The thermal atomic layer etching (ALE) of SiO was performed using sequential reactions of trimethylaluminum (TMA) and hydrogen fluoride (HF) at 300 °C. Ex situ X-ray reflectivity (XRR) measurements revealed that the etch rate during SiO ALE was dependent on reactant pressure. SiO etch rates of 0.027, 0.15, 0.20, and 0.31 Å/cycle were observed at static reactant pressures of 0.1, 0.5, 1.0, and 4.0 Torr, respectively. Ex situ spectroscopic ellipsometry (SE) measurements were in agreement with these etch rates versus reactant pressure. In situ Fourier transform infrared (FTIR) spectroscopy investigations also observed SiO etching that was dependent on the static reactant pressures. The FTIR studies showed that the TMA and HF reactions displayed self-limiting behavior at the various reactant pressures. In addition, the FTIR spectra revealed that an AlO/aluminosilicate intermediate was present after the TMA exposures. The AlO/aluminosilicate intermediate is consistent with a "conversion-etch" mechanism where SiO is converted by TMA to AlO, aluminosilicates, and reduced silicon species following a family of reactions represented by 3SiO + 4Al(CH) → 2AlO + 3Si(CH). Ex situ X-ray photoelectron spectroscopy (XPS) studies confirmed the reduction of silicon species after TMA exposures. Following the conversion reactions, HF can fluorinate the AlO and aluminosilicates to species such as AlF and SiOF. Subsequently, TMA can remove the AlF and SiOF species by ligand-exchange transmetalation reactions and then convert additional SiO to AlO. The pressure-dependent conversion reaction of SiO to AlO and aluminosilicates by TMA is critical for thermal SiO ALE. The "conversion-etch" mechanism may also provide pathways for additional materials to be etched using thermal ALE.

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.

Thermal atomic layer etching (ALE) is an important technique for the precise isotropic etching of nanostructures. Thermal ALE of many materials can be achieved using a two-step fluorination and ligand-exchange reaction mechanism. Most previous thermal ALE processes have used HF as the fluorination reactant. Alternative fluorination reactants may be needed because HF is a weak nucleophilic fluorination reactant. Stronger fluorination agents may be required for the fluorination of some materials. To demonstrate the usefulness of SF 4 as an alternative to HF, thermal Al 2 O 3 ALE was compared using SF 4 or HF together with Sn(acac) 2 as the metal precursor for ligand exchange. SF 4 and HF were observed to behave similarly as fluorination reactants during Al 2 O 3 ALE. The mass gains during the initial SF 4 and HF exposures on Al 2 O 3 atomic layer deposition (ALD) films at 200 °C were comparable at 35 and 38 ng/cm 2 , respectively, using quartz crystal microbalance measurements. In addition, the etch rates were similar at 0.20 and 0.28 Å/cycle for Al 2 O 3 ALE using SF 4 and HF, respectively, at 200 °C. Thermal VO 2 ALE was also performed for the first time using SF 4 or HF and Sn(acac) 2 as the reactants. There was evidence that SF 4 is a stronger fluorination reactant than HF for VO 2 fluorination. The mass gains during the initial SF 4 and HF exposures on VO 2 ALD films were 38 and 20 ng/cm 2 , respectively, at 200 °C. Thermal VO 2 ALE also had a higher etch rate when fluorinating with SF 4 compared with HF. Etch rates of 0.30 and 0.11 Å/cycle were measured for VO 2 ALE using SF 4 and HF, respectively, together with Sn(acac) 2 at 200 °C. Fourier transform infrared experiments were also used to monitor fluorination of the Al 2 O 3 and VO 2 ALD films by SF 4 or HF. FTIR difference spectroscopy was used to observe the increase of Al−F and V−F stretching vibrations and the loss of the Al−O and VO/VO stretching vibrations for Al 2 O 3 and VO 2 , respectively, versus SF 4 or HF exposure at 200 °C. Additional absorbance features after fluorination of the Al 2 O 3 ALD films by SF 4 were consistent with SF x surface species. SF 4 is a useful fluorination agent for thermal ALE processes and can be used as an alternative to HF. In addition, SF 4 may be necessary when fluorination requires a stronger fluorination reactant than HF.

