Jackson M.J. Micro and Nanomanufacturing
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Principles of Micro- and Nanofabrication 31
The cantilever beam and tip are controlled by certain physical
laws and as such possess a spring constant, k, and a resonant fre-
quency, fo. The tip and cantilever experience a displacement force
that is dominated by surface tension in ambient conditions (AFM
mode),
and chemical, van der Waals, and electrostatic forces in vac-
uum (STM mode). In summary, the modes of operation of the AFM
are static and dynamic. In static AFM, the force between tip and
surface is variable and non-linear compared to the surface-tip dis-
tance. Figure 1.33 shows the basic principle of operation in the
static mode. The figure shows the contact mode, non-contact mode,
and the intermittent dynamic modes and explains the differences be-
tween them.
Advances in the use of atomic force microscopy are reviewed by
Giessibl [12] who presents an overview of the development of the
technique and how it can be used for imaging and manipulation of
small-scale features.
Cantilever beam+Tip
^•P'
SAMPLE
^p§
Repulsive Attractive
Modes of Operation: Static AFM
I. Summary
• Force
Fj-g
between tip and sample IS
imaging signal
• Force varies nonlinearly with tip-sample separation
• Detected as deflection
q
'=Fj^k
of
cantilever
II.
Contact Mode-Static AFM
• Tip is in repulsion regime
• Tip exerts large normal and lateral force
• Force is kept constant during scan (feedback)
• Process subject to low-frequency noise
III.
Non-Contact Mode-Dynamic AFM
• Tip oscillates above sample is in attractive regime
• Avoids force and noise problems of contact mode
• Subject
to
jump-to-contact if k<hr^°^js
IV. Intermittent Contact Mode-Dynamic AFM
• Tip oscillates above sample is in attractive regime
• Tip contacts
("taps")
sample for a short time each
cycle
Franz J. Giessibl, "Advances in atomic force microscopy,"
[Online]:
xxx.lanl.gov, arXiv:cond-mat/0305119vl 6May2003.
Fig. 1.33. Modes of operation of the atomic force microscope [12]
32 Micro- and Nanomanufacturing
Contact
AFM
Atomically resolved
image
of KBr
(001)
in
contact
AFM
mode.
The
small
and
large
protrusions
are
attributed
to
K+-
and
Br?
-ions,
respectively.
Giessibl, F. J. and G. Binnig, 1992b, "True atomic resolution on KBr with a low-temperature
atomic force microscope in ultrahigh vacuum", Ultramicroscopy 42-44, 281-286.
Fig. 1.34. Atomic resolution of KBr surface using an AFM in ultrahigh vacuum
[16]
Non-Contact AFM
Non-contact AFM image of a cleavage face of
KQ
(001)
with mono- and
double steps
of
a height of 3.1 and 6.2A respectively. Image
size
120
nmx
120
nm
Qessibl, F. J. and R M Trafes, 1994, "Hezoresistive cantilevos utilized for scanning tunneling
and scanning force rrricroscqjc in ultrahi^ vacuum", Rev. Sci. Instrum. 65,1923-1929.
Fig. 1.35. Non-contact AFM image of a cleavage face of potassium chloride
showing distinctive variations in height [17]
Principles of Micro- and Nanofabrication 33
The contact mode shows provides images of atomically resolved
surfaces such as the potassium bromide surface (001) shown in Fig.
1.34. Here, the small and large protrusions are attributed to K"^ and
Br" ions. The tip exerts a large normal and lateral force in the repul-
sive regime when the tip is scanning the surface of the potassium
bromide layer. Figure 1.35 shows the image generated by the non-
contact dynamic AFM imaging regime. The tip oscillates above the
sample in the attractive force range, which tends to avoid the noise
and force deflections associated with the repulsive range. The image
shows distinctive variations in height that can be used to manipulate
features at the nanoscale in three dimensions. This technique can be
used to manipulate nanotubes and other forms of carbon to construct
nanoproducts. There are two modes of operation for the dynamic
AFM (Fig.
1.36):
AM-AFM; in which excitation of the cantilever-
tip is done at a fixed amplitude and frequency, and FM-AFM where
excitation is performed at fixed amplitude and a varying frequency.
AM-AFM provides near atomic resolution (Fig.
