Gas- versus Liquid-Phase Silylation as a Way to Inhibit Thin-Film Atomic Layer Deposition (ALD) on Silicon Oxide Substrates

The effectiveness of silylation treatments of silicon oxide surfaces using <i>N</i>-(trimethylsilyl)-dimethylamine (TMSDMA) for the passivation of atomic layer depositions (ALDs) was evaluated by using X-ray photoelectron spectroscopy (XPS). It was determined that such silylation does indeed block the silanol (Si-OH) nucleation centers where the ALD precursors react and therefore inhibits film growth, but only temporarily; after a few ALD cycles, deposition becomes evident. By testing this chemistry on two types of SiO₂ surfaces, prepared by plasma-enhanced chemical vapor deposition (PE-CVD) and by chemical (RCA) treatment of Si(100) wafers, it was concluded that the nature of the initial substrate does not play a crucial role in the silylation or ALD blocking processes. The material being deposited, on the other hand, does make a difference: TiO₂ film growth can be blocked for almost 10 ALD cycles, whereas HfO₂ starts building up on the surface after less than 5 ALD cycles. Moreover, the steady-state deposition rate reached for TiO₂ on the silylated surfaces is lower than that seen for the untreated substrate, whereas with HfO₂ not only is that not the case, but the new films may in fact grow as 3D nanostructures. One of the key findings of this work is that the silylation can be carried out using either gas- or liquid-phase treatments. It was found that the extent of silylation and the inhibition of the subsequent ALD were comparable in both cases, but the gas-phase method was determined to be cleaner and to deposit less carbon contaminants. This silylation-based ALD inhibition is expected to be selective with respect to the nature of the substrate and therefore useful for the design of area-selective ALD (AS-ALD) processes.