INFICON XTM/2 Thin Film Deposition Monitor - Manual (Page 94)

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XTM/2 Operating Manual
For multiple layer deposition (for example, two layers), the Z-value used for the
second layer is determined by the relative thickness of the two layers. For most
applications the following three rules will provide reasonable accuracies:
Š If the thickness of layer 1 is large compared to layer 2, use material 1’s
Z-value for both layers.
Š If the thickness of layer 1 is thin compared to layer 2, use material 2’s
Z-value for both layers.
Š If the thickness of both layers is similar, use a value for Z-ratio which is the
weighted average of the two Z-values for deposition of layer 2 and
subsequent layers.
5.5 Measurement Theory
5.5.1 Basics
The Quartz Crystal deposition Monitor, or QCM, utilizes the piezoelectric
sensitivity of a quartz monitor crystal’s resonance to added mass. The QCM
uses this mass sensitivity to control the deposition rate and final thickness of a
vacuum deposition. When a voltage is applied across the faces of a properly
shaped piezoelectric crystal, the crystal is distorted and changes shape in
proportion to the applied voltage. At certain discrete frequencies of applied
voltage, a condition of very sharp electro-mechanical resonance is
encountered. When mass is added to the face of a resonating quartz crystal,
the frequency of these resonances are reduced. This change in frequency is
very repeatable and is precisely understood for specific oscillating modes of
quartz. This heuristically easy to understand phenomenon is the basis of an
indispensable measurement and process control tool that can easily detect the
addition of less than an atomic layer of an adhered foreign material.
In the late 1950’s it was noted by Sauerbrey
1,2
and Lostis
3
that the change in
frequency, DF = F
q
-F
c
, of a quartz crystal with coated (or composite) and
uncoated frequencies, F
c
and F
q
respectively, is related to the change in mass
from the added material, M
f
, as follows:
[5]
where M
q
is the mass of the uncoated quartz crystal.
1.G. Z. Sauerbrey, Phys. Verhand.8, 193 (1957)
2.G. Z. Sauerbrey, Z. Phys. 155,206 (1959)
3.P. Lostis, Rev. Opt. 38,1 (1959)
M
f
M
q
-------
F
∆ ( )
F
q
------------ =