The Atomic Force
Microscope:

The Atomic Force Microscope (AFM) is a form of scanning probe
microscopy (SPM) involving the detection of interatomic forces that happen
between a probe tip and a sample. The AFM consists of a cantilever with a sharp
tip (probe) at its end that is used to scan the specimen surface. The
cantilever is typically silicon or silicon nitride and is used to scan across a
sample in an effort to obtain information about its surface (topography). The
tip is integrated into a cantilever (figure 2) which moves up and down tracing
the interaction of the surface of a sample. When the tip is brought into proximity
of a sample surface, forces between the tip and the sample lead to a deflection
of the cantilever according to Hooke’s law. The probe is scanned in a similar
pattern to the SEM where it moves in a raster pattern across the sample in
order to generate an image in an x, y and z pattern. The AFM can either use the
probe in a contact or non-contact mode. A great advantage of the AFM is that
the specimen being examined can be non-conducting, unlike for the SEM. In this
manner, the AFM can be used to study almost any sample.

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The AFM generates resolution by calculating the vertical and
lateral deflections of the cantilever. This is completed by reflecting a laser
beam off the cantilever which is reflected to a position sensitive photodiode (PSPD)
that consists of two sections. Consequently, any small change in deflection of
the cantilever produces a magnified reflection of displacement on the
photodiode relative to the cantilever (American society for Microbiology,
2002).

AFM has considerable advantages over the SEM. The ATM
provides more information with regard to the 3D surface of the specimen. Another
major advantage of the AFM is that it does not require any pre-treatment that
may cause damage or disfigurement to the sample. AFM also does not require a
vacuum and can be performed in air and liquid environments. Disadvantages of
the AFM include the single scan image size produced. The AFM has a lower depth
of field compared to the SEM because it only scans an area of micrometres, not
millimetres like the SEM. During analysis, the AFM has a slower scanning speed
to the SEM. Due to the nature of AFM probes, they cannot normally measure steep
walls or overhangs. Specially made cantilevers and AFMs can be used to modulate
the probe sideways as well as up and down to measure sidewalls. These
cantilevers however are considerably more expensive and have a lower lateral
resolution.

What is AFM used for?

· Inorganics, polymers, coatings and bio-samples

· Personal care products, the measuring of the change in
nanoscale mechanical properties (modulus and friction) of hair, teeth and skin.

· Investigation of the force in which is required to remove
nanoparticles from a surface.

· The topography and nano mechanical properties of coatings.

The challenges faced
with measurement

· Calibration, quantification and understanding of AFM modes
(including that of force spectroscopy,

multi-frequency modes, frequency modulation mode, lateral
force and amplitude modulation mode.

· Obtaining important and additional information from AFM (mechanical,
chemical, electrical).

· Imaging soft samples at a high resolution whilst working
on minimising damage.

 

Procedure:

As per manual

Results and
discussion:

For this particular experiment, a calibration grid with
different areas was examined first.

Scan 1a was performed and the results were read as follows:

·        
0.5 second per line

·        
Had 64 points per line

·        
Was tilted

·        
Had poor resolution

·        
Also viewed in 3-D

·        
White line obscured visualisation of scan indicating
dirt/dust. Dirt can be viewed as heights on the screen

Scan 1b un-tilted was performed and the results were read as
follows:

·        
1 second per line

·        
Had 64 points per line

·        
Un-tilted. Measurements can’t be taken unless
tilt plane is removed

·        
Measurement of pillar at 12.5µm (length)
compared to standard length of 10µm and 119.2nm (depth) compared to
standard distance of 100 nm.

·        
Measurement of square holes 12.89µm (length)
and 119.6nm (depth)

Scan 1b 128 points per line was performed and the results
were read as follows:

·        
1 second per line

·        
Had 128 points per line

·        
Un-tilted

·        
Measurement of pillars at 13.28µm
(length) and 116.9nm (depth)

·        
Measurement of square holes at 12.1µm
(length) and 111.7nm (depth

 

Scan 2 was performed and the results were read as follows:

·        
1 second per line

·        
Had 128 points per line

·        
Measurement of circular holes at 7.813µm
(length) and 125.4nm (depth)

·        
Circular holes should have 5µm between them.

·        
Depressions were shown with dark colours.

·        
Elevations were shown with light colours.

Scan 2 3d was performed and the results were read as
follows:

1 second per line
Had 128 points per line
Measurement of circular
holes at 7.0um (length) and 108.0nm (depth)

A microchip was examined

The depth of the valley of the chip was measured.

·        
Depths in the trench – 0.223µm

·        
Length of the hole – 1.362µm

Lastly, Staphylococcus aureus was examined.

The characteristic look resembled tiny bunches of grapes,
some of which were stacked on top of one another, and others higher up on the
scan.

·        
Width of the bacteria = 0.976µm,
0.585µm (very small clusters)

·        
0.2µm flattened

·        
Force 29N