afm vs sem

Svetlana SanterFreiburg, 06.07.2004

SEM and AFM: SEM and AFM: Complementary Techniques for Complementary Techniques for

High Resolution Surface High Resolution Surface Investigation Investigation

Svetlana Santer

Freiburger Freiburger MaterialforschungszentrumMaterialforschungszentrum

Institut für Institut für MikrosystemtechnikMikrosystemtechnik


AFM SEMAFM SEM


ScanningScanning ElectronElectron MicroscopyMicroscopy (SEM)(SEM)

Vacuum: 10-4-10-10 Torr


Principles Principles of SEM of SEM ImagingImaging

When the electron beam hits the sample,the interaction of the beam electrons from the filament and the sample atoms generates a variety of signals.

•secondary electrons (produced byinteraction of primary e with the looselyheld outer electrons of the sample),

•backscattered electrons (beam electrons from the filament that bounce off nuclei ofatoms in the sample(elastic-interaction of the primary electrons with the nucleus of the atom),

•X-rays, light, heat,

•transmitted electrons (beam electrons that pass through the sample).

Secondary electrons: high spatial resolution, good topographic sensitivity

Backscattered electorns: they have more energy and can escape from greater depths, carry some informartion of sample composition


Scanning Electron MicroscopyScanning Electron Microscopy (SEM)(SEM)

The SEM uses a beam of electrons to scan the surfaceof a sample to build a three-dimensional image of the specimen.Major Components of the Scanning Electron

Microscope

All scanning electron microscopes consist of:

1. A column which generates a beam ofelectrons.

2. A specimen chamber where the electron beam interacts with the sample.

3. Detectors to monitor the different signals that result from the electron beam/sample interaction.

4. A viewing system that builds an imagefrom the detector signal.


Generating the beam Generating the beam of of electronselectrons

The electron gun is housed on the top of the column and generates the beam ofelectrons that rushes towards the sample housed in the specimen chamber.

Electrons are very small and easily deflected by gas molecules in the air. Therefore, toallow the electrons to reach the sample, the column is under a vacuum. The vacuum is maintained by two vacuum pumps: a rotary pump and an oil diffusion pump which is housed inside the SEM and is water cooled. Thus, the SEM needs a water cooling line which filters the water before it cools the oil diffusion pump.


Generating the beam Generating the beam of of electronselectrons

Within the electron gun is the filament which is the source of the beam of electrons.The filament is made of tungsten and is heated to generate a fine beam of electrons.

As the filament gets used, it becomes brittle and coated. If the filament is overheated or too old, it will break.


Detectors Detectors of of the the SEMSEM

The SEM has several detectors to view the electron signals from the sample.

(1) secondary electron detector looks like a Faraday cage, and detects secondary electrons.

(2) backscattered electron detector (solid state detector) is located above the sample, consists of a diode with a thin gold conductor across the front surface. Backscattered electrons have sufficient energy to pass through the front surface and produce electron hole pairs which produce a curreent in the diode


Can we see electrons directly by eyeCan we see electrons directly by eye??

The SEM scans its electron beam line by line over the sample.

It's much like using a flashlight in a dark room to scan the room from side to side.

Gradually the image is built on a TV monitor (cathode ray tube or CRT for short). The SEM hasbuttons on the keyboard that control the scan speed. A fast scan which takes a couple of secondsto generate an image can be very grainy - like you're looking at an object in a snow storm. A slow scan is very clear and sharp - but takes a minute or two to get a picture.


Sample Sample preparationpreparation

Samples have to be prepared carefully to withstand the vacuum inside the microscope. Biological specimens are dried in a special way that prevents them from shriveling. Because the SEM illuminates them with electrons, they also have to be made to conduct electricity.

