INFICON XTM/2 Thin Film Deposition Monitor - Manual (Page 98)

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XTM/2 Operating Manual
The “AT” resonator is usually chosen for deposition monitoring because at room
temperature it can be made to exhibit a very small frequency change due to
temperature changes. Since there is presently no way to separate the
frequency change caused by added mass (which is negative) or even the
frequency changes caused by temperature gradients across the crystal or film
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.
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, L
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. 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
r
where F
r
is the reference oscillator’s frequency. The monitor crystal’s
period is (n/F
r
)/m where n is the number of counts in the second accumulator.
The precision of the measurement is determined by the speed of the reference
clock and the length of the gate time (which is set by the size of m). Increasing
one or both of these leads to improved measurement precision.
Having a high frequency reference oscillator is important for rapid
measurements (which require short gating times), low deposition rates and low
density materials. All of these require high time precision to resolve the small,
mass induced frequency shifts between measurements. When the change of a
monitor crystal’s frequency between measurements is small, that is, on the
4.K. H. Behrndt, J. Vac. Sci. Technol. 8, 622 (1961)
M
f
M
q
-------
T
c
T
q
–
( )
T
q
-----------------------
F
∆ ( )
F
c
------------ = =