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.
Introduction: Silver, prized throughout history for its luster and shine, develops a black Ag 2 S tarnish layer that is aesthetically displeasing when exposed to atmospheric pollutants. Tarnishing, and subsequent polishing, leads to irreversible material loss and object damage. Currently, nitrocellulose coatings are often used to prevent silver from tarnishing, however non-uniform coatings and degradation over time limit their effectiveness. Atomic layer deposition (ALD) has been explored as a new method for creating dense, uniform, and conformal coatings on 3-dimensional (3D) objects that are more effective than nitrocellulose in preventing silver from tarnishing. Results:To create high quality ALD coatings on 3D objects, slowing down the ALD process is critical to ensure proper precursor exposure. Non-ideal deposition of organo-oxy-metallic compounds can occur with fast deposition rates that do not allow sufficient flow around 3D objects. The coatings can be removed by dissolving the Al 2 O 3 ALD films in aqueous NaOH. Thicker ALD films prevent defects from occurring on non-ideal surfaces and effectively prevent silver objects from tarnishing under ambient aging conditions. Conclusion:Thick ALD films, deposited with sufficiently long precursor pulse and purge times, may be effective in preventing complex, 3D non-mixed media silver cultural heritage objects from tarnishing.
In this article we report on the characterization of atomic layer deposited (ALD) films on silver alloy objects, both with regard to film porosity, which potentially limits their effectiveness as tarnish barriers, and with regard to color change upon deposition, which affects their visual appearance. We find that the porosity of ALD alumina films decreases with thickness, and shows no clear dependence on surface preparation, nor on multiple ALD oxide layering. We also find that the optimized structures for minimizing color change are sensitive to the composition of the alloy, and must be tuned accordingly.
Although rare among Hopewell horizon artifacts, meteoritic metal represents the most exotic raw material used during the Middle Woodland period in Eastern North America. We demonstrate that Hopewell meteoritic beads recovered from Havana, Illinois can be linked to the Anoka, Minnesota, iron, which fell as a shower of irons across the Mississippi River. The similarity in major, minor and trace element chemistry between Anoka and Havana, the presence of micrometer-sized inclusions of gamma iron in kamacite in both, and the obvious connection via the Mississippi and Illinois Rivers between Anoka and Havana point to the production of the Havana beads from a mass of the Anoka iron. Experiments strongly support the manufacture of the beads via fragmentation of schreibersite inclusions to liberate small pieces of metal. Repeated cycles of heating to temperatures of 600-700C followed by cold-working produced flattened metal sheets. These sheets were subsequently rolled to make the Havana beads. Recovery of the iron mass of Anoka that was used to make the beads likely occurred by local populations who were part of the Trempeleau Hopewell center, with exchange bringing it to the Havana Hopewell center, where the beads were manufactured.
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