Sensor crystal, Crystal health, Sensor – INFICON MDC-260 Thin Film Deposition Controller User Manual

Page 127: Crystal -3, Crystal health -3

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MDC-260 DEPOSITION CONTROLLER

THEORY OF OPERATION 10-3

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-260.

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 10.5.3.

10.3 SENSOR

CRYSTAL

An essential part of the MDC-260 system is the sensor crystal. A good
understanding of its behavior will help minimizing unnecessary problems, and
ensure proper and reproducible measurements. Refer to Section 9.5.1 and read
the following sections carefully to get a full understanding of the sensor crystal.

10.3.1 CRYSTAL HEALTH

The crystal health is calculated and displayed in two different ways, depending on
which sensor oscillator is used. If the SO-100 was used, the Crystal Health is
displayed as a percentage, which decreases from a value of 99% 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. Because the crystal health is determined from
the calculated film mass, the Acoustic Impedance parameter will affect the
displayed crystal health.

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