Low temperature plasma-enhanced atomic layer deposition of metal oxide thin films

Many reported atomic layer deposition (ALD) processes are carried out at elevated temperatures (>150°C), which can be problematic for temperature-sensitive substrates. Plasma-enhanced ALD routes may provide a solution, as the ALD temperature window can, in theory, be extended to lower deposition temperatures due to the reactive nature of the plasma. As such, the plasma-enhanced ALD of Al2O3, TiO2, and Ta2O5 has been investigated at 25–400°C using [Al(CH3)3], [Ti(OiPr)4], [Ti(CpMe)(OiPr)3], [TiCp*(OMe)3], and [Ta(NMe2)5] as precursors. An O2 plasma was employed as the oxygen source in each case. We have demonstrated metal oxide thin-film deposition at temperatures as low as room temperature and compared the results with corresponding thermal ALD routes to the same materials. The composition of the films was determined by Rutherford backscattering spectroscopy. Analysis of the growth per cycle data and the metal atoms deposited per cycle revealed that the growth per cycle is strongly dependent on the film density at low deposition temperatures. Comparison of these data for Al2O3 ALD processes in particular, showed that the number of Al atoms deposited per cycle was consistently high down to room temperature for the plasma-enhanced process but dropped for the thermal process at substrate temperatures lower than 250°C.