INFICON Cygnus Thin Film Deposition Controller - Manual (Page 57)

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Cygnus Operating Manual
2.4 Crystal Life, Starting Frequency and Activity
On the CHANNEL 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
instrument. 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 not
always possible to use a monitor crystal to 100% of crystal life. Useful crystal life is
highly 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. For these
materials, the inherent quality of the quartz has very little to do with the monitor
crystal’s failure.
It is normal for a brand new quartz monitor crystal to display a crystal life anywhere
from 0 to 5% due to process variations in producing the crystal. Naturally, this
invites the question, “Is a brand new crystal indicating 5% life spent inferior to a
crystal indicating 1% life spent?”
If a new crystal indicates 5% life spent, it means that either the quartz blank is
slightly thicker than normal (more mechanical robustness), or the gold electrode is
slightly thicker than normal (better thermal and electrical properties), or both. In
either case, its useful life with regard to material deposition should not be adversely
affected. To verify this assertion, laboratory testing was performed on crystals that
covered the crystal life range in question. Results indicate that a brand new crystal
that indicates 3 to 5% life spent is just as good as, if not better than, a crystal
indicating 0 to 2% life spent. As a consequence, it is important to consider the
change in crystal life (%), not just the absolute crystal life (%) indicated.
The activity value is useful for predicting when a crystal needs to be replaced. If a
crystal is about to fail, its series resistance will increase, allowing less current to
flow through the crystal and hence the activity value will decrease. The closer the
activity value is to zero the more imminent a crystal failure.
It also can be used to gauge the health of the sensor head electrical contacts.
Activity values range from a maximum of 650 (healthiest) to a minimum of 0 (least
healthy). For example, if a new monitor crystal is placed into the sensor head and
has a crystal life of near 0%, but the activity value for this crystal is lower than 400,
the sensor head or in-vacuum cable is in need of repair.