INFICON PLO-10i Phase Lock Oscillator - Manual (Page 38)

PLO-10 PHASE LOCK OSCILLATOR
CRYSTALS, HOLDERS AND FLOW CELL 7-4
(0.081 Hz/ng/cm 2 for a 6 MHz crystal @ 20° C)
(0.181 Hz/ng/cm 2 for a 9 MHz crystal @ 20° C)
∆ m = the change in mass per unit area, in g/cm 2
The minimum detectable mass change is typically a few ng/cm
2 and limited by the noise
specifications of the crystal oscillator and the resolution of the equipment used to
measure frequency shifts. For example, the INFICON RQCM has a frequency resolution of
0.03 Hz @ 6 MHz, therefore, its minimum detectable mass change is 0.37 ng/cm 2 .
The Sauerbrey equation relies on a sensitivity factor, C
f
, which is a fundamental property
of the QCM crystal. Thus, in theory, the QCM mass sensor does not require calibration.
This ability to calculate the mass sensitivity from first principles is obviously a very
attractive feature of these devices. However, it is very important to notice, that the
Sauerbrey equation is only strictly applicable to uniform, thin-film deposits originating
from a low pressure (i.e. vacuum) gas environment 21 . Thick deposits and operation in
liquid environments or in contact with lossy films, relies on the use of more complex
equations relating the frequency shifts to mass loading, and often requires calibration of
the setup for accurate results. Several articles have been published on simple ways to
calibrate the mass sensitivity of QCMs for electrochemical applications 4 and for vacuum
thin-film deposition processes 5 6 7 , and some useful calibration guidelines are also
described herein.
Many studies have shown that the crystal's sensitivity is approximately Gaussian. The
maximum sensitivity is in the center of the crystal and it tapers off towards the edge of
the active area 8 9 10 . The mass sensitivity distribution has also been shown to become
slightly more confined to the electrode region as the mass loading is increased.
7.1.7 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 11 :
♦ 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
♦ Temperature: See section 7.1.9 for data on frequency versus temperature for
INFICON’s crystals.
Other factors that can affect stability are humidity, shock, vibration and change in
pressure. Controlling those conditions is a must to insure accurate measurements of