INFICON IC/5 Thin Film Deposition Controller - Manual (Page 47)

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IC/5 Operating Manual
Needless to say, similar errors occur with the Z-match™ technique under similar
circumstances. However, the automatic Z-ratio estimate is somewhat more prone
to error, because the amplitude distribution of the mode [102] is asymmetric,
whereas that of the mode [100] is symmetric over the active area of the crystal.
In our experience, film-induced stress on the crystal has the most deleterious
effect. This effect is most pronounced whenever there is a presence of gas in the
environment, for example, in reactive evaporation or sputtering processes. In such
cases, if the bulk Z-ratio is already well known, it is better to use the bulk value
instead of the automatically determined Auto Z-ratio. In cases of co-deposition and
sequential layers, automatic Z-ratio estimation is significantly superior.
2.1.7 Control Loop Theory
The instrumental advances in measurement speed, precision and reliability would
not be complete without a means of translating this improved information into
improved process control. For a deposition process, this means keeping the
deposition rate as close as possible to the desired rate. The purpose of a control
loop is to take the information flow from the measurement system and to make
power corrections that are appropriate to the characteristics of the particular
evaporation source. When properly operating, the control system translates small
errors in the controlled parameter, or rate, into the appropriate corrections in the
manipulated parameter, power. The controller’s ability to quickly and accurately
measure and then react appropriately to the small changes keeps the process from
deviating very far from the set point.
The controller model most commonly chosen, for converting error into action is
called PID. In the PID, P stands for proportional, I stands for integral and D stands
for derivative action. Certain aspects of this model will be examined in detail a little
further on. The responsiveness of an evaporation source can be found by
repetitively observing the system response to a disturbance under a particular set
of controller settings. After observing the response, improved controller parameters
are estimated and then tried again until satisfactory control is obtained. Control,
when it is finally optimized, essentially matches the parameters of the controller
model to the characteristics of the evaporation source.
It is quite laborious and frustrating to tune a controller for an evaporation source
that takes several minutes to stabilize. It may take several hours to obtain
satisfactory results. Often the parameters chosen for a specific rate will not be
satisfactory for another. Ideally, it would be nice if a machine could optimize itself.
INFICON’s IC/5 can do this. In an operator initiated mode that is used during initial
setup the instrument will measure the source characteristics. Slow sources are
characterized by having significant dead time, whereas fast sources have no dead
time.
For a slow source, the IC/5 will use a PID model to calculate optimum source
control parameters. For a fast source, the IC/5 will use an integrating control loop
model to calculate optimum source control parameters