Georgia Institute of Technology Materials Science and Engineering

Committee Members:

Prof. Mark D. Losego, Advisor, MSE

Prof. Kyriaki Kalaitzidou, Advisor, ME/MSE

Prof. Lauren Garten, MSE

Prof. Asif I. Khan, ECE

"Phase Transformations in Atomic Layer Deposited (ALD) Metal Oxide Thin Films”

Abstract:

Atomic layer deposition (ALD) is a widely employed chemical vapor deposition (CVD) technique designed to grow stoichiometric and highly conformal thin films, even on high aspect ratio structures and porous substrates. With ALD, deposition occurs via sequential dosing of a precursor and co-reactant into the reaction vessel to grow a thin film on the substrate layer-by-layer. These complementary reactants are dosed in a time-separated or space-separated manner such that once the precursor or co-reactant reacts with the substrate, a “purge” step is employed to remove any excess species and by-products from the deposition region. ALD can be used to functionalize surfaces, build multi-layer stacks, or develop nano-structured composites. As such, ALD is a crucial thin film deposition process suitable for application in semiconductor processing, microelectromechanical systems (MEMS), stabilization of photovoltaics and energy storage devices, and development of catalysts and sensors.    

The hallmark of ALD, compared to other CVD techniques, is the self-limiting surface half-reactions that ensure uniform substrate saturation. These self-limiting reactions, performed in the proper temperature window, ensure ideal layer-by-layer deposition but lengthen the overall process time with the required purge steps. As such, emphasis has been placed on minimizing total cycle time in the effort to minimize total process time to be more competitive with other deposition processes. But this manner of studying ALD does not consider the effect of process time on deposited thin film structure. The proposed research will fill this literature gap by investigating the microstructural phase transformation kinetics during an ALD cycle.   

In this work, I propose to probe the effects of cycle time on structural transformations to introduce process time as an additional ALD design parameter. While it is known that surface diffusion plays a significant role in thin film deposition, it is lesser known for ALD specifically; the industrial need for process optimization has not incentivized studies on potential for thin film structural rearrangement during the ALD cycle. As such, this work will probe crystal nucleation and growth kinetics in ALD on a layer-by-layer basis. Phase One will probe phase transformation conditions at the low processing temperatures often employed in atomic layer deposition via understanding the kinetics of phase transformation during post-deposition annealing with titanium dioxide as the model system. This will help deconvolute process temperature effects from process time effects. Phase Two will use ALD purge time as a design parameter to probe the degree of structural rearrangement in titanium dioxide during the ALD cycle. Finally, Phase Three will leverage time as a process parameter and investigate the ability to use targeted heating and time during the ALD cycle to develop enhanced microstructural control in vanadium oxide films.