Paper PS+NS+SS+TF-FrM10
Thermally-Driven Atomic Layer Etching of Metallic Tungsten Films Using O₂ and WF₆

Friday, November 3, 2017, 11:20 am, Room 23

The semiconductor industry is facing the challenge of manufacturing transistor devices with sub-10 nm high aspect ratio features. Understanding and developing self-limiting etching processes that allow precise control over the thickness of materials removed is essential for enabling the manufacturing complex transistor structures. In this work, we investigated chemical vapor etching of tungsten films using oxygen (O₂) as the oxidant source and tungsten hexafluoride (WF₆) as the etchant.

We propose that etching of tungsten proceeds in two steps: 1) oxidation of the tungsten film to form WO_x surface species and 2) formation and removal of volatile metal fluoride species upon reaction with WF₆. Using quartz crystal microbalance (QCM), we found that the oxidation step with O₂ is required for etching to occur during WF₆ exposure. In addition, etching of O₂ treated tungsten films showed saturation towards WF₆ exposure. This indicates that etching of tungsten using oxygen and WF₆ is a self-limiting process, making it promising as an atomic layer etching process. QCM results also showed that the rate of etching depends on the temperature. Minimal amount of etching was measured at temperatures less than 275 °C. Ex-situ characterization techniques were applied to analyze the etching of tungsten films deposited on SiO₂ substrates. Scanning electron microscopy (SEM) results revealed the change in morphology of tungsten films after different number of O₂-WF₆ ALE cycles. The tungsten film on SiO₂ started out as a coalesced film, which transformed into disjointed nuclei, and the nuclei appeared completely removed as the number ALE cycle increased. Lastly, X-ray photoelectron spectroscopy (XPS) analyses further confirmed etching of tungsten film and showed a minimal amount of fluorine remained on the surface after the O₂-WF₆ ALE process.