Atomic layer deposition of tantalum oxide thin films using the precursor tert-butylimido-tris-ethylmethylamido-tantalum and water: Process characteristics and film properties

Abstract

In this work, the precursor tert-butylimido-tris-ethylmethylamido-tantalum (TBTEMT) was applied for the atomic layer deposition (ALD) of tantalum oxide (Ta2O5) thin films for the first time. Water was used as the second reactant. A self-limiting, and hence, ALD-like film growth was confirmed in the temperature range from 100 to 300°C. The temperature window of this process extends from 250 to 300°C and features a growth rate of about 0.56Å/cycle. For lower temperatures, the growth rate increases gradually up to 0.92Å/cycle at 100°C. At a deposition temperature of 200°C, the process showed perfect layer-by-layer growth with 0.64Å/cycle and without any noticeable incubation period on both silicon with native oxide and hydrogen-terminated silicon. In addition, the conformal coating of structures with an aspect ratio of 40:1 is demonstrated as well.According to XPS analyses, the films are oxygen rich (Ta:O ratio around 0.34±0.01 for films grown at 150–300°C) and contain a significant amount of carbon (6±2at.%) and some nitrogen (<3at.%). The film densities, refractive indices and dielectric constants increase with increasing deposition temperature and are as high as 8.0±0.1g/cm3, 2.25 and 31, respectively. Films grown at 200°C are amorphous and smooth. They exhibit a film density of 7.8±0.1g/cm3, a refractive index of 2.17 (at 550nm) and a dielectric constant of 26±1. However, the films suffer from high leakage currents (>10−4A/cm2). In addition, electrical measurements revealed the formation of an interfacial layer between the Ta2O5 films and bare silicon. By using substrates with a thin thermally grown silicon oxide, the leakage could be reduced by three orders of magnitude.Post-deposition annealing at 800°C in nitrogen resulted in the crystallization of the Ta2O5 films, which is also accompanied by an increase in film density and refractive index. Moreover, the crystallized films exhibit an enhanced dielectric constant of 48±2. Electrical measurements revealed the growth of an interfacial layer with an equivalent oxide thickness of around 2.4nm due to the 800°C annealing. While this interfacial layer degrades the effective permittivity of the dielectric (e.g. 20.5±0.5 for a 20nm Ta2O5 film), it also causes a reduction of the leakage currents by more than three orders of magnitude (e.g. to 1·10−7A/cm2 for a 20nm Ta2O5 film).