Scanning Probe Microscopy

Danny Porath 2003

(Eigler et. al.@IBM)

Introduction to Scanning Probe Microscopy and applications.

Julio Gómez Herrero. LNM. UAM

“SPM - the eyes to the Nano world”.

With the help of…….

1. Yosi Shacam – TAU2. Yossi Rosenwacks – TAU3. Julio Gomez-Herrero - UAM4. Serge Lemay - Delft5. …

Outline SEM/TEM:1. Examples, links and homework

2. STM principle, lab, Images

3. Tunneling

4. Instrumentation

5. Artifacts

6. Spectroscopy

7. Lithography

Books and Internet Sites“Scanning Probe Microscopy and Spectroscopy”, R.

Wiesendanger (Cambridge U. Press)

http://www.embl-heidelberg.de/~altmann/http://www.eng.tau.ac.il/~yosish/courses.html

http://www.eng.tau.ac.il/~yossir/course/http://www.chembio.uoguelph.ca/educmat/chm729/STMpage/stmtutor.htmhttp://www.weizmann.ac.il/surflab/peter/afmworks/index.html

http://www.almaden.ibm.com/almaden/media/image_mirage.html...

Homework 51. Read the paper by:

Crommie, Lutz & Eigler, Science 262, 218 (1993)] - Emphasize the “lithography” part.

2. Read the paper: Scanning Tunneling Microscope Instrumentation” –Kuk & Silverman 60, 165 (1989).

3. Find on the web, in a paper or in a book the 3 most impressive STM images:a. 1 - Technicallyb. 1 - Scientificallyc. 1 - Aesthetically

Explain your choice. If needed compare with additional images.

Scanning Tunneling Microscope (STM)

Sample

Piezo

Electronics(Current+Feedback)

Computer(Control)

Matrix ofheights(Image)

Tip

I(V) ~ Ve-(ks)

Tunneling between a sharp tip and conducting surface.Piezo enables xy and z movement.Working modes: constant current and constant height.The feedback voltage Vz(x,y) is translated to height (topographic) information.

STM-Principle

STM Head

דגם

בידוד

בסיס

STM - ראש ה

חוד

בורגמיקרו-מטרי

גבישפייזו-אלקטרי

רכיבי היחידה המרכזית

Low Temperature STM

Low Temperature STM

בורג מיקרומטרי

מוט קפיץ

ממברנהגביש

פייזואלקטרי

חוד

רכיבקרוב גס

מחזיקהדגם

מחזיקהדגם

STM Laboratory

STM

חיפויעץ

אלקטרוניקה

מערכתהמחשב

בסיס

STM Laboratory

STM Images

Graphite – atomic resolution

Supercoiled DNA

Gold monolayers

STM image of a Single-Wall Carbon Nanotube

Si (111) Surface (7x7 reconstruction)

STM Images Combination GaSb/InAsOnly every-other lattice plane is exposed on the (110) surface, where only the Sb (reddish) and As (blueish) atoms can be seen

This color-enhanced 3-D rendered STM image shows the atomic-scale structure of the interfaces between GaSb and InAs in cross-section. A superlattice of alternating GaSb (12 monolayers) and InAs (14 monolayers) was grown by molecular beam epitaxy. A piece of the wafer was cleaved in vacuum to expose the (110) surface, and then the tip was positioned over the superlattice about 1 µm from the edge. Due to the structure of the crystal, only every-other lattice plane is exposed on the (110) surface, where only the Sb (reddish) and As (blueish) atoms can be seen. The atoms are 4.3 Å apart along the rows, with a corrugation of <0.5 Å From work of W. Barvosa-Carter, B. R. Bennett, and L. J. Whitman..

Inspection

Tip techniqueswith CCD-camera for on-chip inspectionStylus (α-step)

Height resolution 5 nmLateral resolution ≥ 15 µm

AFMHeight resolution monolayerLateral resolution ≤ nm

MicroscopyOptical microscopy (1 µm) Dark field, Interference contrast, luminescence Scanning Electron M. (≤ 1 nm)

20-25% of fabrication time!

