Period Measurement Technique - Inficon Cygnus 2 Operating Manual

Figure 14-3 Thickness shear displacement
displacement node

14.1.2 Period Measurement Technique

Although instruments using
had a very limited range of accuracy, typically holding accuracy for DF less than
0.02 F
M
f
------- =
M
q
where T
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
= 1/F
q
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. Since the frequency of the
reference is stable and known, the time to accumulate the m counts is known to an
accuracy equal to ± 2/F
4.K. H. Behrndt, J. Vac. Sci. Technol. 8, 622 (1961)
. In 1961 it was recognized by Behrndt
q
 
T – T
  
F
c q
---------------------- = -----------
T F
q c
and Tq are the periods of oscillation of the crystal with film (composite)
c
. Electronically the period measurement technique uses a second crystal
where F
r
E
X
2
X
X
3
equation [2] were very useful, it was soon noted they
4
that:
, and the period of oscillation, T
q
is the reference oscillator's frequency. The
r
Cygnus 2 Operating Manual
1
[3]
q
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