1.37),
whereas FM-
AFM provides absolute atomic resolution (Fig.
1.38).
Cantilever beam+Tip
^PSAMPLEI^PI
TS
Modes of Operation: Dynamic AFM-
Two Modes
1.
AM-AFM: Excitation at fixed amplitude &
frequency-^changes in vibration amplitude
& phase as tip approaches sample are
detected as signal in time tj^ ? 2Q/fo
{slow)
-Provides near atomic resolution.
2.
FM-AFM: Excitation at fixed amplitude &
varying
fi'equency->
changes in frequency as
tip approaches sample are detected as signal
in time
Xpj^
? lliQ.ifast)
-Provides atomic resolution.
Repulsive Attractive
Franz J. Giessibl "Advances in atomic force microscopy,"
[Online];
xxx.lanlgpv, arXiv:cond-mat/0305119 vl 6 May
2003.
Fig. 1.36. Modes of operation of the dynamic AFM [12]
34 Micro- and Nanomanufacturing
AM-AFM
AFM image of the silicon with AM mode.
Image size 100°Ax 100°A. The grey
scale in the image corresponds to a height
difference
of lA
Erlandsson,
R, L
Olssai, and
P.
Martensson, 1997, "Inequivalent atoms and imaging mechanisms
in ac-mode atomic-force microscopy of Si(l 11)7x7", Hiys. Rev.
B
54, R8309-^8312.
Fig. 1.37. AFM image of silicon surface (111) in the AM-AFM mode [18]
The coupling
of
the FM-AFM mode
of
operation with the ability
to deposit colloids
of
nanoconducting particles allows
the AFM to
take part
in
constructing three-dimensional nanoscale products
through fabrication. This
can be
performed
by
using
a
process
called electrostatic trapping.
The
fabrication
of
single-electron tran-
sistors using the AFM
is
shown
in
Fig.
1.39. In
this figure, nanopar-
ticles
are
pushed into position using
the
cantilever beam
and tip to
deposit the particles
in
the correct position to form the conductive
is-
land between drain, gate
and
source.
The
attraction
of
the particles
is aided
by
electrostatic trapping (Fig.
1.40).
A
commercial instru-
ment that can be used for nanofabrication using the AFM principle
is
one provided
by
the Veeco Metrology Group (Fig.
1.41).
This piece
of equipment
can be
used
to
fabricate nanoscale features such
as
single-electron transistors with
a
little modification. The instrument
can also
be
used
to
manipulate carbon nanotubes
to
construct con-
ducting three-dimensional devices (Figs.
1.42 and
1.43).
Principles of Micro- and Nanofabrication 35
FM-AFM
6 nmx6 nm constant frequency shift
image (f = ?38 Hz, rms error
1.15Hz,
scan speed 2 nm/s). The labels 1, 2,
and 3 indicate the position of
frequency distance measurements
Lantz, M., H. J. Hug, R. Hoffmann, P. van Schendel, P. Kappenberger, S. Martin, A.
Baratoff,
and
H.-J. Guntherodt,
2001,
"Quantitative Measurement of Short-range Chemical Bonding Forces",
Science
291,
2580-2583.
First FM-AFM image of an insulator
(KCl) with true atomic resolution.
Parameters: k = 17N/m,
fo
= 114 kHz,
A = 34nm, f=?70Hzand
Q = 28
000
Patrin, J.,
1995,
Atomic resolution of
an
insulator
by
noncontact
AFM,
Presentation at the 12th
International Conference on Scanning Tunnling Microscopy 1995 (Snowmass, Colorado, USA).
Fig. 1.38. AFM Image of potassium chloride with absolute atomic resolution op-
erating in the FM-AFM mode [19,20]
36 Micro- and Nanomanufacturing
Building a Single-Electron Transistor by AFMNanomanipulation
Requicha," Nanorobots, NEMS and Nanoasserrbly,"
Proc.
IEEE,
Nov.
2003,
http:www lmr.usc.edu~lmrhtmlpublications.htni
Fig. 1.39. Construction
of
a
single-electron transistor aided by pushing the parti-
cles using an AFM tip and trapping the particles by electrostatic trapping [21]
Figure
1.39
shows
the
construction
of
a single-electron tran-
sistor, which
is
aided
by
physically pushing
the
particles using
an
atomic force microscope
tip and
trapping
the
particles
by
electro-
static trapping.
