Biennial Report 2016/2017

Reports
less energetic or less numerous), Fig. 3 shows
the orientation dependent excess power density,
i.e. corrected for the background, for nitrogen gas
and steel substrate using two passive thermal
probes with different acceptance angles.
Additionally, calculated values for a point source
are provided. As can be clearly seen, the
measured flux is arriving directly from the
direction of the substrate with the angular
dependence following a cosine function. Thus, a
point source is present and any power density
arriving not directly from the substrate is rather
weak (5 mW/cm 2 or less) and could be well within
variations of the local plasma environment.
As the plasma is generated at the top of the
chamber above the substrate and is diffusing out
from there, local density gradients will be present,
which will translate into a variation of directional,
local power density from the plasma. Rotating the
probe without HV leads to a similar variation of
the baseline intensity by 5 – 10%.
For a next step, the variation of the pulse voltage
for different pulse frequencies (nitrogen, Cu
substrate) was performed. An increase faster than
linear is observed for the increasing pulse voltage,
especially for the highest voltage of 5 kV which
corresponds to a combination of increased
secondary electron energy with reduced collisions
and a higher ion flux during the PIII pulses as the
plasma sheath is expanding faster for higher
voltages.
The combined data of such experiments for four
different gas-substrate combinations are shown in
Fig. 4. The variation observed with substrate
material and gas precursor employed in PIII is in
agreement with available literature data [5]. For
the nitrogen gas, a plasma composition of about
Power Density (mW/cm 2 )
Pulse Voltage (kV)
400 0 1 2 3 4 5 6
300
200
100
0
N 2
, 15 µs, 5 kHz
distance 26 cm
Al
Cu
stainless steel
45 Cu, 15 µs, 5 kHz
distance 26 cm
30
15
N 2
Ar
0
0 1 2 3 4 5 6
Pulse Voltage (kV)
Figure 4: Power density for different gases and substrate
materials; as a guide for the eye, straight lines representing
a linear dependency are indicated. The measurements were
performed with 15 µs pulses at 5 kHz.
90% molecular ions and 10% atomic ions is
present, thus the effective ion energy leading to
the emission of secondary electrons corresponds
to about 50% of the pulse voltage.
Using a homogeneous substrate in PIII allows a
direct correlation of the primary current through
the pulse generator (as well as the current
through the substrate) with the emission of
secondary electrons. For fundamental
investigations, this setup is ideal. In contrast,
direct measurements of a variety of materials
necessitate a flexible substrate holder for
changing substrates where no secondary
electrons from the holder (comprising of a
different material) can reach the PTP.
The experiment for measuring secondary electron
emission coefficients is ideally suited for dynamic
investigations as the ion flux of about 10 15 /cm 2 s
leads to typical sputter rates of 0.5 – 1.0 µm/h.
Combining this value with the time resolution of
1 minute, a conservative depth resolution of
10 nm can be realized. As PIII experiments are
naturally conducted from 5 to 50 kV, the incident
ion flux will lead to excessive heating at higher
voltages. Thus, reduced voltages between
0.5 and 10 kV are more suitable for dynamic
measurements, with the lower energies being of
considerable interest in magnetron or HIPIMS
discharges. The lower limit is normally given by
the specifications of the employed pulse
generator for the PIII experiments.
Typical results for the primary current through the
pulse generator, the measured power density
from the secondary electrons and the substrate
temperature are shown in Fig. 5 for the
magnesium alloy AZ91. The primary current and
the power density show an almost constant
plateau after an initial phase, followed by a nearly
Current // Power Density
100
80
60
40
20
Ar PIII into alloy AZ91
3 kV, 3 kHz, 15 µs
current (mA)
power density
(mW/cm 2 )
temperature
0 15 30 45 60 75 90 105 120
Time (min)
400
320
240
160
Figure 5: Temporal evolution of primary current, power
density and sample temperature for oxidized alloy AZ91
during sputtering with Ar ions using 3 kV, 5 kHz, 15 µs PIII
pulses.
80
Temperature (°C)
28