INFICON STM-2XM 2-Channel Rate/Thickness Monitor - Manual (Page 135)

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STM-2XM Operating Manual
The second set of improvements was to contour one face of the crystal and to
reduce the size of the exciting electrode. These improvements have the effect of
trapping the acoustic energy.
Reducing the electrode diameter limits the excitation to the central area.
Contouring dissipates the energy of the traveling acoustic wave before it reaches
the edge of the crystal. Energy is not reflected back to the center where it can
interfere with other newly launched waves, essentially making a small crystal
appear to behave as though it is infinite in extent.
With the vibrations in the crystal restricted to the center, it is practical to clamp the
outer edges of the crystal to a holder, and not produce any undesirable effects.
Contouring also reduces the intensity of response of the generally unwanted
anharmonic modes; hence, the potential for an oscillator to sustain an unwanted
oscillation is substantially reduced.
Figure 8-2 Frequency response spectrum
The use of an adhesion layer has improved the electrode-to-quartz bonding,
reducing “rate spikes” caused by micro-tears between the electrode and the quartz,
as film stress rises. These micro-tears leave portions of the deposited film
unattached, and therefore unable to participate in the oscillation. These free
portions are no longer detected, which prevents the consequential wrong thickness
from being inferred.
The “AT” resonator is usually chosen for deposition monitoring, because at room
temperature it can be made to exhibit a very small frequency change, due to
temperature changes.
5.
98
1 M
H
z 15
oh
m
6.
15
3 M
H
z 50
oh
m
6.
19
4 M
H
z 40
oh
m
6.3
33
MH
z 1
42
ohm
6.3
37
MH
z 1
05
ohm
6.3
48
MH
z 3
22
ohm
6.4
19
MH
z 3
50
ohm
17
.79
2 MH
z
278
oh
m
17
.95
7 MH
z
31
1 o
hm
18
.13
3 MH
z
350
oh
m
Log of
r
e
lat
iv
e
in
te
nsity
(
A
dmitt
ance
)
Frequency (in MHz)
1
10
1
100
1
1000
6 7 17 18