As a deposition system manufacturer, we have the opportunity to investigate what causes process problems, and how best to fix or at least address them. When you invest in a Techne IBAD system from Plasma Process Group, you also benefit from our insight into these problems that have plagued deposition processes since their inception. Here is a piece of that insight:
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 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
The slope of the line comes from your existing data starting when the measured deposition rate becomes constant (derivative of the qcm measurements becomes constant). Using the data above, this is illustrated graphically in the chart below. The slope should be close to your expected
deposition rate, and will be the true deposition rate calculated by the QCM.
Getting the slope from your QCM measurements
After calculating the slope, you can determine the offset (see chart below). This offset is how much overshoot you would have had if you ignored the thermal effects to the thickness measurement.
QCM Measurement Offset
Graphically, shifting the line upward until it passes through (0,0) will allow you to see how long the layer should have been run to meet your desired thickness (chart below). Alternatively, the layer could be terminated early by the offset thickness.
A QCM is capable of fantastic levels of precision. By correcting for the impact of thermal variation, your process can begin to take better advantage of the true capacity of your QCM.