INFICON MDC-370 Thin Film Deposition Controller - Manual (Page 127)

MDC-370 DEPOSITION CONTROLLER
Each sensor also has a Weight factor so you can establish how much each sensor's
measurement affects the average rate and thickness. Each sensor's affect on the
average rate and thickness is the ratio of that sensor's weight divided by the total
weight for all enabled sensors. For example, if two sensors are enabled for
measurement with sensor #1 rate = 10.0 and its weight is set to 100.0 and sensor
#2 rate = 20.0 and its weight = 200.0 then
Average Rate = (10*100+20*200)/(100+200) = 16.67
Of course the same formula applies for the average thickness.
See section 4.3.2.1 for sensor tooling setup, sensor weight setup and details on
how to enable/disable sensors for measurement. Also see section 10.6.2 for
sensor tooling calibration.
10.6 EMPIRICAL CALIBRATION
For many film materials the film density and acoustic impedance is known to
sufficient accuracy that the values can be used directly, and empirical calibration
of these parameters is not necessary. A library of material names, density and
acoustic impedance of the more commonly deposited materials is stored in the
MDC-370 memory. These materials are also listed in Table 9.1.
If the values of the density and acoustic impedance are not known they can be
calibrated empirically as described below.
Calibration requires the establishment of the film density, the tooling factor and
the acoustic impedance in this order. If the approximate value of the parameters
is known they should be used initially. If the acoustic impedance is not known
use the value 8.83, the value for quartz.
10.6.1 FILM DENSITY
Establishing the film density can be accomplished by depositing a trial film on
several test substrates placed around and as close as possible to the sensor crystal
and in the same plane. The trial deposition should be thick enough to allow an
independent measurement of the film on the test substrates to be made with
adequate precision using an optical interferometer or surface-measuring device.
When making the trial deposition, use a fresh crystal and remember to write down
the final thickness reading displayed by the MDC-370, as this will be needed in
the calculation. If the acoustic impedance parameter has been accurately
established previously, a fresh crystal is not required.
Determine the average film thickness on the test substrates and use the following
equation to calculate the material's density:
Density = (Displayed Thickness/Average Measured Thickness) * Density(test)
Where Density(test) is the density parameter setting used during the calibration
run.
Once the calibration procedure is complete, the programmed film density is
correct for this particular film.
THEORY OF OPERATION 10-5