SCBD

Supersonic Cluster Beam Deposition (SCBD) is a technique consisting in the production of a supersonic beam of inert gas seeded by clusters and in the deposition of those aggregates by intercepting the beam with a substrate.
If the clusters in those beams have low kinetic energy (below 0.2 eV/atom) they are not subjected to relevant fragmentation during the deposition, so that the obtained films retain the memory of the precursor clusters structure. The resulting material is therefore characterized by a low density compared to that of films assembled atom-by atom, with a nano- and mesostructure that can be tailored by selecting the clusters prior to deposition.

The Pulsed Microplasma Cluster Source (PMCS) is a pulsed source producing supersonic beams of carbon, metal and oxides clusters having kinetic energy below 0.2eV, and therefore it enables to deposit nanostructured thin films.
The PMCS consists of a ceramic cubic body, in which a cylindrical cavity is drilled. A channel is drilled perpendicular to the axis of the cavity and hosts two rods (electrodes) of the material to be vaporized facing each other and separated by a small gap. At the back of the source a solenoid pulsed valve is placed, enabling to inject helium at the pressure of several bars inside the cavity. The gap between the electrodes is off axis, in such a way that the gas flushes on the cathode surface. A removable nozzle with cylindrical shape closes the front of the source.

Three dimensional view of the PMCS cluster source (not to scale). The source body is sectioned in order to show the thermalization cavity and the electrodes arrangement.

The principle of operation of the cluster source is the following: the valve delivers an intense He pulse with an opening time of a few hundreds of ms, thus forming a small high gas density region at the cathode surface (1). After that, a very intense discharge (hundreds of amperes) lasting for a few tens of ms is fired between the electrodes by applying a voltage ranging from 500 up to 1500 V (2). Ionized helium sputters a small area of cathode surface at the point where the He flux impinges on the electrode. The mixture of helium-vaporized atoms quenches and cluster nucleation takes place (3). The clusters are then carried out through the nozzle by the supersonic expansion (4).
By exploiting aerodynamic effects on the supersonic beam it is furthermore possible to reduce the beam divergence (leading to high deposition rates) and to control the clusters mass distribution.

Schematic representation of the PMCS working principle.

The main advantages of the PMCS as compared to other cluster sources are the following:

• It provides extremely high rates: for carbon a typical rate is 100um/h for a covered area of ~0.8cm² (obtained at a distance of 500mm between the source nozzle and the substrate);
• It can be interfaced with UHV characterization apparatus (STM, electron spectroscopy facilities, synchrotron beamlines) to perform in situ characterizations;
• Exploiting aerodynamic separation effects, it allows to select and spatially spread clusters with different size;
• It enables to deposit nanocomposites carbon-metal (by using composite cathods);
• It does not need any substrate polarization or heating: it thereby allows using any kind of material as a substrate (metallic, semiconductor, ceramic, polymeric, membranes, etc.);
• It is compatible with planar technologies used in microelectronics;
• The high collimation of the supersonic beams produced allows the deposition of patterned films, with sub-micrometric resolution, through the use of suitable masks not in contact with the substrate.