Atomic layer deposition for energy and semiconductor applications

In this thesis, I contribute to two instrumental sectors in our current society -- semiconductor and energy -- by developing novel materials through advanced nanoscale engineering through atomic layer deposition (ALD). ALD is a deposition technique in vacuum, which can make highly uniform films with a thickness range of 1-100 nm on both 2-D and 3-D structured substrates. ALD chemistries enable the deposition of a wide diversity of materials. Furthermore, performing diverse ALD chemistries in single deposition is possible, leading to fabrication of both homogenous mixtures and heterogeneous structures (e.g. nanolaminate and metal/metal-oxide composite). First, I will discuss about the use of ALD to fabricate barium titanates and silicon nitrides for the applications in key semiconductor components in Chapters 2 and 3. The continuous downscaling of integrated circuits (ICs) necessitates the development of ultrathin oxides and nitrides, mainly used for dielectric materials and sidewall spacers in transistors, respectively. Oxide films with high dielectric constants (i.e. high-k) and low electrical leakage currents are essential for the applications of information storage devices such as dynamic random-access memory (DRAM). Using ALD, ultrathin (< 7 nm) barium titanates (BaxTiyOz) with different cation stoichiometries are fabricated and the tunability on electrical properties by changing Ba-to-Ti cation ratio is successfully demonstrated. Also, aluminum incorporation into BaxTiyOz shows it can further decrease the electrical leakage current with a marginal sacrifice of dielectric constants. Another contribution on semiconductor of this thesis is ALD of Nanolaminates of silicon nitride-aluminum nitride (SiN-AlN) for sidewall spacer. The development of superior wet-etching resistant and electrically insulating films is very crucial for modern transistor fabrication processes with a process temperature limitation below 350 °C. The developed SiN-AlN nanocomposites meet industry requirements by sufficiently lowering the wet etch rates as well as the electrical leakage currents. This dissertation also describes the use of ALD on energy materials in Chapter 4. In the past, ALD has been a key manufacturing process in the semiconductor industry. Transferring this mature technology to energy applications has recently gathered much attention since ALD is capable of precisely controlling loadings and morphologies of both metal and metal oxides, already demonstrated in several semiconductor devices. These capabilities can bring about technical innovation in a wide range of energy devices with nanoscale components such as fuel cells, solar cell, and batteries. Specifically, I will showcase a novel synthesis route for Pt-Ti alloy nanomaterials through ALD of metal/metal-oxide composite in conjunction with post thermal annealing in hydrogen. The developed Pt-Ti alloys successfully prove their capability as cathode electrocatalysts in Polymer electrolyte fuel cells (PEFCs), i.e. oxygen reduction reaction (ORR). PEFCs, regarded the most promising implementation of fuel cells in the automotive sector, still require the significant reduction of platinum used in electrocatalysts. One of the most effective ways to minimize platinum is the utilization of platinum alloys. Among several Pt3M alloys (M = Y, Ni, Co, and Fe), Pt3Ti alloy is one of the most promising candidates. In this chapter, a simple and scalable synthesis route of Pt3Ti through ALDs of Pt and TiO2, followed by thermal annealing at 800 °C in hydrogen for intermixing of Pt and TiO2 layers, is successfully demonstrated. The best performing Pt-Ti alloy catalysts show excellent stability and more than 2-fold improvement in ORR Pt-based mass activity in comparison to commercial Pt powder catalyst. I envision that these aforementioned ALD materials introduced in this thesis will be actually utilized in semiconductor and energy applications, which will eventually accelerate the innovations of ICs and contribute to change the landscape of electrical energy production toward a more efficient and eco-friendly direction.

Keywords: Mechanical Engineering.
Advisor: Prinz F. B. 
Kenny Thomas William 
Schladt Thomas D.
Zheng Xiaolin