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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.
2
3
4
5
6
7
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
8
STM LaboratorySTM 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!
9
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-IntroductionThe 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-HistoryThe 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
10
STM-TunnelingSTM-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 –האנרגיה הכללי ת של האל קטרון , k- וקט ור הגל של הא ל קטרון
m – מסת האל קטרון
א. בור פוטנ ציא ל א ינס ופי חד -מימדי
מימדי -פוטנ ציא ל סו פי חד מחסום. ב
≥
<=
0
00)(
XV
XxV
o
0)()( 2
2
2
=+ XkdX
Xd ψψ
( )[ ] 2121 VEmk −=h
E- האנרגיה הכל לי ת של האל קטרון , k- וקט ור הגל של הא ל קטרון m- מסת האל קטרון
V - אנרגיה פו טנציא ל ית
V(x)
xמתכת אויר
e
V0
x=0
) המשך (ממדי-פוטנ ציא ל סופ י 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=
11
xikxik BeAexx 11)( ;0 −+< =ψxx
EVmEVm
DeCexsx hh
)0(2)0(2
)( ;0−−− ++<< =ψ
xikFexxs 1)( ; =< ψ
Continoussx
Continoussx
==
==
′ ),0(
),0(
ψψ
: תנאי שפה
sEVm
xik eFFexxs h
)0(22
;1 2)(;−−
=< αψה הסתברות הדועכת בעל ערך מוחלט של אמ פלי טודת) אלק טרון (קבלנו גל
aע ם ה גדלת רוחב המחסו ם אקס פונ נציא לית
מחסום פוטנ ציא ל צר
)תתקיים בנ קודות אל השרדי נ גר כדי שמשוואת (
e
x
V(x)
V0
מתכתאויר מתכת
e
x=0 x=s
מינהור -מחסום פוטנ ציא ל צר
ses
EVmxik eFFexxs καψ 2
)0(22
;1 2)(; −=
−−
=< h
כל פעם שמ תרחקי ם eהסתברות למצו א את האלק טרון יורדת פ י : מרחק של
)(2221
0 EVm −−
=h
κ
Å 1בצד השני של מחסו ם ברוחב של הסתברות ל מצוא אל קטרוןה e/1הי א Vo-E =1.0 eVכאשר
מרחק זה הוא בקי רוב (1. ההס תברות היא Å 2~רוחב מחסום של עבור ) המרחק הבין א טומ י
e
x
V(x)
V0
מתכתאויר מתכתe
x=0 x=s
Ψ (x)
x
מינהור
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.
12
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
13
c) Single atom interacting tip
d) Adsorbed atom on a metal electrode (Jellium model - free-electron metal substrate))
Common Tips ModelsConstant height vs constant current imaging
STM 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
14
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”
15
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 characterizationImportant 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 formationIon milling: tip radii of 4 nm, cone angle ~ 10o.
In air : cutting of Pt-Ir wire
Tip Preparation – Ion Milling & FIB Cutting
16
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 artifactsTip artifacts
Imaging Errors and Tip Convolution1) 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 !)
17
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
18
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 TechniquesScanning 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
19
STM NanolithographySTM 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"