Niobium metal CVD deposition was examined primarily in the context of depositing super-conducting Nb3Ge and Nb3Ga phases. For this application, niobium halides NbHal5 (Hal =Cl, Br) were investigated as potential niobium CVD precursors. (both compounds are commercially available).
Synthesis of NbHal5:
Nb2O5 + 5 C → 2 Nb + 5 CO
Nb + 2.5 X2 → NbHal5 (Hal = Cl, Br)
Niobium pentachloride NbCl5 is yellow solid, melting at 203.4 °C, boiling at 274.4 °C.
Niobium pentachloride has dimeric structure [NbCl5]2, i.e. Nb atom has coordination number 6
[NbCl5]2 was reported to be useful as CVD precursor for the growth of Nb3Ge and Nb3Ga alloy by CVD, as well as Nb carbonitride deposition by CVD.
Niobium pentabromide NbBr5 is orange solid, melting at 254 °C, boiling at 365 °C, The structure of niobium pentabromide is dimeric [NbBr5]2 (similar to NbCl5)
Deposition conditions:
Lowest deposition temperature is 700 °C (usually 1200-1400°C), i.e. low-temperature CVD is not possible. However, the high quality Nb films or alloys with excellent electrical properties can be obtained.
In general films were grown in presence of H2:
NbCl5 + 5/2 H2 → Nb + 5 HCl
[The Chemistry of Metal CVD, International Research Training Group, “Materials and Concepts for Advanced Interconnects”, Prof. Dr. Heinrich Lang]