AFM and double pass techniques - unipr.it › html › Ghidini.pdf23/04/2009 M. Ghidini, Grenoble,...

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



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



General Physical principle of scanning probes : interaction between probe and sample

STM



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



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


f1 f2 f3

f

A

DC: zDC= F/k QzmFQ

zDC2

0

AC


23/04/2009 M. Ghidini, Grenoble, Institut Néel

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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 :


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


9

Displacement sensors



10

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



11

Operation modes


12

Contact Mode


13

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...


14

...And its limitations


15

Operation modes


16

Operation modes

Contrast formation : non-contact (dynamic) vs. Contact (static)


17

Fts term : relevant forces


18

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.


19

Tapping mode (semicontact, intermittent contact...)


20

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


21

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


22

Tapping AFM dynamics

23/04/2009 Massimo Ghidini , Parma, CIM 23

Resolution

23/04/2009 M. Ghidini , Grenoble, Institut Néel

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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


25

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

;


26

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

23/04/2009 M.Ghidini, CIM, Parma 27

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.


28

23/04/2009 M. Ghidini , Grenoble, Institut Néel

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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


30

Hard Materials: CoPt


31

M. Ghidini et al, J. Appl. Phys. 2006

Soft Materials: flux closures in Fe (110) dots


32

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

23/04/2009 M. Ghidini Grenoble, Institut Néel

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Tip-sample interaction

Electrostatic Force Microscopy

23/04/2009 M. Ghidini, Scuola CIM 35

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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AFM and double pass techniques - unipr.it › html › Ghidini.pdf23/04/2009 M. Ghidini, Grenoble,...

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