Analysis of Microsystems (AFM) 2010 - cpi.uni-freiburg.de

OutlineScanning Probe Microscope (SPM)

•A family of microscopy forms where a sharp probe is scanned across a surface and some tip/sample interactions are monitored

•Scanning Tunneling Microscopy (STM)

•Atomic Force Microscopy (AFM)

contact mode

non-contact mode

tapping mode

•Other forms of SPM

lateral force

magnetic or electric force

thermal scanning

phase imaging

AFM Probe Construction

Low spring constant (k - 10-2 to 102 N/m)

Sharp protruding tip (r=5-50 nm)

High resonance frequency mk

πω

21

=

Three common types of AFM tip

normal supertip ultralever

Common types of cantilevers

Si3N4 Si

Diamond

Fabrication of cantilevers

Calibration of cantilever

Theoretical method

Static methodDynamic method

•Measuring of thermal response of the cantilever

•Measuring of the change of resonance frequency caused by the addition of known masses

( ) ctstt kZkZZ ' =⋅−

E=300 GPa

E=238 GPa

Superposition of two geometries

Reconvolution of the tip shape

D=dreal

r

d

D

I II

rDd4

2=

Deconvolution of the tip shape

Tobacco Mosaic Virus (TMV)

d~18 nm

r-?

Calibration of the tip shape

hL

2rL

rLR4

2= h

LR2

2=

Useful formulas

nmd)Rd(

dz

384

2

>+

=∆

nm.dR

dz

828

2

≥

=∆

( )hzzRd ∆∆∆ ++= 2

AFM Tip Artifacts

We start off with an example of a „good“ AFM image of 300 nm

polystyrene spheres.....

AFM Tip Artifacts

Similar spheres imaged with a supposedly sharp tip

AFM Tip Artifacts

This image should only contain images of large polysterene spheres

AFM Tip Artifacts

Classical example of tip artifact, showing DNA and debris

AFM Tip Artifacts

Here is a cute example of some serious artifacts. The obtained image looks like the three bears

Different types of forces relevant to AFM

2

3

3

89251

34

sec/m.gnmr

cm/g

grmgFg

=

=≈

==

ρ

ρπ

nmrnm.D

JAD

rAFvdW

2530

10619

2

===

⋅=

−( )

energysurfacepolymermJ/mγ

energysurfacesilicamJ/mγ

/γγγ

γπradhF

p

s

ps

225

2100

212

4

=

=

=

⋅=

nNFg910−≈

nN~FvdW 5nN~Fadh 30

(b) Capillary forces

nN~cosRFcap 224 1 θγπ=(d) Deformation forces

radiuscontact typical thenm, 5 ~a 63

,nN~R

KaFd =

(a)

Blind Reconstruction

AFM profile of a single „bump“

What does this single scan line tell us about the topography of the tip and sample?

The tip geometry can be no bigger than the obtained profile

Blind Reconstruction

Line scan having two „bumps“

What does this tell us about the shape of the tip?

Case 1: Tip with single apex

Case 2: double tip

True three-dimensional scanning?

One of the drawbacks of typical AFM is that the images obtained are not truely three-dimentional. No matter how sharp the tip, the data collected can never access the underside of the sample.

„Petticoat“ effect-all images of objects having steep walls or undercut regions appear to have flared sides

Method for imaging sidewalls by AFM

Martin, Wickramasinghe, Appl. Phys Lett 1994, 64, 2498

Can we get a similar image using a typical AFM and the boot-shaped tip?

No!

Titanium thin films

2 µm per 2 µm

The tip is not too bad, it just is not very sharp

A subtle example of a double tip

Here is an image taken by a multiple tip

This is an image of a triple tip

Sometimes you just can‘t figure ou what is wrong with the tip- it just doesn‘t look right!

The tip broke or had a large contaminant dislodged while scanning down

„TGT“ spikes

„delta-like“ function: 1 µm high, 1.5 µm apart

„TGT“ spikes

„delta-like“ function: 1 µm high, 1.5 µm apart

A school of fish, maybe? The“eyes“ are rounded tips about 100nm in radius

The return of space invaders

A formation of B2 bombers?

