Atomic layer deposition (ALD) is a vapor phase thin film deposition technique based on self-limited surface reaction. ALD processes consist of two (or more than) half-reactions. The first half-reaction is the self-limiting adsorption of precursor molecules which contain core metal atoms; the second half reaction is the self-limiting reaction between surface adsorbed precursor molecules and reactants. Since ALD could deposit thin films with high quality, good uniformity, high conformality, and sub-nanometer thickness controllability, [1-3] ALD has been regarded as one of the most suitable deposition technologies for semiconductor device fabrication.

Since thin films of alumina (Al₂O₃) have wide range of applications such as high-k dielectric material for electronic devices, mechanical and chemical protective coatings, diffusion barriers, and optical coatings, [4-7] ALD of Al₂O₃ process is the mostly and thoroughly studied. For the deposition of Al₂O₃, trimethylaluminum (TMA) have been the most widely used for Al precursor due to its high vapor pressure and reactivity. And water (H₂O) is widely utilized as the oxygen source in ALD Al₂O₃ processes, because since H₂O often shows facile ligand exchange reaction during ALD. However, ALD Al₂O₃ processes with H₂O reactant showed undesirable substrate oxidation issue. For example, there was an unwanted interface oxide between ALD deposited Al₂O₃ film and Si substrate. The interface oxide could reduce the dielectric constant of the deposited thin films and increase leakage current density. [8-9] Especially, the oxidation of substrate is critical issue for 2 dimensional (2D) transition-metal dichalcogenides (TMDCs) based field effect transistors (FETs). From our previous results, the oxidation of MoS₂ by H₂O considerably degraded device performance. [10-11] To avoid the oxidation of substrates and improve device performance, it is necessary to develop a new ALD process by using oxidants with lower oxidation potential than that of H₂O.

Despite its technical importance, ALD Al₂O₃ processes with weaker oxidants such as alcohols have rarely been investigated. [8] For this reason, the chemical reaction mechanism between surface adsorbed precursor and reactant has not been clearly identified.

In this work, we fundamentally investigated ALD process Al₂O₃ on Si substrate, using TMA and various alcohol oxidants (methanol (MeOH), ethanol (EtOH), and n-propanol (n-PrOH)). Furthermore, we investigate the reaction mechanism of various alcohol oxidants during ALD of Al₂O₃ with TMA. Density functional theory (DFT) calculations at B97D3 level of theory were performed using Gaussian 09 suite of programs.

Our developed ALD processes showed typical ALD growth characteristics. The saturated growth rates with MeOH, EtOH and n-PrOH were 0.10, 0.96, and 0.74 Å/cycle, respectively. From the calculation results, we revealed that the beta-hydrogen transfer reaction of EtOH and n-PrOH could easily oxidize surface methyl group into surface hydroxyl. The results could be applicable to highly integrated semiconductor devices fabrication and 2D TMDC based FET fabrication processes.

References

[1] S.M. George, Atomic layer deposition: an overview, Chem. Rev. 110 (2010) 111–131

[2] H. Kim, I.-K. Oh, Review of plasma-enhanced atomic layer deposition: Technical enabler of nanoscale device fabrication, Jpn. J. Appl. Phys. 53 (2014) 03DA01.

[3] S. Seo, Molecular oxidation of surface –CH₃ during atomic layer deposition of Al₂O₃ with H₂O, H₂O₂, and O₃: A theoretical study, Applied Surface Science 457 (2018) 376–380

[4] W. Vandervorst, High-k dielectrics for future generation memory devices, Microelectron. Eng. 86 (2009) 1789–1795

[5] S.B. Mendes, Low-loss optical waveguides for the near ultra-violet and visible spectral regions with Al₂O₃ thin films from atomic layer deposition, Thin Solid Films 518 (2010) 4935–4940.

[6] M. Ritala, Low-temperature atomic layer deposition of Al₂O₃ thin coatings for corrosion protection of steel: Surface and electrochemical analysis, Corros. Sci. 53 (2011) 2168–2175

[7] T. Nam, A composite layer of atomic-layer-deposited Al2O3 and graphene for flexible moisture barrier, Carbon 116 (2017) 553–561

[8] Choon-soo Lee, Atomic Layer Deposition of Al2O3 Thin Films Using Trimethylaluminum and Isopropyl Alcohol, Journal of The Electrochemical Society, 149 (6) C306-C310 (2002)

[9] Woo-Byoung Kim, Improved Interface and Electrical Properties by Inserting an Ultrathin SiO₂ Buffer Layer in the Al₂O₃/Si Heterojunction, Adv. Funct. Mater. 2018, 1807271

[10] Whang Je Woo, Bi-layer high-k dielectrics of Al2O3/ZrO2 to reduce damage to MoS2 channel layers during atomic layer deposition, 2D Mater. 6 (2019) 015019

[11] Jeong-Gyu Song, Effect of Al2O3 Deposition on Performance of Top-Gated Monolayer MoS2‑Based Field Effect Transistor, ACS Appl. Mater. Interfaces 2016, 8, 28130−28135