23/04/2009 M. Ghidini, Parma, CIM 1
AFM and double pass techniques
Outline
1. Introduction to Scanning Probe Microscopy : basic concepts.
2. Force Microscopy • concept and Instrumental Aspects
• relevant forces
• operation modes in force microscopy
3. Tapping AFM
4. Magnetic Force Microscopy
5. Electrical Force Microscopies
Massimo Ghidini, Dip.to di Fisica dell’Universitàdi Parma
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1. Introduction to Scanning Probe Microscopy : basic concepts.
deflection sensor
approach
Data acquisition
tipfeedback
force sensor
vibration damping
sample
G. Binnig, Ch. Gerber and C.F. Quate, Phys. Rev. Lett. 56, 930 (1986)
The birth of the AFM
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1. Introduction to Scanning Probe Microscopy : basic concepts.
General Physical principle of scanning probes : interaction between probe and sample
STM
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1. Introduction to Scanning Probe Microscopy : basic concepts.
Scanning and Control : Actuators
PZT-5H d31= 0.262 nm/V
Horizontal scanning obtained by the bending of the tube
Dh
Uldx x
2
3122
Vertical movement : elongation of a tube
1. Introduction to Scanning Probe Microscopy : basic concepts.
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Problems limiting position performance of scanners
•Non –linearity
•Hysteresis in scanning
•Creep
•Noise and drift in high voltage supply
•Thermal drift of mechanical set up
Force Microscopy: concept and instrumental aspects
Mesurement of forces between a sharp tip and sample: static vs. dynamic
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f1 f2 f3
f
A
DC: zDC= F/k QzmFQ
zDC2
0
AC
Force Microscopy: concept and instrumental aspects
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Change in resonance
curve can be detected by:
Lock-in (A or ) (as in Tapping™
mode)
FM detection (PLL, f)
Albrecht, Grutter, Horne and RugarJ. Appl. Phys. 69, 668 (1991)
Force gradient F’ :additional
stiffness in series
F’= 2k f/f
(if d2V / dz2 constant)
AC :
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Force sensors and measurements : static (or DC) vs. dynamic (or AC modes)
Atomic Forces :1 nN; Magnetic:1-10 pN; Precessional forces: 1fN
Tkzk B2
1
2
1 2
Force resolution and thermal noise
Q
TkkzkF Bstatic
0
2
min
2
DC: Off-resonance bandwidth convenient
Ultimate force resolution determined by thermal noise (ideal case)
Q
Tkkz
Q
kF Bdyn
0
2
min
2
0
AC: resonance bandwidth convenient
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Displacement sensors
Force Microscopy: concept and instrumental aspects
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3
3
4l
Ewtkn
For silicon: E=1.69 1011 N/m2
Cantilever
type
L
(micr)
W
(micr)
T
(micr)
K
(N/m)
f0
(kHz)
NSC18 230 3.0 2-5.5 75
NSC 21 290 40 2.0 1 25
CSC38/Pt (3 levers per chip)
350
300
250
35 1.0
0.03
0.05
0.08
12
Force Microscopy: concept and instrumental aspects
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Operation modes
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Contact Mode
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file:///E:/Documents%20and%20Settings/Massimo/Desktop/constforcemode.SWF
file:///E:/Documents%20and%20Settings/Massimo/Desktop/constant_height_mode.s
wf
file:///E:/Documents and Settings/Massimo/Desktop/constforcemode.SWFfile:///E:/Documents and Settings/Massimo/Desktop/constforcemode.SWFfile:///E:/Documents and Settings/Massimo/Desktop/constant_height_mode.swffile:///E:/Documents and Settings/Massimo/Desktop/constant_height_mode.swffile:///E:/Documents and Settings/Massimo/Desktop/constant_height_mode.swf
Point Mass Modeling in the harmonic approximation...
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...And its limitations
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Operation modes
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Operation modes
Contrast formation : non-contact (dynamic) vs. Contact (static)
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Fts term : relevant forces
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Relevant Forces....
...are ultimately of electromagnetic origin. But different effects give rise to specific distance dependences. So the following contributions can be identified:
Long-range acttractive Van der Waals arising from e.m. Fluctuations. Therefore they are universally present regardless of tip/sample system.
Contact and short-range repulsive forces : Pauli exclusion principle and ionic repulsion. However if contact area involoves 10-100 atoms usually microscopic picture is replaced with continuum models of between elastic bodies.
Capillary : meniscus or liquid bridge formed between tip and surface. This implies actractive force.
Electrostatic forces : due to charges trapped on dielectric surfaces.
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Tapping mode (semicontact, intermittent contact...)
