INFICON XTM/2 Thin Film Deposition Monitor - Manual (Page 102)

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XTM/2 Operating Manual
5.5.6 ModeLock Oscillator
INFICON has created a new technology
7
that eliminates the active oscillator
and its limitations. This new system constantly tests the crystal’s response to
an applied frequency in order to not only determine the resonant frequency, but
also to verify that the crystal is oscillating in the desired mode. This new system
is essentially immune to mode hopping and the resulting inaccuracies. It is fast
and accurate, determining the crystal’s frequency to less than 0.05 Hz at a rate
of 4 times per second. Because of the system’s ability to identify and then
measure particular crystal modes, it is now possible to offer new features that
take advantage of the additional informational content of these modes.
This “intelligent” measurement system uses the phase/frequency properties of
the quartz crystal to determine the resonant frequency. It operates by applying
a synthesized sine wave of specific frequency to the crystal and measuring the
phase difference between the applied signal’s voltage and the current passing
through the crystal. At series resonance, this phase difference is exactly 0
degrees; that is, the crystal behaves like a pure resistance. By separating the
applied voltage and the current returned from the crystal and monitoring the
output of a phase comparator it is possible to establish whether the applied
frequency is higher or lower than the crystal’s resonance point. At frequencies
well below the fundamental, the crystal’s impedance is capacitive and at
frequencies slightly higher than resonance it is inductive in nature. This
information is useful if the resonance frequency of a crystal is unknown. A quick
sweep of frequencies can be undertaken until the output of the phase
comparator changes, marking the resonance event.
For AT crystals we know that the lowest frequency event encountered is the
fundamental. The events slightly higher in frequency are anharmonics. This
information is useful not only for initialization, but also for the rare case when
the instrument loses track of the fundamental. Once the frequency spectrum of
the crystal is determined the instrument’s task is to follow the changing
resonance frequency and to periodically provide a measurement of the
frequency for subsequent conversion to thickness.
The use of the “intelligent” measurement system has a series of very apparent
advantages when compared to the previous generation of active oscillators;
namely immunity from mode hopping, speed of measurement, precision of
measurement, and the ability to measure heavily loaded (damped) crystals.
7.US Patent 5,117,192 (May 27 1992)