Transcript Surface Analysis Microscopy and Spectroscopy

Surface Analysis
Microscopy and Spectroscopy
Fundamentals of Electrochemistry
CHEM*7234
CHEM 720
Lecture 11
Areas of Application
ELECTROCHEMISTRY
ANALYSIS
SYNTHESIS
Microscopy
Spectroscopy
Current Microscopy Example
“Advanced Plating Chemistry for 65 nm Copper Interconnects”
Semiconductor International May 2003
Mounding when
filling trenches
with electrochem
deposited Cu
using two
component
electrolyte
system.
No mounding with three
component system.
Utilized Fast Ion Beam (FIB) Microscope for these images.
Current Spectroscopy Example
“Structural Studies in Lithium Insertion into SnO-B2O3 Glasses and Their Applications
for All-Solid-State Batteries”, Katada et al., JES 150, A582 (2003)
XRD
NMR
Mössbauer
Ex Situ vs. In Situ
Ex Situ Experiment: An experiment performed on a sample after it has
been removed from the location wherein it was formed.
• wider range of experimental techniques available.
In Situ Experiment: An experiment performed on a sample while it is still
located in its native environment.
• less risk of altering the sample’s true properties.
Challenges Analyzing
Electrochemical Samples Ex Situ
• loss of electrochemical control
• loss of solvent, ion atmosphere
• risk contamination, oxidation
Remove sample ?
Challenges Analyzing
Electrochemical Samples In Situ
• the electrolyte
solution can
strongly absorb
the various probe
particles which
might be used to
perform different
analyses.
• cell design
needs to account
for refraction
photons
ions
electrons
Microscopy
• What is the structure of the surface of the sample?
• Resolution: Lateral, Vertical
• Contrast mechanism
• Dynamic Range
• Ex situ or In situ
Atoms
1Å
1 nm
Molecules
Red Blood
Cells
Viruses
1 µm
Computer
Circuits
1 mm
Hair
1 cm
Lateral Resolution
Here are some of the techniques we will examine and a comparison of their
lateral resolution capabilities.
AFM
1Å
1 nm
OM
SAM
SEM
STM
1 µm
IM
1 mm
1 cm
Vertical Resolution
Here are the same techniques, comparing their vertical resolution.
AFM
1Å
STM
SAM
1 nm
IM
1 µm
SEM
OM
1 mm
1 cm
Dynamic Range
Not only do we need image small things, but large things too. What is the
largest field of view that the instrument can provide? What is the range of
features, largest to smallest, that can be observed?
OM
AFM
1Å
STM
1 nm
1 µm
IM
SEM
SAM
1 mm
1 cm
Diffraction vs. Scanning
Two approaches to image formation
Diffraction:
Scanning:
Incident wave scatters from surface
features, interfering with itself and
forming a diffraction pattern. When
diffracted wave is refocused, it
produces an image of the surface.
Incident wave focused to a small
point and rastered across surface.
Signal is acquired from each point on
surface.
•Entire image formed simultaneously
•Image formed sequentially
•Resolution limited by wavelength
•Resolution determined by spot size
Vibration Isolation
Buildings vibrate (motors, air conditioners, walking, vehicles).
Resonances between 1 and 100 Hz. Amplitudes in micron range.
• build microscope rigid
• couple to building loosely
• provide multiple stages with alternate
rigid/loose coupling
• shield acoustically for very precise
measurements
Optical Microscopy
• a diffraction experiment
• basic lens components
• coarse/fine focus
• Mon/Bin/Tri ocular schemes
• working distance
• adjust interpupillary distance
• quantitation with reticle
• image recording
A good web site for a brief introduction to
optical microscopes can be found below.
http://www.olympusmicro.com/primer/opticalmicroscopy.html
http://www.greatscopes.com/important.htm
Optical Microscopy
Select the correct combination of
lenses for your task.
continued
Optical Microscopy Resolution
• Rayleigh equation
d = 0.61 (l / N.A.)
