INFICON IC6 Thin Film Deposition Controller - Manual (Page 65)

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IC6 Operating Manual
S Total (Crystal Stability value field)
This field displays the positive total frequency shift currently accumulated in the
Crystal Stability counter,
see page 5-12 .
3.3.3.2 Crystal Life and Starting Frequency
On the Sensor Information RATE/XTAL display, crystal life is shown as a
percentage of the monitor crystal’s frequency shift, relative to the 1.50 MHz
frequency shift allowed by the IC6. This quantity is useful as an indicator of when
to change the monitor crystal to safeguard against crystal failures during
deposition. It is normal to change a crystal after a specific amount of crystal life
(% change) is consumed.
It is usually not possible to use a monitor crystal to 100% of crystal life. Useful
crystal life is dependent on the type of material being deposited and the resulting
influence of this material on the quartz monitor crystal. For well-behaved materials,
such as copper, at about 100% crystal life the inherent quality, Q, of the monitor
crystal degrades to a point where it is difficult to maintain a sharp resonance and
therefore the ability to measure the monitor crystal’s frequency deteriorates.
When depositing dielectric or optical materials, the life of a gold, aluminum or silver
quartz monitor crystal is much shorter—as much as 10 to 20%. This is due to
thermal and intrinsic stresses at the quartz-dielectric film interface, which are
usually exacerbated by the poor mechanical strength of the film. Many organic
materials deposit as non-rigid films which causes the crystal’s Q to fall rapidly. For
these materials, the %Life of the quartz has very little to do with the monitor
crystal’s failure.
3.3.3.3 Percent Life Auto-Zero
If a new crystal has a starting frequency above 5.945 MHz and is more than
0.04 MHz different from the last valid crystal reading, its % Life will be set to 0. This
facilitates using crystals to a uniform known % Life point without wasting usable
crystal life. In all cases, reaching 4.50 MHz results in a crystal life fail even though
% Life may not have reached 100%. For example, a crystal with a starting
frequency of 5.95 MHz will have a % Life reading of 97% when it reaches 4.5 MHz.