Quartz-Crystal Microbalance
A quartz-crystal microbalance (QCM) and a quadrupole mass spectrometer (QMS) both monitor a process chamber's environment at extremely fine sensitivity, but measure completely different things — QCM weighs deposited material directly by tracking a vibrating crystal's resonant frequency shift, while QMS identifies which molecular species are actually present in the chamber's gas by sorting ions by mass.
QCM's sensitivity comes from a beautifully direct physical relationship: adding mass to a vibrating quartz crystal lowers its resonant frequency in a precisely predictable way. As material deposits onto a QCM sensor placed inside or near the deposition zone, the added mass shifts the crystal's natural vibration frequency downward by an amount that converts directly to deposited thickness or mass — sensitive enough to detect deposition rates far too slow to measure any other way, which is why QCM sensors are standard equipment for calibrating and monitoring thermal evaporation and sputtering deposition rates in real time.
QMS's mass-sorting capability catches contamination and process gas problems a simple pressure gauge would miss entirely, because total chamber pressure says nothing about composition. A pressure gauge reports how much gas is present but can't distinguish process gas from an unexpected contaminant or leak species at the same total pressure; a QMS sorts ions by their mass-to-charge ratio and identifies exactly which molecular species make up that pressure reading, catching a water-vapor leak, an unexpected reaction byproduct, or outgassing contamination that a total-pressure reading alone would never reveal.
QCM and QMS both push chamber-condition monitoring well beyond what a simple pressure or temperature gauge can tell you — one by weighing deposited material directly at sub-monolayer sensitivity, the other by identifying the actual chemical makeup of whatever gas is present — and fabs deploy both as the finer-grained diagnostic layer beneath routine process monitoring.