Atomic layer deposition of metal oxide thin films and nanostructures

Abstract

With the continuing scaling down of microelectronic integrated circuits and increasing need for three-dimensional stacking of functional layers, novel or improved growth techniques are required to deposit thin films with high conformality and atomic level thickness control. As being different from other thin film deposition techniques, atomic layer deposition (ALD) is based on selflimiting surface reactions. The self-limiting film growth mechanism of ALD ensures excellent conformality and large area uniformity of deposited films. Additionally, film thickness can be accurately controlled by the number of sequential surface reactions. Gallium oxide (Ga2O3) thin films were deposited by plasma-enhanced ALD (PEALD) using trimethylgallium as the gallium precursor and oxygen plasma as the oxidant. A wide ALD temperature window was observed from 100 to 400 °C, where the deposition rate was constant at ~0.53 Å/cycle. The deposition parameters, composition, crystallinity, surface morphology, optical and electrical properties were studied for as-deposited and annealed Ga2O3 films. In order to investigate the electrical properties of the deposited films, metal-oxide-semiconductor capacitor structures were fabricated for a variety of film thicknesses and annealing temperatures. Ga2O3 films exhibited decent dielectric properties after crystallization upon annealing. Dielectric constant was increased with film thickness and decreased slightly with increasing annealing temperature. As an additional PEALD experiment, deposition parameters of In2O3 thin films were studied as well, using the precursors of cyclopentadienyl indium and O2 plasma. Initial results of this experiment effort are also presented. Accurate thickness control, along with high uniformity and conformality offered by ALD makes this technique quite promising for the deposition of conformal coatings on nanostructures. This thesis also deals with the synthesis of metal oxide nanotubes using organic nanofiber templates. Combination of electrospinning and ALD processes provided an opportunity to precisely control both diameter and wall thickness of the synthesized nanotubes. As a proof-ofconcept, hafnia (HfO2) nanotubes were synthesized using three-step approach: (i) preparation of the nylon 6,6 nanofiber template by electrospinning, (ii) conformal deposition of HfO2 on the electrospun polymer template via ALD using the precursors of tetrakis(dimethylamido)hafnium and water at 200 °C, and (iii) removal of the organic template by calculation to obtain freestanding HfO2 nanotubes (hollow nanofibers). When the same deposition procedure was applied on nanofibers with different average fiber diameters, thinner HfO2 wall thicknesses were obtained for the templates having smaller diameters due to insufficient exposure of precursor molecules to saturate their extremely large surface area. Thus, “exposure mode” was applied to obtain the desired wall thickness while coating high-surface area nanofibers. We present the experimental efforts including film deposition parameters, structural, elemental, and morphological properties of HfO2 nanotubes.