Sputter coater


ExamplesExamples


HistoryHistoryMax Knoll and Ernst Max Knoll and Ernst RuskaRuska --19311931

electron microscopyelectron microscopy


HistoryHistory

••1938 1938 –– first first SEM SEM by by von Ardennevon Ardenne

••1942 1942 –– first first SEM SEM for bulk samples for bulk samples by Zworkinby Zworkin

••1965 1965 –– first commercila instrument first commercila instrument (Cambridge)(Cambridge)

Resolution:Resolution:

50 nm in 194250 nm in 1942

0.7 nm 0.7 nm todaytoday


Basics of AFMBasics of AFMAFM provides very high resolution images of various sample properties

50 nm

Piezo

Sample

Cantilever

Tip

PSDLaser

Three basic components:

• Piezoelectric scanner

• Cantilever with a sharp tip

• Position sensitive detector (PSD) coupled with a feed-back system

Digital Instruments (DI) MultiMode Nanoscope IIIa


Historical steps Historical steps of of developmentdevelopment

•1981-invention of STM

•1985-invention of AFM

•1986-Nobel Price

Christoph GerberChristoph Gerber

It

Vt

1

20

12

−

−

=

=

o

ΑΦ ~h/mk

,eI)z(I kzt

RestrictionRestriction: : conductive samplesconductive samples

IBM‘s Zurich Research Center in Rüschlikon


General General componentscomponents and and their functionstheir functions


Control Control of of the cantilever deflectionthe cantilever deflection

Optical Lever

Tunneling sensor (Binnig, Rohrer)

Optical interferometer detection system

Piezoresistive detection

It

Vt

STM tip

•special design of cantilever

•changing of resistivity with the applied stress


Piezoelectric scannerPiezoelectric scanner•SPM scanners are made from a piezoelectric material that expands and contracts proportionally to an applied voltage

•Whether they expand or contract depends upon the polarity of the applied voltage. Digital Instruments scanners have AC voltage ranges of +220 to –220V

0 V - V + V

No applied voltage Extended Contracted

•In some versions, the piezo tube moves the sample relative to the tip. In other models, the sample is stationary while the scanner moves the tip

Solenoid

Cantilever

PZT


Piezoelectric scannerPiezoelectric scannerMaterial Material PropertiesProperties

•Piezoelectric ceramics are a class of materials that expand or contract when inthe presence of a voltage gradient

•Lead (plumbum) zicronate titanate (PZT) crystallites exhibit tetragonal or rhombohedric structure

•Due to their permanent electrical and mechanical asymmetry, they exhibit spontaneous polarization and deformation

Poling, an intense electric field (>2000V/mm) is appliedPerovskite-type PZT unit cell (1) in the

symmetric cubic state, (2) distorted


Geometry Geometry of PZT of PZT scannerscanner

Tube scanner The tripod

Not stable

•The outer electrode is segmented in four equal sectors of 90 degrees

•The inner electrode is driven by the z signal

5 µm125 µm 125 µmJ

2.5 µm10 µm 10 µmE

0.4 µm0.4 µm 0.4 µmA

Vertical RangeScan SizeModel

V/nm~K,VKx 3∆∆ =

Bipolar configuration


Triangular patternTriangular pattern

Slow

sca

n di

rect

ion

Fast scan speed

Slow scan speed

Hzf lvv 2=

Nvl

v Hzs

⋅=

Fast scan direction


Feedback Feedback looploop


AFM Probe ConstructionAFM 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 normal supertip ultraleversupertip ultralever


A A few requisites for cantileversfew requisites for cantilevers

1. Must be soft zkF ∆= Minimize k

For rectangular cantilevers

Example: t=10 µm, w=1mm, l=4mm k~1N/m

k (C-C stretch.)~500N/m k (C-C-H bend)~50 N/m

2. Must be insensitive to external vibrations Maximize eigenfrequencies

mk=ω Minimize m

Ex: Si or Si3N4 L=140µm, w=40µm, t=1.5 µm, k~0.7 N/m, ~60 kHz


Common Common types types of of cantileverscantilevers

Si3N4 Si

Diamond


Fabrication Fabrication of of cantileverscantilevers


Calibration Calibration of of cantilevercantilever

Theoretical method

E=300 GPa

E=238 GPaStatic method Dynamic method•Measuring of thermal responseof the cantilever

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

( ) ctstt kZkZZ ' =⋅−


Superposition of Superposition of two geometriestwo geometries


Reconvolution Reconvolution of of the tip shapethe tip shape

D=dreal

IIIIII

r

d

D

rDd4

2=


DeconvolutionDeconvolution of of the tip shapethe tip shape

Tobacco Mosaic Virus (TMV)

d~18 nm

r-?


