INFICON IC6 Thin Film Deposition Controller - Manual (Page 316)

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IC6 Operating Manual
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.
Figure 16-6 Heavily loaded crystal
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 and precision of
measurement. The technique also allows the implementation of a sophisticated
feature that cannot even be contemplated using the active oscillator approach. The
same capability that allows the new technology to sweep and identify the
fundamental can be used to identify other oscillation modes, such as the
anharmonics and the quasiharmonic. Not only can the instrument track the
Phase (degrees)
-90
Log |Z| (ohms)
Frequency (MHz)
4.541500 4.512500 4.543600 4.544500 4.544500
1000
100
Phase
Impedance
Series resonance
+90
1000
100
0