INFICON XTC/3 Thin Film Deposition Controller Operating Manual - Manual (Page 203)

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XTC/3 Operating Manual
into a greater frequency shift than that which would be produced with a new crystal.
In the extreme, the basic phase/frequency shape is not preserved and the crystal
is not able to provide a full 90 degrees of phase shift.
The impedance, |Z|, is also noted to rise to an extremely high value. When this
happens it is often more favorable for the oscillator to resonate at one of the
anharmonic frequencies. This condition is sometimes short lived, with the oscillator
switching between the fundamental and anharmonic modes, or it may continue to
oscillate at the anharmonic. This condition is known as mode hopping and in
addition to annoying rate noise can also lead to false termination of the film
because of the apparent frequency change. It is important to note that the controller
will frequently continue to operate under these conditions; in fact there is no way to
tell this has happened except that the film’s thickness is suddenly apparently
thinner by an amount equivalent to the frequency difference between the
fundamental and the anharmonic that is sustaining the oscillation.
Figure 8-5 Crystal frequency near series resonance point
8.1.5 ModeLock Oscillator
INFICON has created a new technology 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 .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 new “intelligent” measurement system
Phase
Impedance
Series resonance
Frequency (MHz)
Log |Z| (ohms)
1CO
1C
Phase (degrees)
+90
0
-90
5.965500 5.966500 5.967500 5.968500