d is distance between objects that can still be distinguished
l is wavelength of light
N.A. is numerical aperture of lens = n sin(Qvertex)
Q
Scanning Electron Microscopy
Electron Gun
Secondary
Electron
Detector
Vacuum Chamber
SEM Focusing Column
Thermal
Lens 1
Field
Emitter
Extractor
Beam
Acceptance
Aperture
Suppressor
Assembly
Steering
Quadrupole
1
Steering
Quadrupole
2
Lens 2
Beam
Blanking
Plates
Sample
Deflection
Octupole
SEM Experiment
Trochodiscus longispinus in OM and SEM. Note improved depth of field
and resolving capability of the SEM experiment.
Electron Reemission
Elastically scattered
Backscattered
SEM
Inelastically scattered
Secondary electron emission
Relative Intensity
e–
Fraction of Incident Beam Energy
SEM
BSE vs. 2° Detection
Both can be used, different information, different detection scheme.
BSE
2° Electrons
Specular reflection
Isotropic emission
Higher energy
Very low energy
Encode some chemical information
Better structural contrast
Excitation Depth Profile
Incident electron beam penetrates 100 - 200 mm into sample.
Different emission mechanisms arise from different depths.
BSE
SED
CL
Atomic Number Dependence
The probability of an incident electron being scattered varies as the square
of the atomic number of the atom and inversely as the incident kinetic
energy.
• Greater depth of penetration for
low Z materials (e.g. Al vs. W)
• BSE emission branch increases
with Z
Low Z
High Z
Equation d prop to W V2/Z rho
SEM Example
Microstructural Development and Surface
Characterization of Electrodeposited
Nickel/Yttria Composite Coatings, Cunnane et
al., JES 150, C356 (2003)
Changing the Y content in the Ni
electrolyte bath from 1 to 5 g/L.
Preferential growth directions are
altered as the nucleation rates are
changed by the co-depositing
material.
Scanning Auger Microscopy
Uses same e-beam source as SEM. Energy analyzes electrons emitted in
100 - 1000 eV range (higher than secondary, lower than backscattered).
Provides unique atomic identity information.
Very surface specific (10 nm)
Chemical maps of surfaces
Auger process. Chemical maps. Hemispherical Electron Analyzer.
Secondary scattering in samples.
The Auger Process
Incident
Electron
Beam (5 kV)
Measure kinetic energy of
ejected electron.
Vacuum Level
M
L
Higher electron
falls into hole
Another higher level
electron is ejected to
carry away excess
energy.
Eject core electron
K
Ekinetic = E(K) - E(L) - E(L)
Auger Spectra
SAM Resolution
BS electrons are also scattered into the neighbouring regions of the sample
with sufficient energy to further excite atoms not in the original excitation
volume.
Spatial resolution degraded 2 to 5 times over that of the corresponding SEM
resolution.
Scanning Tunneling Microscopy
Tunneling gap ~ 5 Å
Probe Tip
Tunneling Current
10 pA - 10 nA
Tunneling Electron Current
Sample
Tunneling Mechanism
Tip
Sample
DOS
DOS
EF
EF
0
VBias
d
IT  exp(-2kd)
Density of States
Every substance has a complex electronic structure. At every energy,
there are a certain number of electronic states. The number is so large for
bulk material, that one reports the number of states per unit energy – the
Density of States or DOS.
Tunneling can occur between states of the same energy; the electron’s
energy does not change during the tunneling event.
Control Electronics
Feedback Electronics
Error Signal
Z-piezo
Set Point
—
Sample
Current
Amplifier
Logarithmic
Amplifier
Difference
Resolution
Lateral
∆x
R
Vertical
In Situ Electrochemical STM
There’s still a vacuum gap, even in water!
Shield tip to minimize faradaic processes. Melted wax or plastic to coat
shank of tip. Expose last few nanometers only. Tunneling current must be
large compared to faradaic current.
STM Example #1
Adlayer of 1,10-phenanthroline on Cu(111) in acidic solution
Itaya, et al. J.E.S. 150 E266 (2003).
