atomic force microscopy

Natural Sciences Tripos Part II

MATERIALS SCIENCE

II

MATERIALS SCIENCE

C20: Atomic Force Microscopy

Dr R. A. Oliver

Michaelmas Term 2014 15

Name............................. College..........................

Michaelmas Term 2014-15

C20:C20:Atomic force microscopypy

Lecture 1Lecture 1

1

Course synopsis Lecture 1

AFM: basic principles Imaging modes

Lecture 2 Resolution and artefacts in AFM

Horizontal and vertical resolutionImaging modes Imaging formation Key instrumentation Calibration issues

Horizontal and vertical resolution Tip-related artefacts Other artefacts Image processing and removal of

Forces acting on the tipImage processing and removal of artefacts

Lecture 3 Lecture 4Lecture 3 Magnetic measurements by AFM

MFMElectrical measurements by AFM

Lecture 4 Mechanical property measurement

Friction measurements Phase imaging Electrical measurements by AFM

EFM and KPFM CAFM

SCM

Phase imaging Force-volume imaging Nanoindentation

Nanofabrication by AFM SCM SSRM Electrical case studies

Nanofabrication by AFM AFM in biology

Lecture 1 Atomic force microscopy 2

Books and other resources Scanning Probe Microscopy: The Lab on a Tip, E Meyer, HJ Hug, R

Bennewitz Springer (intended as an introductory book for graduate students)Bennewitz, Springer (intended as an introductory book for graduate students)

http://www.doitpoms.ac.uk/tlplib/afm/index.php (New teaching and learning package on the basics of AFM, developed in this department in 2009)

http://web.mit.edu/cortiz/www/AFMGallery/PracticalGuide.pdf, http://www.padova.infm.it/torzo/Mironov_SPM.pdf(online books giving detailed discussions of many of the topics in the first 2 l t )lectures)

http://www.veeco.com/pdfs/library/SPM_Guide_0829_05_166.pdf(online book from Veeco covers most topics on the course, but with a bias ( p ,towards Veeco instruments)

Atomic Force Microscopy: Biomedical Methods and Applications PC Braga, R. Davide, Humana Press. (Basic principles of AFM and its uses in aga, a de, u a a ess ( as c p c p es o a d ts usesbiomedical and life sciences)


Why study scanning probe microscopy?microscopy?

Without GerdWithout Gerd Binnig's Nobel Prize-winning microscopes,

Scanning tunnelling microscopy was

w g c oscopes,nanotechnology would not exist Scanning tunnelling microscopy was

invented by Gerd Binnig (right) and Heinrich Rohrer (left) in 1981. They were awarded the Nobel Prize in 1986. Binnig

Harry GoldsteinIEEE Spectrum

2004galso invented the atomic force microscope. http://www.spectrum.ieee.org/may04/3984


What is atomic force microscopy for?py

Atomic force microscopyAtomic force microscopy (AFM) is a method of measuring themeasuring the topography of a surface at a micrometre to

5 m

0 mat a micrometre to nanometre scale.

It has also been

-5 m

It has also been extended to allow measurement of manymeasurement of many other surface properties.


AFM capabilities: Example 1p p This image shows aThis image shows a

terraced GaN surface. The steps on the surface

h thi khave thickness corresponding to one layer of gallium and nitrogen atoms. These steps are about 0.25 nm high.g

Sub-nm vertical resolution is genuinely straightforward in AFMstraightforward in AFM measurements.


AFM capabilities: Example 2p p

By choosing the right By choosing the right probes, extremely high lateral resolution can also be achieved.

In this image, individual carbosilane dendrimerscan be seen, each of diameter 9 nmdiameter 9 nm.

Image from:Image from: http://www.spmtips.com/hires30 nm


AFM capabilities: Example 3p p Unlike scanning electronUnlike scanning electron

microscopy, for example, AFM can be used to look at insulating samples suchat insulating samples, such as polymers, ceramics or biomaterials without

i l lspecial sample preparation.

This image shows a gcollagen fibril the spacing between the bands is 64 nm.bands is 64 nm.

Image from: Jess Gwynne



Its even possible to image It s even possible to image in a liquid environment.

This image shows DNAThis image shows DNA plasmids imaged in fluid under physiologically relevant conditions.

400 nm400 nm

Image from: http://www.veeco.com/library/nanotheater/



Topography SCM phase SCM amplitude

N t h i h b d l d t ll t i l ti t Numerous techniques have been developed to allow materials properties to be imaged simultaneously with topography.

Here a GaN sample is imaged in cross section to examine its sub-surface l t i l ti i i it ielectrical properties using scanning capacitance microscopy.

The phase data tells us about the type of carriers in the sample; the amplitude data tells us about the concentration of carriers.



AFM can also be used for AFM can also be used for micro- and nano-lithography.g p y

Here, Si has been patterned using anodic oxidation.

11 m

Image from: http://www.veeco.com/library/nanotheater/


AFM capabilities: Example 7p pMartian dust

particle An AFM was sent to Marsparticle An AFM was sent to Mars on the Phoenix mission which landed on 8th May 20082008.

The AFM will be used toThe AFM will be used to help with studies of the geological history of Mars.

On August 14th 2008, it sent back the first AFM image of a Martian dust particle.

From: http://phoenix lpl arizona edu/images php?gID=21266&cID=222


From: http://phoenix.lpl.arizona.edu/images.php?gID=21266&cID=222

Why study AFM?y y Probably the most commonly used nano-

h t i ti t h icharacterisation technique. Applicable to imaging almost any material. Provides information about topography and

materials properties.p p Also applicable to rapid prototyping of nanoscale

structures and devicesstructures and devices. Even if you never use an AFM, you will see AFM

data in the literature and at conferencesdata in the literature and at conferences You need to be able to interpret, assess and (if

necessary) criticise that data!necessary) criticise that data!Lecture 1 Atomic force microscopy 13

Why study AFM? An exampley y pData from

(a mostly excellent paper)( y p p )

A schematic from the paper based on this data:

artefacts

A schematic, from the paper, based on this data:

based on artefacts

14

artefactsLecture 1 Atomic force microscopy

AFM SchematicDetector

PCPiezo-

scanner

xy

z

m

o

t

i

o

n

Tip

Sample

i

o

n

z

m

Tip

Fixed sample stage

Voltage ramps

x

,

y

m

o

t

Controller

ramps

Feedback


Basic principles of operationp p p A tiny tip at the end of a flexible micro-cantilever is

d i t tt th fscanned in a raster pattern over the surface. This relative motion is performed with sub-Angstrom

accuracy by a piezoelectric actuator (usually a tube, y y p ( y ,sometimes a tripod).

The tip-sample interaction leads to cantilever deflection which is monitored using a laser beam reflected off thewhich is monitored using a laser beam reflected off the back of the cantilever

The beam is reflected towards a split photodetector, hi h it d lifi th til d fl tiwhich monitors and amplifies the cantilever deflection.

In almost all operating modes, a feedback circuit, connected to the deflection sensor, keeps tipsample , p p pinteraction at a fixed value by controlling the tipsample distance.


Contact mode AFM Tip is scanned across the sample surface while monitoring

the change in cantilever deflection with the detectorthe change in cantilever deflection with the detector. The tip contacts the surface through the adsorbed fluid layer

on the sample surface. In the simplest version of contact mode AFM, the cantilever

would be scanned over the surface, and allowed to deflect when it encounters a change in height (like the stylus on a g g ( yrecord player). This deflection would then be measured for each pixel of the image, and a surface topography may built up.

However, large changes in topography would lead to large deflections, and hence to large forces on the tip and sample, which could damage the tip or the sample.

Hence a feedback circuit is used to try and maintain a constant cantilever force (= a constant deflection).


Contact mode AFM with no feedback

This isnt how we usually operate the microscope!Lecture 1 Atomic force microscopy 18

This isnt how we usually operate the microscope!

Why is feedback necessary?y y

We need feedback to reduce the tip-sample forces to protect both

the sample and the tip!the sample and the tip!


Schematic operation of AFM with feedbackfeedback

Detector senses change in laser signal due to bending of cantilever.Detector senses change in laser signal due to bending of cantilever.

Signal sent to feedback circuit.

Height of cantilever adjusted by z-piezo so that cantilever deflection returns to g j y pset-point value.

Voltage applied to z-piezo converted to height through which cantilever has moved using built in calibration data


moved using built-in calibration dataMore animated illustrations available at

http://www.doitpoms.ac.uk/tlplib/afm/feedback_circuit.php

Signals from contact mode AFM using feedbackusing feedback

T h i lTopography signal

(Heights at which z-piezo has placed cantilever during scan)p g )

Deflection error signal

Feedback working poorly

Deflection error signal

Feedback working well


g p y g

Making use of the deflection i lerror signal

The error or deflection signal gives a measure of how well th f db k t i i t i i th d i d d fl tithe feedback system is maintaining the desired deflection setpoint.

