Area-Selective Atomic Layer Deposition of Two-Dimensional WS₂ Nanolayers

With downscaling of device dimensions, two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs) such as WS₂ are being considered as promising materials for future applications in nanoelectronics. However, at these nanoscale regimes, incorporating TMD layers in the device architecture with precise control of critical features is challenging using current top-down processing techniques. In this contribution, we pioneer the combination of two key avenues in atomic-scale processing: area-selective atomic layer deposition (AS-ALD) and growth of 2D materials, and demonstrate bottom-up processing of 2D WS₂ nanolayers. Area-selective deposition of WS₂ nanolayers is enabled using an ABC-type plasma-enhanced ALD process involving acetylacetone (Hacac) as inhibitor (A), bis(tert-butylimido)-bis(dimethylamido)-tungsten as precursor (B), and H₂S plasma as the co-reactant (C) at a low deposition temperature of 250 °C. The developed AS-ALD process results in the immediate growth of WS₂ on SiO₂ while effectively blocking growth on Al₂O₃ as confirmed by in situ spectroscopic ellipsometry and ex situ X-ray photoelectron spectroscopy measurements. As a proof of concept, the AS-ALD process is demonstrated on patterned Al₂O₃/SiO₂ surfaces. The AS-ALD WS₂ films exhibited sharp Raman (E _2g ¹ and A _1g) peaks on SiO₂, a fingerprint of crystalline WS₂ layers, upon annealing at temperatures within the thermal budget of semiconductor back-end-of-line processing (≤450 °C). Our AS-ALD process also allows selective growth on various TMDs and transition metal oxides while blocking growth on HfO₂ and TiO₂. It is expected that this work will lay the foundation for area-selective ALD of other 2D materials.