INFICON XTC/C Thin Film Deposition Controller - Manual (Page 120)

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XTC/C - XTC/2 Operating Manual
4.6.2 S-Factor (Stability)
Normally, as material is deposited on a crystal its operating frequency
decreases. It is from this change in frequency (over the measurement time
period) that the instrument derives its thickness measurement and rate control
functions.
There are times when the crystal may become unstable and will experience a
positive frequency shift over the measurement time period. The S-Factor can
then be used as a measure of the crystal’s instability.
When the S-Factor is activated, and a positive frequency shift occurs, the
magnitude of the positive shift is accumulated in the S-register. A limit is placed
on the total cumulative positive frequency shift by programming the S-Factor
between 1 and 9. When the limit is exceeded the instrument will fail the crystal
and effect a CrystalSwitch, Complete on Time Power, or STOP, depending on
the instrument configuration.
Maximum accumulations for selected S-Factors are listed in Table 4-4 . To
prevent random noise from accumulating in the S-register a minimum positive
frequency shift of 25 Hz is required.
There are many reasons for a crystal to exhibit a positive frequency shift. For
example, when a crystal is near the end of its life it is prone to instabilities that
may result in a temporary increase in crystal frequency. Also positive frequency
shifts may occur due to film stress relieving or a film tearing off a crystal.
Additionally, temperature effects may cause positive frequency excursions. A
crystal subjected to temperatures over 100 °C is more sensitive to small
changes in temperature inducing frequency changes. When heat is applied
inside a chamber and/or when the shutter is opened (exposing the crystal to the
hot source), the crystal frequency will shift higher until thermal equilibrium is
obtained. When the active process ends and/or the shutter closes, the crystal
frequency will shift in a negative direction due to cooling.
Table 4-4 Maximum Accumulations for Selected S-Factors
S-Factor Pos. Frequency Accumulation
0 Disabled
1 5000 (max single shift 1250)
2 1000
3 500
4 400
5 200
6 200 (max single shift 100)
7 100
8 100
9 25