Biomedical Engineering Reference
In-Depth Information
(a) A two-dimensional representation of the image shows the topography of the
specimen being scanned. To correct for tilt between the probe and sample, the height
image is typically line levelled in real time (see Section 5.1.1 for an explanation of
levelling). Without real time line levelling, the image will only show the tilt between
the probe and sample. Usually multiple channels can be shown simultaneously.
(b) An 'oscilloscope window' displays a two-dimensional scan line such as the
Z
signal
versus the
x
axis motion. An oscilloscope window is very helpful for optimizing the scan
parameters and ensuring that the probe is tracking the sample's surface. Typically, data
collected in the forward and backwards directions data can be overlaid.
(c) As discussed in Section 2.3, there are a number of data channels that may be
monitored in the AFM instrument. They include the
Z
piezo voltage,
Z
error signal,
Z
motion sensor, and phase and amplitude signals. The AFM control software will allow one
or more of these signals to be displayed on the screen. They may be available as either
images, or scan line data or both. In addition to the height data, viewing the
z
error signal
while optimizing acquisition parameters can be particularly helpful.
2.4.2 Stage control
Making an AFM practical to use requires motion control including at least one stepper
motor to move the probe relative to the sample in the
Z
axis. Additional motion control is
used for moving the sample in the
X-Y
axis relative to the probe as well as controlling the
zoom and focus of an optical microscope.
(a)
Z
motion control: Probe approach is a very important function in the AFM (see
Section 2.2.3 for the hardware implementation of this and Section 4.2 for precautions for
the user on applying
z
approach). The
Z
-approach software should be rapid, but it should
not allow the probe to touch the surface in an uncontrolled manner. Properly optimized,
probe approach takes less than a minute.
The approach software typically has several options for controlling the rate at which
the probe moves toward the sample's surface. Software algorithms are also critical for
setting the threshold signal levels associated with the probe interacting with the surface.
Once the threshold is met, the approach is stopped and the AFM is put into feedback.
Properly implemented, a fully automatic approach system prevents the user accidentally
inputting a threshold value that would crash the tip into the surface. If there is an
automated video microscope, the software algorithm for tip approach can be augmented
to shorten the time required for probe approach. This is achieved by focusing the
microscope on the probe, then the sample. The relative positions of the probe and sample
are compared. Then the
Z
motors are driven rapidly until the probe is less than 100
microns from the surface.
(b)
X-Y
motion control: Because the
x-y
motion using the scanner in the AFM usually has a
range of less than 100 microns, an
X-Y
motion control system is required that is able to move the
probe to within a few microns of the features that are to be scanned. An
X-Y
positioning table
driven with stepper motors is often used. Software is then used to move the translation stage.
The software typically is activated by mouse control within the software or by a track ball.
Advanced software functions may be added to microscopes with automated
X-Y
stages.
Functions include an ability to measure many images adjacent to each other, and to
