivity. After a brief survey of the available diffusion barriers designed to suppress the Al interaction with Si, I concentrate on the sacrificial barrier structure. The generalized layered structure approach, utilizing Si/silicide/sacrificial barrier/Al, is analyzed with Ti, V, or Cr as the sacrificial barrier material. A study of Cr as a barrier between Al and NiSi, Pd2Si, or PtSi reveals that impurities in the as-deposited Cr film determine the barrier properties. The concept of the sacrificial barrier is critically reexamined in the light of these results. A study of a thin W layer, which is probably a stuffed barrier, as a barrier between NiSi and Al is reported. An outlook to future trends and approaches concludes this part of the thesis. A major attribute of silicides as interconnection material is their capability to form SiO2 upon thermal oxidation. A study of the oxidation characteristics of near-noble metal silicides (Co, Ni, Pd, and Pt) is presented in the second part of this thesis. The oxidation kinetics and the mass transport through the silicide during oxidation are explored. The role of mass transport in the oxidation kinetics is reviewed in the light of all the reported experimental results. The effect of oxidation on: a) the epitaxial registration of a silicide on Si substrate (NiSi2, CoSi2, and Pd2Si), and b) the electrical resistivity (NiSi2 and CoSi2), is explored. Properties of SiO2 grown on different (Ti, Co, Ni, Pd and Pt) silicides are found to be the same as for SiO2 grown on Si substrates, except for the lower (~1.5 x 106 V/cm) dielectric breakdown. A preliminary implementation of interconnections with NiSi2 demonstrates the applicabilty of this silicide for VLSI. Finally, a novel interconnection scheme is advanced as a possible method to produce self-confined metal interconnection lines.