Growth kinetics and microstructure in two precursor chemical vapor deposition of borides

Canova, Kinsey Lee

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https://hdl.handle.net/2142/117642 Copy

Description

Title

Growth kinetics and microstructure in two precursor chemical vapor deposition of borides

Author(s)

Canova, Kinsey Lee

Issue Date

2022-11-09

Director of Research (if dissertation) or Advisor (if thesis)

Abelson, John R

Doctoral Committee Chair(s)

Abelson, John R

Committee Member(s)

• Girolami, Gregory S
• Krogstad, Jessica A
• Flaherty, David W
• Perry, Nicola H

Department of Study

Materials Science & Engineerng

Discipline

Materials Science & Engr

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• CVD
• chemical vapor deposition
• conformal
• superconformal
• morphology
• diboride
• oxidation
• thin films

Abstract

Chemical vapor deposition (CVD) of alloys from two precursors alters growth kinetics to control thin film morphology and composition, and my focus is on coatings of alloyed transition metal diborides. In CVD at low temperatures, where film growth rates are primarily determined by surface reaction rates, I demonstrate unique kinetic effects in two precursor CVD in the context of their utility as protective coatings (Chapters 2-3), area selective growth (Chapter 4), and interconnect metallization in nanoscale devices, in the form of experimental results and a kinetic model (Chapter 5). Finally, a computational study shows how nucleation and growth rates, which may be engineered by these two precursor CVD methods, affect film morphology at the point of coalescence (Chapter 6). Below, we briefly summarize these results. Chapter 2: We demonstrate growth of Hf1-xAlxBy from the combination of the single-source HfB2 precursor, Hf(BH4)4, and Al precursor, AlH3-NMe3 (Me = CH3), as a route to growing oxidation-resistant protective coatings. Alloy film growth exhibits exceptionally fast growth rates and improved crystallinity compared to film grown from each precursor alone, and the alloy composition is stable around 6:4 Hf:Al. Analysis of growth rates indicate that the incorporation rate of Al positively correlates to the incorporation rate of HfB2. We hypothesize that Al overcomes the rate-limiting step in HfB2 deposition by reacting with BH4 ligands: Formation and volatilization of B2H6 to remove the excess boron during HfB2 deposition is likely slow due to the surface instability of diborane reported in literature. When Al is present, it may react with BH4 groups to form Al-B based products, removing the need to remove excess boron as diborane. This effect also has implications for improving crystallinity (without increasing temperature) from the amorphous, unalloyed HfB2 to a substitutional solid solution of crystalline Hf1-xAlxBy. Chapter 3: In close collaboration with Samyukta Shrivastav, we analyze the alloy films in terms of oxidation resistance and stability at elevated temperatures. Alloy films are homogeneous in composition and have equiaxed grains as-deposited. When annealed to 700 °C, alloy films do not crack, and little grain growth is observed. Finally, oxidation of the films shows that a small amount of aluminum segregates to the surface oxide, comprised primarily of aluminum and boron oxides, and the composition in the un-oxidized layer largely unchanged. The surface oxide in alloy films is continuous up to ~ 700 °C; at higher temperatures, the mixed oxide forms acicular crystals of aluminum borate, and this transformation allows complete oxidation of the entire film. Chapter 4: Area selective deposition is desirable for reducing the number of patterning steps in integrated device processing that may cause device failure due to pattern misalignment, and these processes often use an inhibitor to improve selectivity. Inhibition for area selective growth depends closely on surface chemistry, where small, unreactive molecules such as ammonia or silane-based compounds passivate non-growth surfaces against precursor species for the desired film growth. With two precursor CVD, I show that some metal precursors may be used as inhibitors against the deposition from another, more reactive metal precursor. Selectivity is imparted by adsorption and nucleation behavior for the less reactive precursor used as inhibitor: metal precursors that adsorb strongly on oxides may block those preferentially to metals for area-selective growth. Chapter 5: Modern integrated device processing requires uniform filling in high aspect ratio features for metallic interconnects and dielectric isolation, and this is challenging for even conformal processes like atomic layer deposition, which yield a low density seam along the centerline. I show that the vanadium precursor, V(NMe2)4, inhibits the overall growth rate of Hf1-xVxBy with the HfB2 precursor, Hf(BH4)4, and I apply this inhibition to fill narrow trenches to meet the processing challenge in device metallization. As the vanadium precursor adsorbs and reacts slowly towards the opening of the trench, its pressure declines towards the bottom of the trench, leading to less inhibition to growth and a faster overall growth rate from the HfB2 precursor. Kinetic modeling, based on competitive adsorption in the growth rate and transport by molecular flow, supports that filling hypothesis. This method is generalizable to any CVD growth using a consumable inhibitor, which may be another precursor or plasma-generated atomic species. Chapter 6: The effect of nucleation and growth rates on film morphology has been shown in many experiments over the years using growth inhibition, nucleation enhancement, and the combination of the two to obtain ultra-smooth films. In collaboration with Diana LaFollette, we develop computational models that show the power law relationship between nucleation rates, growth rates, and morphology. Comparison with experimental film morphologies shows that the models are reasonable, and the simplicity of the model provides a convenient benchmark for identifying other surface kinetics that appear as deviations from the model.

Graduation Semester

2022-12

Type of Resource

Thesis

Copyright and License Information

Copyright 2022 Kinsey Lee Canova

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