Jingwei Shi PhD Thesis Defense

Jingwei Shi
PhD Candidate
Chemical Engineering
Academic advisor: Professor Stacey F. Bent

Abstract: Developing New Chemistries and Applications in Molecular Layer Deposition

Recent advancements in renewable energy, health and medicine, semiconductor fabrication, and water purification have resulted from nanotechnology. Our existing nanomaterials synthetic toolkit must continue to expand to meet the increasing demand from existing and emerging industries for smaller and more precise features, more complex architectures, and reduced defects and imperfections. Atomic layer deposition (ALD) and molecular layer deposition (MLD) are methods that can expand that synthetic toolkit. Both are vapor-phase processes that deposit thin films in a layer-by-layer approach using self-limiting surface chemistry. The key distinction between the two related methods is the type of materials deposited: ALD deposits inorganic thin films ranging from metals to oxides to nitrides, whereas MLD deposits organic polymeric thin films. Although five decades of research have already gone into ALD and one decade of research into MLD, there still exist significant knowledge and technological gaps in both techniques. Particularly for MLD, a significant limitation to the technique is the small number of polymers that can be grown, being limited to primarily acyl and thionyl backbone polymers like polyureas and polyesters. For ALD, the temperature deposition conditions and harsh reactants of certain processes preclude it from being used for more sensitive applications. Furthermore, more exploration of new applications for both ALD and MLD is needed to expand their potential use. This dissertation presents work to better understand ways to address these fundamental limitations of ALD and MLD, as well as work that explores new applications of ALD and MLD by understanding the electron-responsive and opto-electronic properties of these materials.

The first part of this work develops modified ALD and MLD processes to enable the growth of more varied materials. To achieve this goal, an ultrathin solvent layer of ionic liquid is deposited onto the substrate surface prior to performing ALD or MLD. By performing the surface reactions of ALD and MLD within the ionic liquid solvent, solvent effects are recreated inside the deposition chamber, enabling the use of solvent-mediated surface reactions. Using this solventassisted approach, the ionic liquid assisted MLD (IL-MLD) of polyetherketoneketone is demonstrated with the precursors diphenyl ether and isophthaloyl dichloride, catalyzed by vapordosed aluminum chloride. This high-performance, industrially relevant thermoplastic had not been grown previously with any type of vapor-phase deposition method. Notably, we used the same precursors and catalyst in our vapor-phase deposition method as those used in the solution synthesis of polyetherketoneketone; this suggests that it is possible to more easily adapt existing solution-based polymerization chemistries to vapor-phase deposition methods via IL-MLD.

As an extension of the previous study, the ionic liquid assisted ALD (IL-ALD) of tin oxide from tin acetylacetonate and water is investigated. In reported ALD processes, tin acetylacetonate is unreactive with both water and oxygen co-reactants and requires ozone for growth to occur. However, for applications that cannot withstand harsh oxidizing environments, it would be ideal to develop ALD processes with milder deposition conditions. In this work, with the use of an ionic liquid solvent layer, a solvent-mediated surface reaction is allowed to take place between the tin acetylacetonate and water that is energetically prohibitive in the vapor-phase. As a result, we report the deposition of SnO with a growth rate of ~0.7 Å per ALD cycle. The composition and morphology are characterized with x-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). The reaction mechanism is probed and demonstrated to be an additionelimination type mechanism that is commonly reported for solvent-mediated reactions with acetylacetonate.

In the second part of this dissertation, two studies on applications of hybrid ALD/MLD are performed. First, MLD is used to deposit a hybrid inorganic-organic hafnium-containing thin film to serve as an electron-beam resist and photoresist. The e-beam resist characteristics of the “hafnicone” resist thin film are studied, showing a negative tone behavior with 3M HCl as the developer with a sensitivity of 400 μC/cm2 and 50 nm line width resolution. Characterization via XPS and infrared (IR) spectroscopy helps to determine that upon e-beam and deep ultraviolet (DUV) irradiation, the C-O bonds in the material are broken; subsequently, the inorganic Hf-O centers in the material aggregate to form inorganic hafnium oxide nanoparticles, which are observed using scanning electron microscopy (SEM). This compositional change leads to a decrease in solubility in the developer and the observed negative resist behavior. This work serves as a first investigation of MLD for the deposition of inorganic resist materials, which possess high etch resistance, high sensitivity to extreme ultraviolet (EUV), and structural stability against pattern collapse.

Second, a study of the composition-property relationships of molybdenum organosulfur MLD films, termed “molybdenum thiolates”, is performed. Because composition strongly impacts film properties, we seek to identify how the optoelectronic properties of the molybdenum thiolate MLD films change with respect to their composition. To this end, three closely related molybdenum thiolates are synthesized via MLD and their properties are studied. In combination with molybdenum hexacarbonyl as the molybdenum source, ethanedithiol, butanedithiol, or benzenedithiol are used as the organic sulfur source to synthesize three different molybdenum thiolate films that differ by their organic motif. The work shows that the three different compositions of Mo-thiolate differ not only in their MLD growth characteristics such as the growth rate, but also in their optoelectronic properties; for example, the benzenedithiol containing Mothiolate exhibits ~400 times lower resistivity relative to the other two compositions. Additionally, ultraviolet-visible spectroscopy shows the three different compositions all have similar optical bandgaps between 2.3 and 2.4 eV and as a result, a mild photoconductivity response to visible light. This work addresses how the organic component of hybrid ALD/MLD processes can influence the material properties of the resultant material.

Finally, a conclusion is presented with an outlook on the future direction of the use of ALD and MLD. The continued development of these vapor-phase thin film techniques is essential for expanding the fabrication and synthetic toolbox of nanoscale technologies to meet future global challenges. This dissertation serves to advance not only the material capabilities of ALD and MLD, but also their applied use-cases.