7 nontraditional applications, 1 etching, 2 immersion in liquids – INFICON XTM/2 Thin Film Deposition Monitor User Manual
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XTM/2 Operating Manual
2.7 Nontraditional Applications
In addition to their normal application as a deposition monitor/controller, quartz
crystal microbalances have significant utility as generalized mass sensors. This
particular instrument family is capable of measuring mass increases or
decreases on the face of the monitor crystal to an accuracy of +/- 1.23
nanograms/cm
2
in a single 250 ms measurement (density = 1.00, z = 1.00). As
always, it is imperative that the mass be well adhered to the face of the crystal
or improper readings will be taken. It is especially important to recognize this
requirement for measurements of liquids or other non-rigid materials.
INFICON’s 6MHz crystal holders have an open area of ~0.535 cm
2
. For the
highest accuracy possible, it is suggested that the individual crystal holder be
measured with a traveling microscope to determine the exact opening area.
2.7.1 Etching
The instrument may be configured to display the thickness or mass removed
from the face of a crystal. It is imperative that the material be removed uniformly
over the active area of the crystal or improper readings will be taken. This
inaccuracy occurs because of radial mass sensitivity differences across the
face of the monitor crystal.
The etch mode is established by setting a configuration switch (refer to
) on the back of the instrument.
The unit is operated normally, with the ZERO or START keys used to zero the
displayed thickness. The FINAL THICKNESS parameter may be programmed
to terminate the process.
2.7.2 Immersion in Liquids
Measurement of mass change in liquids is a relatively new field, consequently
application information is limited. The energy loss from the vibrating crystal into
the liquid environment is high, limiting the accuracy of the measurement in
some cases. The ModeLock oscillator again provides superior performance,
allowing operation in liquids of higher viscosity than an active oscillator system
would provide. The presence of bubbles on the face of the crystal as it is
immersed will drastically change the noted frequency shift and alter the
sensitivity of the technique from immersion to immersion. Special transducers
may be required as many liquids can electrically short the crystal’s drive voltage
and induce a crystal failure.
NOTE: It is not recommended to use standard INFICON transducers in liquids
without modification.