Due to the finite response time of the feedback loop, high spatial-frequency signals cannot be completely compensated.p

Broad-range, gradual changes in surface height often do not contribute much to the error signal image but make thenot contribute much to the error signal image, but make the topography image difficult to interpret

Hence, high spatial-frequency details may be sometimes be seen most clearly in the error or deflection signal.


Making use of the deflection error signal: an examplesignal: an example

20 m20 m

Matzke R, Jacobson K and Radmacher M (2001) Direct, high-resolution measurement of furrow stiffening during

Lecture 1 Atomic force microscopy 23division of adherent cells Nat. Cell Biol. 3, 607

Tapping mode (or Intermittent Contact mode) AFMContact mode) AFM

TappingMode AFM operates by scanning a tip attached to the end of an oscillating cantilever across the sample surface.

The cantilever is oscillated at or slightly below its resonance frequency with an amplitude ranging typically from 20nm to 100nm. The tip lightly taps on the sample surface during scanning, contacting the surface at the bottom of its swingcontacting the surface at the bottom of its swing.

Changes in the tip-surface separation, lead to changes in the resonant frequency of the cantilever given by:

zF

o

11

In addition, the amplitude of the oscillation is damped when the tip is closer to the surface.H h i th ti ill ti lit d b d t Hence changes in the tips oscillation amplitude can be used to measure the distance between the tip and the surface.

Here, the feedback circuit maintains a constant vibration amplitude: the amplitude setpoint


the amplitude setpoint

Achieving cantilever oscillationg In general, we drive the cantilever at a fixedIn general, we drive the cantilever at a fixed

oscillation frequency. Two possible methods: Two possible methods:

The most common method (used on the Departmental AFM) is acoustic oscillation:Departmental AFM) is acoustic oscillation:

the cantilever is oscillated by a piezo-actuator in contact with the cantilever supporting chip,

Alternatively, magnetic oscillation may be used: the cantilever is oscillated by means of an alternating

ti fi ld hi h t ti ll tibl filmagnetic field which acts on a magnetically susceptible film deposited on the backside of the cantilever.


Cantilever tuning curveg Cantilever tuning involves

finding the resonantfinding the resonant frequency of the cantilever, and then adjusting the oscillation voltage so thata m

p

l

i

t

u

d

e

oscillation voltage so that the cantilever vibrates at an appropriate amplitude.

a

For tapping mode we usually use a frequency below the resonant

frequency

frequency at which the oscillation amplitude is reduced by 5 10 %.p h

a

s

e

reduced by 5 10 %.


How does a tip tap? (1)p p ( ) Consider a vibrating cantilever being brought

down towards the sample surface, with constant excitation at a constant frequency. Three regimes may be observed:

Distance - approaching surface Lecture 1 Atomic force microscopy 27

Distance - approaching surface

How does a tip tap? (2) Far away from the sample, the

cantilever vibrates with a constant

o does a t p tap ( )

amplitude which is determined by the damping losses in the air and elsewhere.

At a certain separation (point A) the tip begins to tap against the sample, limiting the amplitude. Distance - approaching surface

The amplitude then reduces approximately linearly with decreasing separation (suggesting that the amplitude is limited by the distance between the cantilever rest position and the sample surface).

As the cantilever is brought closer still to the sample, the amplitude of the cantilever motion drops abruptly at a certain separation (point B).

At this point the cantilever no longer has sufficient energy to break away from the surface adhesion, and the tip becomes stuck to the sample. Th id l ib ti lit d i di t th ti f th ti

28 The residual vibration amplitude indicates the motion of the tip

relative to the root of the cantilever.Lecture 1 Atomic force microscopy

Consequences of tapping mode behaviourbehaviour

To take a good tapping mode AFM image the following conditions are required:

The cantilevers energy must be high enough that it is not captured by the sample adhesionnot captured by the sample adhesion i.e. it must either be stiff or driven at high amplitudes.

Sample properties other than topography may effect theSample properties other than topography may effect the image. To remain to first order independent of the samples properties,

the setpoint amplitude should not be too lowthe setpoint amplitude should not be too low. Read more about tapping mode: N A Burnham et al (1997) How does a tip tap?N.A. Burnham et al. (1997) How does a tip tap?

Nanotechnology 8, 67.


Feedback in tapping mode AFM (i)pp g ( )

Detector senses decrease in amplitude of cantilever oscillation.Si l t t f db k i it Signal sent to feedback circuit.

Height of cantilever adjusted by z-piezo so that oscillation recovers Voltage applied to z-piezo converted to height through which cantilever has moved using built in calibration data


cantilever has moved using built-in calibration data

Signals from tapping mode AFM using feedbackusing feedback

Topography signal

(Heights at which z-piezo has placed cantilever during scan)placed cantilever during scan)

Amplitude signal

Feedback working poorly

Amplitude signal

Feedback working wellLecture 1 Atomic force microscopy 31

Feedback working poorly Feedback working well

Some typical images in Tapping M d (i)Mode (i)

600 nm 600 nm

Topography Amplitude

0.4

0.6

0.8

1

1.2

1.4

1.6

Z

[

n

m

]

-0 02

-0.01

0

0.01

0.02

0.03

Z

[

V

]

320 100 200 300 400 500

0

0.2

X[nm]

0 100 200 300 400 500

-0.03

-0.02

X[nm]Lecture 1 Atomic force microscopy

Some typical images in Tapping M d (ii)Mode (ii)

600 nm600 nm

0.110

Topography Amplitude

-0.05

0

0.05Z

[

V

]

2

4

6

8

Z

[

n

m

]

330 50 100 150 200

-0.1

X[nm]

0 50 100 150 2000

2

X[nm]Lecture 1 Atomic force microscopy

Non-contact mode Non-contact mode employs an oscillating cantilever (similar to

tapping mode)tapping mode). The tip is oscillated with a small amplitude usually at slightly

above its resonant frequency. The tip does not contact the surface. The cantilever's resonant frequency is decreased by the van

der Waals forces and by other long range forces which extend y g gabove the surface. The decrease in resonant frequency causes the amplitude

of oscillation to decrease. In many cases, the fluid contaminant layer is substantially

thicker than the range of the van der Waals force gradient, and therefore attempts to image the true surface with NCand therefore, attempts to image the true surface with NC-AFM fail as the oscillating probe becomes trapped in the fluid layer or hovers beyond the effective range of the forces it tt t t


attempts to measure.

Advantages and disadvantages f diff t dof different modes

Mode Advantages DisadvantagesContact Highest scan speeds

Easiest mode to set up for very rough samples

Usually lower resolution than other modesHigh normal and lateral forcesy g p gPossibility of damage to soft samples due to tip scraping over surface

Tapping Higher lateral resolutionReduced normal forcesLateral forces almost

Lower scan speeds

eliminated no scratchingLess damage to samples

Non- Minimal lateral and normal Limited resolution in air due to fluidNoncontact

Minimal lateral and normal forcesAtomic resolution in UHV

Limited resolution in air due to fluid contaminant layer on most surfaces.Slowest scan speeds.Slowest scan speeds.

Lecture 1 Atomic force microscopy 35See also interactive activity at http://www.doitpoms.ac.uk/tlplib/afm/modes_operation.php

Summary of image formation h i i AFMmechanism in AFM

For each pixel in an image:For each pixel in an image: Detector senses deviation of cantilever

d fl ti / ill ti f t i t ldeflection/oscillation from setpoint value. Feedback circuit responds to this deviation. Bias applied to z-piezo to adjust height of

cantilever to that setpoint value is recovered. Applied bias converted into change in height

using calibration data This process is repeated for all pixels in an

image as the tip is rastered over the surface. g pLecture 1 Atomic force microscopy 36

Key components: F db k i itFeedback circuitp

i dt d

controllerSet

point++++

-Power

This is a PID control system. If you want to know more then

wikipedia has a reasonabled

dtd

systemsensor

Amp wikipedia has a reasonable introduction: Search for

PID controller

Dont worry about the details of the feedback circuit shown above!

The key point is that we generally control the gains of two The key point, is that we generally control the gains of two amplifiers: The proportional amplifier (gain: p)

The integral amplifier (gain: i) The integral amplifier (gain: i) Larger gains imply that the error () in the signal is amplified

more strongly, making the feedback circuit more sensitive to deviations from the setpointdeviations from the setpoint.

Lecture 1 Atomic Force Microscopy 37

Non-optimised feedback circuit: Poor trackingPoor tracking

Increase gainsDecrease

amplitude setpointFeedback not properly adjusted Feedback well adjusted

amplitude setpoint (in tapping mode)

Decrease scan speedspeed


Poor tracking: forward and reverse scansscans

Good tracking

Poor tracking (gains set too low)


Image artefacts related to feedback Poor tracking: gains set too low

i = 2, p=20

i = 1.75, p=17.5, p

i = 1.5, p=15

i = 1.25, p=12.5

i = 1, p= 10

i = 0.75, p= 7.5

i 0 5 5i = 0. 5, p= 5

i = 0.25, p= 2.5

i = 0.1, p= 1i = 2, p=20

Topography Amplitude error


Raising the gains...g g As we raise the gains the error (or deviation g (

from the setpoint) decreases. You might think we could just keep raising

the gains until the error was zero... However, if the gains are set too high the

t b t bl d ill tsystem becomes unstable and oscillates. One reason for this is the existence of noise

i th t th d thin the system: as the error decreases, the influence of noise increases, and the noise causes random deviations from the setpointcauses random deviations from the setpoint, which may be amplified by the feedback circuit....


