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

Abstract

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 BO layer while forming volatile WOCl products. Subsequently, HF spontaneously etches the BO layer producing volatile BF and HO 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.

Anisotropic atomic layer etching of W using fluorine radicals/oxygen ion beam

“…For the oxygen ion beam, 300 W of RF power was applied to the ICP source at 1 mTorr of O 2 while applying + 30– + 100 V to the first grid, −100 V to the second grid, and 0 V (grounded) to the third grid. The formation of WO x was estimated by the increased W thickness after the oxygen ion beam exposure using a surface profilometer because, during the oxygen exposure, the thickness of W is increased by the formation of WO x . (In fact, because it is difficult to measure the thickness of a self‐limiting oxidation step of a few angstroms with a profilometer, the oxidation time was increased until the oxide thickness can be measured by the profilometer and the data were extrapolated to the lower oxidation time.)…”

“…The formation of WO x was estimated by the increased W thickness after the oxygen ion beam exposure using a surface profilometer because, during the oxygen exposure, the thickness of W is increased by the formation of WO x . [33] (In fact, because it is difficult to measure the thickness of a self-limiting oxidation step of a few angstroms with a profilometer, the oxidation time was increased until the oxide thickness can be measured by the profilometer and the data were extrapolated to the lower oxidation time.) Figure 2(b) shows the changes in WO x thickness on the W surface measured by the surface profilometer with increasing first grid voltage from + 30 to + 100 V and O x + ion exposure time from 0 to 60 s. The first grid voltage is directly related to the oxygen ion energy from the ion source and the ion energy distributions measured by a retarding grid ion energy analyzer for the different first grid voltages are shown in Figure S1.…”

“…For the application of W to next generation device interconnection, ALE technology of W is known to be required . For the W ALE method, a W thermal ALE obtained through an oxidation‐conversion‐fluorination etch mechanism, which is a three step ALE composed of oxidation using O 3 , the conversion to a volatile oxychloride compound using BCl 3 , and the final removal of the surface reside compound by fluorination by HF, is reported . In addition, a thermal ALE of W using sequential exposure to O 2 (or O 3 ) and WF 6 , where, W is oxidized using O 2 or O 3 to form WO 3 (s), and the WO 3 is removed by forming volatile WO 2 F 2 (g) by the reaction with HF has been investigated .…”

“…[31,32] For the W ALE method, a W thermal ALE obtained through an oxidation-conversion-fluorination etch mechanism, which is a three step ALE composed of oxidation using O 3 , the conversion to a volatile oxychloride compound using BCl 3 , and the final removal of the surface reside compound by fluorination by HF, is reported. [33] In addition, a thermal ALE of W using sequential exposure to O 2 (or O 3 ) and WF 6 , where, W is oxidized using O 2 or O 3 to form WO 3 (s), and the WO 3 is removed by forming volatile WO 2 F 2 (g) by the reaction with HF has been investigated. [34] Even though, selflimited precise etching of W with the rates of a few Å/ cycle could be obtained with the ALE methods investigated until now, all of these methods are isotropic W ALE methods and an anisotropic W ALE method which can form anisotropic etch profiles of W has not been investigated yet.…”

Mechanism of Thermal Atomic Layer Etch of W Metal Using Sequential Oxidation and Chlorination: A First-Principles Study

“…Indeed, progress toward the isotropic thermal ALE of metallic W has been reported by the groups of George and Parsons. The work reported by George et al ( 13 ) has shown the quasi-ALE of metallic W via a conversion etch mechanism utilizing ozone (O 3 ), BCl 3 , and anhydrous HF vapor. In this system, ozone was used to oxidize the W surface to WO 3 in a diffusion-limited process, which then reacts with BCl 3 to generate a volatile etch product in the form of WO x Cl y while tandemly generating a B 2 O 3 surface layer, which is then susceptible to self-limiting etch by HF.…”