@article{saare_xie_parsons_2023, title={Comparison of BCl3, TiCl4, and SOCl2 chlorinating agents for atomic layer etching of TiO2 and ZrO2 using tungsten hexafluoride}, volume={41}, ISSN={["1520-8559"]}, url={https://doi.org/10.1116/6.0002708}, DOI={10.1116/6.0002708}, abstractNote={Recent advances in the semiconductor industry have created an exigency for processes that allow to deposit and etch material in conformal matter in three-dimensional devices. While conformal deposition is achieved using atomic layer deposition (ALD), conformal etching can be accomplished by thermal atomic layer etching (ALE) which, like ALD, proceeds via a binary sequence of self-limiting reactions. This study explores ALE of TiO2 and ZrO2 using WF6 as a fluorinating agent, and BCl3, TiCl4, or SOCl2 as a co-reactant. The effect of co-reactant chemistry was studied using atomic force microscopy, in situ ellipsometry, and in vacuo Auger electron spectroscopy measurements along with thermodynamic modeling. All three co-reactants exhibited saturation and etch rates increasing with temperature. At 170 °C, TiO2 can be etched using WF6 with BCl3, TiCl4, or SOCl2, and the etching proceeds at 0.24, 0.18, and 0.20 nm/cycle, respectively. At 325 °C, ZrO2 ALE can occur using these same reactants, proceeding at 0.96, 0.74, and 0.13 nm/cycle, respectively. A higher temperature is needed for ZrO2 ALE versus TiO2 because the ZrCl4 product is less volatile than the corresponding TiCl4. During ZrO2 and TiO2 etching using BCl3 or TiCl4, boron oxide or titanium oxide intermediate layers, respectively, were formed on the surface, and they were subsequently removed by WF6. In contrast, for ALE of TiO2 using SOCl2, a similar intermediate layer is not observed. This study broadens the understanding of co-etchants role during thermal ALE and expands the range of reactants that can be used for vapor etching of metal oxides.}, number={4}, journal={JOURNAL OF VACUUM SCIENCE & TECHNOLOGY A}, author={Saare, Holger and Xie, Wenyi and Parsons, Gregory N.}, year={2023}, month={Jul} }
 @article{saare_dianat_parsons_2022, title={Comparative In Situ Study of the Initial Growth Trends of Atomic Layer-Deposited Al(2)O(3 )Films br}, volume={126}, ISSN={["1932-7455"]}, url={https://doi.org/10.1021/acs.jpcc.2c01033}, DOI={10.1021/acs.jpcc.2c01033}, abstractNote={In this work, we compare the initial growth trends of atomic layer-deposited aluminum oxide (Al2O3) using three different Al precursors and H2O as the oxygen source on hydroxyl-terminated silicon (Si-OH) and hydrogen-terminated silicon (Si-H) surfaces. Trimethylaluminum (TMA), triethylaluminum (TEA), and dimethylaluminum chloride (DMAC) are chosen as the Al precursors due to comparable variations between their structures. Thickness evolution obtained from in situ ellipsometry exhibits similar behavior for all three precursors with initially accelerated growth during the first cycle on the Si-OH starting surface, which then proceeds in a steady manner characteristic of atomic layer deposition (ALD). In situ Fourier transform infrared spectroscopy (FTIR) shows that at 200 °C both TEA and TMA react with above 85% of −OH ligands present on the initial Si-OH substrate and the subsequent H2O dose reacts with only ∼50% of the surface C–H groups, indicating incomplete removal of the methyl or ethyl ligands on the surface. Al2O3 growth on the Si-H surface exhibits a delay due to the lack of surface hydroxyl groups, leading to formation of Si-Me or Si-Et groups. A lower reactivity of DMAC compared to TMA and TEA results in a lower initial selectivity fraction. The results provide vital insight into the importance of precursor selection for area-selective ALD applications and open a pathway for realizing selective Al2O3 deposition based on inherent substrate selectivity.}, number={16}, journal={JOURNAL OF PHYSICAL CHEMISTRY C}, publisher={American Chemical Society (ACS)}, author={Saare, Holger and Dianat, Golnaz and Parsons, Gregory N.}, year={2022}, month={Apr}, pages={7036–7046} }
 @article{saare_song_kim_parsons_2020, title={Effect of reactant dosing on selectivity during area-selective deposition of TiO2 via integrated atomic layer deposition and atomic layer etching}, volume={128}, ISSN={["1089-7550"]}, url={https://doi.org/10.1063/5.0013552}, DOI={10.1063/5.0013552}, abstractNote={A key hallmark of atomic layer deposition (ALD) is that it proceeds via self-limiting reactions. For a good ALD process, long reactant exposure times beyond that required for saturation on planar substrates can be useful, for example, to achieve conformal growth on high aspect ratio nanoscale trenches, while maintaining consistent deposition across large-area surfaces. Area-selective deposition (ASD) is becoming an enabling process for nanoscale pattern modification on advanced nanoelectronic devices. Herein, we demonstrate that during area-selective ALD, achieved by direct coupling of ALD and thermal atomic layer etching (ALE), excess reactant exposure can have a substantially detrimental influence on the extent of selectivity. As an example system, we study ASD of TiO2 on hydroxylated SiO2 (Si–OH) vs hydrogen-terminated (100) Si (Si–H) using TiCl4/H2O for ALD and WF6/BCl3 for ALE. Using in situ spectroscopic ellipsometry and ex situ x-ray photoelectron spectroscopy, we show that unwanted nucleation can be minimized by limiting the water exposure during the ALD steps. Longer exposures markedly increased the rate of nucleation and growth on the desired non-growth region, thereby degrading selectivity. Specifically, transmission electron microscopy analysis demonstrated that near-saturated H2O doses enabled 32.7 nm thick TiO2 patterns at selectivity threshold S > 0.9 on patterned Si/SiO2 substrates. The correlation between selectivity and reactant exposure serves to increase fundamental insights into the effects of sub-saturated self-limiting surface reactions on the quality and effectiveness of ASD processes and methods.}, number={10}, journal={JOURNAL OF APPLIED PHYSICS}, author={Saare, Holger and Song, Seung Keun and Kim, Jung-Sik and Parsons, Gregory N.}, year={2020}, month={Sep} }
 @article{song_saare_parsons_2019, title={Integrated Isothermal Atomic Layer Deposition/Atomic Layer Etching Supercycles for Area-Selective Deposition of TiO2}, volume={31}, ISSN={["1520-5002"]}, url={https://doi.org/10.1021/acs.chemmater.9b01143}, DOI={10.1021/acs.chemmater.9b01143}, abstractNote={New approaches for area-selective deposition (ASD) are becoming critical for advanced semiconductor patterning. Atomic layer deposition (ALD) and atomic layer etching (ALE), that is, “inverse ALD”, are considered important for ASD, but to date, direct integration of ALD and ALE for ASD has not been reported. This work demonstrates that self-limiting thermally driven ALE, using WF6 and BCl3, can be directly coupled with self-limiting thermal ALD, using TiCl4 and H2O, in a single isothermal reactor at temperature 0.9, nearly a 10× improvement over previous reports of inherent TiO2 ASD. After ALD/ALE (=30/5) 14 supercycles at 170 °C, X-ray photoelectron spectroscopy da...}, number={13}, journal={CHEMISTRY OF MATERIALS}, publisher={American Chemical Society (ACS)}, author={Song, Seung Keun and Saare, Holger and Parsons, Gregory N.}, year={2019}, month={Jul}, pages={4793–4804} }