INFICON STM-2XM 2-Channel Rate/Thickness Monitor - Manual (Page 137)

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STM-2XM Operating Manual
Electronically, the period measurement technique uses a second crystal oscillator,
or reference oscillator, not affected by the deposition, which is usually much higher
in frequency than the monitor crystal. This reference oscillator is used to generate
small precision time intervals, which are used to determine the oscillation period of
the monitor crystal. This is done by using two pulse accumulators:
 The first is used to accumulate a fixed number of cycles, m, of the
monitor crystal.
 The second is turned on at the same time and accumulates cycles from the
reference oscillator until m counts are accumulated in the first.
Since the frequency of the reference is stable and known, the time to accumulate
the m counts is known to an accuracy equal to ± 2/F
r
where F
r
is the reference
frequency of the oscillator.
The period of the monitor crystal is (n/F
r
)/m, where n is the number of counts in the
second accumulator. The precision of the measurement is determined by the
speed of the reference clock and the length of the gate time (set by the size of m).
Increasing one or both of these leads to improved measurement precision.
Having a high frequency reference oscillator is important for rapid measurements
(which require short gating times), low deposition rates, and low density materials.
All of these require high time precision to resolve the small, mass-induced
frequency shifts between measurements.
When the change of frequency of the monitor crystal between measurements is
small, that is, on the same order of size as the measurement precision, it is not
possible to establish quality rate control. The uncertainty of the measurement
injects more noise into the control loop, which can be counteracted only by longer
time constants. Long time constants cause the correction of rate errors to be very
slow, resulting in relatively long term deviations from the desired rate.
These deviations may not be important for some simple films, but can cause
unacceptable errors in the production of critical films such as optical filters or very
thin-layered superlattices grown at low rates.
In many cases, the desired properties of these films can be lost if the layer-to-layer
reproducibility exceeds one, or two, percent. Ultimately, the practical stability and
frequency of the reference oscillator limits the precision of measurement for
conventional instrumentation.