Biomedical Engineering Reference
In-Depth Information
Fig. 1.2. A schematic diagram of Young's topografiner (left), and one of the first images collected
with the instrument (right). Reprinted with permission from [12].
initial position above the surface to approach contact with the surface. Becker remarked
that when using this vibrating profile method for measuring images, the detail of the
images would depend on the sharpness of the probe. Stylus profilers are still in use today,
and have developed considerably. However, fundamental problems with this sort of
instrument persist, notably that the probe touches the surface in an uncontrolled way,
which can lead to probe damage in the case of a hard sample, and sample damage in the
case of a soft sample. Either of these problems would reduce the fidelity of the image
obtained, as well as the resolution achievable.
In 1971 Russell Young demonstrated a non-contact type of stylus profiler [12]. In his
profiler, called the topografiner, Young used the fact that the electron field emission
current between a sharp metal probe and a surface is very dependent on the probe sample
distance for electrically conductive samples. In the topografiner (shown in Figure 1.2), the
probe was mounted directly on a piezoelectric ceramic element which was used to move
the probe in a vertical direction ( z ) above the surface. Further piezoelectric elements
moved the probe in the other axes over the sample.
An electronic feedback circuit monitoring the electron emission was then used to drive
the z -axis piezoelectric element and thus keep the probe-sample distance at a fixed value.
Then, with the x and y piezoelectric ceramics, the probe was used to scan the surface in the
horizontal ( X-Y ) dimensions. By monitoring the X-Y and Z position of the probe, a 3-D
image of the surface was constructed. The resolution of Young's topografiner was limited
by the instrument's vibrations.
In 1981 Binnig and Rohrer, working at IBM, were able to improve the vibration
isolation of an instrument similar to the topografiner such that they were able to monitor
electron tunnelling instead of field emission between the tip and the sample. This instru-
ment was the first scanning tunnelling microscope (STM) [13-15]. A schematic diagram
of the STM is shown in Figure 1.3. The STM works by monitoring the tunnelling current
and using the signal, via a feedback loop, to keep the STM tip (a sharp metal wire) very
close to the sample surface while it is scanned over the surface in the X and Y axes in a
 
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