Superior Crystals for Harsh Process Film Thickness Monitoring
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. For this reason, Colnatec invented two types of monitor crystals that operate stably above 100°C, are stress insensitive, and can operate in plasmas–the HT™ quartz crystal, good to 300°C, and the SuperQuartz™ (SQ™) crystal, a revolutionary crystalline material good to 500°C+. Both crystals now enable measurement of ALD, CVD, OVPD, high temperature PVD and thin film furnace processes such as selenization and indiffusion.
SQ™ crystals can be used in place of standard AT-cut quartz in all commercially available film thickness monitors and controllers, but given their sensitivity and expense, we recommend them specifically for environments where quartz can no longer operate. They are available in 5 or 6 MHz versions, with gold, aluminum, or platinum electrodes, sized in 14mm diameters.