Abstract
Thermal atomic layer etch (ALE) of W metal can be achieved by sequential self-limiting oxidation and chlorination reactions at elevated temperatures. In this paper, we analyze the reaction mechanisms of W ALE using the first-principles simulation. We show that oxidizing agents such as O2, O3, and N2O can be used to produce a WOx surface layer in the first step of an ALE process with ozone being the most reactive. While the oxidation pulse on clean W is very exergonic, our study suggests that runaway oxidation of W is not thermodynamically favorable. In the second ALE pulse, WCl6 and Cl2 remove the oxidized surface W atoms by the formation of volatile tungsten oxychloride (WxOyClz) species. In this pulse, each adsorbed WCl6 molecule was found to remove one surface W atom with a moderate energy cost. Our calculations further show that the desorption of the additional etch products is endothermic by up to 4.7 eV. Our findings are consistent with the high temperatures needed to produce ALE in experiments. In total, our quantum chemical calculations have identified the lowest energy pathways for ALE of tungsten metal along with the most likely etch products, and these findings may help guide the development of improved etch reagents.
Highlights
Atomic layer etch (ALE) processing has gained considerable attention in recent years within the semiconductor industry.[1−4]The continued scaling of semiconductor devices demands the use of ever-thinner and higher-performing materials, which drives the development of gentle etch process technologies with atomic-level precision and high degrees of selectivity toward surrounding materials
All calculations reported in this paper are based on spinpolarized density functional theory (DFT) using VASP v5.4.48 Core electrons are represented by projector augmented wave (PAW) potentials,[49,50] and valence electrons are treated explicitly by expanding their wave functions in a plane wave basis set with an energy cutoff of 400 eV
The proposed W from the surface (Ws)-etch species, which originate from the surface for this ALE process, have been determined to be either WOCl4 or WO2Cl224−26 depending on the etch gas stoichiometry
Summary
Atomic layer etch (ALE) processing has gained considerable attention in recent years within the semiconductor industry.[1−4]. The oxidizing plasma produces a diffusion-limited WO3 surface layer, which can be removed by exposure to WF6 yielding WO2F2 as a volatile etch byproduct They have reported results on an oxidation plus WCl6-based W ALE process[25,26] in which they used a thermodynamic analysis to rationalize the experimental data. Ground state spin polarized DFT was used to calculate the energies of surface species along several proposed reaction pathways From these results, a full atomic layer etch chemical pathway for W metal can be mapped yielding a feasible reaction mechanism featuring sequential oxidation and chlorination steps
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