Adsorption of Sulfur on Ag/Al2O3 and Zn/Al2O3 Surfaces: Thermal Desorption, Photoemission, and Molecular Orbital Studies

Inherent area-selective atomic layer deposition of ZnS

Atomic layer deposition processes with inherent substrate selectivity are more straightforward for area-selective atomic layer deposition (AS-ALD) than approaches using surface passivation or activation with self-assembled monolayers (SAMs), small molecule inhibitors (SMIs) or seed layers. Here, ALD of ZnS using elemental zinc and sulfur as precursors is reported to have excellent inherent selectivity. At 400-500 °C for 250 cycles, substantial ZnS growth was observed on Ti and TiO₂ surfaces while no growth was measured on native SiO₂ and Al₂O₃ surfaces. On TiO₂, the ZnS growth rate remains constant at 1.0 Å per cycle at temperatures of 400-500 °C. On Ti, in contrast, the initial growth rate increases significantly from 1.2 Å per cycle at 350 °C to 6.2 Å per cycle at 500 °C. The high growth rates on Ti are believed to be caused by CVD-like growth during the early ALD cycles, arising from the reservoir effect of the Ti layer for Zn atoms. After the first 100 cycles, the growth rate decreases from 3.5 to 1.0 Å per cycle, the same as the growth rate on TiO₂. Selective adsorption of sulfur on TiO₂ over Al₂O₃ and SiO₂ is assumed to be the selectivity mechanism on TiO₂. Self-aligned deposition of ZnS was successfully demonstrated on a micrometer-scale Ti/native SiO₂ pattern and on a nanometer-scale TiO₂/Al₂O₃ pattern at 450 °C for 250 cycles; ZnS films with a thickness of ∼80 nm were selectively deposited on Ti over native SiO₂, and ZnS films with a thickness of ∼23 nm were selectively deposited on TiO₂ over Al₂O₃.