Atomic layer processing such as atomic layer deposition (ALD) and thermal atomic layer etching (ALE) is usually described in terms of sequential, self-limiting surface reactions. This picture for ALD and thermal ALE leaves out the possibility that the metal precursor in ALD and thermal ALE can also convert the surface material to another new material. This perspective introduces the previous evidence for conversion reactions in atomic layer processing based on a variety of studies, including Al2O3 ALD on ZnO, growth of Zn(O,S) alloys, “self-cleaning” of III-V semiconductor surfaces, and thermal ALE of ZnO and SiO2. The paper then focuses on the reaction of Al(CH3)3 [trimethylaluminum (TMA)] on ZnO as a model conversion system. A variety of techniques are utilized to monitor ZnO conversion to Al2O3 using TMA at 150 °C. These techniques include FTIR spectroscopy, quadrupole mass spectrometry (QMS), x-ray reflectivity (XRR), gravimetric analysis, x-ray photoelectron spectroscopy (XPS), and quartz crystal microbalance (QCM) measurements. The various studies focus on ZnO conversion to Al2O3 for both hydroxyl-terminated and ethyl-terminated ZnO substrates. FTIR studies observed the conversion of ZnO to Al2O3 and provided evidence that the conversion is self-limiting at higher TMA exposures. QMS studies identified the volatile reaction products during the TMA reaction with ZnO as CH4, C2H4, C2H6, and Zn(CH3)2. The CH4 reaction product preceded the appearance of the Zn(CH3)2 reaction product. XRR investigations determined that the thickness of the Al2O3 conversion layer on ZnO limits at ∼1.0 nm at 150 °C after larger TMA exposures. A gravimetric analysis of the conversion reaction on ZnO nanoparticles with a diameter of 10 nm displayed a percent mass loss of ∼49%. This mass loss is consistent with an Al2O3 shell of ∼1 nm on a ZnO core with a diameter of ∼6 nm. XPS studies revealed that ZnO ALD films with a thickness of 2 nm were almost completely converted to Al2O3 by large TMA exposures at 150 °C. QCM investigations then measured the mass changes for lower TMA exposures on hydroxyl-terminated and ethyl-terminated ZnO films. More mass loss was observed on ethyl-terminated ZnO films compared with hydroxyl-terminated films, because TMA does not have the possibility of reacting with hydroxyl groups on ethyl-terminated ZnO films. The mass losses also increased progressively with temperatures ranging from 100 to 225 °C on both hydroxyl-terminated and ethyl-terminated ZnO films. The perspective concludes with a discussion of the generality of conversion reactions in atomic layer processing.

The spontaneous etching of boron oxide (B 2 O 3 ) by hydrogen fluoride (HF) gas is important during thermal atomic layer etching after BCl 3 converts the surface of various metal oxides to a B 2 O 3 layer. In this study, the chemical vapor etching (CVE) of B 2 O 3 by HF was experimentally monitored using Fourier transform infrared (FTIR) spectroscopy and quadrupole mass spectrometry (QMS). The spontaneous etching of B 2 O 3 by HF gas was also analyzed using density functional theory (DFT). B 2 O 3 films were grown using B 2 O 3 atomic layer deposition with BCl 3 and H 2 O as the reactants at 40 °C. FTIR spectroscopy then observed the CVE of B 2 O 3 by HF at 150 °C. B 2 O 3 etching was monitored by the loss of absorbance for B-O stretching vibration in B 2 O 3 films. FTIR spectroscopy studies also observed B-F stretching vibrations from BF x species on the B 2 O 3 surface after HF exposures. In addition, the QMS analysis was able to identify the etch products during the spontaneous etching of B 2 O 3 by HF gas at 150 °C. The QMS studies observed the main volatile etch products as BF 3 , BF 2 (OH), and H 2 O. Additional volatile etch products were also detected including B 3 O 3 F 3 and other boroxine ring compounds. The DFT predictions were consistent with the spontaneous etching of B 2 O 3 by HF gas. DFT confirmed that CVE was likely because the energetics of the spontaneous etching reaction B 2 O 3 (s) + 6HF(g) → 2BF 3 (g) + 3H 2 O(g) were more favorable than the self-limiting reaction B 2 O 3 (s) + 6HF(g) → 2BF 3 (s) + 3H 2 O(g). The spontaneous etching of B 2 O 3 was predicted at temperatures above −163 °C for an HF reactant pressure of 0.2 Torr and BF 3 and H 2 O product pressure of 0.01 Torr.