Self-Limiting Temperature Window for Thermal Atomic Layer Etching of HfO₂ and ZrO₂ Based on the Atomic-Scale Mechanism

Abstract

HfO 2 and ZrO 2 are two high-k materials that are important in the down-scaling of semiconductor devices. Atomic level control of material processing is required for fabrication of thin films of these materials at nanoscale device sizes. Thermal Atomic Layer Etch (ALE) of metal oxides, in which up to one monolayer of the material can be removed, can be achieved by sequential self-limiting fluorination and ligand-exchange reactions at elevated temperatures. However, to date a detailed atomistic understanding of the mechanism of thermal ALE of these technologically important oxides is lacking.In this paper, we investigate the hydrogen fluoride pulse in the first step in the thermal ALE process of HfO 2 and ZrO 2 using first principles simulations.We introduce Natarajan-Elliott analysis, a thermodynamic methodology, to compare reaction models representing the self-limiting (SL) and continuous spontaneous etch (SE) processes taking place during an ALE pulse. Applying this method to the first HF pulse on HfO 2 and ZrO 2 we found that thermodynamic barriers impeding continuous etch are present at ALE relevant temperatures. We performed explicit HF adsorption calculations on the oxide surfaces to understand the mechanistic details of the HF pulse. A HF molecule adsorbs dissociatively on both oxides by forming metal-F and O-H bonds. HF coverages ranging from 1.0 ± 0.3 to 17.0 ± 0.3 HF/nm 2 are investigated and a mixture of molecularly and dissociatively adsorbed HF molecules is present at higher coverages. Theoretical etch rates of -0.61 ± 0.02 Å /cycle for HfO 2 and -0.57 ± 0.02 Å /cycle ZrO 2 were calculated using maximum coverages of 7.0 ± 0.3 and 6.5 ± 0.3 M-F bonds/nm 2 respectively (M = Hf, Zr).

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

“… 57 − 59 An account on the temperature dependence of the oxidation of W using O 2 and O 3 relevant to atomic layer etching has been discussed by George and co-workers. 13 From the N-E analysis 47 given in section S3 of the Supporting Information, comparing the continuous and self-limiting reaction models of the first ALE pulse, we predicted a minimum (thermodynamic) energy barrier of about 2.3 eV to cause bulk oxidation. We could expect that the predicted thermodynamic barrier to be breached below 800 °C as W oxide, formed on the surface, is reported to become volatile beyond that temperature.…”

“…Second, WCl 6 and Cl 2 were examined as potential etch reagents to generate volatile WO x Cl y etch species. Spin-polarized DFT coupled with density functional perturbation theory (DFPT) for a thermochemical analysis 47 were used to address self-limiting versus continuous etching pathways. At the same time, ground state spin polarized DFT was used to calculate the energies of surface species along several proposed reaction pathways.…”

Origin of Enhanced Thermal Atomic Layer Etching of Amorphous HfO2

“…Our previous study examined the difference in thermal ALE for the fluorination step for crystalline HfO2 and ZrO2 using HF. 44 In the present paper, the HF pulse in the first step in thermal ALE of amorphous HfO2 is examined in detail with first-principles DFT calculations of the fluorination mechanism. HF molecules adsorb at the surfaces of metal oxides by forming metal-F bonds and they may remain intact or dissociate.…”

“…45 If HF dissociates it may form Hf-F and O-H bonds, and release water, similar to previous studies on etch modelling for crystalline HfO2, ZrO2 and Al2O3. 44,45 The amount etched (etch rate) is determined by how much of the oxide surface is fluorinated; a larger fluoride film thickness after fluorination can lead to more fluoride removed during the ligand-exchange step and high etch rates. 41 The Natarajan-Elliott analysis 44 (N-E) is used to predict the conditions at which a self-limiting (SL) or spontaneous etching (SE) reaction becomes thermodynamically favourable and can therefore be used to direct experimental studies of thermal ALE.…”

“…44,45 The amount etched (etch rate) is determined by how much of the oxide surface is fluorinated; a larger fluoride film thickness after fluorination can lead to more fluoride removed during the ligand-exchange step and high etch rates. 41 The Natarajan-Elliott analysis 44 (N-E) is used to predict the conditions at which a self-limiting (SL) or spontaneous etching (SE) reaction becomes thermodynamically favourable and can therefore be used to direct experimental studies of thermal ALE. Selflimiting reactions are a necessary part of thermal ALE and allow the degree of etching to be well controlled and defined.…”