Why STM ?The electronic microscopes gives ‘volume images’ (penetration depth)In STM-no use of external particlesPrinciple: Electrons tunnel between an atomically sharp tip and a surface

STM-Introduction

The STM combines three main concepts:

• Scanning• Tunneling• Tip-point probing

• Uniqueness:

STM-Introduction

Animation: http://www.iap.tuwien.ac.at/www/surface/stm_gallery/stm_animated.gif

In March 1981, Gerd Binning, H. Rohrer, Ch. Gerber and E. Weibel observed electrons tunneling in vacuum between W tip and Pt; this in combination with scanning marked the birth of STM.

The breakthrough: atomic-scale surface imaging in real space

The development of STM paved the way for a new family of techniques called : “scanning probe microscopy”.

1986-Nobel prize to G. Binnig and H. Rohrer.

STM-History

The first STM image

Comparison of Characterization TechniquesAnalyticalTechnique

TypicalApplication

Signal DetectedElements

DetectionLimits

DepthResolution

Lateral Probe Size

TXRF Metalcontamination

X-rays S - U 109-1012Atoms/cm2

10 mm

RBS Thin filmcomposition

He atoms Li - U 1 - 10 at%(Z<20)0.01 - 1(Z>20)

2-20 nm 2 mm

XPS Surface analysisDepth profiling

Photo-electrons

Li - U 0.01 - 1 at% 1-10 nm 10 µm – 2 mm

EDAX(EDS)

elementalmicroanalysis

X-rays B - U 0.1 - 1 at% 1 – 5 µm 1 µm

QuadSIMS

DopantprofilingSurfacemicroanalysis

Secon-dary ions

H - U 1014-1017Atoms/cm3

<5 nm 1 µm (Imaging)30 µm (D Profiling)

TOFSIMS

Surfacemicroanalysis

Secon-dary ions

H - U 108Atoms/cm2

<1monolayer

0.1 µm (Imaging)

Comparison of Characterization TechniquesAnalytical Technique

Typical Application

Signal Detected Elements

Detection Limits

Depth Resolution

Lateral Probe Size

AES Surface analysis and depth profiling

Auger electrons

Li - U 0.1 - 1 at% <2 nm 100 nm

HRAES Surface analysis, micro area depth profiling

---“--- Li - U 0.01 - 1 at% 2 - 6 nm <15 nm

SEM Surface imaging Secondary & backscattered electrons

3 nm

AFM Surface imaging

Atomic forces

0.01 nm 1.5 - 5 nm

HRSEM High resolution surface imaging

Secondary & backscattered electrons

0.7 nm

STM Surface imaging Tunneling currents

0.01 nm 0.1 nm

STM-Tunneling

STM-Tunneling

STM-Tunneling Models

Elastic vs. Inelastic (energy loss to phonons etc) tunneling processes

One dimensional vs. three dimensional

Barrier shape

Elastic Tunneling through One-Dimensional Rectangular Barrier

X

V→∞ V→∞

X = LX = 0

≥

<=

0

00)(

XV

XxV

o

0)()( 2

2

2

=+ XkdX

Xd ψψ ( )[ ] 2121 VEmk −=h

E –ה אנרגיה הכל לי ת של האל קטרון , k-וקט ור הגל של ה אלקט רון

m –מ סת הא לקטרון

א. בור פוט נצ י אל אי נ סופ י ח ד-מימדי

מימדי -פוט נצ יא ל סופ י ח ד מחסום. ב

≥

<=

0

00)(

XV

XxV

o

0)()( 2

2

2

=+ XkdX

Xd ψψ

( )[ ] 2121 VEmk −=h

E- האנרגיה הכללי ת של האל ק טרון , k-וקט ור הגל של ה אלקט רון m- מ סת הא לקטרון

V - אנרגיה פ וטנציאלי ת

V(x)

xמתכת אויר

e

V0

x=0

)המשך (ממדי-פוט נצ יאל סו פי חד מחסום

xikxik BeAexx 11)( ;0 −+=< ψ

xEVm

xEVm

xVEm

ixVEm

i

DeCe

DeCexx

hh

hh

)(2)(2

)(2)(2

00

00

)( ;0−

+−−

−−

−

+=

=+=≥ ψ

מתכת אויר

e

V0

X=0

[ ] )21( 211 mEk

h=

xikxik BeAexx 11)( ;0 −+< =ψxx

EVmEVm

DeCexsx hh

)0(2)0(2

)( ;0−−− ++<< =ψ

xikFexxs 1)( ; =< ψ

Continoussx

Continoussx

==

==

′ ),0(

),0(

ψψ

: ת נאי שפ ה

sEVm

xik eFFexxs h

)0(22

;1 2)(;−−

=< αψה ה סתברות הדועכת בעל ערך מוחלט של אמ פ ליטודת) אלקטרון (קבלנו גל

aעם הגדל ת רוחב ה מ חסום אקס פוננציאלי ת

מחסום פוט נצ יאל צר

)תתקי ים בנקודות אלהשרדי נגר כדי ש משוואת (

e

x

V(x)

