Annealing effects of tantalum films on Si and SiO2/Si substrates in various vacuums

Abstract

The annealing effects of 550 nm thick β-Ta films sputtered on Si and SiO2 substrates have been investigated under various vacuum conditions. Phase transformation from the tetragonal β-Ta into body-centered-cubic α-Ta of much higher conductivity occurred at annealing temperatures lower than 500°C and 80% of β-Ta transformed into α-Ta after annealing at 600°C for Ta on a Si substrate. For Ta on a SiO2 substrate, no phase transformation was observed at 500°C annealing, and only 20% of β-Ta transformed into α-Ta at 600°C. Oxygen diffusion into the Ta film at the interface of Ta/SiO2 could hinder β-Ta to α-Ta transformation. Both Ta on Si and Ta on SiO2 samples have smooth surfaces after annealing in 2 X 10-5 Torr. After annealing in a vacuum lower than 2 X 10-4 Torr, surface oxidation of the Ta thin films was detected. The increase of oxygen content in the Ta films caused higher compressive stress, and resulted in the film peeling in a serpentine pattern during annealing at 500°C in 2 X 10-2 Torr for Ta on a SiO2 substrate. The Ta films cracked and detached from the SiO2 substrate after being annealed at 750°C in 2 X 10-2 Torr. In contrast, no crack was found in Ta on Si, probably because of the relief of film stress due to more β-Ta being transformed into α-Ta during annealing. The residual oxygen and moisture in low vacuum may build up stress in Ta thin films during thermal processes, which can cause major reliability problems in electronic and other applications. © 2001 American Institute of Physics.