INFICON MDC-260 Thin Film Deposition Controller - Manual (Page 128)

MDC-260 DEPOSITION CONTROLLER
THEORY OF OPERATION 10-4
10.3.2 CRYSTAL STABILITY
A sensor crystal cannot distinguish the difference between a frequency shift due
to deposited material or that due to other disturbances. Thus any extraneous
factors, other than the deposited mass, which may cause the quartz crystal to
change its resonant frequency, must be properly controlled. Factors that can
influence the stability of a sensor crystal are categorized as follows:
♦ The crystal itself: Improper design, localized stress, damage to the crystal
♦ The crystal holder: Improper seating of the crystal, large mechanical
coupling between the crystal and the holder
♦ Thermal input: Radiation from evaporation source, radiation from substrate
heater, bombardment by charge particles, energy released by condensates
♦ Stress: Thermal stress, stress release in the deposited materials
Other factors that can affect stability are humidity, shock, vibration and change in
pressure. Controlling those conditions is a must to insure accurate measurements.
10.3.3 TEMPERATURE COEFFICIENT OF THE CRYSTAL
The temperature coefficient of quartz crystals is normally specified in units of
parts per million per degree of temperature change. A one part per million change
in frequency of the sensing crystal corresponds to an indicated thickness change
of approximately 7.4 Å for a material with a density of 1.0 gm/cm³. For
Aluminum with a density of 2.7 gm/cm³, this is equivalent to approximately 2.7Å.
This intrinsic dependence of resonance frequency of a sensor crystal on
temperature is generally small in applications in gas phase when operating at or
near its “turn-around-point”. The “turn-around-point” is where the temperature
coefficient of the crystal is zero. That is, there is no change in resonance
frequency due to a change in the temperature of the crystal at the turnaround
point. INFICON vacuum crystals are optimized for operating temperature of 70ºC.
These crystals have very good temperature stability when operating close to their
specified temperature.
Over the temperature range of 20
° to 100° C, the worst case temperature
coefficient is equivalent to approximately 2 Å of Al per degree C of crystal
temperature change. However, because the temperature coefficient is not
constant, the worst case drift in indicated thickness over a crystal temperature
range of 20° to 100° C, is approximately
± 100 Å of Al.
The typical worst-case drift in indicated thickness over the range of 20° to 100° C
(68° to 212° F) is on the order of 50 Å. The indicated thickness drift may be
either positive or negative depending on the actual operating temperature and the
actual crystal cut.
Once the turnaround point is exceeded, the temperature coefficient will always be
negative with respect to indicated thickness (positive with respect to frequency
change). That is, as the temperature of the crystal continues to increase, the
indicated thickness will slowly decrease even though the actual thickness is
constant.