8 - 3
XTC/3 Operating Manual
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 crystal’s vibrations
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 and the wrong thickness consequently 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. Since there is presently no way to separate the frequency
change caused by added mass (which is negative) or even the frequency changes
caused by temperature gradients across the crystal or film induced stresses, it is
essential to minimize these temperature-induced changes. It is only in this way that
small changes in mass can be measured accurately.
5.
98
1
MH
z 15
oh
m
6.
15
3
MH
z 50
oh
m
6.
19
4
MH
z 40
oh
m
6.
33
3
MH
z 14
2 o
hm
6.
33
7
MH
z 10
5 o
hm
6.
34
8
MH
z 32
2 o
hm
6.
41
9
MH
z 35
0 o
hm
17
.7
92
MH
z 2
78
oh
m
17
.9
57
M
H
z 31
1 o
hm
18
.1
33
MH
z 3
50
oh
m
L
og of re
la
tive
int
e
ns
ity (
A
d
m
it
ta
n
ce)
Frequency (in MHz)
1
10
1
100
1
1000
6
7
17
18