SPM Fundamentals for the MultiMode
Control Parameters and Feedback
Rev. B
MultiMode SPM Instruction Manual
37
To better understand what is being viewed when selecting different
Data type
s, consider the
diagram below:
As this diagram shows, different portions of the feedback loop are being accessed, depending upon
what is selected at the
AFM mode
and
Data type
parameters. During STM, the signal coming into
the feedback controller (at line In
0
) is
Current
. In contact AFM, the same line now conveys
De
fl
ection
voltage. In TappingMode, the same line conveys
Amplitude
data. In addition, the
auxiliary channels utilize line In
A
through In
D
.
Note:
Although there are four separate auxiliary lines (In
A
through In
D
), the
controller can only access one at a time, plus the In
0
line.
The controller is designed to handle two input signals simultaneously (In
0
and one auxiliary line).
Because it is extracted from a different (output) portion of the feedback loop, a third channel can be
used to simultaneously extract
Height
data without affecting the input signals. This allows a
maximum of three
Data types
for any one sample. For example, a sample being imaged with
contact AFM can show
Height
,
De
fl
ection
and
Friction
all at the same time.
2.4
Control Parameters and Feedback
The feedback scheme described in
above provides a general platform for imaging
sample surfaces; however, vast differences in samples require additional controls to obtain optimal
images. The NanoScope imaging system utilizes over 1000 parameters. The vast majority of these
are never seen by users, but those that are common should be reviewed. This section provides a
brief description of some of the more commonly used parameters.
2.4.1 Reexamining the Control Loop
Recall that the NanoScope control system performs two main functions: 1) it generates drive
voltages to control the X-Y scans of the piezoelectric transducer; 2) it maintains an incoming
analog signal from the microscope detection circuitry at a constant value. This is done by way of a
closed-loop feedback control system. The computer is programmed to read voltage from a
Signal out (to Z piezo)
Signal in (In0)
Feedback Controller
deflection, amplitude, current
height
auxiliary: phase, frequency, deflection during
Microscope
Signal in (Inaux)
TappingMode, friction.
