by Scott Grimshaw, Colnatec USA
The "Achilles heel" of thin film deposition process monitoring and control is the quartz crystal microbalance (QCM). Since its advent in the 1960's, QCM's have been an integral part of most commercial thin film coating systems. Unfortunately, the limitations of QCM's for processes such as optical coating, ion beam sputtering, MBE and CVD have not been adequately addressed.
A new class of QCM sensor, designed for elevated temperatures, high stress dielectrics and extremely thin coatings is now available. This new sensor embodies an advanced crystallographic cut, "smart" sensor housings with integrated heaters, and novel crystal materials to extend the range of thin film monitoring to up to 250°C.
INTRODUCTION
Quartz crystal microbalances operate in a relatively simple fashion. The QCM consists of a disc of quartz cut at a specific angle and shape from a bar of synthetically grown quartz. This quartz disc is then coupled into an electrical circuit and caused to vibrate at its natural resonance frequency. The resonance changes (decreases in frequency) whenever a thin film coating collects on the crystal surface. If the density of the film material is known, an algorithm can be used to compute the film thickness.