INFICON XTM/2 Thin Film Deposition Monitor - Manual (Page 101)

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XTM/2 Operating Manual
The active oscillator circuit is designed so the crystal is required to produce a
phase shift,
θ, of 0 degrees, which allows it to operate at the series resonance
point. Long- and short-term frequency stabilities are a property of crystal
oscillators because very small frequency changes are needed to sustain the
phase shift required for oscillation. Frequency stability is provided by the quartz
crystal even though there are long term changes in electrical component values
caused by temperature or aging or short-term noise-induced phase jitter.
As mass is added to a crystal, its electrical characteristics change. Figure 5-6
is the same plot as Figure 5-5 overlaid with the response of a heavily loaded
crystal. The crystal has lost the steep slope displayed in Figure 5-5 . Because
the phase slope is less steep, any noise in the oscillator circuit translates 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 5-6 Heavily Loaded Crystal