Effect of HF Pressure on Thermal Al₂O₃Atomic Layer Etch Rates and Al₂O₃Fluorination

Abstract

Thermal Al2O3 atomic layer etching (ALE) can be accomplished using sequential fluorination and ligand-exchange reactions. HF can be employed as the fluorination reactant, and Al­(CH3)3 can be utilized as the metal precursor for ligand exchange. This study explored the effect of HF pressure on the Al2O3 etch rates and Al2O3 fluorination. Different HF pressures ranging from 0.07 to 9.0 Torr were employed for Al2O3 fluorination. Using ex situ spectroscopic ellipsometry (SE) measurements, the Al2O3 etch rates increased with HF pressures and then leveled out at the highest HF pressures. Al2O3 etch rates of 0.6, 1.6, 2.0, 2.4, and 2.5 Å/cycle were obtained at 300 °C for HF pressures of 0.17, 0.5, 1.0, 5.0, and 8.0 Torr, respectively. The thicknesses of the corresponding fluoride layers were also measured using X-ray photoelectron spectroscopy (XPS). Assuming an Al2OF4 layer on the Al2O3 surface, the fluoride thicknesses increased with HF pressures and reached saturation values at the highest HF pressures. Fluoride thicknesses of 2.0, 3.5, 5.2, and 5.5 Å were obtained for HF pressures of 0.15, 1.0, 4.0, and 8.0 Torr, respectively. There was an excellent correlation between the Al2O3 etch rates and fluoride layer thicknesses versus HF pressure. In addition, in situ Fourier transform infrared spectroscopy (FTIR) vibrational studies were used to characterize the time dependence and magnitude of the Al2O3 fluorination. These FTIR studies observed the fluorination of Al2O3 to AlF3 or AlO x F y by monitoring the infrared absorbance from the Al–O and Al–F stretching vibrations. The time dependence of the Al2O3 fluorination was explained in terms of rapid fluorination of the Al2O3 surface for initial HF exposures and slower fluorination into the Al2O3 near surface region that levels off at longer HF exposure times. Fluorination into the Al2O3 near surface region was described by parabolic law behavior. The self-limiting fluorination of Al2O3 suggests that the fluoride layer on the Al2O3 surface acts as a diffusion barrier to slow the fluorination of the underlying Al2O3 bulk. For equal fluorination times, higher HF pressures achieve larger fluoride thicknesses.

Effect of Annealing Temperature for Ni/AlOx/Pt RRAM Devices Fabricated with Solution-Based Dielectric

“…Among these materials, AlO x has been widely applied in gate insulator layers [15,16,17,18] and has attracted extensive attention in the RRAM field owing to its wide band gap (~8.9 eV), high thermal stability with Si and Pt, high dielectric constant (~8) and large breakdown electric field [10,14,19,20,21,22] as Kim et al has reported [19,20,23,24,25,26]. In addition, the superior elasticity [27] and high toughness [28] make it possible for AlO x to be applied under various conditions including vibration and pressure environments [29,30,31]. Cano et al reported that AlO x -based dielectric layer showed superior stability under environments with hydrofluoric acid pressure [29] and Choi et al reported large-scale flexible electronics application with AlO x thin film [31], which have demonstrated that the AlO x thin film has great potential as a metal oxide layer in RRAM devices.…”

“…In addition, the superior elasticity [27] and high toughness [28] make it possible for AlO x to be applied under various conditions including vibration and pressure environments [29,30,31]. Cano et al reported that AlO x -based dielectric layer showed superior stability under environments with hydrofluoric acid pressure [29] and Choi et al reported large-scale flexible electronics application with AlO x thin film [31], which have demonstrated that the AlO x thin film has great potential as a metal oxide layer in RRAM devices.…”

Isotropic plasma atomic layer etching of Al2O3 using a fluorine containing plasma and Al(CH3)3

“…Thermal ALE processes often include fluorination, chlorination, or oxidation reactions, which readily proceed at elevated substrate temperatures without the need for a plasma. 10,11,[14][15][16][17][18][19][20][21] As an emerging technique, the research on isotropic thermal ALE has focused on finding viable precursors and co-reactants to achieve etching of a variety of materials. A selection of these isotropic ALE processes can be seen in Table I.…”

“…21,25 ALE of Al 2 O 3 has previously been achieved by using fluorinating reactants, such as hydrogen fluoride (HF) derived from HF pyridine or sulphur tetrafluoride (SF 4 ), combined with tri-methyl aluminium [TMA, Al(CH 3 ) 3 ], dimethyl aluminium chloride [DMAC, AlCl(CH 3 ) 2 ], or tin acetylacetone [Sn(acac) 2 ] as an etchant. [14][15][16][17][18][19][20] One of the most documented approaches for ALE of Al 2 O 3 is the use of HF and TMA. 10,[14][15][16][17][18][19] Half-cycle A of this thermal ALE process consists of the formation of an AlF 3 /AlO x F y surface layer by exposure to HF gas.…”

“…[14][15][16][17][18][19][20] One of the most documented approaches for ALE of Al 2 O 3 is the use of HF and TMA. 10,[14][15][16][17][18][19] Half-cycle A of this thermal ALE process consists of the formation of an AlF 3 /AlO x F y surface layer by exposure to HF gas. This modified layer can then be removed by dosing TMA in half-cycle B, which produces volatile Al-containing species [namely AlF(CH 3 ) 2 ] via a transmetalation ligand exchange reaction.…”