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Principle: amplitude modulation- cantilever excited near resonance. Oscillation amplitude used as feedback parameter to measure the topography
Tapping (or intermittent contact) is usually associated with measurements in ambient conditions
starting idea was to use high amplitudes(100 nm) and stiff cantilevers (40 N/m);
file://E:/Documents%20and%20Settings/Massimo/Desktop/semicontact_mode.swf
file:///E:/Documents and Settings/Massimo/Desktop/semicontact_mode.swffile:///E:/Documents and Settings/Massimo/Desktop/semicontact_mode.swf
Comparison with Non-Contact
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Tapping Non-contact
file://E:/Documents%20and%20Settings/Massimo/Desktop/semicontact_mode.swf
file:///E:/Documents and Settings/Massimo/Desktop/semicontact_mode.swffile:///E:/Documents and Settings/Massimo/Desktop/semicontact_mode.swf
Tapping AFM dynamics
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Tapping AFM dynamics
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Resolution
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Thermal noise: for k= 40 N/m -> 0.01 nm
Vertical resolution 0.1 nm or better
Lateral resolution : difficult to define . Presence of elangerment
DNA
Magnetic Force Microscopy
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dVHME sampletip 0 stipz HzMF
0
If modifications induced by tip can be neglected
Co tip
Ni sample
F=qtipHsample= 8.65.10-10
N~1 nN
Upper limit in ideal case 1 nNTypically force are smaller 1-10 pN
If force and tip are aligned along z:
z
FFkk
k
Fz
magnonmag
leff
eff
mag
mag
;
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Amplitude and phase variation in the presence of a magnetic forces gradient
F m z zH xe x z
H z e z
fz
F z m z
2
z2
H z
Close to resonance
Lift Mode
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file:///E:/Documents%20and%20Settings/Massimo/Desktop/ac_mfm.swf
Most used method of tip/sample distance control: topography can be made dominant contribution even if tip is magneticOn the other hand the 2° pass method technique can be either static or dynamic
file:///E:/Documents and Settings/Massimo/Desktop/ac_mfm.swffile:///E:/Documents and Settings/Massimo/Desktop/ac_mfm.swf
Contrast categories
Negligible modification : Mtip and Hsample do not change with tip-sample position, and so does contrast
Reversible modification: contrast is a function of tip/sample distance only
Hysteretic or irreversible modification: Mtip and Hsample are changed irreversibly during scan. Contrast depends also on the history of tip-sample position.
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When life is “easy”…...
ẑz)ndz(
z)1(tM4
x̂z)ndx(
ndx)1(tM4)z,x(H
22n
n
r
22n
n
r
d
t
D.Rugar et al. J. Appl. Phys, 68,1169, 1990
Hard disks
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Hard Materials: CoPt
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M. Ghidini et al, J. Appl. Phys. 2006
Soft Materials: flux closures in Fe (110) dots
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Compact 3D dots
300K 500K 700K 900K
~750K ?
1AL
2AL
3AL
4AL
>6AL
Deposition temperature, T (K)S
Not explored
Nom
ina
l co
ve
rage (
ato
mic
layers
, A
L)
T =700K, [2AL,6AL]t=6AL (for Mo)
rQ T 6AL
(for Mo)r
Q T >400K, >6AL (for Mo)r
Q
Flat islandst~1nm
(for Fe/Mo)
Compact 3D dotst>30nm
[-110]
[001]
(110)
Q~3.5AL
1 m
5m
2m
500n
m
O. Fruchart et al., J. Phys.: Condens. Matter 19, 053001, Topical Review (2007). Presented at JEMS08
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Types of contrast
A
B
A
B
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Tip-sample interaction
Electrostatic Force Microscopy
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file:///E:/Documents%20and%20Settings/Massimo/Desktop/efm/efm.swf
KPFM
SCM
file:///E:/Documents and Settings/Massimo/Desktop/efm/efm.swffile:///E:/Documents and Settings/Massimo/Desktop/efm/efm.swf
Cristian Staii et al., Quantitative analysis of Scanning Conductance Microscopy, Nanoletters, 2004, 4 (5), 859-862
Figure 1. (a) Schematic of SCM. In the interleave scan, theDCvoltage-biased AFM cantilever is driven at its resonant frequency at afixed height h above the sample. The phase of the cantilever oscillation isrecorded as a function of tip position. (b) 30 ím 30 ím SCM image of carbonnanotubes. The inset shows a line scan along the black line. SWNTs andother small diameter conducting nanowires show a negatiVe phase shift inSCM. (c) 30 ím 30 ím SCM image of an insulating PEO nanofiber (diameter10-100 nm) with a line scan along the black line. Insulating nanofibers showa positiVe phase shift in SCM. (d) 20 ím 20ím SCM image of a conductingPAn.HCSA/PEO nanofiber (diameter 100 nm). The inset is a line scan alongthe black line showing a negatiVe-positiVe-negatiVe contrast in SCM.
The model
)(tan
22
0
z
F
k
10
2
211
)(
2
1tipV
z
hCF
2
2
1
2
0
)(
2tan tipV
z
hC
k
Q
2/
2
2
2
2
2
1
2
0
)()(
2)tan( tipV
z
hC
z
hC
k
Q
EFM on graphene
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