Gains set too high: forward and reverse scansreverse scans

Gains a little too high

Gains a lot too high


Image artefacts related to feedback Instability: gains set too highInstability: gains set too high

i = 2, p=20

i = 3, p=3i = 4, p=40

i = 5, p=50

i = 6, p= 60

i = 7, p= 70

i = 8 p= 80i = 8, p= 80

i = 9, p= 90

i = 10, p= 100i = 2, p=20

Topography Amplitude error


Key components: the scannery p Consists of a hollow tube

of a piezoelectric material, usually PZT (lead zirconium titanate(lead zirconium titanate, PbxZr1-xTiO3).

Piezoelectric materials expand or contract proportionally to an applied voltage The scanner is applied voltage.

Whether the material expands or contracts

constructed by applying independently operated l t d f X Y & Z t

pdepends on the polarity of the voltage applied.

electrodes for X, Y, & Z to a single tube.


Scanner hysteresisy Because of differences in the

material properties and dimensionsmaterial properties and dimensions of each piezoelectric element, every scanner responds differently to an applied voltageapplied voltage.

The relationship between scanner extension (or contraction) and applied voltage is non linear, with more movement-per-volt at the beginning of a scan line than at the g gend.

This non-linearity gives rise to hysteresishysteresis.

This can cause distortions to AFM data.

Lecture 1 Atomic force microscopy 4510 m pitch grating

Scanner calibration To achieve linear scanner movement in x and y a non-linear voltage

waveform is thus required, which is calculated (for a range of scan i ) d i th lib ti dsizes) during the calibration procedure:

Similarly, calibration in S a y, ca b at othe z-direction is required in order that for a given change in applied voltagechange in applied voltage, the change in extension of the piezo is known.

However, details of the calibration procedure are b d th f thibeyond the scope of this course.


Key components: the cantilever (1)y p ( )

For contact mode AFM we need aFor contact mode AFM we need a cantilever with : a low spring constant which will a low spring constant, which willbe deflected by very small forcesa high resonant frequency to a high resonant frequency to

avoid vibrational instabilities Typical cantilevers are thus short, to

provide a high resonant frequency, and p g q y,thin, to provide a small spring constant.


Key components: the cantilever (2)y p ( )

For tapping mode AFM :For tapping mode AFM : We use cantilevers with a higher spring

constant in order to reduce noise andconstant in order to reduce noise and instabilities.


Key components: detectory p A four-sector photodetector is used p

to monitor the deflection of the laser.SUM signal ( sed in all modes) is SUM signal (used in all modes) is the light detected by all 4 sectors.

In contact mode AFM the AFM In contact mode AFM the differential signal given by the difference between the top and

signal

bottom halves of the detector gives the deflection of the cantilever.In lateral force microscopy the

LFM signal

In lateral force microscopy the difference between the left and right halves is used.


g t a es s used

Key components: vibration isolationisolation

Given the mechanical nature of the AFM imaging process, the microscope is extremely sensitive to external vibrations, including acoustic noise.

Although the departmental AFM rests on an air table, and has an acoustic hood, , ,loud speech, doors banging etc visibly effect its performance.

50 Hz electrical supply, mobile phones and other electrical signals also causeand other electrical signals also cause problems.


Forces acting on the tip: basic ideasg p

R l i F Repulsive Forces When the tip is very close to

the sample, the electronic Repulsive forces short pwave functions of tip and sample start to overlap, resulting in a strong

range Coulumb interaction

Coulombic repulsion. Attractive forces

Longer range van der Waals Attractive forces van Longer range van der Waals forces lead to attractive tip-sample interaction. The effects of the van der Waals

der Waals interaction

effects of the van der Waals force may be seen for tip sample separations of 10 nm or even more.


or even more.

Forces acting on the tip: some commentscomments

The relatively long range of the van derThe relatively long range of the van der Waals force means that not only the apex atoms of the tip are involved in imageatoms of the tip are involved in image formation.

In contrast with STM where the tunnelling current falls In contrast with STM where the tunnelling current falls off exponentially with distance, and so the effect of atoms beyond the tip apex is negligible.atoms beyond the tip apex is negligible.

In air, the sample (and possibly the tip) will be coated in a thin layer of fluid and capillarycoated in a thin layer of fluid and capillary forces may exceed the van der Waals forces.


Force-displacement curves (1)p ( )

F di l t h i th h i

F(z)The force is calculated from Hooke's Law:F = - kx

F F Force-displacement curves, showing the change in the force acting on the tip as it approaches the surface can help us to understand tip-sample interactions

F = Forcek = spring constant

x = cantilever deflection.

interactions.

z

For animations of tip-sample interactions during measurement of a force-displacement curve go to: http://www doitpoms ac uk/tlplib/afm/tip surface interaction php


http://www.doitpoms.ac.uk/tlplib/afm/tip_surface_interaction.php

Force displacement curvesp1: The tip is far from the sample and the cantilever defection is nearly zero.

F(z)Approach2: The cantilever deflects towards the sample due to van der Waals forces

3: The tip snaps into contact with the sample This3: The tip snaps into contact with the sample. This event (the jump-to-contact) occurs when dF/dz exceeds the cantilever spring constant.

z123

4

4: After the contact, a positive deflection of the cantilever arises due to repulsive forces. This is the contact region of the force curve, where elastic properties of the sample


, p p pcan be measured.

Force displacement curvespF(z)Withdrawal

5: Initially, the behaviour of the cantilever during withdrawal follows the trajectory described during approachapproach.

6: Adhesion between tip and sample prevents separation. If a fluid film exists on the sample it will b l ff t th tt ti f b t th f ti fbarely effect the attractive forces, but the formation of a meniscus will contribute significantly to adhesion during withdrawal due to its high surface energy.

ztiptip

7

6liquid

surface

7

7: Once the adhesion force is overcome by the cantilever restoring force the contact breaks (the jump-

6


cantilever restoring force the contact breaks (the jumpoff-contact). The overall behaviour is hysteretic.

Key points from this lecturey p The imaging mechanism in AFM employs a

f db k l hi h t t t d i t ifeedback loop which acts to try and maintain a constant force on the cantilever. The action of the feedback loop must be optimised to getof the feedback loop must be optimised to get good images.

Scanner behaviour exhibits non-linearity and hysteresis which must be compensated for in calibrationcalibration.

A range of force may effect the interaction ofA range of force may effect the interaction of tip and sample. Capillary forces in particular may limit resolution.



Lecture 2Lecture 2

1

Todays topicsy p Resolution

vertical and horizontal resolution limits Different types of tip

Ti l d f Tip-related artefacts Other artefacts

Streaks Streaks Reminder: scanner bow Tip changes Periodic noise

Image processing and removal of artefactsA il bl i i ft Available image processing software

Reducing and removing common artefacts Image display and image enhancement


g y g

Vertical resolution The absolute limit of the vertical resolution is determined by

the resol tion of the ertical scanner mo ement hich is

Lateral resolution Lateral resolution is primarily determined by the radius of

curvature of the end of the tip. The sidewall angles of the tip will also determine its

ability to probe high aspect ratio featuresability to probe high aspect ratio features. Typical tip radii quoted by the manufacturers for tapping

mode tips are around 5 15 nm, but these may increasemode tips are around 5 15 nm, but these may increase quickly with tip wear.


Lateral resolution and pixellationp The number of lines in an AFM scan, and the ,

number of samples per line may be set in most AFM software.

Regardless of the pixel size, the feedback loop is sampling the topography many times at each pixel. Th d t di l d t h i l i th f The data displayed at each pixel is the average of the sampling iterations by the feedback loop over the pixel areathe pixel area.

Obviously, features smaller than the pixel size will not be properly resolved.p p y

Increasing the sampling density gives improved resolution but results in slower scanning.


Errors in width measurementsAssuming a hemispherical tip: h > R(1-cos)

Rh

h R(1 cos)

tanRw ww2

2

tan2

R

R

h < R(1-cos)

wR cot)2(2

hhRhw w2

h

w

Not only the tip radius but the feature height and its likely shape must be known to accurately determine the true width


must be known to accurately determine the true width.

Dense nanostructure arraysy

For densely packed features the tip size can also cause errors in determining the height of the islands or the overall appearance of the surfacethe height of the islands, or the overall appearance of the surface

Blunt tip Sharp tip

200 nm 200 nm

Lecture 2 Atomic force microscopy 8See animated illustration at: http://www.doitpoms.ac.uk/tlplib/afm/tip_related.php

Terraced surfaces

hemispherical

Rtip

hw

Single atom-high steps may be routinely imaged by AFM.

