QCM (quartz crystal microbalance) sensors measure the mass variation per unit area by measuring the changes in the resonator frequency of a quartz crystal using its piezoelectric properties. Frequency changes linearly with the mass (or thickness). Quartz crystal monitoring systems are an essential component of the thin film deposition vacuum chambers, usually used in mass production.
It’s very important to control the temperature, since the resonance frequency can change a lot with the temperature too. There are different types of crystals depending on its tolerance with the temperature.
Usually, work at the Ångström level (Order of 10-10m). Little thickness deviations such as 10 or fewer Ångströms can have a major effect on the product performance. QCM sensor measurements have been improving more and more in the past years. Nowadays advanced optical, electrical, and magnetic devices require a process sensor capable of measuring layer thicknesses with 1 Ångstrom resolution reliably.
How does a typical QCM system look?
Inside the vacuum chamber, the sensor is placed near the substrate to measure the thickness of the film. It’s important to know its operating frequency range and the electrode quality of this crystal. The vacuum chamber must have a temperature controlled housing with the function of regulating the temperature of the sensor and its mechanical integrity.
Outside the chamber, an electric monitor creates a current which forces the crystal into vibration and tracks this frequency over time. It’s really important that this monitor can track accurately the crystal frequency changes and temperature characteristics. An internal microprocessor can convert this frequency changes in time to the actual thickness of the film.
Principal characteristics of the quartz crystals:
The crystal can have different electrode shape depending on the model system you are purchasing
Physical Specifications of the crystal:
- Material: Pure Silicon Dioxide monocrystal.
- Dimensions: 0.550” +. 000/.002” OD
- Contour: 2.5 – 3.0 diopter (212 – 177 mm radius of curvature) plano-convex
- Finish: 7 microns rms roughness plano and contour sides
- No chips, scratches or etch marks within a 13.95 mm diameter clear aperture
Electrical Specifications of the crystal:
- Available Frequencies: 5 and 6 MHz, fundamental series resonance
- Resistance: <10-15 Ohms
- Contact Resistance: <10 Ohms edge to edge on pattern side