Thermal atomic layer deposition of aluminum nitride thin films from AlCl3

Aluminum nitride (AlN) is of interest in many high frequency electronic devices due to its high thermal conductivity, wide band gap, high dielectric constant and resistivity, low dielectric loss, and good piezoelectricity. Atomic layer deposition (ALD) has emerged as a promising candidate for growing AlN thin films which enables conformal deposition and nanometer scale thickness control. So far, the most widely used precursors for AlN thin film growth in thermal ALD are ammonia (NH3) with trimethylaluminum (TMA). However, the process can lead to carbon (C) impurities due to decomposition of TMA at temperature higher than 300℃, which is needed for NH3 to react effectively. Plasma enhanced ALD can decrease the deposition temperature but it usually leads to surface damage and less conformal films. AlCl3 has also been used as precursor for AlN thin film growth, and deposition temperature of 500 ℃ and higher is needed for AlCl3 and NH3 to react effectively, but very few reports have been published. In this work, AlN thin films were deposited by thermal ALD from AlCl3 and NH3 as precursors in Beneq P400 reactor at growth temperature 500 ℃ or higher. The effect of growth temperature on many properties of AlN was studied, including refractive index, residual stress, surface morphology as well as film composition. Polycrystalline hexagonal AlN thin films with (002) orientation and good uniformity were obtained. However, the XRD results suggested a tilt of the c-axis larger than ±15° to the surface normal of the AlN films. The GPC rate of AlN increased from 0.68 Å/cycle at 500 ℃ to around 0.76 Å/cycle at 550 ℃, and the refractive index increased from 1.92 at 500 ℃ to around 2.05 at 550 ℃. The obtained AlN films were nitrogen-rich with a composition of Al0.48N0.52, and higher temperatures were found to result in purer AlN films with less impurities. At 550 ℃, 3.7 % of O and 0.7 % of Cl impurities were measured, compared to 4.3 % of O and 1.8 % of Cl at 500 ℃. The residual stress of the AlN thin films in this work was quite high (> 500 MPa), which was mostly likely because of the different thermal expansions of AlN and the substrates. TiN from ALD and sputtered Mo were tried as seed layers for growing highly (002) oriented AlN films, but no orientation enhancement was observed in this work. To grow highly (002) oriented polycrystalline AlN films, substrates or seed layers with hexagonal symmetry, such as Pt (111), Ti (001) and Al (111), should be investigated. In addition, different in-situ or ex-situ annealing processes could be further investigated to reduce the residual stress