Undergraduate Physics
Research - PHYS 4805
Spring 2011
Operation of Scanning Force Microscope in
Atomic Force Microscopy Mode
James Aldridge
Advisor: Dr. Tom N. Oder
Introduction
Methods of process control extend
beyond having the ability of solving
advanced mathematical statements to arrive
at solutions involving mass and energy
balances. The amount of information an
individual retains in an academic setting is a
priceless tool; however, the need to logically
handle expensive research equipment and
keeping its time of operation to a minimum
has become exceedingly valuable to
employers and future entrepreneurs. At
Youngstown State University (YSU), the
Physics department is providing students
with the opportunity to become trained in
operating an Agile
5500 Scanning Probe Microscope (SPM).
The objective of this research credit is to
familiarize students with software that is
unique to the SPM and to induce an
appreciation for the hand manipulated
features of this device to prevent an
academic scholar from exceeding the
limitations of equipment that he or she may
come across during future employment.
The model 5500 is used frequently to
analyze various materials in the Physics’
laboratory. Scanning probe microscopy
techniques give researchers the ability to
produce high-resolution imaging of a
relatively smooth surface observed by the
naked eye. The curvature of a relatively
smooth surface observed on a nanometer
scale has the potential of revealing jagged
features that hinder the prospects of
employing new materials into industrial
settings. For example, the characterization
of Schottky diodes are commonly executed
techniques under the advisement of Dr. Tom
Nelson Oder. The curvature of the
photolithographic imprints made in the
semi-conductor laboratory effect the quality
of the metals that will be deposited upon
them. Relatively smooth clean surfaces on a
nanometer scale are also vital parameters.
Dirt and unintentional structures bound to
these substrates lead to complications when
conducting current/voltage measurements.
The SPM has various modes of
operation. Most of which have compiling
similarities; however, depending on the
application, choosing a particular mode for
uniquely constrained parameters may save
the investigating research an incredible
amount out time allowing scientific
advancements to reach their full potential in
improving existing applications and those
yet to be discovered.
Current modes of operation included
in the 5500 SPM are:
1.
Atomic Force Microscopy (AFM)
2.
Scanning Tunneling Microscopy (STM)
3.
Current Sensing (AFM)
4.
Lateral Force Microscopy (LFM)
5.
Electrostatic Force Microscopy (EFM)
6.
Kelvin Force Microscopy (KFM)
Becoming proficient at every mode
of operation associated with the 5500 SPM
is beyond the scope of fulfilling this
research assignment. Becoming proficient
with the Atomic Force Microscopy (AFM)
mode is the primary objective of this
assignment.
Background
Gerd Binnig, a German physicist
who became employed at IBM worked in a
research group called the Zurich research
group in 1978. In 1986 he shared the Nobel
Prize in physics with another researcher over
the development of the Scanning Tunneling
Microscope (STM). The STM is a
microscope that contains a cantilever arm
like an old fashioned record player that
lowers its playing needle onto a spinning
record. In contrast to the record playing
metaphor the scientific STM lower its
cantilever onto a fixed substrate. After the
STM cantilever tip makes contact with the
substrate, electrons travel through its needle
to the conductive surface of the substrate
allowing the data processing equipment on
the receiving end to construct a
topographical image of the areas that
indicate a difference in electrical potential.
This very day, the STM mode of
operation is still popular amongst the
various scientist interests in surface patterns
existing on a nanometer scale. The
limitations of the STM require the sample to
be conductive. Also with current biomedical
applications, testable solids at times need to
be submerged in an aqueous environment.
In 1989 the Atomic force microscope
(AFM) was made commercially available by
Binnig and his researchers. The AFM mode
of operation is a derivative of the STM; but
fortunately, due to its design, the atomic
force microscopy escapes the confinement
of using a conductive substrate.
Currently, another advantage of
analyzing samples in AFM mode is that the
analysis is optionally conducted in either dry
or aqueous environment. At this current
date, when seeking to visit structural
surfaces on a nanometer scale, atomic force
microscopy is at the forefront of mankind’s
most advanced technology and quoted from
the distributor Agilent to be the “flagship of
Agilent’s product line.”
