INFICON RQCM - Quartz Crystal Microbalance Research System - Manual (Page 42)

RQCM – RESEARCH QUARTZ CRYSTAL MICROBALANCE
CRYSTALS, HOLDERS AND FLOW CELL 4-4
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.
4.1.7 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 4.1.9 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 small mass changes
over long periods of time.
4.1.8 CRYSTAL LIFE EXPECTANCY
It is difficult to predict the useful life of a crystal since it depends on many factors. Some of these
factors are 11 :
♦ The quality of the quartz
♦ The amount of deposited material
♦ The stress generated in the crystal due to deposited material