SPM Fundamentals for the MultiMode
Control Parameters and Feedback
42
MultiMode SPM Instruction Manual
Rev. B
indication of a good engagement is a distinct jump of about 1V from the
Vertical De
fl
ection
(A-B)
voltage to the
Setpoint
voltage.
The displayed image is an average of the corrections made to Z in a given display period (number
of samples menu item). The two gains are set to values to effectively “tune” the feedback response
to the particular sample topology. This will set response time of the system so that there is no
difference between the SPM's signal and the setpoint reference during scanning.
Proportional gain
—The computer multiplies this number times the value read from the
comparison circuit every time the A/D converter is read. It is the high frequency feedback control.
Integral gain
—This number is multiplied times an accumulated average of A/D readings. This is
the low frequency feedback control.
One of the easiest ways to set the gains properly is to view the input of the feedback loop. This
means displaying the STM current, the AFM de
fl
ection, or the TMAFM amplitude signal. Then
raise the gain values until the input of the feedback is minimized. Note that this will result in an
image that shows only large transitions in Z; this is normal. There will always be a time lag
between the input and the output (height data) of the feedback loop. Usually, the
Integral gain
is
the most sensitive control. Raise the gains together until the input signal (current, de
fl
ection or
amplitude) is minimum. Don't set them so high as to cause oscillations in the image. Oscillations
are an indication of too much feedback correction voltage sent to Z. This is generally known as
feedback oscillation. The
Proportional gain
can usually be set about 20 percent higher then the
Integral gain
, but it is not required.
