AVS 61st International Symposium & Exhibition
    Thin Film Wednesday Sessions
       Session TF+MS+PS-WeM

Invited Paper TF+MS+PS-WeM10
ALD in High Aspect Ratio Structures and Nanoporous Materials

Wednesday, November 12, 2014, 11:00 am, Room 307

Session: Applied ALD: Nanoelectronics and Emerging Applications
Presenter: Jolien Dendooven, University of Ghent, Belgium
Authors: C. Detavernier, University of Ghent, Belgium
J. Dendooven, University of Ghent, Belgium
Correspondent: Click to Email
Atomic layer deposition (ALD) is known to be an excellent technique for the deposition of thin films with uniform thickness over micro- and nanoscale 3D structures. The superior conformality of ALD is a direct consequence of the self-saturated surface reaction control and makes the technique increasingly useful in the rapidly growing field of nanotechnology. The successful ALD-based processing of nanostructured materials requires however a careful optimization of the growth parameters. In this work, we present an extensive study on the conformality of ALD in high aspect ratio structures and nanoporous materials.

A first experimental approach was based on the use of macroscopic, trench-like structures in combination with low precursor pressures. In this way, the transport of the precursor molecules in the test structures was governed by molecular flow, as in microscopic trenches under standard ALD conditions. This method allowed us to quantify the conformality of the trimethylaluminum (TMA)/H2O process as a function of the aspect ratio and the TMA exposure time. Our experimental data indicated that the sticking probability is a determining factor in the conformality of ALD [1]. A better understanding of the effect of this parameter on the conformality was obtained via kinetic modeling and Monte Carlo modeling.

As a second substrate, porous titania thin films with pore sizes in the low mesoporous regime (< 10 nm) were considered in order to get insights on the minimum pore diameter that can be achieved by ALD. Novel in situ characterization techniques were developed to monitor the pore filling by ALD. Synchrotron-based x-ray fluorescence and scattering techniques provided cycle-per-cycle information on the material uptake and densification of the porous film, while ellipsometric porosimetry was used to quantify the pore size reduction. This study nicely demonstrated the ability of ALD to tune the diameter of nanopores down to the molecular level [2].

Finally, we performed ALD of TiO2 into a 3D ordered silica powder with two types of mesopores [3]. By varying the Ti-precursor exposure time, we investigated the introduction of TiO2 into the differently sized mesopores. A TEM study revealed the diffusion limited nature of the TiO2 ALD process, leading to anisotropic penetration profiles in this specific pore structure. We observed a systematic deeper penetration of the deposition front along the main channels compared to the narrower mesopores. These results were corroborated by modeling work.

[1] J. Dendooven et al., J. Electrochem. Soc. 156, P63, 2009. [2] J. Dendooven et al., Chem. Mater. 24, 1992, 2012. [3] S. P. Sree et al., Chem. Mater. 24, 2775, 2012.