XRR characterization of ALD TiO2/Al2O3 nanolaminates with ultra-thin bilayers

Atomic layer deposited (ALD) nanolaminates have many prospective uses in mechanical, electrical and optical applications. ALD nanolaminates consist of a structure with repeated thin layers of alternating materials. A series of Al2O3/TiO2 nanolaminate samples with varied bilayer thicknesses was studied by X-Ray Reflectivity (XRR). In this study, a bilayer denotes a combination of one Al2O3 and one TiO2 layer with equal target thicknesses. The studied samples were grown with a PicosunT ALD reactor from Me3Al and H2O at 200 °C on Si substrates. Nominal bilayer thicknesses ranged from 0.1 to 50 nm and the bilayers were repeated in each sample to yield a nominal total thickness of 100 nm. It is expected that as the bilayer thickness decreases, there will be a cross-over thickness where the intended multilayer structure decomposes to a single TixAlyOz layer. XRR results show that all samples with nominal bilayer thicknesses greater or equal to 1.0 nm consisted of a repeated bilayer structure. Samples with intended bilayer thicknesses 0.5 nm or less did not show multilayer characteristics and the cross-over layer thickness is therefore in between 0.25 and 0.5 nm. XRR results further reveal precisely controlled thicknesses and sharp interfaces of individual layers. The sharpness of the multilayer peak suggests the layers maintain constant thickness and small interface roughness throughout the multilayer stack.