INFICON MDC-370 Thin Film Deposition Controller - Manual (Page 125)

MDC-370 DEPOSITION CONTROLLER
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 film 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 the above equation is implemented in the MDC-370.
The basic measurement is period, which can be thought of as a measurement of
equivalent quartz mass.
The actual film mass on the crystal is then found by applying the acoustic
impedance correction factor.
At the beginning of the deposit, or when the thickness indication is zeroed, the
initial equivalent quartz mass and the initial corrected film mass are stored. For
each subsequent measurement the new corrected total film mass is calculated, and
the film mass deposited since the start of deposit is determined by subtracting the
initial corrected film mass from the total corrected film mass.
The film thickness on the crystal is calculated by dividing by the film mass by the
film density.
The film thickness on the substrates is then calculated by multiplying the film
thickness on the crystal by a tooling factor.
If the acoustic impedance parameter is changed following a deposition both the
total and the initial film masses are recalculated. This allows the effect of the
changed parameter value to be immediately displayed and provides a relatively
straightforward method of empirically determining the acoustic impedance if it is
not available. See section 10.6.3.
10.3 CRYSTAL HEALTH CALCULATION
Crystal Health decreases from a value of 100% for an uncoated crystal blank to 0
at a total deposited aerial mass of 25 mg/cm
2
. This value corresponds to a crystal
frequency shift of approximately 1.5 MHz, or an aluminum thickness of 925 KÅ.
Very few materials can be deposited to this thickness without producing a crystal
failure, so that a crystal health of zero will not normally be achieved and indeed
for some materials the crystal health may never get below 90%.
In order to establish the point at which the crystal should be changed, several trial
runs should be made to determine the point at which the crystal fails and
subsequent crystals should then be replaced well in advance of this point.
Because the crystal health is determined from the calculated film mass, the
Acoustic Impedance parameter will affect the displayed crystal health.
THEORY OF OPERATION 10-3