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XTC/3 Operating Manual
Figure 8-3 Thickness shear displacement
8.1.2 Period Measurement Technique
Although instruments using
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:
[3]
where T
c
and Tq
are the periods of oscillation of the crystal with film (composite)
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 accumulates cycles from the reference oscillator
until m counts are accumulated in the first.
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
-----------
=
=