Monitored molecular orientation on surface in real time
Scanning Electrochemical
Microscope (SECM)
http://www.msstate.edu/dept/Chemistry/dow1/secm/secm.html
Create an ultramicroelectrode and use the faradaic current as the control
signal.
Signal modulated by proximity to surface.
Scanning Force Microscopy
Depends on forces (repulsive or attractive) between atoms.
Reflected light
To Position
Sensitive Detector
Sharpened Cantilevered Tip
Diode laser
Position Sensitive Detector
4-Quadrant Photodiode
(current in each quadrant changes
with light intensity)
1
2
1+2-(3+4) = 0
1+2-(3+4) < 0
1+2-(3+4) < 0
and
1+3-(2+4) > 0
3
4
Contact Mode SFM
Repulsive force between
surface atoms and tip atoms,
lead to cantilever deflection,
altering of relected beam path.
Sample is rastered and moved
vertically to maintain constant
cantilever deflection.
Can damage delicate samples.
Lateral Force Mode SFM
Frictional force measurement. During scan, frictional forces on surface
will tend to twist the cantilever.
Use Signal = 1+3 - (2+4) as feedback/imaging signal.
Chemically sensitive: –CH3 covered surface vs. –COOH covered surface
Non-Contact Mode SFM
Important when dealing with delicate samples.
Can achieve atomic resolution.
Vibrate tip at resonant frequency (100’s of kHz).
As tip approaches surface, the attractive forces between the substrate and
the tip alter the resonance condition.
For feedback/imaging
• frequency shift
• phase shift
• damping
Cantilevers
For contact mode
For LFM and non-contact mode
SFM Example
The Electrochemical Reaction of Lithium with
Tin Studied By In Situ AFM, Dahn et al., JES
150, A419 (2003).
Li is driven into Sn electrochemically
which leads to a swelling of the Sn
grains. SFM images were used to
measure the grain sizes as the
potential changed, contributing to a
model rgarding Li incorporation in the
Sn film.
Interference Microscopy
Visible wavelength optical
microscope.
Also called Non-contact
Profilometry.
Nanometer resolution vertical to
surface.
Uses interferometry to measure
surface profile.
Large dynamic range.
Instrument. Interference technique. Computational process. VSI mode.
PSI mode. Angle of acceptance. Terraced surface vs. rough surfaces.
Interference Fringes
In-phase reflections are bright; out-ofphase are dark
Top view
First reflecting surface
Structured reflecting surface
Side view
Imaging Process
Interferometer
Objective
Lens
Recombined, reflected light is
directed to image plane of CCD
camera.
Points on surface that are
separated from lens by an
integer number of wavelengths is
bright; those a half-integer are
dark.
Imaging Process
continued
Interference is strong only when reflected light is in focus; the samplelens distance is at the focal position.
Scan sample-lens distance around the focal length.
Each pixel will strongly modulate its intensity when the lens reaches
the focal position corresponding to each point on the surface.
High resolution position information comes from a linear variable
differential transformer (LVDT) connected to the lens scanning drive.
VSI and PSI Modes
Vertical Scanning
Interferometry
Phase Shifting
Interferometry
1.
Scan objective over range of
µm.
1.
Alter optical path length in
series of steps.
2.
Record image frames
sequentially.
2.
This causes fringe pattern
to shift laterally.
3.
Search each pixel through
frames and locate frame where
intensity modulation is greatest.
3.
4.
Assign height information by
correlating frame number to
LVDT.
The series of shifted
fringe patterns are
combined to form
interferograms from
which height information
is calculated
Rough vs. Terraced Surfaces
Interference can occur only if light is reflected back into objective lens. If
surface angle is inclined beyond acceptance angle of lens, no interference
is observed.
Lens
Angle
O.K.
2.5x obj.
2°
10x obj.
10°
50x obj.
25°
Terraced surface
Missed data
IM Example
Preparing Au substrates on mica
for use in forming nanostructured
electrodes from self-assembled
monolayers. Heat treatment
created mounds on surface.
