Patented RC™ Quartz Crystals for Precision Film Thickness Monitoring
“Thank you for suggesting the RC™ crystals to us. We have started using them in our integral QCM for ALD and they are superb! The data we are getting is very smooth – considerably better than what we saw with the AT-cut crystals under these conditions.” – Shannon C. Riha, Ph.D., Materials Science Division, Argonne National Laboratory
The measurement of film thickness during a vacuum deposition process can be accomplished with great accuracy and precision using a quartz crystal microbalance, or QCM. Under controlled conditions, it is possible to achieve angstrom level resolution of the fi lm thickness. In practice, however, this is rarely achieved.
New data! RC proven most accurate sensor in the world!
A film thickness monitor measures the change in resonance frequency of an oscillating quartz crystal while a thin fi lm coating is collecting on its surface. As the coating builds up, the resonance frequency decreases in a very predictable fashion. If the density of the deposited film is known, the thickness of the fi lm can be calculated in real-time. A film thickness monitor works on the underlying assumption that any change in the resonance frequency is solely a result of fi lm build-up. Unfortunately, quartz crystals can also change resonance frequency when exposed to a thermal gradient or mechanical stress.
In a typical thin fi lm deposition, both of these phenomena exist due to either the deposition source radiation, highly energetic species (as in sputtering) or stresses caused by fi lm condensation. Often these factors exist in concert.
For ultra-thin fi lm thickness measurements, in the 10 to 100 angstrom range, a combination of frequency shifts caused by heat, stress and fi lm build-up can lead to thickness errors of 100% or more. As a result, the vacuum process becomes uncontrollable. We developed a new quartz crystal that mitigates these factors. Called the RC™ crystal, this recently patented design is insensitive to frequency shifts caused by source radiation or film stress. This is accomplished by adjusting the stress coeffi cients of the quartz plate using advanced fabrication methods.
The RC™ crystal will not show a rate spike when the deposition source shutter is opened. This exciting fact was independently verified in 2011 by BeamTec GmbH, a thin film test lab in Germany. See the results here (pdf). Said the test director:
“It is quite impressive that the RC™ quartz doesn’t show any negative effect on the shutter action. Also there’s no drift when the shutter is closed. Really impressive.”
Typically, opening the source shutter causes a frequency shift of up to 100 Hz, which translates for films such as aluminum, to rate changes of 50 angstroms. Further, the noise associated with the intense energy of impinging atoms in sputtering is dramatically reduced, owing to the stress insensitivity of the crystal. These are very real advantages in the measurement of nanometer fi lms used in the manufacture of OLEDs, precision optical interference fi lms, or next generation electronic devices.
RC™ crystals can be used in place of standard AT-cut quartz in all commercially available film thickness monitors and controllers. They are available in 6 MHz versions, with gold, silver or aluminum electrodes, currently sized in 14 mm (0.550″) and 12.5mm (0.490″) diameters.
For additional information, refer to the following datasheets:
RC™ Crystals Datasheet (pdf)
To place an order for RC™ crystals, please contact sales@colnatec.com or call +1 480-634-1449.
