WO 3 and W Thermal Atomic Layer Etching Using "Conversion-Fluorination" and "Oxidation-Conversion-Fluorination" Mechanisms

• Published in: ACS applied materials & interfaces Vol. 9; no. 39; pp. 34435 - 34447
• Main Authors: Johnson, Nicholas R, George, Steven M
• Format: Journal Article
• Language: English
• Published: United States 04.10.2017
• Subjects: etching; W; fluorination; oxidation; WO3; atomic layer etching; conversion
• ISSN: 1944-8244; 1944-8252
• DOI: 10.1021/acsami.7b09161

The thermal atomic layer etching (ALE) of WO 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. WO ALE was achieved by a "conversion-fluorination" mechanism using an AB exposure sequence with boron trichloride (BCl ) and hydrogen fluoride (HF). BCl converts the WO surface to a B O layer while forming volatile WO Cl products. Subsequently, HF spontaneously etches the B O layer producing volatile BF and H O products. In situ spectroscopic ellipsometry (SE) studies determined that the BCl and HF reactions were self-limiting versus exposure. The WO 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 BCl 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 WO layer using O /O . Subsequently, the WO layer is etched with BCl and HF. SE could simultaneously monitor the W and WO thicknesses and conversion of W to WO . 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 BCl 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.