A density functional theory study of alumina atomic layer deposition on alumina and silica

Atomic layer deposition (ALD) is a thin film deposition process based on alternating exposure of precursor materials on a substrate surface. If the former are chosen in an appropriate way, alternate sequential exposure of precursors results in self-terminating reactions and thus allows a controlled layer-by-layer growth. Therefore ALD has been applied for deposition of high-k dielectric material such as alumina in the micro-electronics industry. In this work, the alumina atomic layer deposition process has been investigated. The success of ALD rests on its chemistry. It is thus important to identify and understand the underlying elementary reactions responsible for ideal film growth and those reactions that can lead to undesired impurities in the films. Experimental investigations have been conducted in an attempt to explain the mechanism of ALD. However, atomic scale simulations can be an effective way to examine mechanisms in more detail. To this end, in this work, Density Functional Theory (DFT) calculations have been used to explore the elementary reactions and the possible intermediates have been validated by comparing calculated vibrational frequencies with experimental values. In this way, a more detailed understanding of alumina ALD mechanism is obtained

Ab initio investigation of surface chemistry of alumina ALD on hydroxylated γ-alumina surface

Atomic layer deposition (ALD) of alumina using trimethyl aluminum (TMA) and water on amorphous alumina is analyzed using periodic-dispersion corrected DFT calculations. The energetics of the investigated reactions suggest that monomethyl aluminum (MMA) is the most abundant reaction intermediate at ALD operating conditions. The dominant reaction path toward the methylation of the surface is found to be adsorption of TMA on bridge oxygen via Lewis acid-base complex formation followed by ligand exchange reactions (LERs) with hydroxyls and surface water in its vicinity. Further adsorption and LERs of TMA leads to a saturated methylated surface (similar to 6.4 CH3 nm(-2)) which is in agreement with experimental observations and infrared spectra. The surface restructuring that is observed in almost all the reactions investigated seems to play an important role in the formation of conformal alumina films

DFT investigation into alumina ALD growth inhibition on hydroxylated amorphous silica surface

Alumina (Al2O3), a suitable replacement for silica (SiO2) as gate oxide in metal oxide semiconductor field effect transistors (MOSFET), is deposited on the amorphous silica layer of the semiconductor substrate by atomic layer deposition (ALD) using trimethylaluminum (TMA) and water as precursors. A computationally efficient model for the hydroxylated amorphous silica surface is obtained by means of molecular dynamics and is used to investigate the reason behind the observed growth inhibition during alumina ALD. The reactions of TMA are investigated by periodic DFT calculations on surfaces with hydroxyl coverage of 3.38 OH nm(-2) and 5.07 OH nm(-2). The formation of SiCH3 surface species is found to be possible only on the less hydroxylated surface during the first TMA half-cycle, while the subsequent reaction of water with the SiCH3 surface species is found to be highly activated (E-a = 196 kJ mol(-1)). Since these SiCH3 surface species are rather unreactive toward water, fewer hydroxyls are regenerated during this first water half-cycle, resulting in the observed initial growth inhibition. Moreover, alumina growth can continue over the aluminum surface species, trapping the unreactive SiCH3 species at the interface between deposited alumina and silica. Such carbon impurities at the interface should be avoided nonetheless, since they can create undesirable tunneling currents in MOSFETs

DFT Investigation into Alumina ALD Growth Inhibition on Hydroxylated Amorphous Silica Surface

Alumina (Al₂O₃), a suitable replacement for silica (SiO₂) as gate oxide in metal oxide semiconductor field effect transistors (MOSFET), is deposited on the amorphous silica layer of the semiconductor substrate by atomic layer deposition (ALD) using trimethylaluminum (TMA) and water as precursors. A computationally efficient model for the hydroxylated amorphous silica surface is obtained by means of molecular dynamics and is used to investigate the reason behind the observed growth inhibition during alumina ALD. The reactions of TMA are investigated by periodic DFT calculations on surfaces with hydroxyl coverage of 3.38 OH nm^–2 and 5.07 OH nm^–2. The formation of SiCH₃ surface species is found to be possible only on the less hydroxylated surface during the first TMA half-cycle, while the subsequent reaction of water with the SiCH₃ surface species is found to be highly activated (<i>E</i>_a = 196 kJ mol^–1). Since these SiCH₃ surface species are rather unreactive toward water, fewer hydroxyls are regenerated during this first water half-cycle, resulting in the observed initial growth inhibition. Moreover, alumina growth can continue over the aluminum surface species, trapping the unreactive SiCH₃ species at the interface between deposited alumina and silica. Such carbon impurities at the interface should be avoided nonetheless, since they can create undesirable tunneling currents in MOSFETs