INFICON Cool Drawer Single Sensor - Manual (Page 31)

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Cool Drawer Single and Dual Sensors Operating Manual
2.2.1 Sensor Installation
The sensor head must be installed such that the face of the crystal is at 90° relative
to the source. That is, the crystal must be square to the vapor stream. Two effects
may arise if the sensor head is not square to the vapor stream:

The deposit is not 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 somewhat 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 look onto the
spurious mode of oscillation, causing a mode hop.
The combination of these effects will have a negative effect on crystal life and will
increase the probability of mode hops.
Install the sensor as far as possible from the evaporation source (a minimum of
25.4 cm or 10 in.) while keeping the sensor in a position to accumulate thickness
at a rate proportional to accumulation on the substrate.
Figure 2-2 shows proper
and improper methods of installing sensors.
NOTE: For best process reproducibility, rigidly support the sensor so that it cannot
move during maintenance and crystal replacement.
Figure 2-2 Sensor installation guidelines
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Incorrect
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Obstruction
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