INFICON MDC-260 Thin Film Deposition Controller - Manual (Page 130)

MDC-260 DEPOSITION CONTROLLER
THEORY OF OPERATION 10-6
become cracked and the deposited film, even the electrode, starts to show
scratches and tears.
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.
10.3.5 CRYSTAL ELECTRODE TYPE RECOMMENDATION
There are generally three film deposition categories, low stress, high stress, and
dielectric films. Recommended crystal electrode types for each category are
explained below.
Low Stress Films : Gold electrode crystals are recommended for deposition of
low stress materials like aluminum, copper, gold, silver, etc. These materials
generally soft and produce very little or no tensile stress. You can also use silver
or aluminum electrode crystals for these materials.
High Stress Films : Materials like chromium, inconel, molybdenum, nickel,
nickel-chrome, zirconium, titanium, etc., will produce high stress films. These
films often "pull" on the crystal's sensing electrode surface, which may sometimes
alter its oscillation, creating rate noise, oscillation failure, etc. We recommend
that silver electrode crystals to be used for these films. The silver electrode
surface yields and absorbs the stress on the quartz. Aluminum electrode crystals
are also recommended for these materials.
Dielectric Films : Aluminum electrode crystals are recommended for use in
deposition of dielectric materials like aluminum oxide, magnesium fluoride,
silicon monoxide and dioxide, titanium dioxide, thorium fluoride, etc. However,
the trick is to get these materials to stick to the aluminum electrode. Usually,
heating up the crystal (and the substrates) would dramatically enhance the
adhesion.
10.4 RATE CALCULATION
The deposition rate for each sensor is calculated by dividing the change in the
measured thickness by the time between measurements. The rate is then filtered
by a three pole digital filter to filter out quantizing and sampling noise introduced
by the discrete time, digital nature of the measurement process. The above filter
has an effective time constant of about 2 seconds. Following a step the displayed
rate will settle to 95% of the final value in 5 sec. The individual sensor rates are
displayed in the Source/Sensor status screen.
If only one sensor is enabled for measurement, then the displayed rate is the
measured rate for only that sensor. If multiple sensors are enabled for
measurement, then the displayed rate is the weighted average of all of the sensors
enabled for measurement.
10.5 EMPIRICAL CALIBRATION
For many film materials the film density and acoustic impedance is known to
sufficient accuracy that the values can be used directly, and empirical calibration