INFICON PLO-10i Phase Lock Oscillator - Manual (Page 39)

PLO-10 PHASE LOCK OSCILLATOR
CRYSTALS, HOLDERS AND FLOW CELL 7-5
small mass changes over long periods of time.
7.1.8 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
♦ 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,
splitting 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.
7.1.9 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