However, as step-edges become closer together o e e , as s ep edges beco e c ose oge e(e.g. on vicinal substrates) it may become impossible to see separate terraces.


p p

Small pitsp

Rdmeasured

dpit

When imaging small pits, such as pits associated with threading

w200nm

g g p p gdislocation terminations, one is unlikely to see the true pit depth.

The measured width will actually by more accurate than the measured depth, which will depend on the angle , and the tip radiusradius.

The pit will not be visible if the measured depth is less than the noise level.


Images with very sharp featuresg y p

1.5

1

Z

[

n

m

]

200nm200nm200150100500

0.5

0

X[nm]

1.2

1

X[nm]

200nm200nm

0.8

0.6

0.4

0.2

Z

[

n

m

]

2001501005000

X[nm]

Given a realistic tip size these very sharp features cannot be real!


Given a realistic tip size these very sharp features cannot be real!

Tip sidewall angles: Silicon nitride probesprobes


Tip sidewall angles: Etched silicon probesprobes


Tip sidewall angles and trench measurementsmeasurements

Scan direction

Scan directionScan direction

1010

55 90 73 73


Specialist tips: trench measurmentp p

Various high aspect ratio tip designs have been realised (usually using FIB) to try

Lecture 2 Atomic force microscopy 15and overcome the trench measurement problem.

Critical dimension AFM for trench measurementmeasurement

Critical dimension AFM (CD-AFM) is a tool for the semiconductor fabrication industry for accurate trench ymeasurement including assessment of sidewall roughness, overhangs etc. Flared or boot-shaped tipsroughness, overhangs etc.

In addition to the specialist tips shown, it requires additional instrumentation providing tip

p pz

instrumentation providing tip oscillation and feedback in the x direction as well as the z di ti i d t i

x

direction, in order to image trench sidewall features.


Specialist tips: improved nanostructure resolution (1)nanostructure resolution (1)

Supersharpsilicon probes employ nano-machining of standard tips to achieve a tip radius of less than 5 nm. They are very expensive and not at all robust

HiRes probes are produced by the growth of diamond-like carbon whiskers on the ends of silicon tips. Sometimes several whiskers grow on one tip, leading to highly distorted images.


Specialist tips: improved nanostructure resolution (2)nanostructure resolution (2)

Tips with carbon nanotubes at the apex may be made by

bringing tip close to a bringing tip close to a bundle of nanotubes, until one (or more) ( )tubes sticks.

using nano-manipulators in amanipulators in a FEG-SEM.

Direct growth using anDirect growth using an ethylene precursor and an iron catalyst. C.L. Cheung et al. PNAS, 97, 3809 (2000)


Atomic resolution in AFM

From: Erlandsson RFrom: Erlandsson R, Phys. Rev. B, 54, R8309 (1996)

Non-contact AFM STM image of Si(111)

Atomic resolution AFM not achievable in air due to effective b d i f ti b d d t f

Non contact AFM image of Si(111)

STM image of Si(111)

broadening of tip by condensed water on surface. Operating in fluid or under ultra high vacuum (UHV) solves

this problem. van der Waals forces operate over a much longer range than

the tunnelling current in STM, so that for UHV operation, additional instrumentation aspects also had to be addressed b f t t i l ti ld b hi d

Lecture 2 Atomic force microscopy 19before true atomic resolution could be achieved.

Tip shape artefacts: damaged tipp p g p

Normal tip

200 nm

Worn or damaged tip: several features have the same shape.

In fact the small nanostructures areIn fact the small nanostructures are effectively imaging the tip shape, rather than vice versa.


200 nm

Damaged tip: more severe examplesexamples


Tip shape artefacts: double tipp p p

Double or multiple tip images are formed when the tip has two or more end points which contact the sample while imagingend points which contact the sample while imaging.

Individual features may appear sharp and small but may appear in pairs or larger groupsLecture 2 Atomic force microscopy 22

or larger groups.

HiRes tips and multiple tip imagesp p p g

600nm 600nm

Image taken using normal Si probe Image of same sample taken usingImage taken using normal Si probe Image of same sample taken using HiRes probe: this particular probe appears to have multiple whiskers involved in imaging.


involved in imaging.

Tip damage scale effectsp g

Tips which give good images on flat surfaces may still show artefacts when much rougher features are imaged.


Tip artefacts on large islands: exampleexample

1.4

1.6

New tip 0 60.8

1

1.2

1.4

Z

[

m

]

4.0m4.0m

New tip

0 0 5 1 1 5 2 2 5 3 3 5 40

0.2

0.4

0.6

0 0.5 1 1.5 2 2.5 3 3.5 4

X[m]

1.4

1.6

Damaged tip 0.6

0.8

1

1.2

Z

[

m

]

4.0m4.0m

tip

0 0.5 1 1.5 2 2.5 3 3.5 40

0.2

0.4


X[m]

So, whats up with this image?, p gData from

(a mostly excellent paper)( y p p )

A schematic from the paper based on this data:A schematic, from the paper, based on this data:

26Lecture 2 Atomic force microscopy

Checking for tip artefactsg p For features with a clear orientationFor features with a clear orientation

Rotate the sample by 90: if the features still have the same orientation as in the previous image, its a p g ,tip artefact.

Dont be tempted to rotate the scan direction instead. This is NOT equivalent in this case.

Use a different tip. If the features now have a pdifferent shape, then the last tip was probably damaged. g If in doubt, try a third tip and go with the majority

verdict!

Lecture 2 Atomic force microscopy 27See animated illustration at:

http://www.doitpoms.ac.uk/tlplib/afm/tip_related.php

Sources of changes to tip shapeg p p Tip shapes may change during scanning due to:p p y g g g

Tip wear as the tip is rubbed over the surface Particularly in contact mode if the material imaged is of

i il h d t th ti G N di dsimilar hardness to the tip - e.g. GaN, diamond. Damage if the tip hits a large surface feature without

the feedback circuit having time to respondthe feedback circuit having time to respond This may occur when the scan rate is too fast over a rough

surface, or when the feedback parameters have not been carefully setcarefully set.

Picking up loose debris from the surface Particularly if samples have been left around in air to gather

dust or have been cleaved. Avoid this by dusting your samples with clean N2 before use.


Scanner-related artefacts: S bScanner bow

Because scanners are attached at one end andBecause scanners are attached at one end and move the sample or tip on the other, the free end does not move in a level plane.does not move in a level plane.

The mechanical properties of the piezo, as well as the p ,kinematics of motion, may result in 2nd order or 3rd order deviations from an ideal plane.

This artefact is known as bow, and increases with

iLecture 2 Atomic force microscopy 29

scan size.

Scanner-related artefacts: S (1)Scanner creep (1)

Creep is the drift of the piezo displacement after p p pa DC offset voltage is applied to the piezo.

When a large voltage is applied, the scanner does not mo e the f ll distance req ired all atdoes not move the full distance required all at once.

It initially moves the majority of the distance It initially moves the majority of the distance quickly, and then slowly moves over the remainder.

If normal scanning resumes before the scanner has moved through the full distance then the image will be distorted by this residualimage will be distorted by this residual movement.


Scanner-related artefacts: S (2)Scanner creep (2)

Creep is likely to occur whenCreep is likely to occur when large changes in the X & Y offsets have been applied using the frame up and frame down commandsusing the frame up and frame down commands,

when the piezo travels over most of the scan area to restart the scan

the scan size is changed abruptly by a large amount.X offset of 10 m

Y

applied at this point

X

Lecture 2 Atomic force microscopy 31More examples at: http://www.doitpoms.ac.uk/tlplib/afm/scanner_related.php

Minimising the effects of creep and driftdrift

Effective sample drift

Resulting image (for

Effective sample drift direction

Resulting image (for retrace i.e. right-to-left scan lines)

Fast scan direction

Slow scan directiondirection

First feature only impinges on tip for part of a scan line

Tip misses second feature entirely


Minimising the effects of creep and driftdrift

Resulting image (for retrace i.e. right-to-left scan lines)

Effective sample drift direction

left scan lines)Fast scan direction

Slow scan di ti

Although features are clearly distorted, both are visible, and the spacing between them is

direction

the spacing between them is preserved.

Always scan perpendicular to step edges and other long thin features where possible to minimise the impact of creep and drift on your image. This also makes image processing easier


makes image processing easier.

Closed-loop scannersp To correct for scanner creep or drift and to minimise the

i t f li iti l d limpact of scanner non-linearities closed-loop scanners have been developed.

As well as using calibrated changes in voltage to adjust g g g jthe scanner position, closed-loop scanners have an additional sensor (either optical or capacitive) which monitors the scanners position.monitors the scanner s position.

An additional feedback loop then adjusts the voltage applied to the scanner so that it reaches the desired positionposition.

