This document summarizes an internship project involving characterization of atomic force microscopy (AFM) tip radius using two different methods. The internship involved learning AFM operation and scanning sample images. For the project, tip radius was quantified before and after imaging using both the capacitance method from electrostatic force microscopy measurements and relating the critical oscillation amplitude to tip radius. Comparing the results found the capacitance method was less accurate for small tips while the tips did not significantly change after imaging. The internship provided valuable experience learning AFM fundamentals and practical techniques.

1. Khalifa University of Science, Technology and Research
Electronic Engineering Department
Electronic Engineering Internship- ELCE399
Final Internship Report
Tip radius characterization and imaging using AFM
Done by:
Isra Lababidi 100020272
Supervised by:
Dr. Baker Mohammad
Dr. Murat Yapici
Summer 2013

2. Table of Contents
Abstract.........................................................................................................................................................3
Introduction ..................................................................................................................................................4
Literature review...........................................................................................................................................5
AFM (Atomic Force Microscopy) ..............................................................................................................5
EFM (Electrostatic Force Microscopy) ......................................................................................................6
Starting project: Scanning Images ................................................................................................................7
Internship project: Comparison between two methods to detect tip radius in situ....................................8
a. Project goal .......................................................................................................................................8
b. Methodology.....................................................................................................................................8
c. Results and Discussion....................................................................................................................10
Conclusion...................................................................................................................................................11
Works Cited.................................................................................................................................................12

3. Abstract
Research and industry interests are growing more towards nano technology; using nano
devices allow higher efficiency and less energy consumption. One of the important tools that are
used in nano industry is AFM, Atomic Force Microscopy, which is a powerful tool for
topographic imaging that’s capable of attaining true atomic resolution. This report shows the
product of a 6 weeks period internship that’s based on a learning process of the basics of AFM.
The report includes an overview about AFM, EFM (Electrostatic Force Microscopy), the work
performed using AFM machine, and the final project which involved a comparison between two
methods to quantify tip radius used in AFM. Results and discussion are included as well which
are considered the main outcome of the work, and finally a conclusion.

4. Introduction
The summer internship at the Laboratory of Energy and Nano-sciences in Masdar
Institute of Science and Technology located in Masdar City was an exciting and beneficial
experience. The faculty there was great help and excellent guiders and tutors, and the laboratory
was equipped with what I needed to get the full experience. The main objective for me was to
learn about AFM and how to operate it in order for me to get the full benefit for my recent and
future work in nano field. AFM is a powerful tool that captures images on a nano scale which
leads to a great understanding and improvement in different nano devices. The theoretical
concepts and the wide applications of AFM cannot be all gathered and understood within this
short period, hence, I was required to focus and comprehend well the most basic theoretical
concepts and work more on the practical side. Overall, I enjoyed my internship experience, it
was full with useful information that is related to my field of interest and that is not only
beneficial for me in the present but also in the future.

5. Literature review
AFM (Atomic Force Microscopy)
AFM uses a cantilever with a very sharp tip in the order of nanometers to scan a sample
surface by measuring the interaction between the tip and the surface; Figure 1 shows a schematic
of the main parts of the AFM. There are two main types of interaction forces between the sample
and the tip depending on the separation distance between the tip and the surface, the van der
Waals short ranges force and the electrostatic long range force as illustrated in Figure 2. These
interactions lead to a deflection of the cantilever while a laser beam is used to detect the
deflections towards or away from the surface. The Z-servo driver and feedback system in the
scanner is used to control the height of the tip above surface to maintain a constant position of
the tip as illustrated in Figure 1. After that, the sample is scanned depending on the mode of
scanning or in other words on the type of force of interactions between the tip and the shown in
Figure 2.
Figure 1: Atomic Force Microscopy block diagram
source: Wikipedia, the free encyclopedia

6. Figure 2: force-distance curve characteristics of the interaction between the tip and the sample
source: www.nanosience.com
There are two main modes of AFM; static and dynamic modes AFM. In static mode AFM,
repulsive force is more dominant as the tip is in contact with the surface of the sample; while in
dynamic mode AFM both regimes of force occur. There are types of dynamic AFM, however, in
the project, the main type used was the dynamic AFM; details to be provided later in the project
section. Next is a brief description about EFM which is one of the many types of AFM.
EFM (Electrostatic Force Microscopy)
EFM is a type of dynamic non-contact AFM which maps the electric properties on the
sample’s surface by measuring the electrostatic force between the surface and the biased AFM
cantilever. EFM applies a voltage between the tip and the surface which leads to a change of the
oscillation amplitude and phase of the cantilever. The voltage applied forms a capacitance
between the tip and the surface which depends on their geometry. EFM allows a higher
understanding of tip surface interactions which leads to having better results and scanning
process. The bias voltage between the tip and the surface forms a capacitance which depends on
their geometry. EFM not only gives information about the capacitance, but also about the energy
stored, the electrostatic force, and the amplitude and phase components. The process of EFM is
more complicated than AFM, however, since it gives more information about the tip surface
system it was used in the final project as will be shown later in the report.

7. Starting project: Scanning Images
The first project assigned for me was scanning project for different samples; this was to
get used to using the AFM machine and to have a better understanding of the how it works side
by side with the theoretical course I was taking.
This starting project was mainly to comprehend the importance of the major project – to be
discussed in the next section -. Throughout this project, the ultimate goal for me was to get the
best resolution for the images; this was indeed a difficult one for me as a first user. Although the
scanning resolution can be controlled by different parameters using the AFM software (called
IGOR) such as the set point, gain, scan size, and scan rate, however, the scanning resolution
depends as well on the life and sharpness of the tip. Hence raises the need of studying the tip
geometry before and after the scanning process and how the tip is being affected by the
interaction forces which can affect the images resolution. Figure 3 shows a scan of a calibration
sample provided by Asylum Research (AR) flipped, the scan I did was on a micro scale.
Figure 3: A Scanning image of a calibration sample
It’s worth mentioning that it was very important when the scanning is in process to avoid
touching any part of the machine to minimize the noise effect. .

