Chemistry Reference
In-Depth Information
20
148 K
0
3
6
9
12
15
18
21
24
2
40
173 K
3
6
9
12
15
18
21
24
10
198 K
0
3
6
9
12
15
18
21
24
20
223 K
0
3
6
9
12
15
18
21
24
50
248 K
25
3
6
9
12
15
18
21
24
263 K
2
40
2
40
3
6
9
12
15
18
21
24
297 K
3
6
9
12
15
18
21
24
Height (nm)
FIGURE 8.70
Height histograms with log-normal fits for the indicated wall temperatures of
the chamber. (From Shyjumon, I. et al.,
Thin Solid Films
, 500, 41, 2006.)
microscopy (AFM) height histograms obtained under different temperature condi-
tions are shown in Figure 8.70. Log-normal fits reasonably reproduce the measured
histograms. The mean height of clusters decreases from about 14 to 8 nm when the
temperature increases from 200 to 250 K. Simultaneously, the surface coverage also
decreases with increasing wall temperature.
Bombardment of the cathode by plasma ions causes emission of metal atoms.
When the number density of metal atoms in the buffer gas exceeds the saturation
vapor density, nucleation in the vapor can proceed. Growing clusters are located in a
restricted aggregation region where they capture free metal atoms. Clusters leave the
aggregation region as a result of diffusion or flow of the buffer gas (argon) and tend to
spread uniformly over all the space inside the magnetron chamber. Ultimately, some
clusters depart from the magnetron chamber through an orifice and are deposited
on a substrate. The cluster flux onto a substratum surface as well as the cluster size
can be restored from the measurement of the cluster area density on the surface at a
given deposition time. This allows for an estimate of the cluster number density in
the magnetron chamber.
Formation of metal clusters starts from free metal atoms. The three-body collision
process [450,451]
2
M
+
A
→
M
2
+
A
(8.80)
is the bottleneck for cluster formation. Small clusters grow by atom attachment to
the cluster surface:
M
n
+
M
→
M
n
+
1
,
(8.81)