The
process
can be
used
on
larger scale features
where there
is a
requirement
to
specifically engineer
a
surface
to
provide
a
known functionality such as
a
non-stick surface. AFM
is a
process capable of reproducing small, but highly functional surfaces.
Principles of Micro- and Nanofabrication 37
(a) Pt electrodes (white) separated
by a 14
nm
gap.
(b) After
ET,
the same electrodes are bridged
by a
single
17
nmPd particle.
(c) Another example where three Pd colloids are trapped across
a 26 nm
gap.
A. Bezryadin, C. Dekker, G. Schmid, "Electrostatic trapping of single conducting nanoparticles between
nanoelectrodes," Appl. Phys. Lett. 71 (9), 1 September 1997
Fig. 1.40. Electrostatic trapping of palladium colloids between two nanoelec-
trodes [22]
Instrument optimized for high resolution
force measurements and topographic
imaging.
By C.B. Prater, P.O. Maivald, K.J. Kjoller, M.G. Heaton, "Probing Nano-Scale Forces with the Atomic Force
Microscope," Application Note 08, Digital Instruments/ Veeco Metrology Group.
Fig. 1.41. Veeco instrument for probing nanoscale features (Courtesy of Veeco
Metrology Group)
38 Micro-
and
Nanomanufacturing
Carbon Nanotube Manipulation
p.
Avouris et al., Appl.
Surf.
Sci 141, 201-209 (1999).
Fig.
1.42.
Carbon nanotube manipulation using the atomic force microscope
[23]
Single Electron Transistor
aTEtiErTioat
Thwxiori:
.\.
i'lilum :ttkl (it-nkl
J.
]
Xilaii. ikil
\.'i\v{\
'W7)
s
Fig.
1.43.
Single-electron transistor constructed using carbon nanotubes (Cour-
tesy
of
Bell Laboratories)
Principles of Micro- and Nanofabrication 39
The atomic force microscope
can be
adapted
to
perform dip
pen
nanolithography
by
adding
a dip pen
nanoprobe
to the tip of the
AFM. Figure
1.44
shows
the
schematic diagram that allows nano-
fabrication
of
three-dimensional products
to
occur using
self-
assembled molecules (SAM)
or
colloids of nanoparticles.
DpffenLithqgsphy
2«tafe
AFM
scanning
h^ad
jr
it
AschaiBticdi^amofAEMsetupfirllN
d
J^y
Use? adJu^ra^RT lev»f
u
Safrnpi*
XY««ig«
X
^^&^
D BLilo^
J.
Z^
KI^
C IJu
S-WOirg
ardC A Mridn
'linear
RXIEAT;^
fcrESp-ftnMnDlifliagspiy
Fig.
1.44.
Schematic diagram showing
the AFM
adapted
for dip pen
nanohtho-
graphy
[24]
40 Micro- and Nanomanufacturing
AFM Tip
Self-Assembled
Monolayer
Moiecutar transport
m
wummm.
'WiWiWiWiiiHl HiliMil 1W1WM
Solid Substrate
Writing >30 iini lines on Au surface
using l-othodecaneOiiol as ink
Van Crocker, Jr, The Dip Pen Nanolithography''^ (DPN"*) Process, Nanoinkpres
Fig. 1.45. Principle of forming a self-assembled monolayer using dip pen nano-
lithography
In the dip pen mode, the AFM is operated in the ambient condi-
tion so that the monolayer film of SAMs is deposited from tip to
substrate in a controlled manner. It seems logical that reservoirs of
self-assembled molecules are provided in order to speed up the
process of nanofabrication. Figure 1.46 shows the type of nanolay-
ers deposited using the dip pen nanolithographic technique.
AFMs are used in contact mode in ambient
conditions, a monolayer film of adsorbed
water usually connects the tip and sample.
Some researchers have thought of using
this property to transfer SAMs from
reservoirs down the tip to the substrate
ig,
J. Zhu, C.A. Mirkin, Science 286, pp. 523-525, 1999.
Fig. 1.46. Dip pen nanolithography of self-assembled monolayers