Precursor Design, Characterization, and Applications of Atomic Layer Deposition of Nano Ceramics
Creators: name: Shahmohammadi, Mina
Publisher: name: University of Illinois at Chicago
Year: 2022
Subjects: Uncategorized

description: Atomic layer deposition (ALD) is a vapor phase technique to deposit thin films on various substrates through sequential and self-limiting surface reactions. ALD consists of alternating pulses and purges of the precursor(s) and oxidant(s), resulting in deposition of the desired film with an expected thickness and composition. ALD has widespread applications from semiconductor industries to biomedical fields. However, ALD has been mostly employed to produce binary oxides, while other areas could hugely benefit if different materials are combined to provide synergistic effects. Moreover, despite the merits of this deposition technique mentioned above, being a slow process is one of the main drawbacks of ALD. Yet, spatial ALD (SALD) has been developed to overcome this drawback. SALD can be sensitive to ambient if performed in open-air; highly volatile precursors are needed for SALD, and there are not many available precursors for SALD. Hence, this thesis focuses on developing and optimizing ternary oxide nanoceramic thin films on different biomaterials to enhance engineering, biomedical, and dental applications. ALD of ternary oxides is designed by combining the process of two binary oxides with various cycle ratios, resulting in desirable thickness ratios. Thus, before developing the ternary oxide process, the ALD of each binary oxide should be investigated thoroughly. Moreover, a computational tool is developed for the in-silico design of optimal and novel precursor materials with an enhanced ALD rate of nanoceramics. First, TiO2 thin film was deposited on Polymethyl methacrylate (PMMA) via ALD. A spectroscopy ellipsometry model was developed for the first time to measure the film thickness on PMMA. The reactions affecting the TiO2 growth rate were investigated on both PMMA and silicon reference, indicating that the film thickness on inorganic reference substrate is not always representative of the film thickness on organic substrate. Initially, the growth rate on PMMA was ~3.5 times higher than that on stand-alone silicon; this was attributed to cyclic chemical vapor deposition of precursor and moisture within PMMA, concomitant with TiO2 ALD from precursor and ozone. However, after the formation of ~30-nm-thick film on PMMA, the TiO2 growth rate became similar to that on stand-alone silicon. The proposed growth regimes were corroborated with different techniques. Moreover, the TiO2 coating increased PMMA wettability by ~70% and surface hardness by 60%. Then, TiO2, ZrO2, and TiO2/ZrO2 mixed oxides nanofilms were deposited with ALD on PMMA substrates to explore any synergistic effects of mixed oxides over either of the single oxides. The coatings protected PMMA surface from thermal and brushing challenges by maintaining wettability, surface roughness, and film integrity. Furthermore, compared to control, the mixed oxides with 1:2 cycle ratio of TiO2:ZrO2 significantly reduced all bacterial and fungal initial adhesion and biofilm formation. Thus, TiO2/ZrO2 ALD on PMMA could enhance its surface, mechanical, and biological properties, preparing it for biomedical and engineering applications. Furthermore, ALD of TiO2, ZrO2, and TiO2/ZrO2 mixed oxides nanofilms was investigated on a widely used artificial implant material, i.e., Ti6Al4V (Ti(V) to improve its corrosion behavior. Artificial saliva was used as the solution to simulate mouth conditions. According to electrochemical and surface studies, TiO2/ZrO2 mixed oxide thin film effectively protected Ti(V) surface against corrosion in artificial saliva. Hence, Ti(V) applications are extended, due to enhanced robustness in an aggressive environment, leading to increased patient satisfaction. Finally, a computational framework was developed based on Adsorbate Solid Solution Theory to design novel precursor materials for ALD with enhanced properties. For this purpose, a new Group Contribution Method (GCM) was employed to predict the properties of the functional groups present in the precursor. Then, using the thermodynamic properties as obtained from GCM, a computer-aided molecular design framework was developed to design optimal novel precursors with enhanced deposition properties for the ALD of metal oxides and metals. All designed molecules resulted in a higher growth rate than existing precursors while satisfying the operating conditions. The best possible designed precursor predicted a 40% higher growth rate than the known one. Additionally, the amount of impurities in an ALD reactor was calculated using the same model.