INFICON STM-2 USB Thin Film Rate/Thickness Monitor - Manual (Page 90)

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STM-2 Operating Manual
6 Using the Density value from step 1 and the recorded values for F
co
and F
c
,
adjust the Z-Ratio value in thickness equation [5] to bring the calculated
thickness value into agreement with the actual thickness.
 If the calculated value of thickness is greater than the actual thickness,
increase the Z-Ratio value.
 If the calculated value of thickness is less than the actual thickness,
decrease the Z-Ratio value.
[5]
where:
T
f
= Thickness of deposited film (kÅ)
F
co
= Starting frequency of the sensor crystal (Hz)
F
c
= Final frequency of the sensor crystal (Hz)
F
q
= Nominal blank frequency = 6045000 (Hz)
z = Z-Ratio of deposited film material
Z
q
= Specific acoustic impedance of quartz = 8765000 (kg/(m
2
*s))
p = Density of deposited film (g/cm
3
)
For multiple layer deposition (for example, two layers), the Z-Ratio 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:
 If the thickness of layer 1 is large compared to layer 2, use material 1 Z-Ratio
for both layers.
 If the thickness of layer 1 is thin compared to layer 2, use material 2 Z-Ratio for
both layers.
 If the thickness of both layers is similar, use a value for Z-Ratio which is the
weighted average of the two Z-Ratio values for deposition of layer 2 and
subsequent layers.
T
f
Z
q
10
4

2 zp
--------------------
1
F
co
--------
 
  ATan zTan
F
co
F
q
-----------
 
 
 
  1
F
c
-----
 
  ATan zTan
F
c
F
q
---------
 
 
 
 
–
 
 
 
=