INFICON IC/5 Thin Film Deposition Controller - Manual (Page 64)

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IC/5 Operating Manual
Figure 3-7 Co-deposition Operate Display
3.3.2.1 Crystal Life and Starting Frequency
On the SENSORS 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