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).

Experiments were made to define quantitatively the effects and interactions of the time of calcination, the temperature of calcination, and the particle size of a pure limestone upon the properties of the calcine that may affect the absorption of sulfur dioxide from flue gases. Response surfaces were derived for the degree of calcination, pore volume, density, crystallite size, and capacity to absorb sulfur dioxide as functions of the calcination conditions. It is predicted that wide variation in the degree of utilization will occur over relatively narrow ranges of retention time and injection temperature likely to be encountered in attempts to control sulfur dioxide emissions by injecting limestone in power plant furnaces.

HfO2 is a high- k material that is used in semiconductor devices. Atomic-level control of material processing is required for the fabrication of thin films of high- k materials at nanoscale device sizes. Thermal atomic layer etching (ALE) of metal oxides, in which up to one monolayer of material can be removed, can be achieved by sequential self-limiting fluorination and ligand-exchange reactions at elevated temperatures. First-principles-based atomic-level simulations using density functional theory can give deep insights into the precursor chemistry and the reactions that drive the etching of metal oxides. A previous study examined the hydrogen fluoride (HF) pulse in the first step in the thermal ALE process of crystalline HfO2 and ZrO2. This study examines the HF pulse on amorphous HfO2 using first-principles simulations. The Natarajan–Elliott analysis, a thermodynamic methodology, is used to compare reaction models representing the self-limiting and spontaneous etch processes taking place during an ALE pulse. For the HF pulse on amorphous HfO2, we found that thermodynamic barriers impeding spontaneous etching are present at ALE relevant temperatures. HF adsorption calculations on the amorphous oxide surface are studied to understand the mechanistic details of the HF pulse. An HF molecule adsorbs dissociatively by forming Hf–F and O–H bonds. HF coverages ranging from 1.1 ± 0.3 to 18.0 ± 0.3 HF/nm2 are investigated, and a mixture of molecularly and dissociatively adsorbed HF molecules is present at higher coverages. A theoretical etch rate of −0.82 ± 0.02 Å/cycle for amorphous HfO2 was calculated using a maximum coverage of 9.0 ± 0.3 Hf–F/nm2. This theoretical etch rate is greater than the theoretical etch rate for crystalline HfO2 that we previously calculated at −0.61 ± 0.02 Å/cycle. Undercoordinated atoms and void regions in amorphous HfO2 allow for more binding sites during fluorination, whereas crystalline HfO2 has a limited number of adsorption sites.

HfO2 and ZrO2 are two high-k materials that are crucial in semiconductor devices. Atomic level control of material processing is required for fabrication of thin films of these materials at nanoscale device sizes. Atomic layer deposition (ALD) and thermal atomic layer etching (ALE) allow fabrication of ultra-thin films for semiconductor device processing. ALD is a well-known metal oxide thin film deposition technique in which metal and oxygen containing precursors are added sequentially to the reactor to ensure self-limiting precursor adsorption and reaction to enable a high level of control over film thickness. Thermal ALE, which is a relatively modern technique, uses self-limiting fluorination (using hydrogen fluoride exposure) and subsequent ligand exchange reactions at elevated temperatures to remove up to a monolayer of the metal oxide material. This modern approach for controlled etching is the reverse of ALD and removes only the fluorinated layer. Given that it is difficult to investigate ALD and ALE reactions directly using experimental techniques, first-principles-based atomic-level simulations using density functional theory (DFT) can give deep insights into the precursor chemistry and the reactions that drive the deposition and etch of different materials. This contribution presents first principles density functional theory modelling to examine the growth and etch of thin films of HfO2 and ZrO2. Concerning the ALD reaction, the adsorption mechanism of precursors TDMAHf and TDMAZr are studied at the bare and hydroxylated surfaces of HfO2 and ZrO2 respectively. Stable OH coverages of 0.50 ML and 0.63 ML on the surfaces of HfO2 and ZrO2 were found respectively. Ligand loss from the metal precursor can be achieved by protonation from the OH terminated surface of both metal oxides. Concerning the ALE reaction, HF exposures on the surfaces of HfO2 and ZrO2 are studied. HF coverages ranging from 1.0 ± 0.3 to 17.0 ± 0.3 HF/nm2 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 HfO2 and -0.57 ± 0.02 Å /cycle ZrO2 were calculated using maximum coverages of 7.0 ± 0.3 and 6.5 ± 0.3 M-F bonds/nm2 respectively (M = Hf, Zr).

<div>HfO2 and ZrO2 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 HfO2 and ZrO2 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 HfO2 and ZrO2 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/nm2 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 HfO2 and -0.57 0.02 Å /cycle ZrO2 were calculated using maximum coverages of 7.0 0.3 and 6.5 0.3 M-F bonds/nm2 respectively (M = Hf, Zr).</div>