Historically, when quartz crystal monitors (QCMs) were first used to monitor thin film depositions, the oscillator that drove the sensing crystal was imbedded in the sensing head inside the vacuum chamber. These internal oscillators were designed with discrete components and were large in size, expensive, and generally non-repairable. As oscillator design and integrated circuits improved, new lower cost designs came about which allowed the oscillator circuit to be moved to a more friendly, space available, location outside of the vacuum chamber. This works well for most applications, but since the sensing crystal and the cable connecting it to a remote oscillator are part of the oscillator design, the allowable capacitance (length) of this internal cable is limited. It is generally recommended that this cable length be kept to less than 30 inches for optimum performance and life of the sensing crystal. Longer cable length lowers the useful life of the sensing crystal. For larger systems the 30-inch cable limitation can be a problem.
Now, advanced low-cost hybrid-circuit technology allows the design of a compact in-vacuum oscillator/sensor package, which solves the large system application for QCMs. Sycon has developed a hybrid oscillator in a compact sensor holder for use in large systems where standard sensing heads cannot be used. The oscillator is closely coupled to the sensing crystal for higher reliability and longer sensor life. For large system applications the maximum in-vacuum cable lengths are specified in feet instead of inches. A single water line cools both the sensing crystal and the electronics package.