AFM design
Current design still makes use of a
cantilever arm to become lowered onto the
sample of interest. The actual contact made
with the sample of interest is a needle-like
tip that applies 0.1-1000 nN of force onto
the sample. The cantilever arm is flexible
and extremely sensitive to a small amount of
force applied. The cantilever’s spring
constant is typically 0.001 - 5 nN/nm. This
is part of the reason for naming this
relatively new mode of operation AFM. This
mode of operation applies a force to the
sample as the cantilever arm moves across
the sample in both x any y directions.
Understanding that even at a nanometer
level, a relatively smooth surface to the
human eye but rough on a nanometer scale,
the jagged nano-terrain will cause the
cantilever to bob upwards and downwards in
the z direction. These upward and
downward movements cause error from a
set-point to occur that was established by a
laser once the cantilever arm’s tip made
contact with the sample from its 1st
positioning. Figure 1 depicts the orientation
of the laser beam that deflects due to
cantilever movement in the z direction and
its associated photodetector.
directions. A detailed figure of the major
forces that contribute to pulling and pushing
the cantilever to and from the sample is
represented in Figure 2.
Figure 2: The amount of force applied to the
cantilever tip at different spatial regimes.
Figure 1: Diagram depicting the main
components located in the sample area.
The laser beam is projected on a
reflective surface located at the head on the
side opposite of the cantilever tip. As
mentioned, when the cantilever’s tip makes
the initial contact with the sample’s surface
the laser beam is projected on onto the
reflective surface and reflected into a
photodetector. The initial contact establishes
the initial point in the z direction to become
the set-point for the particular fixed sample.
Any deviations in the z direction will result
in variable distance recorded as a function of
time. The cantilever’s arm continually
applies force while moving in the x and y
direction recording data points along the
way.
The force applied to the cantilever
tip is governed by the internal control
system of the SFM operating system. The
importance of stating this is that the control
actuating system must account for all of the
forces acting upon the cantilever tip, for the
flexibility of the cantilever arm must work
harmoniously with the electrical response
motioning the cantilever arm in the x and y
Observing figure 2, force is plotted as a
function of distance and due to specific
forces acting on the cantilever tip it
approaches the substrate. The situation may
vary depending on the situation, but a
hypothetical path that may account for the
behavior is list below in three separate
regimes starting with the cantilever
positioned at the furthest distance away from
the x axis. These regimes define as the
characteristic behavior present at specific
moments in time.
1.
Free oscillation
2.
Attractive forces
3.
Repulsive forces
When approaching the sample, assume
that a liquid film is present above the surface
of the sample. An adhesive pulling force
will become present once the cantilever’s tip
makes contact with the liquid prior to
making contact with the surface. This is due
to the nature of water wicking around the tip
of the cantilever. Eventually, a sudden
increase in force will result from the
cantilever making contact with the surface.
Prior to making contact with any physical
mass quantity, the flexibility of the
cantilever may produce an oscillating
motion as it ascends downward into the
physical interactions described in regimes 2
and 3.
Experimental Set-up/ procedure
The Main SPM unit is amount the
size of a standard laboratory microscope.
From a 1st glance, the features of the SFM
appear to be relatively the same as a
standard microscope. There are select knobs
that protrude from this unit that are adjusted
manually to adjust the cantilever in concern
to the cantilever approaching the substrate
during analysis or during calibration of the
SFM.
Samples are positioned on a
removable metallic plate near the bottom of
the microscope in the lower region of this
microscope. The sample plate area to where
it is inserted underneath where the cantilever
arm is lower onto it is outlined in Figure 3
and its major components are labeled also.
The major components of the 5500 SFM
are:
1.
Video imaging system
2.
Coarse adjustment knobbing
3.
Microscope
4.
Sample area
Figure 3: Image of the SFM operating
system.