Closed-loop monitoring often increases the noise-level of the system, and hence is not advisable (or indeed

) f ll inecessary) for small scan sizes. It can be very helpful, however, at large scan sizes.


Tip changesp g

Tip change

Tip change

600nm

35Lecture 2 Atomic force microscopy

Flattening to remove scanner bow and tip changesand tip changes

Lecture 2 Atomic force microscopy 36Free AFM image processing software available from: http://www.nanotec.es/products/wsxm/download.php

What does the flatten function do?do?

For each line of the scan (in the fast scan direction), a least-squares-fit polynomial is calculated.

This polynomial fit is subtracted from the original line.Aft fl tt i th Z l f h li i After flattening the average Z value of each scan line is equal to 0V.

The result is that height information in the slow-scan The result is that height information in the slow-scan direction is removed.

The order of polynomial used my be chosen:p y y 0th order = horizontal line 1st order = straight line

2nd order = parabola 2nd order = parabola 3rd order = cubic


Flatten function: IllustrationSurface profile to be measured:

100 nm

a

n

g

e

+ 2.5 m0th order fit

Subtract fitted line

+ 200 nmRaw data

a

n

n

e

r

z

-

r

a

Rescale

S

c

a

- 2.5 m - 200 nm

Most microscopes use 0th or 1st order flattening in real time to make it easier to understand the data whilst it is being recorded.

This is sometimes called real time plane fitting although it actually uses a This is sometimes called real-time plane-fitting, although it actually uses a line-fitting function.

Even if data is displayed in a flattened format, it is usually recorded in a raw


format, since flattening can result in loss of real information.

Loss of information in the slow scan direction (1)scan direction (1)

Fast scan direction

0th order flatten

Fast scan direction

0th order flatten0 order flatten


Loss of information in the slow scan direction (2)scan direction (2)

0th d fl tt0th order flatten

22m22m


Flattening non-uniform surfaces (1)g ( )

0th order fl ttflatten

200nm 200nm


Flattening non-uniform surfaces (2)g ( )


Only flatten when necessary!y y

200nm 200nm200nm 200nm

As-recorded Simple flatten

Flattening only adds false streaks to this image!


Artefacts due to loose matter on surfacesurface

Loose debris on the sample surface can cause loss of image resolution and can produce streaking in the imageproduce streaking in the image.

Debris falls off tip and

c

t

i

o

n

tip, and resolution is recovered

c

a

n

d

i

r

e

c

Piece of debris S

c

attaches to tip feature size increases

Skips and streakingLoss of resolution


Removing minor streaking (1)g g ( )


Removing minor streaking (2)g g ( )

600nm 600nm


Artefacts due to optical interferencep

Sometimes seen on highly reflective samples

Interference between light reflected from the tip and from the sample surface produces periodicproduces periodic oscillations.

This can usually be This can usually be avoided by re-aligning the microscope so that the plaser is more accurately centred on the cantilever.


Periodic noise

200nm 200nm

Periodic noise is usually due to electrical or mechanical sources.

If you change the scan size, but maintain the same scan rate, periodic noise will maintain the same period.


Fourier transform for the removal of periodic noise (1)of periodic noise (1)

FFT

200nm



200nm

40{1/m}

Lecture 2 Atomic force microscopy 50200nm


200nm

40{1/m}

Lecture 2 Atomic force microscopy 51200nm



Fourier transform for the removal of periodic noise (5): Resultof periodic noise (5): Result

200nm

Original image

200nm

Filtered imageOriginal image Filtered image


Image display: Z-scalesg p y Anyone who looks at your Figure 1: AFM y y

data need to be able to interpret the variations in

l / h d i t f

image of a terraced GaN surface. Image height, h = 3 79colour/shade in terms of

real heights.Either

200nm

3.79 nm

Either Give the total height of the

image (lowest point to 3.79 nm

g ( pheighest point) in the Figure caption, orU l b (thi l Use a z-scale bar (this also clarifies the colour scale you are using). 200nm

0 00


0.00 nm

Image display: 3D renderingg p y g

Lecture 2 Atomic force microscopy 553D rendering can distort the z-scale of an image and give a false impression of the aspect ratio of the observed structures

Key points from todays lecturey p y The lateral resolution of the AFM is limited byThe lateral resolution of the AFM is limited by

the size of the probe used. Damaged probes give rise to artefacts Damaged probes give rise to artefacts. Whilst tip shape artefacts are very difficult to

remove other common artefacts may beremove, other common artefacts may beremoved using image processing software, but beware the processing does alter your databeware the processing does alter your data.

When choosing how to display your data, think about which method best expresses the real characteristics of the surface.

Lecture 2 Atomic force microscopy 56Test your knowledge of artefacts using the quiz at: http://www.doitpoms.ac.uk/tlplib/afm/questions.php

Artefacts Summary 1yName Cause Symptoms Solution

Scanner does not move in a False parabolic contrast in x orScanner bow Scanner does not move in a level planeFalse parabolic contrast in x or

y direction Post-processing of image

Scanner creep Large voltage applied to d l d

Features elongated or compressed; recovers with Wait! (Or use a closed loop )p scanner; delayed response. p ;time. scanner)

DriftChanges in scanner

temperature; slight changes in Similar to scanner creep, but generally recovers more slowly

Check sample is well stuck down. Wait for temperature to

sample position generally recovers more slowly. stabilise.

Tracking failure Feedback circuit cannot adjust scanner height quickly enough.

Equiaxed features develop comet tails. Forward and reverse scan do not match

Increase gains, or adjust setpoint to increase interaction,

or decrease scan ratereverse scan do not match. or decrease scan rate.

Feedback instability Oscillation of feedback circuit

Quasi-periodic noise, especially visible in amplitude or

deflection image, and on t l l d i

Decrease gains

steeply sloped regions.

RingingCantilever oscillation increases for both a slight increase and a

slight decrease in height

Rings around features; for small features potentially

contrast inversion

Adjust the drive frequency (usually reduce it). May need to

reduce amplitude setpoint to maintain contact


g g maintain contact.

Artefacts Summary 2yName Cause Symptoms Solution

Di it l li it ti t li E l i d t i Decrease Z-limit (i.e. increase Z-pixilation Digital limitation to sampling resolution in z-directionEqual sized steps seen in very

small features

(sampling resolution by

decreasing sampling range).

Damaged tip Careless use of tips; picking up Repeating features across Sometimes possible to knock

off dirt by increasing scan sizeDamaged tip dirt or debris. image. off dirt by increasing scan size. Usually: change tip.

Double tip As above Features occur in pairs (or threes or fours ) As abovethrees, or fours)

Skips and streaking

Losse debris on surface; moved by tip. Long or short white streaks

Clean sample with dry air or nitrogen.

Optical interference

Stray light from laser reflected from shiny surfaces

Broad, quasi-periodic stripes in height image.

Refocus laser on cantilever so less light spills onto sample

Pieces of material adhering to Clean sample Post processingTip changes Pieces of material adhering to or falling off the tip Horizontal steps in image.Clean sample. Post-processing

with flatten function.

Periodic noise Mechanical vibrations or electrical interference

Periodic stripes; often particular visible in amplitude or

Switch off noise source if possible. Post-processing


electrical interference deflection image. using FFT.


Lecture 3Lecture 3

1

Todays topicsy p Beyond topography: what else can we measure with an

AFM? Electrical and magnetic characterisation with SPM

M ti f i (MFM) Magnetic force microscopy (MFM) Electric force microscopy (EFM) Kelvin Probe Force Microscopy (KPFM)py ( ) Scanning capacitance microscopy (SCM) (including CASE

STUDY) Scanning spreading resistance microscopy (SSRM)Scanning spreading resistance microscopy (SSRM) Conductive-AFM and tunnelling AFM (TUNA) (including CASE

STUDY)Pi f i (PFM) Piezo-force microscopy (PFM)

Artefacts specific to electrical measurements


Electric and Magnetic techniques:LiftMode or interleave scanningLiftMode or interleave scanning

LiftMode allows the imaging of relatively weak but long-rangerelatively weak but long-range magnetic and electrostatic interactions while minimizing the influence of topography.Fi t LiftM d d First, LiftMode records topographical data in TappingMode on one trace and retrace. The tip then raises to the lift h i ht d d h d ti ti llift scan height, and a second trace and retrace is performed while maintaining a constant separation between the tip and

= charged or magnetic particle

p plocal surface topography.

During this second trace/retrace the electrical or magnetic data is recorded

Main scan: Topography data

Lift scan: electrical or magnetic datarecorded. Used in EFM, MFM and surface

potential microscopy

Lift scan: electrical or magnetic data


MFM: Magnetic force microscopyg py For an oscillating cantilever, the natural g

frequency of oscillation varies as:F 11

For MFM the tip is coated with a ferromagnetic thin film

zo 1thin film.

The magnetic field between the tip and the film is dependent on the tip sample separation andis dependent on the tip sample separation and on the magnetic properties of the sample.

