PhD Thesis_RuiMSMartins.pdf - RUN UNL

In-situ XRD studies during growth of Ni-Ti SMA films and their complementary ex-situ characterization
is one of the main parameters for explaining the observed morphology. One of them is an
excess of positive charge carriers in the space near to the negative cathode. This is related to
the attraction (repulsion) of the ions (electrons) and leads to a sheath voltage that is about the
same as the electrode voltage, V CA . The total difference in a potential between the electrodes
is confined to this sheath close to the cathode, which is named the cathode sheath or Crooke’s
dark space. The Ar + ions are accelerated through this sheath, gaining an energy of eV CA , and
create secondary electrons upon colliding with the target material. The electrons are
accelerated back through the Crooke’s dark space before they ionise or excite the Ar atoms in
the negative glow space. Only parts of the cathode area in the low-current end of the normal
discharge region are covered by the negative glow discharge space. This coverage increases
as the applied voltage is increased until the entire cathode-area is covered, which is the end of
the normal discharge region. Throughout the lateral expansion of the negative glow discharge
the current density stays constant. An increase of the voltage further increases the current
density and the abnormal discharge region is entered.
A high ion current is necessary in order to have a reasonable sputtering rate, which
explains why the abnormal discharge region is used for sputtering. The collision of Ar + ions
with the target surface causes:
• sputtering of the target material;
• emission of secondary electrons;
• electron capture and backscattering of neutral Ar atoms;
• implantation of Ar + ions into the target.
The number of gas atoms between the sample and the magnetrons should be low
allowing the sputtered atoms to arrive at the sample without any major collisions that would
redirect the sputtered atoms way from the sample and thus decrease the deposition rate. The
mean free path (l) is given by
l = (kT/(√2.σ.p) (1.1)
where σ is the cross section for the collision, k the Boltzmann constant, T the temperature and
p the pressure. This equation shows that the pressure has to be lower than a certain maximum
to fulfil this condition. On the other hand, in order to guarantee that secondary electrons
produce ionising collisions, a constraint for having a self-sustaining plasma, the total
thickness of Crooke’s space and the negative glow space must be less than the electrode
separation. This total thickness increases with decreasing sputtering pressure and thus defines
a minimum pressure.
Chapter 1 - Background and Literature Review 20