INFICON XTC/C Thin Film Deposition Controller - Manual (Page 121)

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XTC/C - XTC/2 Operating Manual
Figure 4-8 shows temperature versus frequency relationship for an AT cut
crystal.
Figure 4-8 Frequency Change vs. Temperature for an AT crystal cut at 35°20'
4.6.3 Determining Q and S Values
The Q and S Factors are used to ensure that the evaporation process is always
under the best possible rate control a crystal can provide. The process engineer
can program values between 0 and 9 for these parameters. Thus, when the
primary crystal reaches a point where its behavior is objectionable it will be
disabled and the proper switch/time-power/stop decision made. The tolerance
of instabilities becomes increasingly smaller as Q and S increase towards 9.
They are independent parameters and may be treated one at a time.
If the crystal fails and no backups are available, the TIME PWR parameter
determines whether the process should stop (N) or complete on time-power (Y),
[or crystal switch if a dual or CrystalSix crystal sensor head is employed].
Q and S can be observed when the display is in the operate mode and the LIFE
key is depressed. The value in the S accumulator replaces the TIME display.
When the life key is released, the Q value replaces the S value in the TIME
display for about 1 second.
With a new crystal, the value in the Q accumulator will usually be one or zero if
the Q parameter is programmed properly. As a crystal deteriorates, larger
values will appear as the Q accumulator builds up or counts down. The switch
point occurs when the Q accumulator equals 50. The designated count of 50
requires that the rate deviation instability be sustained for several seconds.
This is so the algorithm does not trip out for short-lived events. The Q