During the lift scan, the tip-sample distance isDuring the lift scan, the tip sample distance is constant and as the tip passes over different magnetic domains the force gradient changes changing the natural frequency of oscillation

Lecture 3 Atomic force microscopy 4changing the natural frequency of oscillation.

MFM: Instrumentation aspectsp Three possible methods to detect the changes in the

natural frequency of the tip:natural frequency of the tip: Amplitude detection: shift in amplitude at fixed drive frequency Phase detection: shift in phase at fixed drive frequncy Frequency modulation: A feedback loop modulates the drive

frequency to keep the cantilevers phase lag at 90 relative to the drive, corresponding to resonance.

Amplitude detection Phase detection


MFM: an examplep25 m x 25 m images of a glass hard disk (see http://www.veeco.com/nanotheatre/)

Topography MFM

VEECO suggest that phase detection and frequency modulation detection are superior methods for pmagnetic force imaging, offering greater ease of use, better signal-to-noise ratios, and reduced artifact content as compared to amplitude detection.


p p

Resolution and image interpretationg p The interpretation of MFM images is complex because

th ti f d d th ti t t fthe magnetic force depends on the magnetic structure of the tip as well as the sample.

When imaging soft magnetic samples with MFM tips with g g g p pa high magnetic moment, displacement of domain walls may occur during scanning.

If the magnetisation of the sample gives rise to strong If the magnetisation of the sample gives rise to strong stray fields, this may lead to rotation of the magnetisation field of the tip, altering the image.Th hi bl l ti d d th ti d i The achievable resolution depends on the tips domain magnetisation, shape, length, cone angle and distance from the surface: In general, MFM resolution is roughly equal to the lift height, and,

magnetic features smaller than the lift height may not be resolved. The tip also experiences stronger fields close to the surface, giving improved signal-to-noise ratios.


s g a o o se a os

EFM: Electric force microscopy (1)py ( )

For an oscillating cantilever the naturalFor an oscillating cantilever, the natural frequency of oscillation varies as:

zF

o

11

As with MFM we detect changes in the f di t d t l l h iforce gradient due to local changes in materials properties, except here the forces involved are electrostatic rather than magnetic.


g

EFM: Electric Force Microscopy (2)py ( ) Tips coated with a conductive film are used, and a DC

bias may be applied. Attractive forces reduce the cantilever resonant

frequency whilst repulsive forces increase the resonantfrequency, whilst repulsive forces increase the resonant frequency.

As with MFM changes in resonant frequency may beAs with MFM changes in resonant frequency may be assessed using phase detection, amplitude detection or frequency modulation. T i l li i f EFM Typical application of EFM: EFM can map the electrostatic fields of an electronic circuit as

the device is turned on and off. This technique is known as q"voltage probing" and is a valuable tool for testing live microprocessor chips at the sub-micron scale.


Kelvin Probe Force Microscopy (1)py ( ) Also known as Surface Potential Microscopy, KPFM is a

more quantitative version of EFM. Again, LiftMode is used. On the first pass the topography is measured. On the second pass a constant height above the sample

is maintained and the cantilever is NOT vibrated.Instead an oscillation oltage is applied Instead an oscillation voltage is applied:

V = Vacsin(t) This creates an oscillating electric force at the frequencyThis creates an oscillating electric force at the frequency on the cantilever. The oscillating force has the following amplitude:

acdcVVdCF where dC/dz is the vertical derivative of the tip/sample capacitance and V = V -V

acdcVVdzF


capacitance and Vdc = Vtip-Vsample

Kelvin Probe Force Microscopy (2)py ( ) When the tip and sample are at the sameWhen the tip and sample are at the same

DC voltage (Vdc = 0), the cantilever experiences no oscillating force.experiences no oscillating force.

By adjusting the DC voltage on the tip (V ) until the oscillation amplitude equals(Vtip), until the oscillation amplitude equals zero, the effective local surface potential on the sample (V ) may beon the sample (Vsample) may be determined.

The surface potential image is then a map of Vsample over the surface.


p

KPFM: Practical pointsp Since a feedback circuit is used to try to maintain y

a constant (zero) tip vibration amplitude, monitoring the actual tip vibration amplitude during scanning can give a measure of the accuracy ofscanning can give a measure of the accuracy of the potential data.

Unlike in EFM and MFM the cantilever oscillation amplitude should be close to zero. Hence, the tip may be scanned closer to the surface (smaller Lift height) without the tip impacting the sample

Lift heights as low as 5 nm are sometimes used (i e 5 nm lower Lift heights as low as -5 nm are sometimes used (i.e. 5 nm lower than the previous tapping mode scan height).

If the cantilever does impact the surface, streaks appear in the surface potential data


surface potential data

KPFM: Applicationspp

Grain boundary potential barrier heterogeneitiesGrain boundary potential barrier heterogeneities Potential Mapping of electronic devices operated

in situin situ Surface charge extent and dissipation

Topography (left) and surface potential (right) images of a CD-RW. The surface potential image shows the position of the written bits on the tracks.2.0m 2.0m

Thanks to Rob Cork for these images!


Effect of local topography on KPFM measurementsmeasurements

Topography Potential

200 nm 200 nm

Simultaneously acquired topographic and KPFM measurements of a gold film on a silicon surface. The gold film is an equipotential surface, but nonetheless the KPFM data shows features which mirror the topography, due to the changes in tip-sample interaction area.

Efi A d C h SR J V S i T h l B 18 4 1051 2000Lecture 3 Atomic force microscopy 14

Efimov A and Cohen SR J. Vac. Sci. Technol. B, 18, 4, 1051, 2000.

Why does topography effect KPFM data? (1)data? (1)

Amplitude of oscillating force: VVdCF p g

Ampitude of cantilever oscillation is proportional to force:

acdcVVdzF

p p p

acdc VVdzdCA k

Feedback circuit aims to adjust amplitude of oscillation to zero, but noise in system means that this is not possible. , y pActual oscillation amplitude = Amin

min VAV)VV(VdCkA

capacitance gradient

samplemin

tipacsampletipmin V

dzdCk

V)VV-(Vdz

kA gdepends ontip-sample i t ti

Lecture 3 Atomic force microscopy 15interaction

areaEfimov A and Cohen SR J. Vac. Sci. Technol. B, 18, 4, 1051, 2000.

Why does topography effect KPFM data? (2)data? (2)

(1) As the tip follows the

(2)

dotted line which defines a constant distance above the surface the tip samplethe surface the tip-sample interaction area changes, changing the capacitance g g pgradient.


Scanning capacitance microscopyg p py In SCM the tip and a

semiconducting sample, with an oxide layer on the surface form a metal-insulator-semiconductor (MIS or MOS) capacitor.

The capacitance value is monitored using a high

Metal coating

monitored using a high frequency resonant circuit.

By maintaining a constant force b t ti d lbetween tip and sample, simultaneous topographic and capacitive images are

Semiconductor

generated, enabling the direct correlation of local topography with electrical properties.


p p

Schematic capacitance voltage curvecurve

With a positive voltage applied to the tip, electrons are attracted to the surface and accumulate there.surface and accumulate there.

In accumulation, the MIS capacitance is the capacitance across the oxide layer.

For a negative tip voltage, the electrons are repelled from the tip,

This effectively increases the spacing between the capacitor

For a negative tip voltage, the electrons are repelled from the tip, depleting the material of carriers.

spacing between the capacitor plates and lowers the capacitance.

The lower the carrier concentration the greater the decrease ofthe greater the decrease of capacitance with voltage.

Therefore, the slope of the CV curve (i dC/dV) i l f l i(i.e. dC/dV) is larger for lower carrier concentrations.

For p-type material, the CV curve l it i t

Lecture 3 Atomic force microscopy 18polarity inverts.

SCM examplesp

(1) p-n junction in GaN

180(a) (b)Sample edge n-GaN p-GaN155 nm

Topography SCM phase

-180250 nm 250 nm0 nm


SCM examples (2)p ( )n-type dopant staircase in GaN

nid-GaNn-GaN (~1018 cm-3)

G N ( 1019 3)nid-GaN

nid-GaN

n-GaN (~1019 cm-3)

n-GaN (~7.5x1018 cm-3)

G N ( 5 1018 3)

0.7 m

nid-GaN

nid-GaN

n-GaN (~5x1018 cm-3)

n-GaN (~2.5x1018 cm-3)18 3 10

18

1019

1020

1200

1400

1600

S

)

/

c

m

-

3

a

/

m

V

nid-GaN

nid-GaN

n-GaN (~1018 cm-3)

n-GaN (~7.5x1017 cm-3)17 3 10

15

1016

1017

10

400

600

800

1000

a

n

t

c

o

n

c

(

S

I

M

S

a

m

p

l

i

t

u

d

e

d

a

t

a

n-GaN (~5x1017 cm-3)

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

1014

10

0

200

400

D

o

p

a

D istance from surface / m

S

C

M


Distance from surface / m

Electrical measurements byElectrical measurements by AFM: Case Study 1y

Doping in epitaxial lateralDoping in epitaxial lateral overgrowth (ELOG) of GaN

21

Epitaxial lateral overgrowth of GaNp g Initial growth of 500 nm thickseed layer (many dislocations)seed layer (many dislocations).

