Seeing It Happen—Insights Into the Surface Chemistry of HfO₂ and TiO₂ ALD from Operando Ambient Pressure X-Ray Photoelectron Spectroscopy

The development of ALD processes is based on a number of different considerations and factors. One consideration is the envisaged ALD surface chemistry, which has to match not only the desired process outcome and processing conditions, but also the reaction properties of both the precursor and the surface. For many precursors, their surface chemistry is assumed to follow general reaction schemes. For example, the thermal ALD of transition metal oxides from amido complex and water precursors is typically assumed to follow a ligand exchange mechanism. The wide spread of such general reaction schemes results from that they often provide a sufficiently successful prediction of the ALD process outcome, but also because experimental tools are lacking that allow direct insight into reaction mechanisms. Indeed, it has been noted that surface chemistries can be both more complex and varied than general reaction schemes make believe [1,2].<br/>Methods that allow the time-resolved monitoring of ALD processes, such as quartz crystal microbalance measurements, quadrupole mass spectrometry, pyroelectric calorimetry and ellipsometry can provide deepened insight into ALD surface reaction mechanisms. Recently, these methods have been joined by two chemically sensitive techniques for the time-resolved characterisation of ALD processes, namely infrared spectroscopy [3] and ambient pressure x-ray photoelectron spectroscopy (APXPS) (cf., e.g., [4,5]). These two methods are capable of following the ALD surface chemistry in real time and at processing pressures equal or similar to those in an ALD reactor.<br/>Using the metal amido complex- and water-based ALD of HfO₂ and TiO₂ on different surfaces as examples, I would like to demonstrate the usefulness of time-resolved APXPS for the elucidation of surface species and their evolution as well as for the observation of substrate processes such as oxygen transport. Such information allows to formulate ALD reaction mechanisms. Thus, we observe reaction pathways that deviate from the standard models of ALD surface chemistry, including, in particular, bimolecular reaction pathways that are feasible also on non-reactive surfaces. But also on partially hydroxylated surfaces non-standard reactions occur, which draws attention to the fact that full surface hydroxylation cannot always be achieved. Further, for reducible surfaces we find that oxygen ion transport may play a major role in the initial ALD.<br/>I will also demonstrate how the time resolution in <i>operando</i> APXPS experiments during steady-state ALD can be improved so that surface chemistry monitoring under conditions that resemble those in standard ALD reactors becomes feasible. Altogether, APXPS provides us with entirely new information on ALD reaction mechanisms during both the initial phases of ALD as well as steady-state ALD, which is important input for the future optimisation of ALD processes.<br/><br/>F. Zaera, Coord. Chem. Rev. <b>257</b>, 3177 (2013)<br/>N. E. Richey et al., J. Chem. Phys. <b>152</b>, 1 (2020)<br/>B. A. Sperling et al.,J. Vac. Sci. Technol. A <b>32</b>, 031513 (2014)<br/>R. Timm et al., Nat. Commun. <b>9</b>, 1412 (2018)<br/>G. D’Acunto et al., Chem. Mater. <b>35</b>, 529 (2023)