INFICON RQCM - Quartz Crystal Microbalance Research System - Manual (Page 43)

RQCM – RESEARCH QUARTZ CRYSTAL MICROBALANCE
CRYSTALS, HOLDERS AND FLOW CELL 4-5
♦ The acoustic losses in the deposited material
♦ The design of the oscillator circuitry
Other aspects that affect the crystal life include the type of the deposited material, spitting of
source material resulting in non- uniform films, film flakes that landed on the crystal’s active
area, and of course, physical damage to the crystal such as chipping, cracking, or peeling of the
electrode, etc.
In general, a sensor crystal can be used until its frequency drops below 50% of its uncoated value.
However, for the reasons stated above, crystal failures often occur well before a 40% shift in
frequency is reached.
The sensor crystals are considered expendable. However, a crystal may be reused up to 20 times
on average in experiments that don’t physically alter the crystal electrode. In experiments where
a film is deposited, the crystal can be stripped using a chemical etchant . Care must be taken so
only the deposited material is stripped and not the crystal electrodes. The amount of times that a
crystal can be reused greatly depends on its condition after each experiment or stripping.
Needless to say, careful handling and cleaning of the crystal is required to maximize its re-
usability.
Noisy or erratic measurement indicates that the crystal is about to fail. It might even be difficult
to obtain a stable baseline. Spurious signals might become evident in electrochemical QCM
experiments. Visually, traces of consumption and wear can often be seen on the crystal surface.
Edges of the sensor crystal might become cracked and the deposited film, even the electrode,
starts to show scratches and tears.
The crystal motional resistance R does reflect the influence of deposited material on the
performance of a crystal. This resistance is associated with the damping of acoustic waves by the
electrodes, deposited materials, and the supporting structure. This resistance increases as more
material is being deposited onto the crystal 11 . This resistance value can be used to determine
when a crystal reaches a maximum loading.
4.1.9 TEMPERATURE COEFFICIENT
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 experiments 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 1 inch crystals are optimized for two operating
temperatures namely 90ºC and 25ºC. These crystals have very good temperature stability when
operating close to their specified temperature.
Even though AT cut crystals are designed to minimize the change in frequency due to
temperature, the effect of temperature can be significant when attempting to resolve small mass
(frequency) changes over long periods of time. This frequency change due to temperature is
magnified when the sensor crystal is submerged in liquids. This is due to the coupling of the
shear mode oscillation with the temperature dependent viscosity and density of the fluid. For
experiments in liquid phase in which the frequency is to be monitored over long periods of time,
the temperature must be controlled to at least 0.1°C, and preferably better. In electrochemical
experiments this is often achieved with temperature controlled baths and jacketed cells. It is