INFICON XTC/C Thin Film Deposition Controller - Manual (Page 139)

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XTC/C - XTC/2 Operating Manual
induced stresses, it is essential to minimize these temperature-induced
changes. It is only in this way that small changes in mass can be measured
accurately.
Figure 5-3 Thickness Shear Displacement
5.5.3 Period Measurement Technique
Although instruments using equation [6] were very useful, it was soon noted
they had a very limited range of accuracy, typically holding accuracy for
∆F less
than 0.02 F
q
. In 1961 it was recognized by Behrndt
4
that:
[7]
where T
c
and Tq are the periods of oscillation of the crystal with film and the
bare crystal respectively. The period measurement technique was the
outgrowth of two factors; first, the digital implementation of time measurement,
and second, the recognition of the mathematically rigorous formulation of the
proportionality between the crystal’s thickness, I
q
, and the period of oscillation,
T
q
= 1/F
q
. Electronically the period measurement technique uses a second
crystal oscillator, or reference oscillator, not affected by the deposition and
usually much higher in frequency than the monitor crystal. This reference
oscillator is used to generate small precision time intervals which are used to
determine the oscillation period of the monitor crystal. This is done by using two
pulse accumulators. The first is used to accumulate a fixed number of cycles,
m, of the monitor crystal. The second is turned on at the same time and
displacement node
X
X
X
2
1
3
E
4.K. H. Behrndt, J. Vac. Sci. Technol. 8, 622 (1961)
M
f
M
q
-------
T
c
T
q
–
( )
T
q
-----------------------
F
∆ ( )
F
c
------------ = =