INFICON SQM-242 Thin Film Deposition Controller Card Operating Manual - Manual (Page 80)

8 - 4
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SQM-242 Operating Manual
[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 (MKS units)
p = density of deposited film (g/cc)
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:
 If the thickness of layer 1 is large compared to layer 2, use material 1 Z-value
for both layers.
 If the thickness of layer 1 is thin compared to layer 2, use material 2 Z-value 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 values for deposition of layer 2 and subsequent
layers.
8.5 Tuning the Control Loop
The function of the control loop parameters is to match the instrument’s reaction to
an error (between the measured deposition rate and the desired rate) to the time
related characteristics of the deposition source and its power supply. There are
three adjustable parameters; P(proportional), I(integral) and D(derivative) used to
accomplish this. It is convenient to think of sources as falling into two categories
"fast" or "slow". The tuning parameters are affected by source level, rate, sweep
range or beam density, tooling and source condition.
NOTE: If you do not know if the source is fast or slow, it is straight forward to
measure the delay. Using manual power, establish a rate and allow it to
become steady. Increase the source power a few percent (~5% if
possible). Allow the source to again stabilize. If the delay time is greater
than 1 second characterize the source as "slow". Typically, thermal
evaporation sources are considered "slow" and E-beam sources are
considered "fast."
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
---------
 
 
 
 
–
 
 
 
=