INFICON XTM/2 Thin Film Deposition Monitor - Manual (Page 97)

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XTM/2 Operating Manual
response at about three times the frequency of the fundamental is called the
third quasiharmonic. There are also a series of anharmonics slightly higher in
frequency associated with the quasiharmonic.
Figure 5-3 Thickness Shear Displacement
The monitor crystal design depicted in Figure 5-1 is the result of several
significant improvements from the square crystals with fully electroded plane
parallel faces that were first used. The first improvement was to use circular
crystals. This increased symmetry greatly reduced the number of allowed
vibrational modes. 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 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.
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.
displacement node
X
X
X
2
1
3
E