Films minces à base de Si nanostructuré pour des cellules ...

2.1.1 Radiofrequency Magnetron Reactive Sputtering
Principle
tel-00916300, version 1 - 10 Dec 2013
The phenomena of sputtering consists of striking the source material (the target)
with fast moving, inert gas ions (eg. Ar) which transfer momentum to the target
species (electrons, atoms, molecules) and eject them out.
The positive ions of the plasma (from ionized Ar) are accelerated towards the
negatively charged target and impinge on its surface. Some of the bombarding ions
are reected back and are neutralised, while some have sucient energy to reach the
substrate and are back scattered. The secondary electrons are also emitted during
this process and they may cause further ionisation of the neutralised species. Due
to the law of conservation of energy, when these electrons return to ground state,
the energy gained by the neutral gas atom is released as photons which keep the
plasma glowing. The main intention of sputtering is to direct the ejected species towards
the substrate with sucient energy and mobility, because if they collide with
other atoms in the plasma, the energy is diminished and their path is modied. The
incident species with sucient mobility diuse to join with other species at the substrate
and grow until they coalesce to form a continuous lm [Wagendristel 94]. The
mobility of the incident atoms arriving at the substrate is highly dependent on sputtering
parameters such as the power applied, pressure in the chamber, temperature,
distance between the target and the substrate etc.
The energy required to initiate and sustain the plasma was initially provided
by a DC power supply to create a discharge and ionise the gas. But the principle
of DC discharge is ineective for an insulating target, because no current can pass
through it. As a consequence, charge build-up occurs and stops the process. In
order to overcome this shortcoming of a DC power supply, RF power system was
developed. For frequencies up to 50 kHz, the ions are mobile enough to reach the
electrodes at each half cycle of the AC power. This leads to alternative sputtering
of the substrate and the target which cancels the global transport of matter. At
high frequencies typically used for sputtering (13.6 MHz), new phenomena occur:
1. The ions remain practically motionless in the RF eld due to their mass as
compared to the electrons, and do not involve themselves in sputtering.
2. The electrons in high frequency eld acquire sucient energy to cause ionization
of the eective plasma.
3. At such high frequencies, the target behaves as a conductive one.
Thus, the use of RF sputtering enables any material to be sputtered. A matching
impedence network is always required with RF sputtering in order to absorb
30