This document discusses three primary modes of atomic force microscopy (AFM) and their applications in nanolithography. The three modes are contact mode, tapping mode, and non-contact mode. Contact mode allows fast scanning but can damage soft surfaces. Tapping mode provides higher resolution with minimal damage. Non-contact mode exerts very low force but can have lower resolution. Each mode has distinct uses in nanofabrication including patterning polymers, local oxidation, and controlling pattern size.

1. NANO 525 HOMEWROK :
AFM and its application in lithography
Yang He ID:10400425 ;
Advisor: Prof. F. Fisher
AFM are widely used in the characterization of the surface pattern. This paper introduces three primary
modes of AFM from the aspect of my project study -- laser interference lithography. Then the
application of the three modes in nanolithography will be succinctly discussed.
There are basically two imaging modes, that is, Contact Modes and Vibrational modes. While in
vibrational mode there are two sub-modes, they are tapping modes, non-contact modes. To be specific,
the contact modes contain the Lateral Force, Lithography, and shark modes; The vibrating modes
include Close Contact (CC), Intermittent(IC), Magnetic Force (MFM), Electric force (EFM) and Kelvin
Probe (KPM). In this paper I will briefly discuss the contact modes, tapping modes and non-contact
modes to characterize the surface pattern via laser interference lithography and their distinct
application in surface fabrication and reaction.
First, the positions of tip and the sample are different in the three modes. The probe-surface separation
are <0.5 nm, 0.5-2 nm and 0.1-10 nm in contact modes, tapping modes and non-contact mods
respectively. The disparate distance leads to different force range in different mode. We can see the
difference in Fig 1. And this is the most principle part which gives the advantage and disadvantage of the
different modes.
Fig 1. The difference force range of three modes [1]

2. I f we choose the contact modes to image the sample; we can get a really fast scanning. And it is also
good to some rough samples such as, in my research interest, the grooves made by developed
photoresist. Nevertheless, It also needs to notice that the most photoresist is kind of crosslinked
polymer, which means they do not have the relatively high hardness as crystal. Contact mode imaging is
heavily influenced by frictional and adhesive forces, and can damage samples and distort image data. So
at time forces can damage or deform some soft surface. In fact, there is a technique to protect sample –
coating a thin liquid just on the sample.
The tapping mode is similar to contact modes. The cantilever is oscillated at its own resonant frequency.
This mode allows us to see the profile of easily damaged surface. But the scanning speed is slower.
When we want to characterize the delicate materials like vertical nano-tube grown on the surface, we
may use the non-contact modes. In this case, the probe oscillates above the adsorbed fluid layer on the
surface during scanning. So it exerts very low force on the sample which not only protects the sample
but also expands the lifetime of the probe. Non-contact imaging generally provides low resolution and
can also be hampered by the contaminant (e.g., water) layer which can interfere with oscillation.
Contaminant layer on surface can interfere with additional oscillation; usually we need ultra-high
vacuum (UHV) to have best imaging. As for the comparison of the Non-contact modes and the tapping
modes, apparently they both are based on a feedback mechanism of constant oscillation amplitude. The
non-contact modes have the amplitude set as about 100% of the ‘free’ amplitude. However, the tapping
modes can provide roughly 50% to 60% of the ‘free’ amplitude. The tapping modes can give higher
resolution with minimum sample damage. Tapping mode imaging takes advantages of the two above. It
eliminates frictional forces by intermittently contacting the surface and oscillating with sufficient
amplitude to prevent the tip from being trapped by adhesive meniscus forces from the contaminant
layer.
Fig 2. The surface influence with the contacted AFM tip [2]
There are several advanced imaging techniques developed from the contact-mode scanning and the
tapping-mode scanning. We can measure the surface friction via lateral force microscope and the
surface stiffness or elasticity via force modulation microscope in contact-Mode scanning. When we
make some PDMS micro-channel to be the component of micro-fluidic device, we want to know the
elasticity of the material, since this property can be used as a flow pressure sensor function. Moreover,
the friction between the channel wall and the flow is vital to the performance of the micro-fluidic device.

