Thermal Selective Vapor Etching of TiO₂: Chemical Vapor Etching via WF₆ and Self-Limiting Atomic Layer Etching Using WF₆ and BCl₃

Controlled thin film etching is essential for further development of sub-10 nm semiconductor devices. Vapor-phase thermal etching of oxides is appealing for achieving highly conformal etching of high aspect ratio features. We show that tungsten hexafluoride (WF 6 ) can be used to selectively etch amorphous TiO 2 films versus other oxides including Al 2 O 3 . Chemical vapor etching (CVE) of TiO 2 by WF 6 was studied with quartz crystal microbalance (QCM), spectroscopic ellipsometry, X-ray photoelectron spectroscopy (XPS), and thermodynamic modeling. The XPS results show evidence for a WO x F y layer that forms on of the TiO 2 films during the etch process, which may act as a surfactant layer to help enable fluorination of the TiO 2 . Direct CVE of TiO 2 by WF 6 is strongly temperature dependent, where etching proceeds readily at 220 °C, but not at T ≤ 170 °C. This is consistent with thermodynamic modeling showing that the etching rate is determined by the volatilization of metal fluoride and WF 2 O 2 product species. We also show that, at low temperature, BCl 3 can be used as a coreagent with WF 6 to achieve self-limiting atomic layer etching (ALE) of TiO 2 . At 170 °C, the rate of ALE saturates at ∼0.6 Å/cycle, which is ∼2× the rate of TiO 2 ALD at the same temperature. Experimental QCM analysis shows selectivity for TiO 2 ALE vs Al 2 O 3 as predicted by thermodynamic modeling. We also demonstrate and describe how etching reactions during initial cycles can differ from those during steady-state ALE, and we draw a physical analogy between rate evolution in ALE and well-known rate evolution during nucleation in atomic layer deposition (ALD). This work expands understanding of surface reactions in CVE and ALE and the range of reactants and materials that can be active for advanced thermal ALE processing.