Normal tip vs NT-tip

Conclusion:

Golden rule of AFM spectroscopy:

Every time one measures one obtains an image

Not every time one obtains an artifact

Oxide-Sharpened Tipsincreasing aspect ratio

reducing tip radius

•Aspect ration- 10:1

•Radius r~1nmSiO2

HF

etching

Electron beam deposition (EBD)High-aspect-ratio tips

Carbon materials are deposed by the dissociation of background gases in the SEM vacuum chamber

L=(1-5)µm

R=(20-40)nm

Carbon Nanotube Tips•Single-walled carbon nanotubes (SWNT), d=(0.7-3)nm

•Multiwalled carbon nanotubes (MWNT) (nested, concentrically arranged SWNT, d=(3-50)nm

•High-aspect-ration AFM probes

•Very stiff, E=1012 Pa (the stiffest known materials)

•Buckled nanotubes

Labor intensive

Not amenable to mass production

Pick-up Tips

d=0.9nm d=2.8nm

Chemical Vapor deposition (CVD)Direct grow nanotubes onto AFM tip

•Heating of nanocatalyst particle (r~3.5 nm)

•Presipitates carbon nucleates a grow of nanotube

Direct grow of nanotubes

Alumina/iron/molybdenum-powdered catalyst

•2 nm in diameter

•2µm in length

Labor intensive

Not amenable to mass production

Modes of operation

The common AFM modes

contact mode

tapping mode

Contact mode

Non-contact mode

Intermittent mode

Contact mode AFM

•A tip is scanned across the sample while a feedback loop maintains a constant cantilever deflection (and force)

•The force on the tip is repulsive ~ a few nN

•The tip senses lateral and normal forces

•The tip contacts the surface through the adsorbed fluid layer

•Forces range from nano to micro N in ambient conditions and even lover (0.1 nN or less) in liquid

Force curve

Contact mode

Force

Oscillation amplitude

Probe-sample distance, z

AB

CD

Static mode

Dynamic mode

pN~Fnm.z

,m/N..k,kzF

110

10010=

÷==

C"" contact to a into jump is B""at

,kzF>

∂∂

Problems of the contact mode

Large deformation forces ~ 100 nN

Capillary forces

nN~cosRFcap 224 1 θγπ=

To solve the problem operation in liquid

Elimination of capillary forces

Reduction of van der Waals forces

Problems of the contact mode

Large lateral (shear) forces ~ 100 nN

To solve the problem non-contact mode

Non-contact Mode AFM

Highly unstable mode Ultra high vacuum at low temperature

Tapping mode AFM

•A cantilever with attached tip is oscillated at its resonant frequency and scanned across the sample surface

•A constant oscillation amplitude (and thus a constant tip-sample interaction) are maintained during scanning. Typical amplitudes are 20-100 nm

•Forces can be 200 pN or less

•The amplitue of the oscillations changes when the tip scans over bumps or depressions on a surface

Tapping mode AFM

nmAkHz

1001050050

0

0

÷≈÷≈ω

Force

Oscillation amplitude

Probe-sample distance, zAB

C

D

Static modeDynamic mode

Tapping mode AFM

Tapping mode AFM

0AA

r spsp =Three regimes of tapping mode:

(i) Light tapping

(ii) Moderate tapping

(iii) Hard tapping

170 ≤≤ spr.

7030 .r. sp ≤≤

30010 .r. sp ≤≤

Tapping mode AFM

Phase Imaging

Driven force

Actual responce

Different characteristics of the sample different offset the phase

Examples of Phase Images AFM

Advantages and DisadvantagesContact Mode

Advantages

high scan speeds

the only mode that can obtain „atomic resolution“ images

rough samples with extreme changes in topography can sometimes be scanned more easily

Disadvantages

lateral (shear) forces can distort features in the images

the forces normal to the tip-sample interction can be high in air due to capillary forces from the adsorbed fluid layer on the sample surface

the combination of lateral forces and high normal forces can result in reduced spatials resolution and may damage soft samples (i.e. biological samples, polymers) due to scraping

Advantages and Disadvantages

Tapping mode

Advantages

higher lateral resolution on most samples (1 to 5 nm)

lower forces and less damage to soft samples imaged in air

lateral forces are virtually eliminated so there is no scraping

Disadvantages

slightly lower scan speed than contact mode AFM

m/N.k 1010 ÷−

m/N~k 50

Cantilevers used in contact and tapping modes

Contact vs Tapping modes

How to interpret height and phase TM-AFM images of a sample in

terms of its physical and morphological properties?

Analysis of Microsystems (AFM) 2010 - cpi.uni-freiburg.de

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