INFICON XTC/3 Thin Film Deposition Controller Operating Manual - Manual (Page 54)

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XTC/3 Operating Manual
3.3.2.1 Crystal Life and Starting Frequency
On the Operate display, crystal life is shown as a percentage of the nominal 1 MHz
frequency shift, relative to the 5.0 MHz ending frequency allowed by XTC/3. 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 unusual 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. Even for well-behaved
materials, such as copper, 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 before reaching the
theoretical 100% Life point.
When depositing dielectric or optical materials, the life of a gold, alloy or silver
quartz monitor crystal is much shorter—as little 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, Q, of the quartz has very little to do with the monitor
crystal's failure.
3.3.2.2 Choosing The Right Crystal
Gold crystals are recommnded for most applications. However, silver crystals will
provide superior performance in processes with high heat loads, such as
sputtering. Silver crystals may also improve the deposition of oxides, but try gold
crystals first. Keep in mind that silver tarnishes and cannot be kept in storage for
long periods of time. INFICON silver crystals are shipped in a sealed bag filled with
an inert gas to maximize shelf life.
3.3.2.3 Percent Life Auto-Zero
If a new crystal has a starting frequency above 5.92 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 5.0 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 95% when it reaches 5.0 MHz.