SiNX mask deposited thenpatterned using lithography with 5

G N h h h h

patterned using lithography with 5m stripes and 5 m windows(dislocations blocked by mask)

GaN regrowth through thewindows (some dislocations bendover at facets).

Coalescence of GaN stripes(dislocations terminate at void orannhilate each other)

10 mperiod

5 mwindow

10 mperiod

5 mwindow

10 mperiod

5 mwindow

10 mperiod

5 mwindow

Sapphire substrateannhilate each other)

Planar growth. Final structurehas stripes of low dislocation

Magnesium is sometimes used to aid in the coalescence of the stripes.

Sapphire substrate

Lecture 3 Atomic force microscopy 22density material. Magnesium is a p-dopant in GaN.

Cross sectional SCM images: ELOG samplesELOG samples 180n-GaN p-GaNPhase Amplitude

Sample coalesced-180

250 nmppn nnSample coalesced

with Mg to enhance lateral growth rate

Initial GaN regrowth is unintentionally n-

10 m

Phase10 m

Amplitude

doped

Using Mg during coalescence does

Sample coalesced without Mg n

coalescence does result in p-type doping

Variation in

10 m

Variation in doping density is seen within the n-type regions


10 m 10 m type regions.

Calibrating dopant densitiesg p180

Data

V

Additional GaN layers grown with known dopant densities

120

140

160 Best Fit

d

e

S

i

g

n

a

l

/

m

V

80

100

120

S

C

M

A

m

p

l

i

t

u

d

11018 1019

Dopant Conc (SIMS) / cm -3

Cross SCM Amp /mV Carriers /cm-3

1 130 1 7x10183.0m

23

1 130 1.7x1018

2 142 1.1x1018

3 78 9.4x1018

Lecture 3 Atomic force microscopy 24(c)

Why does the dopant density vary?y p y y

10 mperiod

5 mwindow

10 mperiod

5 mwindow

10 mperiod

5 mwindow

10 mperiod

5 mwindow

3.0m

Sapphire substrate

Within the red triangle growth is occurring on (0001) facets

In the yellow triangles growth is occurring on (11-22) facets

Sapphire substrate

y g g g ( )

Dopant incorporation is shown to be facet-dependent in this system

But what is the dopant? Si from the mask?


Why do we see extra features in the sample coalesced with no Mg?the sample coalesced with no Mg?

with Mg

10 m

No Mg Faster dopant i tiincorporation on slanting facets extra trianglestriangles


10 m

Why do we see extra features in the sample coalesced with no Mg?the sample coalesced with no Mg?

with Mg

10 m

No MgI l tIncomplete formation of initial stripes leads to gapsleads to gaps between extra triangles


10 m

Case study 1: Key pointsy y p

SCM can be used to identify both dopantSCM can be used to identify both dopant density and dopant typeC lib ti t t ll tifi ti Calibration structures allow quantification of dopant type

Possible to identify facet-dependence of dopant incorporationdopant incorporation

SCM data can give insight into growth mechanisms as well as into electrical properties


p p

Scanning spreading resistance microscopy (SSRM)microscopy (SSRM)

SSRM measures the resistivity of samples spanning a wide range of conductivity from insulating through semiconductingrange of conductivity, from insulating through semiconducting to metallic.

SSRM is especially useful in two-dimensional mapping of electrical carriers in semiconductorselectrical carriers in semiconductors.

A DC bias is applied between a conductive tip and the sample. Whil i i t t d l ith i t While scanning in contact mode, a logarithmic current amplifier with a range of 10 pA to 0.1 mA senses the current, I, passing through the sample. Th t f th ti d t i th l t l l ti The geometry of the tip determines the lateral resolution, which is roughly equal to the end radius of the tip.

Simultaneous topographic and current images may be generated, enabling the direct correlation of local topography with electrical properties.

As with SCM, calibration structures are often used for

Lecture 3 Atomic force microscopy 29quantification of dopant densities.

SSRM on silicon samplesp On Si structures, high forces

(typically a few N) are required(typically, a few N) are required in order to penetrate the native oxide and to establish a stable electrical contactelectrical contact.

Since standard AFM probes are easily damaged at these high f d d di dforces, doped diamond or diamond-coated silicon probes are employed.

The extreme hardness, high Youngs modulus, and the electrical conductivity obtainedelectrical conductivity obtained through doping make diamond particularly suitable for use as the SSRM tip coating material.

Thanks to Joy Sumner for this image!


SSRM tip coating material.

SSRM data interpretationpR = +106exp(-Vout) f i i li d bifor positive applied bias

R = -106exp(-Vout) f ti li d bifor negative applied bias

Ideally, for an Ohmic contact R = /4awhere a is the contact radiuswhere a is the contact radius

More realistically:R = (/4a) + R ()R = (/4a) + Rbarrier()Rbarrier() (the barrier resistance component) depends on probe shape surface state concentrationshape, surface state concentration, applied contact force etc..


SSRM example 1pTopography SSRM

7 m x 7 m scans

InP test sample - SSRM data shows differences in resistivity between n-and p type regionsand p-type regions.

Note: Unlike SCM, SSRM will not differentiate between n- and p-type material unless they have different resistivities.


SSRM example 2p(a)n-type dopant staircase in GaN

nid-GaNn-GaN (~1018 cm-3)

G N ( 1019 3)0.7 m

nid-GaN

nid-GaN

n-GaN (~1019 cm-3)

n-GaN (~7.5x1018 cm-3)

G N ( 5 1018 3)

1019

1020-2

/

c

m

-

3

nid-GaN

nid-GaN

n-GaN (~5x1018 cm-3)

n-GaN (~2.5x1018 cm-3)18 3

1016

1017

1018

-4

-3

t

c

o

n

c

(

S

I

M

S

)

/

S

R

M

d

a

t

a

/

V

nid-GaN

nid-GaN

n-GaN (~1018 cm-3)

n-GaN (~7.5x1017 cm-3)17 3

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

1014

1015

-5

D

o

p

a

n

Sn-GaN (~5x1017 cm-3)


Distance from surface / m

Three-dimensional carrier profiling in SSRMprofiling in SSRM

Pit formed in InP multilayer by imaging at progressively smaller scan sizes and deliberately damaging surface.

SSRM data at different heights in the pit is recorded and hence a profile of resistivity with depth is achieved

See Xu et al Appl Phys Lett 81 177 (2002)Lecture 3 Atomic force microscopy 34

See Xu et al. Appl. Phys. Lett. 81, 177 (2002).

Conductive AFM (C-AFM) and Tunnelling AFM (TUNA)Tunnelling AFM (TUNA)

C-AFM and TUNA operate in a similar wayC AFM and TUNA operate in a similar way to SSRM, but the output is conductivity rather than resistivityrather than resistivity.

C-AFM can sense currents in the range 1pA to 1A.

TUNA can sense currents in the range TUNA can sense currents in the range 80fA to 120pA and is suitable for use with hi hl i ti lhighly resistive samples.


Applications of TUNA (1)pp ( ) TUNA is especially useful for evaluating dielectric films

subject to breakdown (e g transistor gate oxides)subject to breakdown (e.g. transistor gate oxides). The tip/sample tunneling current depends on film

thickness, leakage paths (possibly caused by defects), g p (p y y )charge traps and tip geometry.

a)20 0 nm

Topography Current Example of analysis of leakage paths in a dielectric film:20.0 nm

10.0 nm 50.0 pA

a dielectric film:

MgO tunnel barrier for a spintronic device.

1 m0.0 nm 1 m25.0 pA

b)8.0 nm

In (a) preparation for barrier growth has not been optimised, and current leakage occurs at impurity particles.

0.0 pA

1 m0.0 nm

4.0 nm

1 m

leakage occurs at impurity particles.

In (b) The impurity particles are absent and current leakage is reduced.


From: Singh et. al. (2007) J. Phys. D. 40, 3190.

Applications of TUNA (2)pp ( ) Other applications include:

Testing the integrity of tribological films (such as Testing the integrity of tribological films (such as Diamond-Like-Carbon (DLC) as used in magnetic recording head/disk interfaces) g )

Determining the conductivity of small structures (such as light-emitting polymer molecules, nanowires and carbon nanotubes)carbon nanotubes).

Example of the application of TUNA in

Measurement geometry Topography Current mappp

assessing nanowire electrical properties:

Mapping of current flowMapping of current flow through Ge nanowiresFrom: D Erts et al. (2006) J Phys Chem B


(2006) J. Phys. Chem. B 110, 820

Spectroscopic measurementsp p Techniques like SCM, SSRM, C-AFM andTechniques like SCM, SSRM, C AFM and

TUNA also allow for spectroscopic measurementsmeasurements i.e. for SCM site-specific dC/dV versus V

curves can be takencurves can be taken For SSRM, C-AFM and TUNA, site-specific I-

V curves may be takenV curves may be taken. These curves can provide extra insight

into the materials properties at specificinto the material s properties at specific locations.


