An experimental and graph theoretic study of atomic layer deposition processes for spacecraft applications
Adomaitis, Raymond A
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Accurate understanding of the atomic layer deposition (ALD) process kinetics is necessary for developing new ALD chemistries to produce novel nanomaterials, and also optimization of typical ALD processes used in industrial applications. Proposing a potential reaction sequence alongside with accurate kinetic data is among the very ﬁrst steps in studying the ALD process kinetics and forms the backbone of further engineering analysis. A valid and proper ALD reaction net work (RN) must be able to reﬂect the self-limiting and cycle to cycle reproducibility behavior experimentally observed for practical ALD processes. Otherwise, the mathematical model built based on it fails to precisely capture and reproduce ALD behavior no matter how accurate the available kinetic data are. In this work, a RN analysis method based on species-reaction graphs and the principles of convex analysis is developed to study the mathematical structure and dynamical behavior of thin-ﬁlm deposition RN models. The key factor in ALD RN analysis is the presence of consistent surface-originated invariant states for each ALD half-cycle. Therefore, the primary focus of the proposed approach is on identifying and formulating physically-relevant RN invariant states, and to study the chemical signiﬁcance of these conserved modes for ALD reaction mechanisms. The proposed method provides a well-deﬁned framework, applicable to all ALD systems, to examine the above criteria of a proper ALD RN without requiring any information on the reaction rates. This method ﬁlls a gap in the procedure of ALD process modeling before the time-consuming step of calculating individual reaction rates which is usually done through ALD experiments in reactors equipped with in-situ measurement instruments or computationally expensive computational chemistry-based calculations such as density functional theory. The presented approach is also extended to study the variant states of a RN. The generalized method provides information on diﬀerent variant states dynamically depending on each individual reaction in the network which facilitates the study and ultimately the formulation of diﬀerent reaction rates in the system. In the second part of this dissertation, an experimental study of ALD of indium oxide and indium tin oxide ﬁlms using the trimethylindium, tetrakis (dimethylamino) tin(IV), and ozone precursor system is conducted to ﬁrst, investigate the potential application of this ALD process for producing high-quality transparent conducting layers; and second, to understand the relationship between the thickness of the deposited ﬁlms and their electrical and optical properties. The optimized recipe was then used to process commercial Z93 heat radiator pigments used in manufacturing spacecraft thermal radiator panels to enhance their electrical conductivity to avoid the diﬀerential charging that may occur due to the interaction with charged particles in Van Allen radiation belts. To this aim a specialized ALD reactor was designed and constructed capable of processing standard ﬂat substrates as well as coating micron-sized particles. The results conﬁrm that the proposed process can be used to coat the heat radiator pigment particles and that the indium oxide ﬁlm can nucleate and grow on their surface. This provides an example from a variety of potential space-related applications that can beneﬁt from the ALD process.