Aluminium Thin Films: How Grain Growth Affects Reflectivity

Aluminium thin films deposited by magnetron sputtering are generally expected to develop a columnar or textured grain structure as they thicken, under deposition conditions like these. 

A 2024 study from the University of Patras and Aristotle University of Thessaloniki, in Greece, found that this didn't happen: the aluminium films they sputtered grew with randomly oriented grains and no measurable texture, through a mechanism the researchers link to low atomic mobility and diffusion along grain boundaries rather than across the film's surface. 

The same films — particularly the two thickest — also showed low surface roughness and reflected more than 90% of visible light, a separate finding the researchers connect to that roughness rather than to the grain orientation itself.

Science made simple

Picture cars parking along a kerb, one after another. If each driver has time to pull forward, reverse and straighten up before the next car needs the space, the row ends up neat and aligned. If each driver just stops wherever the car first comes to rest, with no time to straighten up, you get cars sitting at all sorts of angles instead — all parked, just not aligned.

Something similar happens when aluminium atoms land on a surface during sputtering. In this study, the atoms stuck close to where they landed almost immediately, without settling into line with their neighbours first. That's why the film grew as a jumble of small, randomly oriented grains rather than the aligned, columnar structure often seen in sputtered metal films. It also had a side benefit: the resulting surface was smoother than expected, which is part of why the film reflected light so well.

Why grain structure matters in aluminium thin films

Grain structure shapes a thin film's surface texture and its electrical and optical behaviour. In aluminium coatings, grain size, shape and orientation influence roughness, reflectance and resistivity — which is why controlling how grains form during deposition matters for predictable, repeatable performance.

How magnetron sputtering deposits aluminium films

Magnetron sputtering is a physical vapour deposition technique used to build thin metal films for optics, microelectronics and telecommunications. Inside a vacuum chamber, argon gas is ionised into a plasma, and a magnetic field behind the target concentrates that plasma close to its surface. 

In radio-frequency sputtering, the target develops a negative self-bias relative to the plasma, accelerating argon ions towards it; each impact knocks aluminium atoms loose through momentum transfer, and those atoms cross the chamber and condense onto a substrate, building the film atom by atom. The Patras/Thessaloniki team sputtered from an aluminium target machined in their own laboratory from commercially pure (1000-series) aluminium, onto oxide-coated silicon wafers at room temperature under a low argon pressure of around 0.3 pascals.

What the researchers found

The team deposited six aluminium films, from 25 to 280 nanometres thick, and examined them using atomic force microscopy, transmission electron microscopy, X-ray diffraction and reflectance spectroscopy (400–1100 nanometres). Grain sizes ranged from around 7.5 to 48 nanometres, averaging 22 (± 12) nanometres, with no columnar or textured structure at any thickness measured. No “hillocks” — small surface bumps that typically signal film defects, substrate mismatch or gas contamination — were observed; combined with the X-ray and electron-microscopy results, the researchers concluded the films were free of measurable impurities. The two thickest films (197 and 280 nanometres) reflected more than 90% of visible light, higher than comparable sputtered or evaporated aluminium films reported elsewhere.

That didn't match what established thin-film growth models predict for sputtering conditions like these, which anticipate a fibrous, columnar microstructure. The researchers' explanation: aluminium atoms had very low mobility once they landed on the amorphous oxide surface at room temperature, settling into fixed positions rather than moving far enough to align into larger structures. As the film thickened, growth proceeded mainly through diffusion along the boundaries between neighbouring grains rather than across the surface — an interpretation supported by how closely the measured relationship between grain size and film thickness matched the pattern expected for that mechanism.

Why the films reflected so much light

This is a separate finding from the grain-orientation result. The researchers link the high reflectance specifically to surface roughness, not to grain orientation itself: smoother films reflect light more efficiently, and the roughness measured here was lower than comparable sputtered or evaporated aluminium coatings elsewhere. 
The same low-mobility growth that produced the randomly oriented grains is a plausible contributor to that smoothness, but the paper treats the two as related observations from the same growth conditions, not one directly causing the other.

What this means for aluminium thin-film research

Aluminium thin films of this kind are used across optics, microelectronics and telecommunications, wherever a reflective, low-resistivity, well-adhered metal coating is needed on silicon, glass or other substrates. This study is a reminder that conditions at the earliest stages of growth can significantly affect a coating's microstructure, even when the outcome runs against what established models predict.

Source: Karoutsos, V., Florini, N., Diamantopoulos, N.C., Balourda, C., Dimitrakopulos, G.P., Bouropoulos, N., Poulopoulos, P. (2024). On the Effect of Randomly Oriented Grain Growth on the Structure of Aluminum Thin Films Deposited via Magnetron Sputtering. Coatings, 14(11), 1441. 10.3390/coatings14111441