In Vacuo X-ray Photoelectron Spectroscopy Insights into the Chemical Selectivity of MgO ALD

Area-selective atomic layer deposition (AS-ALD) holds great promise for many applications, such as microelectronics and catalysis, due to its capability to achieve precise material deposition on targeted regions while leaving others uncoated. However, direct observation of the surface chemistry during selective deposition, a critical aspect of AS-ALD, remains limited. In this work, we utilize an integrated X-ray photoelectron spectroscopy (XPS)-ALD system, enabling in vacuo XPS measurements after each half or full ALD cycle to monitor elemental and chemical state evolution. Both homogeneous metal and oxide films (i.e., Ni, SiO2 and Al2O3) and heterogeneous samples consisting of metal nanoparticles on oxide supports (i.e., Ni/SiO2 and Ni/Al2O3) are employed as substrates to investigate the inherent chemical selectivity of MgO ALD from Mg(EtCp)2 and H2O at different deposition temperatures in the range of 100 to 250 °C. These MgO/Ni materials are particularly relevant for CO2 conversion catalysis. In vacuo XPS confirms the thermal reduction of native Ni oxide in high vacuum prior to MgO deposition, with supported Ni nanoparticles requiring higher temperatures than blanket Ni films. Monitoring the growth of MgO with in situ spectroscopic ellipsometry reveals differences in nucleation behavior on homogeneous blanket Ni, SiO2 and Al2O3 films during the initial ALD cycles. The inhibition of MgO ALD growth on Ni and Al2O3 at 100 °C is attributed to the presence of adventitious carbon species on the substrate surfaces, as confirmed with in vacuo XPS. However, the chemical selectivity observed on blanket SiO2 films is not fully replicated on heterogeneous Ni/SiO2 samples, underscoring the complexity of inherent AS-ALD on nanostructured substrates. Overall, this study demonstrates an effective approach for characterizing surface chemistry during (AS-)ALD and provides critical insights into the mechanisms governing inherently selective MgO deposition.