Atomic Layer Deposition of Al2O3 with Alcohol Oxidants for Impeding Substrate Oxidation
Authors: Seo Seunggi , Woo Whang Je , Oh Il-kwon , Kim Hyungjun , Shong Bonggeun 
Published: February 2020

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

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