Precise real-time study of gold cluster growth during sputter deposition

From gold atoms to gold layers

Sketch of the gold-sputtered surfaces.

Sketch of the gold-sputtered surfaces at four different growth regimes with the corresponding μGISAXS pattern (Credit: Authors).

Scientists from DESY and their collaborators from TU München presented the first precise real-time and in situ investigation of the growth kinetics of gold nanostructures from small nuclei to a complete gold layer during magnetron sputter deposition. The results were published in RSC Nanoscale

The remarkable manifold chemistry of gold cluster assemblies is of great interest for fundamental research and promises various potential applications in solar cells, biosensors, reflective or antireflective coatings and especially in heterogeneous catalysis. The adjustment of catalytic, electrical and optical properties of gold cluster assemblies is therefore a very significant topic in modern applied nanotechnology. For an efficient and controlled production of such active nanostructured gold cluster surfaces, the sputter deposition process plays an important role. The tuning of the size-dependent catalytic activity and optoelectronic properties is crucial for industrial and nanostructural processing, however it is mandatory to control but also to understand how the growth kinetics influences the metal film morphology during sputter deposition.

The in situ grazing incidence small-angle X-ray scattering (µGISAXS) measurements were performed at the PETRA III beamline P03. High frame-rate 2D X-ray detectors in combination with the high photon flux of micro beam spot size, available at the MiNaXS beamline, enables a non-invasive in situ and real-time investigation of gold growth kinetics during sputter deposition. With an acquisition throughput of 66.67 frames per second (fps), the scientists were able to produce kinetic growth movies of the fast surface processes in reciprocal space and on the nanoscale. The high time resolution in the millisecond regime, allows the precise determination of kinetics of initial nucleation and subsequent cluster growth during sputter deposition of gold on an amorphous silicon oxide layer. By using X-ray reflectivity (XRR), electron microscopy and µGISAXS, an accurate investigation of the gold cluster growth kinetics was possible under conditions, which can be further developed towards industrial manufacturing.

The research team was able to identify the four different stages of growth including their thresholds with sub-monolayer resolution and to identify phase transitions. Moreover they introduced a new and flexible geometrical model to extract morphological real space parameters, such as cluster size and shape, correlation distance, layer porosity and surface coverage, directly from the reciprocal space scattering data. This approach is a prerequisite for future investigations of the influence of different process parameters on the thin metal film morphology such as surface temperature, deposition rates, catalysis, and optical response. In particular the knowledge of the wetting behavior and contact angle in the nanoscale regime is important for sputter deposition on functional, polymeric and functionalized surfaces.

The presented real-time study provides a better understanding of the cluster growth kinetics in general, which is essential for the optimization of sputter deposition process parameters and could save energy and resources in future.

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