Quartz crystal elements in microbalances (often called quartz crystal microbalance or QCM) have been used to determine layer thicknesses for nearly 50 years. Here, the resonant frequency of an oscillating piezoelectric crystal is measured. In the addition of a layer (or more precisely, a mass), the resonance frequency changes and thus enables the high precision calculation of film thickness. This technology has become an indispensable tool and the industry standard in process control of optical and electronic components. In the past, due to the frequency-temperature characteristics of quartz, QCMs they were limited to low operating temperatures and used with a water-cooling system to achieve reliable results. Many modern coating methods such as Chemical Vapor Deposition (CVD), Atomic Layer Deposition (ALD) or Molecular Beam Epitaxy (MBE) require significantly higher process temperatures and were therefore not measurable with the methods previously available.
With the development of single crystal Gallium Phosphate, crystal elements are now available which allow accurate measurements of layer thicknesses to 800°C. Like quartz resonators, these elements are used with the existing electronic evaluation and thus open up completely new possibilities for the research and process control of thin layers.