Direct observation of half-cycle adsorption of aminosilane precursors by <i>in situ</i> XPS

We investigated the initial adsorption states of three aminosilane precursors, tris(dimethylamino)silane (TDMASi), bis(diethylamino)silane (BDEASi), and di(isopropylamino)silane (DIPASi), on an –OH-terminated ALD alumina surface using in situ x-ray photoelectron spectroscopy with vacuum transfer. After a 0.5 s precursor pulse without oxidant, the scaled Si 2p/O 1s saturation values, corresponding to Si atoms adsorbed per 1000 O atoms in alumina, were nearly identical for all three precursors, indicating that the saturated Si coverage itself in the first half-cycle is essentially the same. Thus, the reported differences in growth per cycle cannot be explained simply by differences in the amount of adsorbed Si during the initial precursor exposure. In contrast, N 1s, C 1s, and compositional analyses revealed pronounced precursor-dependent ligand retention. TDMASi and BDEASi left substantial nitrogen- and carbon-containing species, whereas DIPASi showed no detectable N 1s signal and a much smaller C contribution. Deconvolution of the N 1s spectra showed that the nitrogen species remaining after TDMASi and BDEASi dosing consist mainly of N–C and N–Si components with a smaller N–O component, indicating the coexistence of undecomposed amino ligands bound to Si and nitrogen-containing ligands re-adsorbed on surface –OH sites. Quantitative comparison of the N–Si and N–O components further demonstrated that TDMASi retains more Si-bonded and readsorbed nitrogen-containing ligands than BDEASi, whereas the single bulky diisopropylamino ligand in DIPASi favors reaction with surface –OH groups and suppresses readsorption. These results show that precursor-dependent ligand removal and readsorption, rather than initial Si coverage, govern the first-half-cycle chemistry of aminosilane-based SiO2 atomic layer deposition.