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

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 WOxCly 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 angstrom/cycle at 128 degrees C to 4.19 angstrom/cycle at 207 degrees 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 O-2/O-3. 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 similar to 2.5 angstrom/cycle at 207 degrees C. An oxide thickness of similar to 20 angstrom 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.