Solid state dewetting^[1] is a topic of current research. Besides targeted patterning, research focuses on the mechanisms and its prevention to avoid degradation or failure of e.g. microelectronic devices. While several studies have addressed solid state dewetting of bare metallic films the focus of this study is laid on Al thin films covered with a native surface oxide layer. In order to simplify the complexity of the film microstructure we grew thin Al films by molecular beam epitaxy on (0001) single crystalline sapphire (α-Al₂O₃) substrates.

The microstructure and epitaxial orientation relationships of the Al films were analysed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) methods including electron backscatter diffraction (EBSD), selected area electron diffraction, high resolution TEM (HRTEM) and atomic resolved scanning TEM (STEM). The as-deposited Al films form two orientation relationships (ORI and ORII) both containing two twin-related growth variants: {111} Al || (0001) α-Al₂O₃ with ±<1‾10> Al || <101‾0> α-Al₂O₃ (OR I) and {111} Al || (0001) α-Al₂O₃ with ±<21‾1‾> Al || <101‾0> α-Al₂O₃ (OR II). The “±” indicates the twin related variants which differ by a 180° rotation around the surface normal. Cs-corrected high angle annular dark field (HAADF) HRSTEM micrographs (Fig. 1) of cross-sectional as-deposited samples indicated strain at the twin boundaries of OR I. In addition, a translation in at the twin boundary by 0.91±0.13 Å (HRTEM^[5]: 0.84±0.17 Å) compared to an ideal, non-relaxed sigma three twin boundary was revealed.

After annealing for 1 to 45 hours below the melting point of aluminum (660°C) at 600°C, instead of Al islands^[3] dark appearing features are observed in plan-view SEM micrographs (Fig. 3). This has been observed in literature for different model systems (Ni films on Al₂O₃^[2] and Al films on Al₂O₃^[4]). Two different models^[1][2][4] exist to explain their formation. The capillary energy driven retraction of thin films can be described by classical solid state dewetting and would lead to holes of bare substrate surrounded by a rim slightly higher than the original film thickness.^[1][2][3] In contrast, in the second model only volume and grain boundary diffusion can take place due to the formation of a thin oxide layer on top of the Al film. Film retraction below the oxide layer would result in drum-like voids.^[4] Site-specific cross-sections prepared by focused ion beam and investigation by SEM, TEM and EDS revealed the presence of voids in the Al film with a thin cover layer (Fig. 2). Electron energy loss spectroscopy (EELS) of the surface layer revealed a phase transformation from amorphous alumina (as-deposited state) to γ-Al₂O₃ (after annealing) as proposed in literature from glancing incidence X-ray diffraction measurements.^[4] Although solid state dewetting was done at 600°C, facetted single crystalline sapphire ridges form at the Al/sapphire/void triple phase boundary as a consequence of the capillary energy force component acting perpendicular to the film/substrate interface. The thickness of the Al film increases locally in the region of the sapphire ridge compared the original film thickness (Fig. 2). The drum-like features possess distinct facets and reflect the hexagonal symmetry of the basal plane of the sapphire substrate (Fig. 3). The EBSD investigations indicate that the grain boundaries act as initial points of void formation.

References

[1] C. V. Thompson, Annu. Rev. Mater. Res., 42, 399-434 (2012).

[2] E. Rabkin et al., Acta Mater., 74, 30-38 (2014).

[3] W. Kaplan et al., J. Mater. Sci., 48, 5681-5717 (2013).

[4] S. Dutta et al., J. Am. Ceram. Soc., 95, 823-830 (2012).

[5] G. Dehm et al., Acta Mater., 50, 5021-5032 (2002).

Figures:

Fig. 1 Cs-corrected HAADF HRSTEM micrograph of a Σ3 {21 ̅1 ̅} Al twin boundary at the Al/sapphire interface

Fig. 2 TEM BF micrograph showing an Al/Al2O3/void triple phase boundary of a cross section of an annealed Al thin film (top). EDS maps shows the O and Al distribution of such a triple phase boundary

Fig. 3 Exemplary in-plane EBSD orientation map with image quality map overlay of an annealed Al thin film (left) showing OR I (purple) with twin boundaries (white) next to faceted voids (bright blue) and OR II grains (green), the out-of-plane grain orientation is indicated by the overlaying cubes. Plan-view SEM micrograph (right) containing micron-size voids within an Al thin film next to a void with disrupted surface layer

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