AFMAFM Tip ArtifactsTip Artifacts

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

polystyrene spheres.....



Similar spheres imaged with asupposedly sharp tip



This image should only contain images of large polysterene spheres


Blind Blind ReconstructionReconstruction

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 Blind ReconstructionReconstruction

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 threeTrue three--dimensional dimensional scanningscanning??

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 Method for imaging sidewalls by AFMAFM

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

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

No!No!


Calibration Calibration of of the tip shapethe tip shape

hL

2rL

rLR4

2= h

LR2

2=


OxideOxide--Sharpened TipsSharpened Tipsincreasing aspect ratio

reducing tip radius

•Aspect ration- 10:1

•Radius r~1nm

HF

etchingSiO2


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

L=(1-5)µm

R=(20-40)nm

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


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

•Multiwalled carbon nanotubes (MWNT) (nested, concentrically arrangedSWNT, 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


PickPick--up up TipsTips

d=0.9nm d=2.8nm


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

•Heating of nanocatalyst particle (r~3.5 nm)

•Presipitates carbon nucleates a grow of nanotube


Direct grow Direct grow of of nanotubesnanotubes

•2 nm in diameter

•2µm in length

Alumina/iron/molybdenum-powdered catalyst

Labor intensive

Not amenable to mass production


Modes Modes of of operationoperation


The common The common AFM AFM modesmodes

contact mode

tapping mode

Contact mode

Non-contact mode

Intermittent mode


Contact Contact mode AFMmode 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 Force curvecurve


ContactContact modemode

Force

Oscillation amplitude

Probe-sample distance, z

AABB

CCDD

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 Problems of the contact the contact modemode

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


Different Different types types of of forces forces relevant to AFMrelevant 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

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

(a)

(b) Capillary forces

radiuscontact typical thenm, 5 ~a 63

,nN~R

KaFd =


Problems ofProblems of the contactthe contact modemode

Large lateral (shear) forces ~ 100 nN

To solve the problem non-contact mode


Problems ofProblems of the contactthe contact modemode


NonNon--contactcontact Mode AFMMode AFM

Highly unstable mode Ultra high vacuum at low temperature


Tapping Tapping mode AFMmode 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


TappingTapping mode AFMmode AFM

nmAkHz

1001050050

0

0

÷≈÷≈ω



Force

Oscillation amplitude

Probe-sample distance, zAABB

CC

DD

Static modeDynamic mode



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 ≤≤



Phase Phase ImagingImaging

Driven force

Actual responce

Different characteristics of the sample different offset the phase




Examples Examples of Phase Images AFMof Phase Images AFM




Advantages and Advantages and DisadvantagesDisadvantagesContact 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 Advantages and DisadvantagesDisadvantages

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


Cantilevers used Cantilevers used in in contact contact and and tapping modestapping modes

m/N.k 1010 ÷−

m/N~k 50


Contact vs Tapping modesContact vs Tapping modes


ScanningScanning ProbeProbe MicroscopeMicroscope (SPM)(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


Comparison Comparison of of TechniquesTechniques: AFM : AFM vs vs SEMSEM

Surface structureSurface structure::

••atomically smooth surfacesatomically smooth surfaces

TM-AFM image of 0.14 nm monoatomic steps on epitaxial silicon deposited on (100) Si. 1 µm scan, RMS=0.07 nm

On a On a sample this smoothsample this smooth, , the the SEM has SEM has difficulty resolving difficulty resolving these features due these features due to to the the subtle variations subtle variations in in heightheight




••Thin filmsThin films

Polysilicon thin film at approximately the same lateral magnification. But they differ in the other types of information

AFM provides with roughness and height

SEM provides a large area view

On On most thin filmsmost thin films, , the the SEM SEM and AFM and AFM produce produce a a similar similar representation representation of of the sample the sample surfacesurface

