Area-Selective Atomic Layer Deposition, AS-ALD, can be initiated by local area-activation^1,2 or -deactivation.³ We studied the maskless growth of ZnO from diethyl zinc and water. The method is based on the different nucleation delays that arise from precursor adsorption on OH-terminated versus that on H-terminated Si-substrate.

TEOS (tetraethyl orthosilicate) and H₂O were used to deposit a SiO₂ seed layer by EBID (e-beam-induced deposition) in a pattern of 500x500 nm² dots on H-terminated a-Si:H substrate. After this local activation the substrate was coated with a ZnO film in a conventional ALD reactor using 80 cycles of diethyl zinc and water vapor. In situ spectroscopic ellipsometry (Fig. 1), SEM, cross-sectional TEM and energy dispersive X-ray spectroscopy (EDX) showed good areal selectivity with ZnO growth only occurring on the SiO₂ dots. This observation was corroborated by Density functional theory calculations suggesting a kinetically hindered surface reaction between diethyl zinc and H-terminated Si.

Atomic Layer Etching, ALE, was also tested on a ZnO case. Current ALE technology is emerging in both thermal, isotropic and plasma-based, anisotropic approaches.⁴ In this presentation we will demonstrate a plasma-assisted ALE-process driven by radicals, and therefore being isotropic.

The ALE-process was tested at temperatures between 150 and 250 °C. We used alternating doses of acetylacetone (Hacac) and O₂-plasma intermitted by Ar-purging. The ZnO-layer thickness as measured by spectroscopic ellipsometry decreased linearly with the number of cycles. In a synergy test, we proved that only the alternated dosing of Hacac and O₂-plasma caused ZnO etching, whereas Hacac and O₂-plasma alone did not (Fig. 2). Preliminary infrared studies suggest that Hacac forms volatile complexes by metal oxide surface chelation (e.g. Zn(acac)₂), whereas the O₂-plasma step removes non-reactive Hacac fragments to refresh the surface for the next etching cycle.⁵

TEM- and XPS-inspection indicated no damage of the ZnO surface, good preservation of the ZnO-stoichiometry throughout the etching process, and no distinct contamination. Furthermore, we demonstrated this ALE-process to be selective over SiO₂. We believe that this novel plasma-assisted ALE-concept can be extended to several other materials.

[1] A. J. M. Mackus et al., Nanoscale, 4, 4477 (2012).

[2] R. Chen et al., Adv. Mater., 18, 1086 (2006).

[3] A. Mameli et al., Chem. Mater. 29, 921 (2017).

[4] D. R. Zywotko, et al., Chem. Mater. 29, 1183 (2017).

[5] M. A. George, et al., J. Electrochem. Soc., 143, 3257 (1996).