ALD/ALE 2022 Session ALE-SuP: Atomic Layer Etching Poster Session

In this study, a cyclic isotropic plasma atomic layer etching (ALE) process for aluminum oxide was developed with two steps of plasma fluorination and ligand volatilization with trimethylaluminum (TMA). In the plasma fluorination step, the Al₂O₃ surface was fluorinated to AlOF_x with NF₃ plasma at 100 °C. As the plasma fluorination time increased, the atomic fraction of fluorine on the surface was increased and then saturated to a value of 25% after 50s of plasma fluorination. The formation of the AlOF_x layer was confirmed by X-ray photoelectron spectroscopy analysis. The depths of the fluorinated layers were in the range 0.79–1.14 nm at different plasma powers. In the removal step, the fluorinated layer was removed by a ligand exchange reaction with TMA at an elevated temperature range of 250–480 °C. The etch per cycle (EPC) was 0.20–0.30 nm/cycle and saturated after 30 s in the temperature range of 290–330 °C. EPC increased in the temperature range of 250–300 °C during the removal step with the ligand exchange reaction and reached the maximum at 300 °C. The fluorine atomic fraction on the surface was reduced to 14% after the removal. The average surface roughness of Al₂O₃ was reduced from 8.6 Å to 5.3 Å after 20 cycles of etching. In conclusion, Al₂O₃ was successfully etched at the atomic scale by the cyclic plasma ALE process.

Titanium nitride (TiN) has been used as a metal gate electrode from 2D FinFETs to 3D FinFETs due to its proper mid-gap work function, high thermal stability, and excellent adhesion. Metal gates require a low-temperature process to prevent device degradation. Therefore, atomic-scale etching techniques at low temperatures are required for TiN films in 3D structures. In this work, plasma atomic layer etching (ALE) was developed for TiN using 3 steps of plasma oxidation, plasma fluorination, and thermal removal. In the plasma oxidation step, TiN was oxidized to form TiO₂ with O₂ plasma at 100℃. The TiO₂ thickness was saturated with O₂ plasma after an exposure time of 300s and saturated thickness increased from 0.29 to 1.23nm with increasing temperature and RF power. In the plasma fluorination step, TiO₂ layer was converted to TiO_xF_y with CF₄ plasma at 100℃. The F atomic percentage on the surface was saturated at 12% with RF power below 15W. In the thermal removal step, TiO_xF_y layer was completely removed above 150℃. The removal rate of TiN ranged from 0.24 to 1.71 nm/cycle by controlling the TiO₂ thickness determined earlier. The roughness of TiN surface decreased from 1.27nm to 0.26nm after 50 cycles of ALE process. The suggested TiN ALE process is expected to provide an effective process for atomic-scale three-dimensional structures.

Since plasma had been employed to the etch process of semiconductor manufacturing, etch uniformity has been one of the most important issues, and in this atomic scale era, high etch uniformity is of increasing importance. In achieving atomic-scale uniformity, atomic layer etching (ALE) has arisen as a next-generation etch technique due to its self-limiting characteristic. The long processing time of ALE however hinders the wider employment of ALE in the semiconductor industry. In this presentation, we will propose a novel ALE method where no purging step is required and discuss its applicability with the evaluation of its ability to obtain high etch uniformity over a wafer-scale area.

Sub-10 nm scaling comes with unprecedented challenges for semiconductor fabrication. Atomic Layer Etching (ALE) is a technique that is becoming increasingly important in semiconductor fabrication, however, it has yet to be widely adopted. Many examples of ALE use harsh chemistry such as halogenated compounds or plasma, which result in damaged and non-conformal structures. Copper is an important interconnect material, so scalable copper ALE is of high importance and further research of both metallic Cu activation and etch are required. Oxidation of metallic Cu surfaces with unselective oxidants (H₂O₂, O₃, oxygen plasma etc.) tend to produce multiple oxidation states of Cu (+1 and +2) as well as Cu mobility at elevated temperatures. This study reports a comparison between conventional oxidants as well as milder, more selective oxidizing agents that produce a more controlled oxidation of Cu surfaces.

X-ray absorption spectroscopy (XAS) was used to directly observe Cu oxidation under oxidation conditions.Temperatures from ambient to 145°C were screened with oxidants and oxidation was found to be to Cu₂O to detection limits.The depth was controllable from less than 2nm to complete oxidation under the conditions studied.The oxidation could be controlled by process conditions and, more importantly, by the nature of the oxidation reagent.These data are compared to in situ TEM measurements of Cu particles and measured oxidation thickness as a function of oxidant exposure and found to be comparable.The XAS technique was further benchmarked using conventional XPS measurements.These measurements illustrate that XAS is able to probe surface activation for ALE.In addition, XAS provides metal coordination number, oxidation state, and other mechanistic information about the surface metal state.These measurements combine to provide a better understanding of the relationship between oxidant strength versus depth as a function of various oxidants. These results give insight into the etch-per-cycle as well, Cu surface roughening, and other metrics for the final films.