Development of ALD precursors for low resistivity Zr, Hf, and Nb nitride films

The synthesis, spectroscopic properties, and suitability for use as precursors in atomic layer deposition (ALD) processes are described for the ZrCp₃ and HfCp₃. ZrCp₃ was prepared by a literature procedure entailing reduction of ZrCp₄ with potassium graphite and was isolated as brown crystals in 34% yield. HfCp₄ was isolated in 57% yield as yellow-brown crystals by a similar route employing reduction of HfCp₄ with potassium graphite. ZrCp₃ and HfCp₃ showed broad paramagnetic resonances in the ¹H NMR spectra that were centered at δ 6.2 and 6.0ppm, respectively, which are assigned to the cyclopentadienyl hydrogen atoms. Additionally, the ¹H NMR spectra of the ZrCp₃ and HfCp₃ crystals revealed resonances arising from the diamagnetic complexes Cp₃ZrH and Cp₃HfH, which are proposed to arise from hydrolysis of ZrCp₃ and HfCp₃ by traces of water in the NMR solvent. The identities of ZrCp₃ and HfCp₃ were established by determination of their x-ray crystal structures, which exactly matched previously reported molecular structures. Thermogravimetric analyses of ZrCp₃ and HfCp₃ were conducted to assess their volatilities and thermal stabilities. ZrCp₃ showed the onset of mass loss at about 125 °C and revealed a single-step evaporation up to about 225 °C, at which point decomposition limited further mass loss. Mass loss for HfCp₃ started at about 150 °C and continued up to about 270 °C, at which point decomposition limited further mass loss. ZrCp₃ and HfCp₃ decomposed extensively during preparative sublimation experiments at 0.5 Torr, and do not appear to have promising properties as ALD precursors.