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

Abstract

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 (WF6) can be used to selectively etch amorphous TiO2 films versus other oxides including Al2O3. Chemical vapor etching (CVE) of TiO2 by WF6 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 TiO2 films during the etch process, which may act as a surfactant layer to help enable fluorination of the TiO2. Direct CVE of TiO2 by WF6 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 WF2O2 product species. We also show that, at low temperature, BCl3 can be used as a coreagent with WF6 to achieve self-limiting atomic layer etching (ALE) of TiO2. At 170 °C, the rate of ALE saturates at ∼0.6 Å/cycle, which is ∼2× the rate of TiO2 ALD at the same temperature. Experimental QCM analysis shows selectivity for TiO2 ALE vs Al2O3 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.

Chemical vapor deposition of monolayer-thin WS2 crystals from the WF6 and H2S precursors at low deposition temperature

“…In this case, the reduction of WF6 is enabled by reaction with the surface yielding metallic tungsten. Others reported on the controlled etching of TiO2 by employing WF6 as fluorinating and etching agent for atomic layer etching 54 . In addition to that, W CVD from WF6 using a Si substrate, yields metallic W by dissociative adsorption of WF6 on Si forming volatile SiF2 or SiF4 55,56,57 .…”

Prediction and Validation of the Process Window for Atomic Layer Etching: HF Exposure on TiO2

“…Thermochemical calculations of model reactions have been performed by researchers investigating ALD and ALE processes. ,− One popular approach is to model the reactions using thermochemical tables from databases such as NIST-JANAF with the help of software packages such as the HSC Chemistry . Alternatively, reactions can be modeled from first-principles using density functional theory (DFT) calculations.…”

Recent Progress of Atomic Layer Technology in Spintronics: Mechanism, Materials and Prospects

“…Therefore, the regulation of ion energy is extremely crucial in order to ensure that ALE is kept within its own self-limitation limits. However, for the majority of the thermal ALE reactions that have been described, the EPC is significantly affected by temperature, and the ALE window is difficult to observe due to its short width [110][111][112]. Another essential element is the reactive gas, as different material systems require varied amounts of surface modification gas.…”