8. Internship project: Comparison between two methods to detect tip
radius in situ
a. Project goal
The main objective behind this project was to characterize tip radius by better understanding
the intermolecular interactions between the tip and the sample surface. As mentioned earlier, one
of the factors affecting the scans resolution is the tip life and sharpness; the purpose of the
project is to characterize the tip radius in two different ways to study the changes occurring to
the tip during the scanning process.
b. Methodology
The EFM principle explained above was highly applied in the project as the capacitance
formed between the tip and the surface by the biased tip can be related to the tip geometry which
was the purpose of the project. This was the first method used, the capacitance method resulting
from the biased tip. The capacitance is dependent on the area of interaction in the system, which
is by turn dependent on the tip radius. This capacitance is only related to the long range force
hence, the way that EFM works is by first obtaining the total force then eliminating the
electrostatic force to only the van der Waals force. The latter is done by applying tip voltages
where at one point the potential difference between the tip and the sample would reach the work
function difference between the two materials; at this point the electrostatic force would equal to
zero. After that, the existing force is subtracted from the total forces leading to obtaining only the
related electrostatic force. [1] The electrostatic force F can be written as: F = (½)(dC/dz)V^2
where C is the tip to sample capacitance and the total energy stored is U = ½ C (delta V)^2.
Using EFM, not only the tip radius is obtained using the capacitance method, but also by finding
the critical amplitude which is related to the tip radius by a known function. The critical
amplitude is the second method. The relationship between the critical amplitude and the tip
radius is different for various tip materials. In my practice of EFM, the relationship used to find
the tip radius R using Ac was 4.75Ac1.12
. [2] The critical amplitude is defined as the minimum
free amplitude (cantilever oscillation amplitude for AC mode) at which an observable transition
between the attractive and the repulsive regimes of the cantilever forces of interaction occur, or
in other words where bi-stability occurs. An example of bi-stability is shown in Figure 4.

9. Figure 4: APD curve at bi-stability
The capacitance method included using a Matlab code for reconstructing a relationship between
the capacitance and the tip radius, it included curve fitting and trial and error; an example of
curve fitting for the capacitance method using Matlab in shown in Figure 5. This method was a
bit difficult for me to use as it involved many steps and took much time from me. On the other
hand, the critical amplitude method was straightforward method since the relationship was
known.

10. Figure 5: Curve fitting using Matlab for capacitance method
c. Results and Discussion
The measurements were taken for five different tips; the three tables below show the
measurements taken for the tips radii respectively using the two methods.
Ac before
imaging
Tip Radius
(nm)
0.36 24
0.38 25
0.35 23
0.32 21
0.3 19
Generally, if Ac has increased after getting the capacitance then the tip radius has increased and
vice versa. This allows a better understanding of the changes occurring to the tip during the
experiment, whether it’s been contaminated or blunted.
Ac after
imaging
Tip Radius
(nm)
0.38 25
0.38 25
0.4 27
0.29 18
0.31 20
Tip radius using
capacitance
method (nm)
329
90
395
112
275

11. Comparing between the measurements, the capacitance has the largest deviation from the other
ones. This has led to the conclusion that the capacitance method is not accurate to be used for
very thin tips. This has also led to the conclusion that the tips used to do the scanning were not
highly damaged because the tip radius has not changed a lot after imaging for each tip.
Conclusion
The main outcome from my internship was learning about AFM, the principle of working,
and how to operate it. Furthermore, I was assigned some projects but the most important ones for
me was the ability to scan some samples and to characterize different tip radius using two
methods. The main advantage of the two methods used to characterize the tip radius is the ability
to do it in situ; other methods proposed by the literature involve removing the tip or the sample
during the experiment which leads to damaging either one of them. In situ characterization of the
tip sample minimizes the experimental complexity and provides a non-destructive and efficient
way. Considering that I was learning about AFM for the very first time, I have found it a very
interesting topic to continue researching about.
Although I have fulfilled my objectives, I have been detecting some mistakes and being careful
through my work not to make them again. One of the mistakes I did was detecting a reflection of
the laser on the cantilever instead of the real one; I learned the difference between the two and
was able to recover it. Another mistake was not approaching the surface enough which I realized
was an extremely important step in all the applications of AFM. Obtaining the best results using
AFM requires sensitivity and accuracy in performance, it is very important to revise the steps
over again because every step matters. Each part in the AFM machine has its own function and
each parameter in the software play a role in getting the best resolution; but most importantly, it
needs a lot of practice.

12. Works Cited
[1] C. Maragliano, D. Heskes, M. Stefancich, M. Chiesa and T. Souier, "Dynamic electrostatic force
microscopy technique for the study of electrical properties with improved spatial resolution,"
Laboratory of Energy and Nanosciences, Masdar Institute of Science and Technology, Abu Dhabi.
[2] S. Santos, L. Guang, T. Souier, K. Gadelrab, M. Chiesa and N. H. Thomas, "A method to provide rapid
in situ determination of tip radius in dynamic atomic force microscopy," Laboratory of Energy and
Nanoscience, Masdar Institute of Science and Technology, Abu Dhabi, 2012.
[3] "Atomic Force Microscopy," nanoScience, [Online]. Available:
http://www.nanoscience.com/education/afm.html.
[4] "Atomic Force Microscopy (AFM)," [Online]. Available:
http://chemgroups.northwestern.edu/odom/nano-characterization/AFM%20more%20info.pdf.