A quartz crystal holder, or instrument, is a wand-like device used to monitor a film deposition process. A standard Colnatec quartz crystal instrument consists of a “head” (a compartment containing a QCM, or a quartz crystal microbalance); crystal control cables; a flange (separates vacuum-side from atmosphere-side); and cooling tubes (regulates the temperature inside the crystal compartment). Colnatec offers a quartz crystal instrument that contains a thermocouple, which takes temperature readings around the crystal (see diagram below).
In examining the basics of quartz crystal monitoring systems, it is worth noting that the key to a deposition monitoring and control system is the quartz crystal. By nature, quartz crystal contains reactive elements that respond to electrical stimulation. Due to this “piezoelectric effect”, applying an alternating current to the crystal produces oscillation at a constant, predictable, and measurable rate. As material is deposited onto the crystal, the rate of oscillation frequency is reduced due to an accrual of mass on the crystal. The monitor/controller, which reads the oscillation frequency of the crystal, immediately registers any frequency changes. The frequency change is directly correlatable to the amount of the mass that has been deposited onto the crystal. In this way, thin film thickness can be measured with a high degree of precision – even to a single angstrom!
A variety of crystals are available depending on the deposition process being performed. Different crystal types might produce different oscillation rates or be able to withstand high temperatures.
The basics of quartz crystal monitoring systems involves sealing the quartz crystal instrument inside a vacuum chamber where a deposition process is to occur (see illustration below). Once the crystal-end of the instrument is sealed inside the chamber, the base connectors of the instrument remain accessible to accommodate various cable connections that are involved in the deposition monitor/control process (an optional cooling system would also connect directly to the base).
The quartz crystal instrument is attached to the monitor/controller, which in turn is connected to a PC where the user accesses control software that is used to monitor/control the deposition process taking place inside the chamber.
What are Quartz Crystal Monitoring Systems?
This article provides additional information on the workings of quartz crystal monitoring systems.
A typical QCM-based thin film deposition system