Atomic layer deposition of WNx thin films using a F-free tungsten metal-organic precursor and NH3 plasma as a Cu-diffusion barrier

Abstract

Tungsten nitride (WNx) thin films were deposited on SiO2 substrates by atomic layer deposition (ALD) using a fluorine- free tungsten metal-organic precursor of tris(3-hexyne) tungsten carbonyl, W(CO)(CH3CH2C ≡ CCH2CH3)3 and NH3 plasma as a reactant at a deposition temperature of 250 °C. Important ALD characteristics, such as self-limiting growth and linear dependency of the film growth on the number of ALD cycles, were obtained with a growth rate of 0.045 nm/cycle. The minimum film resistivity of around ~2800 μΩcm (thickness: ~ 13.3 nm) was stable after 3 days air-exposure, indicating the high stability of these WNx films. Rutherford backscattering spectrometry showed that the N-rich WNx thin films (N/W ratio: ~ 1.56) were deposited with negligible impurities of C and O. Both X-ray diffractometry and transmission electron microscopy analysis showed that ALD-WNx films formed a polycrystalline cubic WN phase with an average grainsize of ~ 6 nm. From scanning Kelvin probe analysis, its work function was determined as 4.79 eV. Detail investigations were carried out after post-annealing of the as-deposited films and formation of metallic-W with significantly reduced sheet resistance was observed upon annealing at and beyond 700 °C. Finally, the ultrathin (~5 nm) ALD-grown WNx film effectively prevented diffusion of Cu even up to 550 °C, promising it as an efficient diffusion barrier material for the Cu interconnect.