INFICON Maxtek PM-700 Plating Monitor - Manual (Page 77)

PM-700 SERIES PLATING MONITOR
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where: R
z
= The acoustic impedance ratio which is obtained by dividing the
acoustic impedance of quartz by the acoustic impedance of the deposited layer.
This equation introduces another term into the relationship: the ratio of the
acoustic impedance of quartz to the acoustic impedance of the deposited film.
The acoustic impedance is that associated with the transmission of a shear wave in
the material. Note that if the acoustic impedance ratio is equal to one, quartz on
quartz, equation 3 reduces to equation 2.
Although the above equation still involves a number of simplifying assumptions,
its ability to accurately predict the thickness of most commonly deposited
materials has been demonstrated.
The use of microprocessors allows an equation as complex as equation 3 to be
solved economically and implemented in the Monitor.
The actual film mass on the crystal is then determined by applying the acoustic
impedance correction factor.
At the start of the deposit or at zero the initial equivalent quartz mass and the
initial corrected material mass is stored. For each subsequent measurement the
new corrected total mass is calculated and the mass deposited since the start of
deposit is determined by subtracting the initial mass from the total mass.
The thickness on the crystal is calculated by dividing the material mass by the
material density.
The thickness on the substrates is then calculated by multiplying the thickness on
the crystal by a tooling factor.
If the acoustic impedance parameter is changed following a plating process, both
the total and the initial masses are recalculated. This allows the effect of the
changed parameter value to be immediately displayed.
6.1.4 Rate Calculation
The plating rate is calculated by dividing the change in thickness between
measurements by the time between measurements. The rate is then filtered by a
three-pole digital filter to minimize the quantizing and sampling noise introduced
by the discrete time, digital nature of the measurement process. Following a step
change, the displayed rate will settle to 95% of the final value in 7 seconds and to
99.5% of the final value in 8.5 seconds.
6.2 FINE TUNING WITH EMPIRICAL CALIBRATION
If the density and acoustic impedance of the material being plated is known, the
values should be entered into the Plating Monitor, and a trial plating deposition
should be made. If the displayed thickness does not agree with an independently
measured thickness, the unit should be calibrated as described below.