3. From this aspect, Contact-mode is very useful. The elasticity property can also be detected via phase
mode imaging in tapping mode scanning.
We have discussed the AFM as a characterization tool. The fact is that researchers prefer to use SEM to
observe their samples; even samples do not have conductivity (metal coating). The real significant role
that AFM plays is AFM nanolithography. I will give a brief introduction about those three modes’ real
research use.
Li et al. use the contact mode AFM as the ‘hammer’ to produce controlled two-dimensional nano-
structure arrays in polymer films [2]. We can see the influence from the tip in Fig 2. The repeated
scanning produces small islands. Then those islands can aggregate into larger ridge structures
perpendicular to the scan direction (Fig 3).
Fig 3 The pattern produced by contact-mode AFM.
The tapping mode can be used as the local nano-oxidation. Fig 2 Is shown that a 10 nm resolution can
routinely be achieved using tapping-mode AFM-based anodization of silicon and titanium operated in air
[3]. The common mechanism can be illustrated on Fig 3 [4]. An anodic reaction locally occurred on the
monolayer where the tip of the AFM probed with applied voltage. Lee et al. use this mode to
nanopartten the self-asssembled monolayers on Si-surface [5]. The thichness and the width of oxide
stripes are studied as a function of the applied probe-sample voltage and the the speed of the tip. In my
reseach, I plan make the regular array of silicon particles on the surface of PDMS wafer via laser
interferenec lithography. And I can employ this metod to oxidize desired local part of my sample. Then
use the Lee et al.’s methods to give the substructure of the silica particle array.

4. Fig.4 Fig.5
Fig 4. Nano-oxidation if a (111) oriented silicaon surface[3].
Fig 5. Scheme of some common mechnism to modify surface with AFM [4].
The non-contact modes are also developed as a method of nanolithography [6-8] . Non-contact
lithography is a more robust and reproducibke techniques, which result in little or no tip wear. We can
vary the distance to control the width of the pattern as shown in Fig 6 [7]. This gives a big advantage for
the microfabrication. In my laser interferance lithography study , I need to adjust lens and mirrors to
arrange my laser beams to give the specific pattern. The experiment relates to the design of optical
control which is complicated. In this case, change the distance between probe and sample can easily
give a control of the pattern size.
.
Fig 6. The pattern size controled by the distance between tips and the surface

5. Refereance:
[1] http://www.eng.utah.edu/~lzang/images/Lecture_10_AFM.pdf
[2] Li, Guangming, and Larry W Burggraf. 2007. “Controlled Patterning of Polymer Films Using
an AFM Tip as a Nano-Hammer.” Nanotechnology 18 (24): 245302. doi:10.1088/0957-
4484/18/24/245302.
[3] Dubois, E., and P.a. Fontaine. 1997. “Nanometer Scale Lithography of Silicon and Titanium
Using Scanning Probe Microscopy.” 27th European Solid-State Device Research
Conference 43: 1085–89. doi:10.1109/ESSDERC.1997.194413.
[4] Garcia, Ricardo, Ramses V Martinez, and Javier Martinez. 2006. “Nano-Chemistry and
Scanning Probe Nanolithographies.” Chemical Society Reviews 35 (1): 29–38.
doi:10.1039/b501599p.
[5] Lee, Won Bae, Young Oh, Eung Ryul Kim, and Haiwon Lee. 2001. “Nanopatterning of Self-
Assembled Monolayers on Si-Surfaces with AFM Lithography.” Synthetic Metals 117 (1-3):
305–6. doi:10.1016/S0379-6779(00)00392-1.
[6] Tello, M., and R. García. 2001. “Nano-Oxidation of Silicon Surfaces: Comparison of
Noncontact and Contact Atomic-Force Microscopy Methods.” Applied Physics Letters 79
(3): 424–26. doi:10.1063/1.1385582.
[7] Davis, Z. J., G. Abadal, O. Hansen, X. Borisé, N. Barniol, F. Pérez-Murano, and a. Boisen.
2003. “AFM Lithography of Aluminum for Fabrication of Nanomechanical Systems.”
Ultramicroscopy 97 (1-4): 467–72. doi:10.1016/S0304-3991(03)00075-5.
[8] Irmer, B, M Kehrle, H Lorenz, and J P Kotthaus. 1999. “Nanolithography by Non-Contact
AFM-Induced Local Oxidation: Fabrication of Tunnelling Barriers Suitable for Single-
Electron Devices.” Semiconductor Science and Technology 13 (8A): A79–82.
doi:10.1088/0268-1242/13/8A/024.