Selecting the Best Crystal for
Your Coating Process
A quartz crystal microbalance (QCM) takes
advantage of the piezoelectric effect found in quartz crystals.
Application of an electric potential across the quartz crystal
induces mechanical shear strain in the crystal. If the polarity
of this electric potential is reversed, the strain direction
reverses. Rapid oscillation of the electric potential polarity
leads to vibrational motion of the quartz crystal. Under the
proper conditions, this vibration can induce an acoustic
standing wave between the two crystal faces. The frequency of
the standing wave is proportional to the thickness of the quartz
crystal. If additional material is uniformly deposited on the
face of the crystal, the additional thickness will decrease the
resonant frequency of the acoustic wave. This frequency shift
due to mass deposition may be correlated to the absolute mass
deposited via the following substituted form of the Sauerbrey
equation:
where rq is the density of quartz,
Aq is the area of resonance, Nq is a
frequency constant for AT-cut quartz crystals (1.668 X 105
Hz cm), Fq is the frequency of quartz prior to
deposition, and F is the frequency at any point during the
deposition process. This equation is only valid if the total
frequency shift is kept within 2 percent of the starting
frequency.
In summary, the thickness of the added layer changes the
wavelength of the standing wave resonance. In essence, a
deposited film acts as if the quartz is increasing in
thickness. The thicker the crystal, the longer the
resonance wavelength. This is measured as a frequency shift at
the monitor. The film density value is input in order to
compensate the density difference between the film deposited
and the density of quartz which is 2.648 g/cc.
Our crystals are plano-convex, meaning they are thicker at the
center than the edges.
Given the wide variety of coating technologies available to the
thin film engineer and scientist, it can be difficult to
determine the best monitor crystal type for a given process. We
will attempt to generally categorize processes and the
appropriate crystals in the following:
Low-Stress Metalizing
The most common thin film process is the
deposition of metals such as aluminum, gold, copper, and silver
to provide electrical contacts or optical reflectance. These
films are relatively free of tensile or compressive stress and
are deposited at room temperature. They are soft and easily
scratched but do not tend to flake off or damage substrates.
Such films can easily be
monitored using either gold, silver, or alloy electrode
crystals. One can deposit over 30’000Å of gold or 200’000Å of
aluminum on 6 MHz X-TRONIX Quality Quartz Crystals before
changing to a new sensor.
Recommended Crystal: XQI-8010G (Gold)
High-Stress Metalizing
Thin films of nickel, chromium,
molybdenum, zirconium, nichrome, titanium and inconel among
others develop high stresses when deposited. These films often
flake or crack at a thickness above 1000 Å and in some cases can
even craze or crack the substrates they are coated onto. This
stress is quickly transmitted to the quartz crystal and
manifests itself as a sudden rate jump or a series of rapidly
occurring positive and negative rate spikes. In some processes
this behavior can be tolerated but in others they tend to
negatively impact evaporation source control.
The best choice for these materials is either a silver or alloy
electrode crystal. The electrode compliance (yielding) tends to
reduce the film stress and diminish or even eliminate the
erratic rate changes.
Recommended Crystal: XQI-8009S
(Silver), XQI-8008A (Alloy)
Dielectric (Optical) Material Coating
The most difficult to monitor materials are those in the
dielectric class of materials including magnesium fluoride,
titanium dioxide, silicon monoxide and dioxide, aluminum oxide
and thorium fluoride. Often chosen for their optical
transmission or reflectance properties these films will not
adhere well or give the desired characteristics unless the
substrate is heated to temperatures of 200°C or higher. When
deposited on a water-cooled crystal these films exhibit
tremendous stress upon condensation and can easily cause crystal
failure within the first 1000 Å of coating. Crystal life can be
extended by approximately 50% by raising the crystal head
cooling water temperature to 50°C (from the normal 20°C) in most
cases.
The best choice for these materials is an alloy electrode
crystal. Test results show a usable life increase of 100% for
magnesium fluoride and silicon dioxide. Furthermore positive and
negative rate spikes are diminished by an order of magnitude.
This can often mean the difference between completing a coating
run or aborting.
Recommended Crystal: XQI-8008A (Alloy)
New!
Heated
Crystal Sensor Head
can raise crystal to 90°C. This
doubles crystal life for most dielectrics.
