Introduction to
microscopes
Working is nanosciences
is difficult due to the inherent difficulties in handling and observing the nano-objects
we are trying to fabricate.
In most of the processes involved , the nanoscale compound
is fabricated “blindly” ( so with-out real-time observation), then the objects
obtained are observed using different techniques.
Most often, we use a microscope
to observe out samples.
We can classify the microscopes
in the following categories :
-
Optical
-
Electron microscopes
o Transmission
Electron Microscope (TEM)
o Scanning
Electron Microscope (SEM)
o Reflection
Electron Microscope (REM)
o Scanning
Transmission Electron Microscope (STEM)
-
Scanning probe microscope
o Atomic force microscope
o Scanning tunnelling
microscope
o Electric force
microscope
o Magnetic force
microscope (MFM)
o Near-field scanning
optical microscope
-
Point-projection
microscopes
o
Field emission microscope,
o
Field ion microscope,
o
Atom Probe
-
Other
microscopes
o
Acoustic microscopes
o
Inverse microscopes
This article will focus on
Scanning electron Microscope(SEM) and Atomic Force
Microscope(AFM).
A . Introduction to AFM
-by
Bogdan Butnaru, UPB -
This document describes how Atomic Force Microscopes work. (The name is
abbreviated as AFM, in this document and elsewhere.)
What is Atomic Force
Microscopy?
AFMs are part of a class of instruments called
Scanning Probe Microscopes. These are instruments that investigate properties
of a surface by scanning the surface with a probe, thus
determining how one property (or several) of the surface varies.
A probe is any kind of device (usually very small, of course) that can
measure some property of the surface on a very small spot. For example, a small
needle might determine the height of the surface in a point, or a very small
magnet might measure the magnetic fields on a very small area.
Scanning means that the probe is moved over the surface. This way the property
of the surface is measured in many points over the surface. Usually the
result is an image that "shows" the property over the measured
surface; remember that this is not a real image in the optical sense: it is
really a two-dimensional graph of how the measured property is
distributed on the surface. Sometimes this graph can be used to reconstruct (or
simulate) how the surface would look to an observer, but these are always simulations,
not a "photographic" image.
Scanning is almost always done in a very regular pattern. The surface is
divided in parallel lines, spaced evenly by a small distance; the probe is
moved along each line in turn, and measurements are recorded at regularly
spaced points along the line. The result is a regular matrix of measurements,
arranged like the intersections of a grid. This might be familiar to you,
because that's exactly the reverse of how computer screens, TVs and ink-jet
printers work: they scan the screen or the paper regularly, and pixels
are displayed instead of measuring a property.
(Of course, there are other ways of scanning over a surface, but this is by far
the most common one. You can do the same thing by touching a surface with your
fingers, keeping your eyes close. The idea is to touch over the entire surface
to feel every feature. You can touch the surface in every way you want, but for
microscopes it's just simpler to do this in a regular pattern.)
Back to AFMs for now: they work exactly as described
above, with a specific type of probe: a (really) tiny needle, attached to a
small "arm", called a cantilever, is scanned over the surface. As the
height of the surface varies (for example, when the needle passes over a bump
or a hole), the cantilever bends. The microscope really measures the force
that causes this bending, hence the "Force" part of the name. Because
the forces are very, very small, comparable with the forces between atoms in
the best microscopes, we have the Atomic Force name.
In short, the Atomic Force Microscope measures the "atomic-sized"
forces with which the surface pushes on a probe scanned over it.
So how are AFMs built?
Needless to
say, measuring such small forces over such small surfaces is no easy task. Though the principles
of their workings are simple (as will be described below), they are complicated
by the difficulty of bridging the huge gap between the minuscule scale of
measurements and the macroscopic scale of the instrument.
As we discussed above, the AFM works by scanning a very small
(squares in the range of 10um x10um) area with a sharp needle, measuring the
force with which the needle is pushed back by the surface. How can it
measure such minute forces? It uses a very clever trick:
The needle (called the "tip") is attached perpendicularly to a
cantilever, a long and very thin "arm" (though larger than the tip).
You can imagine the cantilever as a diving board---some are shaped exactly like
one---with the tip like the diver before a dive. (However, the tip of an AFM
usually points downwards, like someone hanging from a diving board.) Because
the cantilever is very thin, it is flexible, and it bends more or less
depending on how hard the tip presses on the surface---exactly the way a diving
board bends depending on how much the diver (or someone hanging from it)
weighs.
