Scanning Probe Microscopy

Danny Porath 2002(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. Hezy Cohen6. …

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.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 41. Read the paper by:

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

2. Find on the web, in a paper or in a book the 3 most impressive SEM and TEM images:

a. 1 - Technicallyb. 1 - Scientificallyc. 1 - Aesthetically

Explain your choice. If needed compare with additional images.3. For “Maskianim” – Read the paper: Scanning Tunneling Microscope

Instrumetation” – Kuk & Silverman 60, 165 (1989).

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

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 tunneling 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

STM-HistoryIn 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.

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

=+ XkdXXd ψψ ( )[ ] 2121 VEmk −=

hE –ה אנרגיה הכל לי ת של האל קטרון ,

k-וקט ור הגל של ה אלקט רוןm –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

=+ XkdXXd 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 modelBased on 1st order perturbation theory, taking into account density of states in tip and sample, assuming:

Spherical symmetrical tip

s-type tip wave function

Small applied bias (unaffected wavefunctions)

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 the Fermienergy ns(EF,ro)-LDOS at the Fermi energy at ro

Like the one dimensional infinite potential well !