Committee Members: 

• Prof. Mark Losego, Advisor, MSE
• Prof. Kyriaki Kalaitzidou, Advisor, ME/MSE
• Prof. Lauren Garten, MSE
• Prof. Asif Khan, ECE
• Prof. Juan-Pablo Correa-Baena, MSE

Phase Transformations in Atomic Layer Deposited (ALD) Titanium Dioxide (TiO2) Thin Films

Abstract:

Atomic layer deposition (ALD) is a widely employed chemical vapor deposition (CVD) technique designed to grow thickness-controlled and highly conformal thin films. ALD can be used to functionalize surfaces, build multi-layer thin film 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 defining feature of ALD is the self-limiting chemistry and surface half-reactions that, followed by sufficient inert gas purges, can ensure conformal, pin-hole free thin films. Once process is tailored to ensure ideal ALD behavior, deposited film properties are primarily governed by process temperature, substrate, film thickness, precursor selection/film impurities, and plasma enhancement, which cause changes in the as-deposited structure and result in shifts in electrical, optical, chemical, and mechanical properties. 

In vapor deposition literature, the impingement flux of the gaseous precursor on the growing film is a key parameter in ensuring depositing film quality and properties. But in ALD literature, the emphasis on self-limiting surface half reactions, precursor chemistry, and industrial requirements to reduce total process time have obscured the potential for structural rearrangement during/after the ALD cycle. As such, the purpose of this work is to improve understanding of as-deposited film structure and introduce cycle time and atmosphere selection as ALD process parameters. 

Selecting titanium dioxide (TiO2) as the model material system, first I study the effect of low-temperature post-deposition annealing on amorphous as-deposited TiO2-ALD thin films. Resultant TiO2-anatase grain size is found to be dependent on ALD deposition temperature and not on post-deposition annealing temperature. This implies a structure difference in as-deposited amorphous films as a function of ALD deposition temperature. Post-deposition annealing (PDA) is performed to study the amorphous to anatase phase transformation kinetics. Anatase is found to form via a two-dimensional growth mode. The phase transformation reaction rate is deconvoluted into nucleation rate and growth rate. Nucleation is found to be the rate-limiting step for the phase transformation. Further, the nucleation rate frequency factor is found to increase with increasing deposition temperature, implying amorphous films deposited at higher temperature have increased vibrational modes. I develop a model for understanding the resultant microstructure with changes in deposition temperature, nucleation rate, and grain growth rate.  

Second, I study what is limiting crystallization during ALD of TiO2. Thermal-ALD of TiO2 films from the alkyl amide precursor and water chemistry grow amorphous for deposition temperatures up to 220 °C while TiO2 films from ALD of the chlorinated precursor are crystalline as-deposited above 150 °C. I introduce an intermittent controlled atmosphere (ICA) annealing step during the ALD cycle to encourage growth of fully crystalline TiO2 thin films at 180 °C and less than 50 nm. As-deposited films without the air anneal are amorphous and sub-oxidized while those with the in situ air anneal are crystalline with fewer Ti3+ states. Additionally, I vary process conditions to highlight the effectiveness of atomic rearrangement during the ALD cycle compared against bulk diffusion during PDA. Finally, I present results regarding the importance of purge time during ALD for crystal formation and regarding TiO2-brookite formation with post-deposition annealing (PDA). Overall, a PDA method is developed to probe as-deposited amorphous film structure, an ALD process variation (ALD-ICA) is introduced to encourage phase transformation during deposition, and I propose that oxidation state is limiting TiO2 crystallization during growth from tetrakis(dimethylamino)titanium(IV) TDMAT/H2O thermal-ALD.