AFM lecture 2

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Outline

• AFM designs• Imaging modes• Cantilevers and tips• Imaging artefacts• References / links

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Deflection detection

as in MHB

light lever

interferometer

electron tunneling

capacitance

piezoresistive

piezoelectric

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Sample scanning

Veeco Multimode – sample scans x,y,z Asylum Research MFP-3D – sample scans x,ytip scans z

from Veeco training manual from AR website

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Tip scanning

Difficulty in maintaining laser position on tip

More flexibility in samples

Harder to make rigid

from Veeco training manual

Veeco Dimension Veeco Enviroscope

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Piezo scanner design

• typical piezo response ~ 0.262 nm / V

• to elongate a bar of 1 cm by 1 m,apply a voltage of 380 V across 1 mm

Veeco SPM guide

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Tube piezo

BUTcross-talkdifficult to integrate with sensors

RIGIDhigh resonant frequencyless sensitive to noisefaster scan rates possible

Veeco SPM guide

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Flexure based

No cross talk,Easier to integrate sensorsLarger scan areas possible

higher mass – lower resonant frequencygreater sensitivity to noiseslower scan speeds

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Controllers

Can be mostly hardware or almost entirely software.

Look for high sampling rates, flexibility, and low electrical noise

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Available AFMs

Asylum Research MFP-3D – sample x-y tip z90 m square closed loop, z range 15 m closed loop, controller - 16 bit variable gain

Enviroscope – tip scanning, tube piezoScan size 90 m square x-y, 5 m zz noise ~ 0.5 - 1.0 A (greater in vacuum)sample size – flexible depending on stage usedNanoscope IV controller – similar specs to Nanoscope IIIa plus Quadrex

Multimode – sample scanning, tube piezoJ scanner ~120 m square and 5.5 m z (z noise < 0.5A)E scanner ~ 5 m square x-y (I think!), and ~ 2.5 m zA scanner ~ 1 m square x-y, z ? (not vertical engage)Picoforce ~ 40 m x-y, and 20 m z closed loopNanoscope IIIa controller – 16 bit variable gainQuadrex – dual phase lock-in amplifiersample size limited to ~ 15 mm square by ~ 5 - 10 mm

Warwick

Nottingham – lots including above and several

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Imaging Modesstatic dynamic

contact

noncontact

contact mode force modulation ultrasonic force

microscopy

tapping mode

‘noncontact’ mode

torsional / shear mode

static deflection

mode

pulsed force mode

EFM, MFM …

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Contact mode

height deflection lateral force

5 m scans of pressure densified polycrystalline alumina, 50 nm height scale. Sample courtesy of Geoff West.

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Force curve

deflection sensitivity (typical values ~ 100 – 200 nm V- 1)

spring constant (in contact typically 0.01 – 1 nN m- 1)

adhesion force (typical values in ambient ~ 10 – 100 nN)

imaging forcelarge contact force => large lateral force when imaging

BAD FOR SOFT SAMPLES AND SHARP TIPS

Def

lect

ion

(V)

z displacement (nm)

constant compliancejump to contact

adhesion force

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‘Tapping mode’

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• dynamic mode• driven oscillator• amplitude modulation• intermittent contact

A0 – free amplitudeA – imaging amplitudeA < A0

A / A0 ~ 1 ‘Soft tapping’A / A0 << 1 ‘Hard tapping’

Typical oscillation amplitudes A ~ 5 – 50 nm

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Imaging in tapping mode

height amplitude phase

5 m scans of pressure densified polycrystalline alumina, 50 nm height scale. Sample courtesy of Geoff West.

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Force curve – tm

150 175 200-1.0

-0.5

0.0βα

Amplitude(V)

z displacem ent(nm )150 175 200

0.00

0.05

0.10

γTM deflection(V)

z displacem ent(nm )

amplitude sensitivitydeflection sensitivity

With Veeco AFM’s usually have 2 V target oscillation amplitude

amplitude sensitivity ~ 10 nm V-1 for 100 m cantilever20 nm V-1 for 200 m cantilever

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TM vs CM

Tapping ModeContact Mode

Lower interaction forces (especially lateral)

Cheap

More versatileSimple

More sensitiveStable

Higher resolution (tip effects)

Easy to implement

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Cantilevers and tips

l

wt

V-shaped cantileverdiving board or rectangularcantilever

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the ‘tip’

scale bar 1 mm scale bar 20 m scale bar 25 nm

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Critical parameters

CANTILEVERspring constant, deflection sensitivity, Q,

0,length,width,thickness,geometry,reflectivity,tolerances,tip positions, materialcoatings

https://www.veecoprobes.com/

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Critical parameters

TIPradius,coneangle,coneheight,tip setback, materialcoatinggeometry

https://www.veecoprobes.com/

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Choosing cantilevers / tips

• contact mode – force (spring constant and length), wear resistance, surface functionality, thermal drift, lateral stability, cost ….

• tapping mode – resonant frequency, Q, k,length, resolution, coatings, cost, imagingunder solution ….

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Tip effects – ‘resolution’

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Tip wear

Damage of the tip

‘picking – up’ contaminants

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Common tip / imaging artefacts

• double tip• blunt tip• streaking• contamination• optical interference• poor tracking• ‘ringing’• ‘noise’

Veeco SPM guide / NW

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Tip deconvolution

• critical for quantitative force measurements• deconvolution sample• drawback – tip wear

www.ntmdt.com

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Imaging artefacts - piezo

Piezo’s should be calibrated in x-y and z regularly

• critical• should be checked with relevant test sample• best ~ few %

Piezo’s without closed loop are liable to• hysteresis• creep• bow• drift• ageing

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Tip manufacturers

• Nanoworld• MikroMasch (www.spmtips.com)• Budget Sensors – available through Windsor Scientific• Veeco• Nanosensors• www.nanoandmore.com• ……

Also do calibration samples etc.

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Instrument manufacturers

• Agilent Technologies• Asylum Research• Jeol• JPK• Omicron• Nanonis and Nanosurf for controllers• Park Systems • Veeco (used to be DI)• ……….

Also usually have application notes and educational materials

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Image processing software

• WSXM freeware available from www.nanotec.es• Nanoscope Software (on CD) for Veeco files• SPIP – from Image Metrology

http://www.imagemet.com/• Image J - general image analysis software with plug-ins

for SPM analysis http://rsb.info.nih.gov/ij/

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References and links

• Veeco ‘A practical Guide to SPM’ available from their website (hopefully on CD as well)

• http://www.novascan.com/education/• http://virlab.virginia.edu/VL/easyScan_AFM.htm• For calibration standards see

http://www.pacificnano.com/standards-references.html• How AFM works

http://stm2.nrl.navy.mil/how-afm/how-afm.html• Advances in AFM

GiessiblRev. Mod. Phys. 75, 949-982, 2003on CD