Access Type

Dissertation/Thesis

Date of Award

January 2019

Degree Type

Dissertation

Degree Name

Ph.D.

Department

Chemistry

First Advisor

Charles H. Winter

Abstract

ABSTRACT

ATOMIC LAYER DEPOSITION (ALD) AND ATOMIC LAYER ETCHING (ALE) OF THIN FILMS: SYNTHESIS AND CHARACTERIZATION OF A NEW CLASS OF ALD PRECURSORS, ALD OF PrAlO3 THIN FILMS, AND THERMAL ALE OF COBALT METAL FILMS

by

WATHSALA LAKMALI IWADUNNA WADUGE

February 2019

Advisor: Professor Charles H. Winter

Major: Chemistry (Inorganic)

Degree: Doctor of Philosophy

ALD is an attractive thin film deposition technique due to its unique self-limiting growth behavior and the ability to achieve Angstrom level thickness control. Alternative thin film deposition techniques such as CVD and PVD are unable to attain the conformal and Angstrom level film growth which are of great significance to future microelectronics device fabrication demands. Because ALD proceeds by surface reactions, the precursors which are used for ALD should have unique properties. Therefore, the design and synthesis of these ALD precursors are challenging. Transition metal ALD has not been well developed due to a lack of good ALD precursor chemistry. Herein, mid-to-late transition metal complexes containing enaminolate and carbohydrazide ligands have been synthesized and characterized. Additionally, the thermal properties and reactivity of these complexes were determined. Enaminolate ligand-containing chromium, manganese, iron, cobalt, and nickel complexes were isolated as pure solids by vacuum sublimation. TGA of these metal complexes shows a single step weight loss with minimum non-volatile residuals. Manganese complexes show lower volatility compared to the other metal complexes. However, the highest thermal stability (303 ºC) was observed for manganese complexes. Interestingly these metal complexes thermally decompose to their metals at high temperatures and show high reactivity towards reducing agents. Similar to enaminolate containing metal complexes, newly synthesized carbohydrazide-containing metal complexes also display promising thermal properties and reactivity to use as ALD precursors. Detailed synthesis procedures, precursor characterizations of these metal complexes are reported in this thesis, Chapter 2.

Following Moore’s Law, the device size in the microelectronics industry is dramatically shrinking. SiO2 has been used as the gate dielectric material in MOSFETs until recently, and it has been partially replaced with HfO2. However, the need for replacing SiO2 with an efficient high-k material is increasingly growing. Binary and ternary lanthanide oxides are potential candidates to use as high-k materials on MOSFETs. Additionally, a polar non-polar system containing ABO3/SrTiO3 is important in creating 2-DEGs. Herein ALD of Pr2O3 and nearly stoichiometric PrAlO3 are developed and discussed. Characterization of the Pr2O3 films was difficult due to their hygroscopic nature. Incorporation of stable Al2O3 minimizes the Pr2O3 film degradation. Additionally, 1, AlMe3, and water were used for the film deposition with 1:1 Pr:Al precursor pulse ratio. The growth rate of Pr2O3 is 0.85 Å/cycles, and PrAlO3 is 1.6–2.0 Å/cycles at 300 ºC on both Si(100) and SiO2 substrates. The PrAlO3 films contain low impurity levels by XPS measurements, and as-deposited films are amorphous by GI-XRD. The PrAlO3 films deposited on TiO2-terminated SrTiO3 substrates can be fully crystallized by annealing at 800 ºC for 3 h to result in (001) highly oriented PrAlO3. Detailed film morphology and properties are discussed in this thesis, Chapter 3.

ALE is a method of removing films in a layer-by-layer manner, in contrast to ALD, which deposits films layer-by-layer manner. Thermal ALE of transition metal films is poorly studied due to the lack of chemistries to overcome thermodynamic and kinetic barriers of etching half reactions. Therefore, scientists have used highly energetic ion bombardment or plasmas to initiate etching reactions. For the Co metal ALE, surface chemical modification followed by ligand exchange reaction mechanism is used. In our Co ALE study, formic acid is used as an oxidant to oxidize the cobalt metal and carbohydrazone or acetylacetone is used for the ligand exchange reactions. The volatile cobalt complexes are formed after ligand exchange reactions and can then be removed with an inert gas purge resulting in etched cobalt films. Different etch rates are observed for different ligands. Etch rates, changes in film resistivities and surface roughness of before and after etching of the cobalt films, are discussed in detail in this thesis, Chapter 4.

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