INFICON TM-400 Thin Film Deposition Monitor - Manual (Page 60)

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TM-350/400 Operating Manual
9.3 Sensor Number (TM-400 Only)
The Sensor Number parameter defines the primary sensor for the film, meaning
the sensor that the film will start with. This parameter is included because it is
desirable to use one sensor/crystal for one material and the other sensor/crystal for
the second material in a multi layer application.
9.4 Density
The Density parameter provides the Monitor with the density of the material being
deposited so that it can calculate and display the physical film thickness. If the film
density is known, it should be used. A list of the more commonly used film densities
is presented in
Table 9-2 on page 9-4 . As a first approximation, bulk material
density can be used in programming this parameter. Empirical calibration of this
parameter is described in
section 9.6.1 on page 9-3 .
9.5 Acoustic Impedance
The shear wave acoustic impedance of the deposited film is required by the
monitor in order to accurately establish the sensor scale factor when the sensor
crystal is heavily loaded. If the acoustic impedance of the film material is known, it
can be entered directly in units of 100,000 gm/sq. cm sec. In most cases the
acoustic impedance of the bulk material can be used and can be obtained from the
Handbook of Physics or other source of acoustic data. The shear wave acoustic
impedance can be calculated from the shear modulus or the shear wave velocity
and the density by using the following equation:
[3]
where:
P = Density (gm/cm
3
).
C = Transverse (shear) wave velocity (cm/sec).
G = Shear modulus (dynes/cm
2
).
A list of the acoustic impedance and density of the more commonly deposited
materials is presented in
Table 9-2 on page 9-4 , and a technique for empirically
determining this parameter is presented in section 9.6.3 on page 9-4 .
In many cases, and particularly if the sensor crystal is not heavily loaded, sufficient
accuracy can be achieved by using the acoustic impedance of quartz:
8.83 X 100,000 gm/sq. cm sec. [4]
acoustic impedance PC PG = =