Quartz Crystal for High-Temp Film Thickness Monitoring (400°C+)
- 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 film thickness. In practice, however, this is rarely achieved.
A film thickness monitor measures the change in resonance frequency of an oscillating quartz crystal while a thin film 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 film can be calculated in real-time.
A film thickness monitor, however, works on the underlying assumption that any change in the resonance frequency is solely a result of film build-up. Unfortunately, quartz crystals can also change resonance frequency when exposed to a thermal gradient or mechanical stress. In a typical thin film deposition, both of these phenomena exist due to either the deposition source radiation, highly energetic species (as in sputtering) or stresses caused by film condensation. Often these factors exist in concert.
For processes that operate at temperatures above 100°C, standard quartz crystals are extremely noisy. This noise prevents accurate and stable film thickness measurements. To solve this problem, Colnatec has invented monitor crystals that operate stably above 100°C, are stress-insensitive, and can operate in plasmas. The most vigorous of these, SuperQuartz (SQ™), maintains operating efficiency at temperatures of 500°C+. The SuperQuartz is ideal for processes requiring intense heat, such as ALD, CVD, OVPD, high temperature PVD, and thin film furnace processes such as selenization and indiffusion.
As an added benefit, SuperQuartz crystals are able to disregard the rate spike caused by temperature variances. This usually occurs when the deposition source shutter is opened or the crystal is exposed to plasmas. Typically, this action causes a frequency shift of up to 100 Hz, which translates to rate changes of 50 angstroms or more for films such as aluminum. 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 films used in the manufacture of OLEDs, precision optical interference films, or next-generation electronic devices (e.g., solar cells).
Although it can be used effectively in place of standard AT-cut quartz in all commercially available film thickness monitors and controllers, the SQ™ crystal is ideally used in Colnatec high temperature sensor heads. SuperQuartz is available in 5 or 6 MHz versions, with gold, aluminum, or platinum electrodes, sized in 14mm diameters.
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SuperQuartz™ crystals (400°C+)
Gold or Alloy
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