Atomic layer deposition (ALD) is now used in semiconductor fabrication lines to deposit nanometre-thin oxide films, and has thus enabled the introduction of high-permittivity dielectrics into the CMOS gate stack. With interest increasing in transistors based on high mobility substrates, such as GaAs, we are investigating the surface treatments that may improve the interface characteristics. We focus on incubation periods of ALD processes on III-V substrates. We try to understand the detailed chemistry of the period when the interface is formed, specifically the interaction between the substrate and the ALD precursor.
We use atomic-scale simulations in the framework of density functional theory (DFT) on the bulk structures, surface models and gas phase molecules. Ab initio thermodynamics provides a bridge between zero-temperature, zero-pressure DFT and real experiments. We analyse the interaction of the precursors with the considered substrates: starting from initial adsorption of the gaseous molecule, through its transformation in contact with native oxides, until formation of surface intermediates that can be lost as by-products. Some kinetic aspects of the incubation mechanisms are also considered. Based on the trends arising from the initial computations, we are able to develop a comprehensive model that allows us to compare the operation of different classes of precursor chemicals, including alkyls, alkylamides and chlorides, and assess their properties for so called ‘clean-up’ or ‘self-cleaning’ ALD.
‘Clean-up’ is the observation that the ALD precursors remove deleterious oxides from the substrate before growth of the dielectric layer. Self-cleaning ALD has been observed especially for organometallic and metalorganic precursors: trimethylaluminium (TMA) or hafnium and titanium amides. On the example of TMA we identified two separate factors governing the clean-up effect: formation of the metal oxide as the driving force and affinity of the precursor ligand to the substrate as the ancillary force. ‘Clean-up’ of an oxide film is shown to strongly depend on the electropositivity of the precursor metal, and thus always results in formation of dielectric film from native oxide. However, self-cleaning ALD does not necessary result in substrate-enhanced growth, as the clean-up effect can be spread over a few cycles. The choice of ligand determines bonding at the interface and the overall type of clean-up. The predominant pathway for a metalloid oxide such as arsenic oxide is reduction, producing volatile molecules or gettering oxygen from less reducible oxides. An alternative pathway is non-redox ligand exchange, which allows non-reducible oxides to be cleaned-up. We predict that the investigated methyl precursors are the best reagents for deposition of dielectrics and performing clean-up. Clean-up with metal chlorides has a fundamentally different mechanism, probably involving removal of the O from the native oxide film and passivation with Cl groups.