SEMSEM

AFMAFM




••Thin filmsThin films: : interpretation interpretation of of heightheight

In In the the SEM image, SEM image, it can be it can be sometimes be difficult sometimes be difficult to to determine whether the feature determine whether the feature is sloping is sloping up up or or downdown

GaP on Si during chemical beam epitaxy deposition




••High High Aspect Aspect Ration Ration StructuresStructures

With With AFM AFM one can measure the one can measure the structure nondestructivelystructure nondestructively, , but but without details without details on on the sides

SEM SEM provides measuring the provides measuring the undercuts undercuts of of these linesthese lines

the sides




••Rough surfacesRough surfaces

SEM has a SEM has a large depth large depth of of fieldfield::

Ability to image Ability to image very rough very rough surfacessurfaces



EnvironmentEnvironment::

SEM SEM is conducted is conducted in a in a vacuum vacuum environmentenvironment

AFM AFM is conducted is conducted in in vacuumvacuum, , gas, liquid, gas, liquid, vapourvapour, and in an , and in an ambient ambient environmentenvironment

Liquid cell AFM


ComparisonComparison ofof TechniquesTechniques: AFM: AFM vsvs SEMSEM

Although Although SEM and AFM SEM and AFM appear very appear very different , different , they they share share a a number number of of similaritiessimilarities

Both techniques raster Both techniques raster a probe a probe across the surfaceacross the surface

Similar Similar lateral lateral resolutionresolution

Both techniques can produce artifactsBoth techniques can produce artifacts

AFM AFM can provide measurements can provide measurements in all in all three three dimensionsdimensions, , with with a a vertical resolution vertical resolution of <0.05 nmof <0.05 nm

SEM has SEM has the ability the ability to image to image very rough surfacesvery rough surfaces

SEM and AFM SEM and AFM are complementary techniques that are complementary techniques that provide provide a a more complete representation more complete representation of a of a surface surface

when used together than if each were the only when used together than if each were the only technique availabletechnique available


ReviewReview ofof Harmonic OscillatorsHarmonic Oscillators

Summing the forces, we get the equation for damped, driven oscillators:

)tcos(Fdtdxkz

dtzdm

)tcos(FdtdzkzFFFF drivingdampingspring

ωγ

ωγ

02

2

0

+−−=

+−−=++=∑

γωω

γ 00 mQ,

Qm

==Using expresion for Using expresion for quality factor:

)cos(00

2

2

tFdtdz

Qmkz

dtzdm ωω

+−−=

)tcos(mF

dtdz

Qz

dtzd

mk

ωω

ω

ω

00202

2

20

+−−=

≡


Modeling the Modeling the AFM AFM cantilevercantileverthe cantilever is essentially a driven damped oscillator

)z,z(F)tcos(Fdtdz

Qm

zkdt

zdm cc ++−−= ωω

00

2

2

kkcc

zzcc

zz

S>aS>a00 S<aS<a00

zz

Z=0Z=0F(zc,z) term inserted to account for the surface interactions. This term depends on whether or not the tip in contact with the surface (i.e., zc+z <ao)

or not (zc+ z > ao)

002/3

0220

02

,)()(33

46

),(

,)(6

),(

azzdtdzzza

hRzzaRE

aARzzF

azzzz

ARzzF

cccc

cc

c

≤+−−−−−−

+−=

>++

−=

πην


Modeling theModeling the AFM AFM cantilevercantilever

,dtdz)zza(

hR)zza(RE

aAR)tcos(F

dtdz

Qm

zkdt

zdm c/

cc −−−−−−

+−+−−= 023

0220

00

2

2

334

6πη

νω

ω


RestrictionsRestrictions

The direct asignment of the phase contrast is hadly possible:

(i) The abrupt transition from an attractive force regime to strong repulsion which acts for a short moment of the oscillation period

(ii) Localisation of the tip-sample interaction in a nanoscopic contact area

(iii) The non-linear variation of both attractive forces and mechanical compliance in the repulsive regime

(iv) The interdependence of the material properties (viscoelasticity, adhesion, friction) and scanning parameters (amplitude, frequency, cantilever position)

The interpretation of the phase and amplitude images becomes especially intricate for viscoelastic polymers

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afm vs sem