Thixoforming : Semi-solid Metal Processing

250j 8 Tool Technologies for Forming of Semi-solid Metals
Figure 8.9 Schematic drawing of a magnetron sputtering PVD process.
temperature and composition of the substrate surface and the ion or electron
bombardment [13]. Temperature and composition of the substrate surface have a
fairly low impact on the arrival process, but can strongly influence the residence time
or mobility of the atoms on the surface, which drastically affects structure evolution
and the properties of the deposited films.
Magnetron sputtering methods are often used to synthesize compound materials
by adding a reactive gas (O2, N2,) to the sputtering process. This process for which
chemical reactions occur between the sputtered atoms and the reactive species at the
substrate, on the chamber walls or at the target itself to create the compound is called
reactive sputtering [13, 15]. As already mentioned, due to the low pressure used during
the sputtering processes, reactions in the plasma are very unlikely. Nevertheless, after
determination of the process window, this method permits reproducible fine control
of the compound film stoichiometry and microstructure. Therefore, numerous
industrial coaters are equipped with this technology.
CVD is a process in which the substrate is exposed to one or more volatile
precursors which react chemically or decompose at the substrate surface to form a
thin film. Often high substrate temperatures are employed to overcome the energetic
barrier to synthesize the compound, which limits the range of suitable substrate
materials. Therefore, plasma-assisted CVD processes have been developed [14] and
were used here. The experimental setup used is shown in Figure 8.10 and discussed
in a recent paper [17]. In the glow discharge, decomposition and activation of the
volatile precursor species take place [14]. Hence chemical reactions are triggered
at considerably lower temperatures than in thermal CVD processes. For example,
a-alumina can be formed at about 1000 C by CVD [16] whereas the synthesis
temperature using PECVD was 550 C [17].
Whatever the plasma-based deposition technique employed, the plasma–surface
interaction during film growth defines the structure evolution and hence the
properties of the coatings. Energy supplied by the arriving particles (neutrals, ions,
clusters, etc.) and by heating the substrate is often considered as one of the main
parameters affecting structure evolution [18].