Atomic layer deposition, characterization, and dielectric properties of HfO₂/SiO₂ nanolaminates and comparisons with their homogeneous mixtures

Nanolaminates of HfO₂ and SiO₂ were prepared using atomic layer deposition (ALD) methods. Successive exposure of substrates maintained at 120 or 160°C to nitrogen flows containing Hf(NO ₃)₄ and (^tBuO)₃SiOH led to typical bilayer spacings of 2.1 nm, with the majority of each bilayer being SiO ₂. The density of the SiO₂ layers (measured using X-ray reflectometry (XRR)) was slightly higher than expected for amorphous silica, suggesting that as much as 10 % HfO₂ was incorporated into the silica layers. Based on cross-sectional transmission electron microscopy (TEM) and XRR, oxidation of the silicon substrate was observed during its first exposure to Hf(NO₃)₄, leading to a SiO₂ interfacial layer and the first HfO₂ layer. Combining the ALD of Hf(NO ₃)₄/(^tBuO)₃SiOH with ALD cycles involving Hf(NO₃)₄ and H₂O allowed the systematic variation of the HfO₂ thickness within the nanolaminate structure. This provided an approach towards controlling the dielectric constant of the films. The dielectric constant was modeled by treating the nanolaminate as a stack of capacitors wired in series. The nanolaminate structure inhibited the crystallization of the HfO₂ in post-deposition annealing treatments. As the HfO₂ thickness decreased, the preference for the tetragonal HfO₂ phase increased.