Effect of nitrogen on the electrical and structural properties of triode-sputtered tantalum films

Abstract

The structural and electrical properties of nitrogen‐doped tantalum films, sputtered in a dc triode system, have been investigated. For constant sputtering voltage and current, the tantalum deposition rate decreased with increasing nitrogen flow rate; at a flow rate of 12 cm³ min⁻¹, the deposition rate was 75% of the rate obtained without the addition of nitrogen. The density of the films decreased from 15.4 to 9.8 g cm⁻³ as the nitrogen flow increased from 0 to 12 cm³ min⁻¹. The net effect on the film thickness is an increase from 2700 to 3800 Å for films on glass substrates. Films were also deposited on ceramic substrates at the same time. Both x‐ray diffractometer and powder camera analysis showed that the films consisted of the bcc Ta phase at nitrogen flows up to 3 cm³ min⁻¹ but the lattice was approximately 3% larger than for bulk Ta. The hcp Ta₂N phase appeared at flow rates above 3 cm³ min⁻¹ and the change from bcc Ta to hcp Ta₂N was accompanied by an increase in the room‐temperature coefficient of resistance from positive to negative values. For films on glass, the resistivity was approximately 250 μΩ cm as the nitrogen flow increased from 4 to 8 cm³ min⁻¹ but the resistivity had a monotonic increase for films on ceramic substrates. The hcp Ta₂N phase was replaced by the fcc TaN phase as the flow rate increased in this range; the latter phase had a preferred (111) orientation on the substrate. As the flow rate increased from 8 to 12 cm³ min⁻¹, the resistivity increased rapidly for both types of substrates and the temperature coefficients became rapidly more negative. It is suggested that this behavior is due to the presence of an insulating nitride phase in the film.