INFICON STM-1 Thin Film Deposition Monitor - Manual (Page 13)

3. Calibration and Theory
3.1 Measurement Theory
The STM-1 uses the resonant frequency of an exposed quartz crystal to sense the mass of deposited films attached to its surface.
There is a known relationship between the mass of such a film and the measured frequency of the sensor crystal. Knowing the
frequency change due to accumulated mass, film thickness is determined by the following equation:
( )
( )
( )
Af =
Nq Dq
Df Z Fc
ArcTan
Z Tan
Fq - Fc
Fq
•
• • •
•
•












•






Π
Π
Where the terms used in the equation are defined as:
Af
Film Thickness, in Angstroms (1a=10-10 Meters)
Nq Frequency Constant for AT Cut Crystal, 1.668 X 1013 Hz/Å
Dq Density of Quartz 2.648 gm/cm3
Π The Constant Pi, 3.14159265358979324
Df Density Of Film Material in gm/cm3
Z Z-Factor of material, is the square root of the ratio [(Dq*Uq)/(Df*Uf)]. Dq and Df are the densities as above and Uq and
Uf are the shear moduli of quartz and the film, respectively. These values are available in several materials handbooks.
Fq
Frequency of sensor crystal prior to depositing film material on it. This value is a manufacturing controlled constant.
Fc Frequency Of Loaded Sensor Crystal.
3.2 Rate Computation
Rate computation is based on the rate of change of these thickness readings, updated ten times per second, and then filtered for
display. Also available from the instrument is the raw measured frequency of the sensor crystal.
3.3 Thickness Reading Calibration
Instrument calibration is affected by three different parameters, material density, material Z-Factor, and tooling. Tooling is a
deposition system geometry correction (location of sensor relative to substrates). Density and Z-Factor are material factors.
3.4 Density Determination
Use of the material bulk density value will normally provide sufficient film thickness accuracy. If additional accuracy is required,
the following procedure may be used:
3.5 Density
Using a new sensor crystal (this eliminates Z-Factor errors) place a substrate adjacent to the sensor so that both sensor and
substrate see the same evaporant stream. Set the instrument density to the bulk value of the material (see the Material Reference
Table in Section 4.3). Set the Z-Factor to 1.000 and the Tooling Factor to 1.00. Deposit approximately 5000 Angstroms of
material on the sensor and substrate. After deposition remove the substrate and measure the film thickness with a profilometer or
multiple beam interferometer. The correct density value may be determined by the formula:
Correct Density = [Trial Value * Measured Thickness (from STM-1)] / (actual thickness (via profile))
After reprogramming with the new value, another run can be made to verify instrument reading against empirical results.
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