Because the tip and the cantilever are very small, they can only bend a little
before breaking. (usually a 1um bump can break the tip)
In fact, so little that it is very hard to measure directly. The AFM uses a
clever technique: A laser beam is shined obliquely on the cantilever, and it
reflects to a photo diode (a sensor that can detect the light shining on it).
Because the laser light is so concentrated, it is strong enough to be detected
despite the small size of the reflecting cantilever. (The top surface of the
cantilever, opposite the tip, is made very smooth to maximize the reflectivity.
Sometimes it is even covered with a thin metallic layer---usually gold---to
make it even more shinny.)
The trick is that although the cantilever moves only slightly in absolute
terms, because of its small size the angle of the bending is
significant. The deflection of the laser beam is proportional to the angle, not
to the vertical movement; because the distance between the cantilever and the
detector is relatively large (around 20 centimeters), the small movement of the cantilever is amplified to an
easily measurable movement of the laser beam.
The detector is a photo diode, a device that transforms light falling on it
into an electric current. In an AFM, the detector is made from two touching
photo diodes (actually, it's a photo diode split in two, built from a single
semiconductor chip). When the microscope is calibrated, the detector is moved
carefully so that the laser beam reflected from the cantilever falls equally on
both photo diodes; the electric current generated by the two is equal. When the
microscope starts scanning, the movement of the cantilever moves the beam more
towards one or the other half; this causes the current given by the two halves
to differ, which can be easily measured. From the difference in currents the
microscopes can determine how much the cantilever moved, and thus the height of
the surface at any point.
The microscope scans the surface with the tip and remembers its deflection in
every point, and this way constructs a height-map of the surface. The
height-map can be displayed directly as a color-coded
image, or can be processed to create simulated 3D views.
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Advanced AFM techniques
The method of
operation described above is the most simple and elementary way of using the
microscope. However, the simple idea of scanning a sharp tip over the surface
can be adapted to measuring many different properties of a surface, and some
different ways of measuring them. Some of them are described below (there are
numerous other variations).
Flexible surfaces
The
standard technique described above works quite well for the surface of rigid
materials, like metals, stone or ceramics, glass and even wood. However, soft,
flexible surfaces are not that easy to measure. Important examples of soft
surfaces are plastics or other polymers, and organic media, like cells. The tip
of the AFM must press slightly on the surface in order to bend the cantilever,
but on soft surfaces this causes the surface to bend, too. This causes the
measurements of surface height to be incorrect (see the next method, tapping
mode, for a solution), but this can also have a useful side effect:
Because the AFM is so sensitive enough to measure very small forces, it is
possible to scan the surface several times and apply different pressures with
the tip. Areas with different softness or elasticity will deform differently on
each pass. This way the composition of the surface---in fact, the differences
of composition---can be detected. This is useful, for example, to see inside
cells. The membrane and the cytoplasm are soft and give way easily to the tip;
cell organites, like the nucleus and the
mitochondria, are more rigid, and can be seen through the membrane because they
resist the tip pressure. Especially when combined with the next technique this
is a very useful way of determining the structure of cells.
Tapping mode
As
mentioned before, the tip of the AFM must press on the surface to be able to
measure its height. This can be a problem in two cases: First, soft surfaces
can be physically damaged by the sharp tip. They can be "molded" by the pressure, eroded by the scanning, or
even scratched. In some cases objects on the surface can be moved, for example
biologic cells, which are hard to "glue" to a substrate. Second, hard
surfaces with sharp reliefs can damage the tip, by
eroding it (making it less sharp), or catching and breaking it altogether. Even
the cantilever can be broken by very high surface features.
The solution is tapping mode: instead of pressing the tip to the surface
when scanning, the tip (together with the cantilever) is made to vibrate
vertically before touching the surface. (In this context, the first method is
called contact mode.) Instead of pressing the tip onto the surface, the
microscope can detect when the vibration of the tip is dampened by the
approach of the surface. This dampening is used to measure the height of the
surface instead of pressing onto it. Instead of being always in contact, the
tip only lightly taps the surface with a high frequency. (It is really possible
to scan the surface without even touching it. Very slight attraction and
repulsion effects between the tip and the surface can dampen the vibration enough
to measure it. In fact, at such small scales "contact" isn't a very
well-defined concept; it is more accurate to consider the repulsive
interactions between the atoms of the tip and the surface.)
The vibration mode solves, or at least diminishes, the two problems. For one,
the pressure on the surface is much lower, which diminishes the damage to
sensitive samples. Second, the tip is farther from the distance, never in
contact with it, so the lateral forces on the tip are very much reduced, which
almost eliminates the risk of breaking it.