LAUSR

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.