If you run quartz crystal microbalance (QCM) controlled processes, you probably have noticed that a quartz crystal can seem very unstable, particularly on short layers. Since you rely on the accuracy of this device for the repeatability of your coatings, instabilities and noise can be a major headache and lead to inaccuracies. Selection of the appropriate crystal material makes a significant difference.
Measurements taken on a Techne IBAD system have shown a significant improvement in crystal stability when using Alloy-coated crystals instead of Gold-coated crystals. Examine the two graphs below for a comparison.
Gold coated crystal
Alloy coated crystal
Both crystals in the graphs are new and running the same process, but the gold crystal shows significantly higher instabilities and noise. The reasons for this are many; however, the primary reason lies in thermal stability. QCMs measure thickness based on resonance frequency, which is strongly effected by temperature. Each crystal-coating combination will carry a different temperature at which the thermal effects are weakest, and operating your QCM at that temperature will give the most stable and highest precision readings. At standard IBAD process temperatures between 70 to 100°C, the Alloy-coated crystal has a much better response than a standard Gold-coated crystal.