V0

מתכתאויר מתכת

e

x=0 x=s

מי נהור-מחסום פוט נצ יאל צר

ses

EVmxik eFFexxs καψ 2

)0(22

;1 2)(; −=

−−

=< h

כל פעם שמ תרח ק ים eהס תב רות למצוא את הא לקטרון יורדת פי : מרח ק של

)(2221

0 EVm −−

=h

κ

Å 1בצד השנ י של מ חסום ברוחב של הס ת ברות למצוא אלקט רוןה e/1ה יא Vo-E =1.0 eVכאשר

מרח ק ז ה הוא בקירוב (1. ה הס תב רות הי א Å 2~רוחב מ חסום של עבור ) ה מרח ק הבין אטומי

e

x

V(x)

V0

מתכתאויר מתכתe

x=0 x=s

Ψ (x)

x

מי נהור

STM-Tersoff and Hamann tunneling model

Spherical symmetrical tip

s-type tip wave function

Small applied bias (unaffected wavefunctions)

Based on 1st order perturbation theory, taking into account density of states in tip and sample, assuming:

STM-Tersoff and Hamann tunneling model

φ - Effective local barrier height

Since: ns α exp(-2κ(s+R)); (s wave function)

⇒ I α exp(-2κs)

h/)2( φκ m=

I α U .nt (EF) exp(2κR) ns(EF, ro)U-Applied bias between tip and sample,

nt(EF)-tip density of states at Fermi energy ns(EF,ro)-LDOS at the Fermi energy at ro

Like the one dimensional infinite potential well !

The interpretation of STM images in light of Tersoff and Hamann tunneling modelThe STM image represents contour maps of constant surface LDOS at EF, evaluated at the center of the curvature of the tip.

Higher wavefunctions (l, ..) gave minor corrections, thus a good model for single atom tip.

Modifications of the simple model Effect of finite bias on tip (zero voltage wave functions):

U-Applied bias between tip and sample, nt(eU±E)-tip density of states at an energy E under bias U, ns(E,ro)-sample density of states at an energy E at the center of curvature of tip. (Ef is taken =0)

dErEnEeUnI os

eU

t ),()(0

±±∫α

Modifications of the simple model

W and platinum-iridium are the most widely used for tip material, the density of states at Ef is dominated by d states.

CCT Imaging with different “tip atoms”

The 2x2 nm image of Au (111) changes during imaging probably due to change of effective orbital of the tip end

Comparison with theory

Theoretical corrugation amplitude for s and dz2

tip state on Al (111)

Conclusion – the orbital at the end of the tip determines the spatial resolution in STM

Negative tip displacement for He: the closed valence shell of He produces a local decrease in density of states near EF;

Reduced tunneling current => negative tip displacement in CCT.

Bumps or holes in CCT may not correspond to presence or absence of atoms !

Constant current of Na tip over Na, S, and He adatoms

a) Spherical potential well (continuum):

Predicted resolution:

Based on experimental results this model was not accurate enough

b) Cluster of atoms or multiatom tip

Common Tip Models

2/1

)(2

+sRA

o

c) Single atom interacting tip

d) Adsorbed atom on a metal electrode (Jellium model - free-electron metal substrate))

Common Tips Models

Constant height vs constant current imagingSTM Working Modes

Constant Current Imaging (CCI)

Pohl, D. W. IBM J. Res. Dev. 30, 417 (1986), Kuk and silverman, Rev. Sci. Instrum. 60, 165 (1989)

STM Instrumentation

1. Mechanical Construction

2. Electronics

3. Data Acquisition

To obtain vertical resolution of 0.01Å,

a tip-sample stability of ~ 0.001 Å is required

Typical floor vibration ~ 0.1-1.0 µm, 0.1-50 Hz,

⇒ Vibration Isolation

Vibration Isolation

• low frequency (<20 Hz, building) ⇒tension wires or springs (resonance frequency ~1-5 Hz) , air table (resonance frequency ~ 1 Hz).

