May 2015 newsletter

Ion Times

QCM Sensitivity with IBAD

Have you ever observed the QCM deposition rate dipping below zero at the beginning of a coating (see the figure below)? Is the QCM losing mass or is the measurement sensitivity just too low? The answer is neither. The QCM measures mass by differences in its resonant frequency. However, that value is also vulnerable to thermal changes. When the QCM is allowed to heat up, it records a negative mass change. This has the secondary effect of creating an offset to your recorded QCM thicknesses, which in turn can throw off the entire layer, particularly if the layer is short!

QCM Thickness Error
QCM thickness measurement during one process layer

The good news is that this effect can be calibrated out. Because the deposition rate during a layer is (very nearly) constant, the QCM measurements will make a straight line, offset by the initial (negative) effect of thermal exposure – the 1st order equation of a line.

Thickness = (Linear Rate)x(Time) + Offset

While the coefficients will change depending on your specific conditions, a calibration run just before your process can provide the data needed to offset this frustrating thermal problem! Visit our get more from your gear web resource center to learn more.