Electrical measurements byElectrical measurements by AFM: Case Study 2y

Gate oxide thickness assessment using TUNA

Olbrich et al. Applied Physics Letters pp y73 (21) 3114-3116, 1998.

39

Aims of Olbrichs studyy

To develop a quantitative method forTo develop a quantitative method for assessment of oxide thicknesses at the nanoscalenanoscale

Important for the assessment of gate oxides in metal-oxide semiconductor field effect transistors (MOSFETs).effect transistors (MOSFETs).


Tunnelling through gate oxidesg g gOxide thickness > 5 nm: Oxide thickness < 3 nm: Fowler-Norheim tunnelling (FNT) Direct tunnelling (DT)

Metal Ox SemiconductorMetal Ox Semiconductor

V

Efm

dox

dFN Electrons tunnel into oxide conduction b d Vg

Efm

dox

Electrons tunnel directly into semiconductor conduction bandVg

Efs

dox band Vg

Efs

conduction band

Efm = metal Fermi level Efm = semiconductor Fermi level

Dox = oxide thickness dfn = tunneling distance

Vg = gate voltage


TUNA I-V curves for uniform oxide layers of different thicknesslayers of different thickness

Oxide thicknesses measured by ellipsometry:ellipsometry: = 3.0 nm = 5.3 nm 7 7 = 7.7 nm

Lines of best fit represent FNT model for different oxide thicknesses: = 3.0 nm = 5.3 nm = 7.8 nm

Good match between data measured by different techniques. Hence this method may be used to measure samples for which

Lecture 3 Atomic force microscopy 42ellipsometry is not appropriate.

Sample with varying local oxide thicknessthickness

Data from a MOSFET

Topography

Tunnelling current Fitting the data from the points shown to the FNT

model gives oxide thicknesses of 20.3 nm () and 17 nm () . For thinner oxides a DT model may be more For thinner oxides a DT model may be more appropriate. Later work in this area addresses this issue.


Case study 2: Key pointsy y p Two possible tunnelling regimes in TUNA:Two possible tunnelling regimes in TUNA:

Fowler-Nordeim tunneling and direct tunnelingtunneling

I-V spectroscopy provides access to the mechanism of current flowmechanism of current flow

By looking at samples without significant l l i h i t d i tlocal inhomogeniety and comparing to a well-understood macroscopic measurement, a new nanoscopic technique was validated.


Artefacts in electrical measurements

The output in SCM, C-AFM and SSRM is highly p , g ydependent on the tip-sample contact area.

This depends not only on the tip size, but on the p y p ,sample morphology.

Topography SCM amplitude SCM phasep g p y p p

600nm 600nm 600nm


Variation in contact area in C-AFM

Large contact area:Large contact area: increased current

Loss of contact: no current


Variation in contact area in C-AFM: ExampleExample

Topography CurrentTopography Current

500 pA500 p

600 nm 600 nm

-500 pA


Extra resources

For more information on using SCMFor more information on using SCM, SSRM, TUNA and KPFM to measure the electrical properties of semiconductorselectrical properties of semiconductors and semiconductor devices see:

Oliver RA (2008) Advances in AFM forOliver RA (2008) Advances in AFM for the electrical characterization of

i d t R t P isemiconductors Reports on Progress in Physics 71, 076501.


Piezoresponse Force Microscopy (PFM) (1)(PFM) (1)

Piezoelectric materials expand or contract upon p papplication of an applied bias NB: All ferroelectric and pyroelectric materials are

also piezoelectricalso piezoelectric where = strain, d = piezoelectric

coefficient, E = electric field. dE

, For a ferroelectric, the piezoelectric coefficient,

d, will depend on the polarisation. Hence polarisation domains in ferroelectrics can

be visualised by looking at the their change in size upon application of an electric fieldsize upon application of an electric field.

(See http://www.doitpoms.ac.uk/tlplib/index.php for more information on ferroelectrics and piezoelectrics)


Piezoresponse Force Microscopy (PFM) (2)(PFM) (2)

In PFM the tip is scanned over the

Sample with ferroelectric domains

In PFM the tip is scanned over the surface in contact mode and an alternating bias is applied.

The average deflection of the tip is used for topography measurements.

The phase and amplitude of the ACThe phase and amplitude of the AC component give the piezoresponse of the sample and are collected by a lockin amplifier, allowing imaging of oc a p e , a o g ag g oferroelectric domains.

Di ti f d i t lDirection of domain controls whether piezo-response is in-phase or out-of-phase with the bias applied by the tip


bias applied by the tip

PFM examplesp

Topography Piezoresponse Topography Piezoresponse

Ferroelectric domains in Pb(Mg1/3Nb2/3)O3PbTiO3 (PMNPT)(From: Zeng et al. Journal of Crystal G th 267 (2004) 194 198)

Lamellar domains within PbTiO3 grains in a thin film.(From: Zeng et al. Journal of El t i 15 135 141 2005)Growth 267 (2004) 194198) Electroceramics, 15, 135141, 2005)


More about PFM A recent review: Nanoscale electromechanics of

ferroelectric and biological systems: A new dimension in scanning probe microscopy S V Kalinin et al ANNUAL REVIEW OF MATERIALSS.V. Kalinin et al., ANNUAL REVIEW OF MATERIALS RESEARCH, 37, 189-238 (2007)

Agilent Technologies Applications Note (S. Wue): http://cp.literature.agilent.com/litweb/pdf/5989-7611EN pdf7611EN.pdf

Animated online tutorial: http://www ntmdt com/spm- Animated online tutorial: http://www.ntmdt.com/spm-principles/view/piezoresponse-force-microscopy


Key points from todays lecturey p y Many different methods have been developed toMany different methods have been developed to

assess electrical properties of materials with AFM (only a subset discussed here!)AFM (only a subset discussed here!)

The finite size of the tip should always be considered in interpretation of electrical dataconsidered in interpretation of electrical data, especially for samples with significant topographytopography.

The use of known calibration structures and spectroscopic techniques provide opportunitiesspectroscopic techniques provide opportunities for quantification of electrical AFM data.



Lecture 4Lecture 4

1

Todays topicsy p Beyond topography: what else can we measureBeyond topography: what else can we measure

with an AFM? Measuring mechanical properties with AFM Measuring mechanical properties with AFM

Friction force microscopyPhase imaging Phase imaging

Force-volume imagingNanoindentation Nanoindentation

Nanofabrication with AFM Biological systems and measurements in fluid


Friction force microscopy (or lateral force microscopy)force microscopy)

If the scanner moves perpendicular toPhoto-detector

Topography: Vertical deflection

If the scanner moves perpendicular to the long axis of the cantilever, friction between the tip and sample causes th til t t i t

Laser beam

the cantilever to twist. The four-sector photodetector can

distinguish the resulting left-and-right Sampledistinguish the resulting left-and-right motion of the reflected laser beam from the up-and-down motion caused b t hi i ti

Friction: Lateral deflectionby topographic variations.

Hence tip-sample friction and sample topography may be measured

Lateral deflection

topography may be measured simultaneously.


Friction forcesThe measurements of cantilever torsion are carried out at a constant force setpoint, i.e. the vertical deflection of the cantilever is constant.

LFM image(trace) LFM image(retrace)


Lateral forces arising from topographyg p g p y

I f ili id ( ithImage of a silicon grid (with5m square pits of depth 180 nm and a pitch of 10m).

S. Sundararajan et al. J. Appl. Phys. 88, 48254831 (2000).


Distinguishing and material-dependent and topographic effects (1)and topographic effects (1)

The lateral force is e ate a o ce shigh at the leading edge of asperities and low at the trailing Retrace (R)low at the trailing edge.

The magnitude of material induced

Retrace (R)

material-induced effects should be independent of the scan direction

T

scan direction whereas topography-induced effects are different between Rdifferent between forward and backward scan directions.

From: B. Bhushan, Nanotribology and nanomechanics, Wear 259, 15071531 (2005)

R

Lecture 4 Atomic force microscopy 6For 3D animations illustrating these effects see:http://www.doitpoms.ac.uk/tlplib/afm/lfm.php

Distinguishing and material-dependent and topographic effects (2)and topographic effects (2)

Since the sign of the f i ti f hfriction force changes when the scanning direction is reversed dditi f th f i ti

T+ R

addition of the friction force data of the forward and backward scan should eliminate material-induced effects while topography-induced p g p yeffects remain.

For more detail about FFM see B Bhushan

T- RFFM, see B. Bhushan, Nanotribology and nanomechanics, Wear 259 15071531 (2005)


259, 15071531 (2005)

Friction force microscopy example (1)py p ( )

Topography Friction

Lecture 4 Atomic force microscopy 8LangmuirBlodgett LB film of C60 -containing polyimide

Friction force microscopy example (2)py p ( )

atomic force microscopy

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