INFICON UHV Bakeable Sensor - Manual (Page 26)

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UHV Bakeable Sensor Operating Manual
The sensor head must be installed such that the face of the crystal is perpendicular
to the evaporant stream from the source (see
Figure 2-2 ). Two effects may arise if
the sensor head is not perpendicular to the evaporant stream, and the combination
of these effects will have a negative effect on crystal life and increase the
probability of mode hops:

The deposit will not be even across the crystal surface. The edge of the crystal
that is angled away from the source is farther away from the source and
receives less material, causing the thickness of the deposit to become wedge
shaped. This wedge shape in the deposited film tends to reduce the activity of
the crystal at its primary resonance.
 The area of the deposit shifts from the center of the crystal. This is due to the
shadowing effect of the crystal aperture. If the crystal is not square to the
evaporant stream, the strength of spurious (non-thickness shear) modes of
vibration are enhanced. If the activity of these spurious modes of oscillation
become strong enough, they cause short-term perturbation of the fundamental
frequency. If they get very strong, the oscillator can lock onto the spurious
mode of oscillation, causing a mode hop.
Figure 2-2 Sensor installation guidelines
To guard against spattering, use a source shutter to shield the sensor during initial
soak periods. If the crystal is hit with only a very small particle of molten material,
it may be damaged and stop oscillating. Even in cases when it does not completely
stop oscillating, the crystal may immediately become unstable, or shortly after
deposition begins, instability may occur.
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