Biomedical Engineering Reference
In-Depth Information
0 µm
3 µm
0 µm
3 µm
0 µm
3 µm
117 mV
0 µm
0.47 µm
0 µm
-80 mV
3 µm
Fig. 3.3. Illustration of the relation between height and deflection images. Left to right: height,
right-shaded height and deflection images of the surface of a mosquito eye. The height image shows
how much the
z
scanner moves to maintain the set-point. The deflection image shows how the
cantilever bends as it passes over the sample, and is the signal used for feedback in contact-mode
AFM. This image is very similar to the shaded height.
simple way to show the shape of the sample, and may even show features not visible in the
height image (which could be an indication that feedback was not optimized). However, it
is worth remembering that where feedback was correctly optimized, the AFM height
image will also contain all the features present in the error signal. One way to show this
is to apply a shading algorithm to the height image - this effectively gives the derivative of
the height image; the resulting image will be very similar to the deflection image.
An example of this is shown in Figure 3.3. Note that in the deflection image shown in
Figure 3.3 the
z
-scale is in volts. The
z
-scale was included here for illustrative purposes, in
such images the
z
-scale is almost completely meaningless scientifically - even if con-
verted to nanometres, the size of the deflection could easily be changed by adjustment of
the feedback parameters, and so should always be removed from the image before
presentation.
The deflection signal is used, as described previously, with the feedback parameters to
determine how the
z
piezoelectric must move to maintain a constant cantilever deflection
(and hence constant tip-sample force). The amount the
z piezo
moves to maintain the
deflection set-point is taken to be the sample topography; this signal, plotted versus
distance, forms the height or topography image in contact-mode AFM. There is a third
signal which is typically available in contact-mode AFM. This derives from the lateral
twisting of the cantilever, and is therefore usually called lateral deflection. This signal is
typically used due to its material sensitivity, rather than as a measure of sample topog-
raphy, and it is therefore covered in the non-topographic modes part of this chapter
(Section 3.2.3.1). The origin of the vertical and lateral deflection signals in a typical
optical lever AFM set-up is shown in Figure 3.4. The photodetector used in optical lever
AFMs usually comprises of four segments. The difference in signal between the top two
and bottom two segments, i.e. (A
þ
B)-(C - D) gives the vertical deflection (measured in
volts, or amps), and the difference between the rightmost two segments and the leftmost
two segments gives the lateral deflection, i.e. (B
C).
It is worth noting here that in contact-mode AFM, like most of the other modes,
two versions of each data type can be available, these being the data collected in the
left-to-right direction, and the reverse set, collected in the right-to-left data direction. By
þ
D)-(A
þ