• Medium frequency (20-200 Hz,motors, acoustic noise) ⇒ mounting on heavy plates.

• External vibration isolation system+rigid STM design can reduce external vibrations by a factor of 10-6-10-7 (“Interference filter”).

Damping

STM design An example for an approach mechanism

Three dimensional movement of tip and sampleTip movement -piezoelectric drives.Sample-piezoelectric, magnetic (magnet inside a coil), mechanical.

STM Positioning Devices - Tripod

STM Scanners - Piezoelectric bars

Vhldl 31=∆

h V

l

Tube Scanners

Design Considerations:• Tube scanner: higher sensitivity due to thin walls, symmetrical vs.

asymmetrical voltage.• Single tube scanner: higher resonance frequency

⇒ scan rate, stability• voltage signals to produce a scan.• Large piezoresponse.• Low cross-talk between x,y and z piezodrives.• Low nonlinearity, hysteresis, creep, and thermal drift.

Sample-Tip Approach Mechanisms:• Step size has to be smaller then the total range of z piezodrive; Step

resolution of 50 Å ,and dynamic range of cm is reuired!“Inchworm” (electrostrictive actuator), motion not limited in distance. Two dimensional “Inchworm”=“louse” (כינה)

STM Scanners (cont.)

Approach mechanism - “Inchworm”

STM Electronics (general)

STM Electronics (cont.)Careful design for low currents nad stable feedback circuits.Sample and hold amplifier ⇒ local I-V characteristicsComputer: 4-5 DAC’s, 2 ADC’s, real-time plane fit etc.

Atomic resolution: single atom termination, For atomic structures: macroscopic shape of little importanceLarge scale structure: macroscopic tip shape is important

1Å difference ⇒ 1 order of magnitude in tunneling current.

Tip preparation and characterization

Important factors:Chemical composition of tip

Oxide layer (jump to contact)For atomic resolution - type of atom (tip material does not ‘dictate’ atom at the tip!)Higher resolution has been obtained by tunneling into or from d-orbitals (W).

Tip material: hard+low workfunction, UHV-W, Mo, Ir Air-Pt, Au (soft) ⇒ Pt-Ir.

Tip preparation: Electrolytic etching (drop-off technique)Cutting (with scissors)

Tip preparation

Tip preparation - Electrolytic Etching (drop-off technique)

The disadvantage of electrolytic etching: oxide formation

Ion milling: tip radii of 4 nm, cone angle ~ 10o.In air : cutting of Pt-Ir wire

Tip Preparation – Ion Milling & FIB Cutting

STM Tips PreparationTips are produced by electron beam deposition (EBD) inside SEM.Dissociation of chamber residual gases (H2, O2, CO, H2O)

Tip Shapes

Mechanically cut Pt Tip

Etched Au Tip

Ion milled W Tip

Tip artifacts

Tip artifacts

Imaging Errors and Tip Convolution

1) Checking the tip• Checking atomic resolution• Chemical purity: dirty tip => lower sensitivity and resolutionChecking vacuum gap: measuring I as a function of distance

Taking STM Images

−−=

h

sEVmAI )(22exp 0 φ••−= smBIh

22)ln(

φh

mds

Id 22)ln(−=

For a barrier height of 4 eV, and ds=2 Å, d(lnI)=4.2, indicates a vacuum gapIf the sample is covered by an insulator, the barrier height and sensitivity decreases (also due to compression of contaminants !)

Samples1) Metals

In air- atomic resolution is easily obtainable (only on thin films), but samples are usually contaminated.

In UHV typical cleaning : ion sputtering, annealing at 400-600 C (checked by AES or LEED)

I-V curves can check cleanliness: at metals no energy gap around Ef

Samples2) Semimetals

Graphite, and other layered transition metal dichalcogenides (WS2, MoS2) can be imaged with atomic resolution in air simply by cleaving or peeling (by adhesive tape).

I-V curves can check cleanliness: at metals no energy gap around Ef

4)Biological Specimens

Usually droplet deposited on conducting substrates and dried.

Freeze drying and then coating with metal film

3)Semiconductors

usually capped with oxides -reduced resolution; atomic resolution is possible only with sub-monolayer oxygen coverage

In UHV: three main preparation methods: Ion sputtering followed by annealing, cleavage, and thermal desorption (Si-flashing the sample for short periods at 1100-1200 C).

