WO3 and W Thermal Atomic Layer Etching Using "Conversion-Fluorination" and "Oxidation-Conversion-Fluorination" Mechanisms

Abstract

The thermal atomic layer etching (ALE) of WO₃ and W was demonstrated with new "conversion-fluorination" and "oxidation-conversion-fluorination" etching mechanisms. Both of these mechanisms are based on sequential, self-limiting reactions. WO₃ ALE was achieved by a "conversion-fluorination" mechanism using an AB exposure sequence with boron trichloride (BCl₃) and hydrogen fluoride (HF). BCl₃ converts the WO₃ surface to a B₂O₃ layer while forming volatile WO_xCl_y products. Subsequently, HF spontaneously etches the B₂O₃ layer producing volatile BF₃ and H₂O products. In situ spectroscopic ellipsometry (SE) studies determined that the BCl₃ and HF reactions were self-limiting versus exposure. The WO₃ ALE etch rates increased with temperature from 0.55 Å/cycle at 128 °C to 4.19 Å/cycle at 207 °C. W served as an etch stop because BCl₃ and HF could not etch the underlying W film. W ALE was performed using a three-step "oxidation-conversion-fluorination" mechanism. In this ABC exposure sequence, the W surface is first oxidized to a WO₃ layer using O₂/O₃. Subsequently, the WO₃ layer is etched with BCl₃ and HF. SE could simultaneously monitor the W and WO₃ thicknesses and conversion of W to WO₃. SE measurements showed that the W film thickness decreased linearly with number of ABC reaction cycles. W ALE was shown to be self-limiting with respect to each reaction in the ABC process. The etch rate for W ALE was ∼2.5 Å/cycle at 207 °C. An oxide thickness of ∼20 Å remained after W ALE, but could be removed by sequential BCl₃ and HF exposures without affecting the W layer. These new etching mechanisms will enable the thermal ALE of a variety of additional metal materials including those that have volatile metal fluorides.

Keywords: W; WO3; atomic layer etching; conversion; etching; fluorination; oxidation.