Furthermore, the SFM is kept in a
rectangular container suspended in air by the
use of bungee cords. This is to prevent
vibrations from the environment from
disturbing the apparatus during a scan. The
remaining components of the SFM are
separate electrical devices that are situated
near the microscope. There is a power
source, computer tower with dual monitors,
and an actuator. The actuator contains a
switch that allows the user to lower the
cantilever a large distance in a short amount
of time. Once the cantilever comes within
millimeters of the sample to undergo AFM,
the using the computer software’s guided
prompts guide the cantilever slowly and
gracefully onto the sample. There is no
specific order to which these units must be
turned on. It is worth mention that it is
important that the sample plate is properly
fixed to the SPM and that the appearance of
the sample to be analyzed should look
relatively smooth to the naked eye. The
remaining assumption a user must make that
only a few micrometers will be scanned in
the x and y axis directions. A distance of
millimeters entered into the computer’s
scanning parameters would take an
incredibly long time to scan. The SPM
operating in any of its modes are meant for
micro to nanoscale differences. Using the
correct computer prompts, the user should
have to make any physical movements
except for removing the sample when the
scan is over. At the end of the scan, the
computers programming raises the
cantilever a large enough distance to where
user won’t damage anything when he or she
removes the sample plate from the device.
Experimental results
Having a cleaned silicon carbide substrate
(SiC), this sample is place on the sample
plate and scanned to identify the surface
roughness in terms of a root mean square
value. No hand calculations are necessary;
these calculations are automatically
provided by the SFM’s software after the
scan is completed. Figure 4 depicts the
topography found by the SFM and a root
mean square of 6 nanometers was found to
be the surface roughness. Figure 5 is the
computer generated tabular version of
pertinent information provides the SFM’s
software including the root mean square
value. The limitation of the SFM in concern
to the amount error it may have is roughly 1
to 2 nanometers. Repeated scans for one
sample are not necessary to get reliable
results.
After removing the sample,
photolithography techniques were applied to
prepare the substrate for metallic plasma
deposition. Another AFM scan was made
after the deposition of nickel and gold onto
the substrate to confirm the intended
curvature and height in metal of the metal
contacts. The intentional aim was to deposit
100 nanometers of nickel and 100
nanometers of gold onto the substrate. AFM
scan results reveal the metal contact to be
185 nanometers. These results not only
confirm that the intended goals were met,
but it also helped to confirm that the plasma
deposition chamber is operating within a
reasonable error margin. Results also reveal
that the edges of the metal contact have a
tiny mound of metal around their square
perimeters. Figures 6 and 7 are the results
provided by the AFM scans after the metal
was deposited onto the SiC.
Figure 4: AFM graphical data of the cleaned
SiC substrate
Figure 6: AFM graphical data scan of
deposited metal.
Figure 5: AFM tabular data showing the root
mean square of the cleaned SiC substrate.
Figure 7: AFM Cartesian graph of deposited
metal.
It was intended to execute further
analysis for this particular sample leading to
determining the rate of plasma etching into
the original SiC creating a new depth at
which the metal contact would be removed
transferring the metal contact pattern into
the SiC. However, a delay in repairing the
existing plasma etching device is preventing
further progress with the atomic force scans.
Conclusions
The SFM is an extremely useful tool
in observing surfaces that appear relatively
smooth to the naked eye. More importantly,
substituting the STM mode of operation (a
use of electrons) with the use of atomic
force microscopy to create topographical
imaging allows for a larger variety of
substrates to become examined by the
account that samples don’t need to be
conductive. Also, an aqueous environment
became plausible since deviations from the
laser beam’s set-point is the source of what
the SFM operating in AFM mode uses to
create the topographical imaging. The most
apparent disadvantage of using the SFM in
any of its modes of operation is that scans
are limited to only micrometers in distance
concerning the x and y plain of operation,
and the distance in the z direction only
works for surfaces that appear relatively
smooth to the human eye. For the samples
that are investigated using the AFM mode of
operation, the limitation for this device is
detecting the actual height in the z plain of
operation between 1 – 2 nanometers of error.
The complexity of the system’s internal
control system and its software interfaces
call for minimal input actions made by the
user operating the SFM.
Using special attachments that need
to be physically changed to execute the
other various modes of operation are
perhaps the most technical issues a user may
face after learning the AFM software. These
attachments may include changing the
cantilever tip or making an environmental
change concerning a temperature sample
mounting plate to heat the sample during
analysis or introducing a special container
issued by the manufacture to submerge a
particular sample in an aqueous environment
during analysis. Future work may include
using these special attachments to further
characterize samples being analyzed at
YSU.
References
Agilent Technologies 5500 Scanning Probe
Microscope User guide