•The reconstruction: reduces the number of dangling bonds from 49 to 19 per 7x7 unit cell

•Reconstruction between atomic steps-depends on

rate of cooling during Surface preparation

Si (111) (7x7)

STM and Spectroscopy

Si (111) : sample @ –2V

Sample @ +2V

General• Information of local electronic structure due to energy-

dependent changes in density of states ⇒1) STM does not reveal position of atoms 2) Spectroscopic information with atomic resolution

Voltage-dependent Imaging

Scanning Tunneling Spectroscopy

Sample Bias (V)

The tunneling from states near EFis dominant

Positive tip bias (in picture) shows tip empty states

Positive sample bias shows sample empty states

Therefore:

More strongly bound electrons have shorter decay lengthTunneling direction depend on bias between tip and sample, most of the tunneling is from electrons near Ef ⇒spectroscopic information can be obtained by voltage-dependent imagingEqulibriumPositive sample bias (Measuring the sample empty surface states)Negative sample bias (‘Measuring the tip’-sam. Surf states)Only electronic states between Ef’s of tip and sample contribute

STM and Spectroscopy

−−=

h

zEVmA )(22exp 0ϕ

Si (111) (7x7)

There are 12 adatoms per 7x7 unit cell

Each adatom ties up 3 dangling bonds from underlying atomic layer

Single dangling bond on each adatom

The dangling bonds are partly filled and therefore contribute to both filled and empty surface states

The positions of the maxima do notdepend on the applied bias

Si (111) (7x7) is a special case where STM image provide position of surface atoms !

Sample =1.5 V Sample =-1.5 V

Si (111) (7x7)

At positive sample bias all atoms are at equal height (empty surface states)

At negative sample bias also rest atoms are observed

Sample =-3 V

At –3V ‘rest atoms’ are visible

STM for Surface State Spectroscopy

[ ]dEeVETrEnEeVnV

dEeVETrEnEeVndVdI eV

ostos

eV

t ∫∫ ±∂∂

=

±⇒00

),(),()(),(),()( mm

Assuming dnt/dV≈0

The disadvantage of succesive imaging (for surface state spec)-thermal drift, tip changes make direct comparison between successive images dificult

The idea-the tunneling current increases if the applied voltage onsets a tunneling into unoccupied surface states⇒dI/dV(V) reflects surface states density

dEeVETrEnEeVnI os

eV

t ),(),()(0

m±∫α

0),(),()0( −+ eVeVTreVnen ost

),(),()0(),(),()(0

eVeVTreVnendEeVEVTrEnEeVn ost

eV

ost +

∂∂

±= ∫ m

STM for Surface State Spectroscopy

In order to eliminate bacground (due to T(eV)), dI/dV is normalized as: dI/dV /I/V

Because T(eV) is a monotonic increasing function of V, structure in dI/dV (V) is mainly due to changes in surface density of states of the sample.

For eV« φ, dT/dV = 0

),(),()0( eVeVTreVnendVdI

ost≈⇒

Experimental TechniqueSTS- scanning tunneling spectroscopy: imposing a small high

frequency Vand measuring with lock-in amp. Measurement gives :

dI/dV at the voltage VDC of the CCT Frequency range: too low-interfere with feedback

electronicstoo high-displacement current

High DC Voltage: high surface barrier forms a “quantum well” for surface electrons ⇒ changes density of states.

Low DC Voltage : surface density of states is obtained

STM Modulation Techniques

Scanning Tunneling Spectroscopy of Si (111) (7x7)

Top: I-V Measured at 3 locations of the Unit cell

Middle: UPS measurement

Bottom: dI/dV/(I/V) averaged on several unit cells

STM Nanolithography

STM Nanolithography

, 524 (1990).344(Nature

Xe on Nickel,

The science of PR……

STM Nanolithography

48 iron atoms in a circular ring – the “quantum corral"The waves are surface state electrons forced into "quantum" states of the circular structure. The ripples in the ring of atoms are the density distribution of a particular set of quantum states of the corral. The artists were delighted to discover that they could predict what goes on in the corral by solving the classic eigenvalueproblem in quantum mechanics -- a particle in a hard-wall box. [Crommie, Lutz & Eigler, Science 262, 218 (1993)]

48 iron atoms in a circular ring – the “quantum corral"