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Scanning Probe Microscopy

Dr. Benjamin Dwir

Laboratory of Physics of Nanostructures (LPN)

Benjamin.dwir@epfl.ch

PH.D3.344

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Outline:

• Introduction: What is SPM, history

• STM

• AFM

• Image treatment

• Advanced SPM techniques

• Applications in semiconductor research

and industry

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What is SPM ? • Scanning Probe Microscopy :

The characterization of a sample by scanning its surface with

a probe, at a small distance

Usually, only surface properties are observable

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How does SPM compare with other

microscopy techniques ? Microscope: Optical Confocal SEM TEM STM AFM SNOM

XY

resolution

400

nm

150 nm 1 nm 0.1 nm 0.1 nm (0.1) 1-10

nm

<50 nm

Z resolution - 100 nm - - 0.01 nm 0.01 nm (0.01nm)

Ambience air

(liquid)

Air

(liquid)

vacuum vacuum Vacuum (air) Air (liquid) Air

Sample

preparation

none none none /

coating

polishing,

ion milling

None / UHV

cleaving

None None

Damage to

sample

none none Contami-

nation

Contami-

nation,

heating

None None

(scratches)

None

Price (kFr) 5-30 50-200 200-500 500-2000 70-300 70-300 70-300

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Advantages of SPM • 3D imaging

• High spatial and

vertical resolutions

• No sample

preparation

• Simple to operate

• Low-cost

• Main disadvantage :

slow (5-20 min/image)

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History of SPM :

An old principle

Scanner

(rotating+

advancing

cylinder)

Tip+reading

Stabilizer

Surface

features

(grooves)

Scanner

(rotating

cylinder) Tip

Surface

features

(grooves)

Source: Wikipedia

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The Stylus Profilometer (“Alpha-step”)

• Scans a line profile of the surface with a tip

• Z-resolution: 5-10 nm

• X-resolution: 1-10 mm

• Scan length: up to 10-100 mm

Source: CMI-EPFL

Source: pc-optimize.com

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How to get nm resolution

in X,Y,Z?

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First attempt: The Topografiner (1972)

• Field-emission tip to generate narrow

electron beam (in UHV)

• Sample current is measured

• Feedback keeps tip distance from

sample by keeping constant current

• Scans the surface of the sample by a

piezo scanner (scan length: up to 8 mm)

Source: CMI-EPFL

R. Young, J. ward, F. Scirer, Rev. Sci. Inst. 43, 999 (1972)

“A noncontacting instrument for measuring the microtopography

of metallic surfaces”

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The Topografiner (1972)

Source: CMI-EPFL

System construction:

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The Topografiner (1972)

• Z-resolution: 3 nm

• XY-resolution: 400 nm

Source: CMI-EPFL

Results: scan of a Pt optical grating replica

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The Topografiner (1972)

Source: CMI-EPFL

Noise:

Thermal drift

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The Topografiner (1972)

• Comparison with other

microscopes (1972):

Source: CMI-EPFL

Resolution:

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The main problem: How to get

nm resolution?

Potential problems:

1. Tip size

2. High-resolution XY scanning

3. Non-destructive

4. Keep distance from sample

5. Vibrations

6. Thermal stability

The first solutions: G. Binnig, H. Rohrer, 1986

Potential problems:

1. Tip size

2. High-resolution XY scanning

3. Non-destructive

4. Keep distance from sample

5. Vibrations

6. Thermal stability

Solutions:

1. Short-range interactions

2. Piezo scanner

3. Non-contact

4. Height feedback

5. Rigid structure, isolation

6. Compensation

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Let’s look at the solutions: 1. Short-range interactions: Do you know any?

• Quantum-mechanical electron tunneling

• Van-der-Waals forces

-1

-0.5

0

0.5

1

1.5

2

0.5 1 1.5 2Z [nm]

F [n

N]

• Nuclear (strong) forces But range is

too short!

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Outline:

• Introduction: What is SPM, history

• STM

• AFM

• Image treatment

• Advanced SPM techniques

• Applications in semiconductor research

and industry

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STM: uses quantum tunneling • STM = Scanning Tunneling Microscope

• The principle of quantum-mechanical tunneling (1928):

• Electron wavefunctions "leak" into vacuum ("tail")

• At short distances, current can flow:

• Wavefunction decay length is very short: ~ 1 Å!

• F = potential difference from vacuum level to Fermi level F

m2

/2deI

Electron wavefunctions

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Tunneling as surface probe: • We approach the sample with a

sharp metallic tip, biased to a

small potential (1-1000 mV)

• At a very close distance,

tunneling current will start to

flow between the tip's atoms

and the samples' surface atoms

• This current is measurable (nA)

at tip-sample distance of 1Å

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Details of quantum Tunneling • When two metals are close enough, electron wavefunctions

y1, y2 can overlap and tunneling current flows:

• E1, E2 are the electron levels,

• M1,2 is the matrix element:

• V = applied voltage

)()(1)(2

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2

2,12,

121

EEMeVEfEfe

IEE

2121

2

2,12

yyyym

M

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Tunneling between metals • At low temperatures, between planar identical metals, we get

an approximate formula:

• Where: ~ 1 Å-1

• Since 1/k~1Å, tunneling is

significant only at very short distances

• STS = Scanning Tunneling Spectroscopy:

• Between metals, the I/V curve looks like :

(no gap)

kdeVm

eI 22

2

2

12

32

2

yy

F

mk

21

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Tunneling between plane and tip • At low temperatures, between plane and spherical tip of

identical metals, we get the tunneling current:

• Where D = density of states at tip,

R = tip radius

• There is still exponential

dependence on distance

• Tip radius plays a secondary role

• DOS can be measured as well

F

sFss

Kd

FT EEreKREDVe

I )()()(32 2

0

242223

y

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Metal-semiconductor tunneling • Tunneling between the metallic STM

tip and a semiconductor shows the

energy gap in the I/V curve (STS)

• In many cases the derivative dI/dV is

plotted vs. V to show more clearly

the DOS, states in the gap etc.

• Surface states (oxidation) can pin

the Fermi level – UHV is needed