Raman Imaging Microscopy
Raman spectroscopy is
molecular vibrational
spectroscopy. Microscope
uses a focused laser beam
as the excitation source.
The detector can be tuned
to look for a particular
spectral peak and this can
be used to produce a
chemical map - now based
on molecular and not just
atomic features.
Raman Effect
Incident laser impinges on sample. Scattered light is shifted slightly to
longer wavelengths; small amount of photon energy is left in molecules to
excite vibrations. This scattered light, looking for loss of energy,
correlates with molecular vibrational spectrum.
Mapping
Distribution of beclamethasone dipropionate (BDP) and salbutamol in
an allergy medication. Particle size is important for effectiveness.
Imaging
Much faster than mapping. Uses bandpass filters instead of dispersive
grating detection. Entire image passes through filter and exposed to CCD
camera at once. Image keyed to the radiation intensity passing through
the bandpass. This is selected for a particular molecular transition.
Raman image can
pick out the 5
differfent layers
very easily. From a
forensics study of a
car.
Raman Example
Fuel cell
development.
Troubled by
contamination
with NO+ in
solid oxide fuel
cell electrolyte,
which poisoned
process. IR is
weak and
overlapped by
CO2.
Spectroscopy
What is on the surface? (Atoms or Molecules or Bulk)
What is the structure of the surface layer?
How are they oriented?
What is their oxidation state?
How do these properties change with potential? with time? with
additional participants in the electrolyte solution?
Energy Dispersive X-ray
Spectroscopy (EDX)
Done in conjunction with SEM. Name shifting to EDS.
Add an X-ray detector. Emitted X-rays identify atomic species in excitation
volume.
Detector analyzes X-ray photons by energy, rather than wavelength.
Can be used to chemically map a surface.
Can also be done in wavelength dispersion mode. Higher resolution (10
eV compared to 100 eV), but more complex. Getting better. Also higher
sensitivity. Order of magnitude better.
EDS Detector
Cooled in LN2 temps,
Si crystal converts Xray photon into charge
by ionization. Charge
is integrated through
the FET and is
proportional to X-ray
energy.
WDS Detector
Concave mirror crystal is key
to the process. Can be LiF,
thallium acid phthalate, or
multilayered structures such
as W/C, W/Si, or Mo/B.
MoS2
EDX Example
A cast iron sample
SEM
C map
Si Map
Fe map
X-ray Photoelectron
Spectroscopy
Irradiate sample with monochromatic X-ray beam and energy analyze the
photoelectrons which are ejected. (Kind of opposite of EDS).
High resolution (< 1eV) allows chemical state identification (Si, Si2+, Si4+,
SiO2 compared to SiTe2.
Vacuum required to be able to detect the electrons.
New instruments can focus X-ray to a few µm in diameter. The beam can
be scanned to do imaging XPS.
X-ray Source: Anode
Electron beam (15 kV) strikes an anode (Mg or Al). Emits x-rays. Tuned
to maximize for narrow emission range (example, Mg Ka).
X-ray Source: Synchrotron
Electron Energy Analyzer
Hemispherical analyzer. Electron lens systems adjusts incoming electron
energy to particular kinetic energy. Only specific energy passes through
the hemispherical path to reach detector.
Detector is electron multiplier.
Can be multichannel.
XPS Spectrum #1
Spectrum for Yttrium
XPS Spectrum #2
Ag spectrum
showing the spinorbit splitting of the
3d peaks. The
instrumental
linewidth of 0.82 eV
is also shown.
IR Spectroscopy
Vibrational information about molecules. Valuable because of surface
selection rules
P-polarized: electric vector
Ep
amplified at surface.
Ep
S-polarized: electric vector
cancels at surface.
Es
Es
Phase shifts 180° upon reflection
SNIFTIRS
Subtractively Normalized Interfacial Fourier
Transform Infared Spectroscopy
R R( E2i )  R( Ed1 )

R
R( Ed1 )
Working Electrode
Thin Film Electrolyte (2 µm)
ZnSe prism
PM FTIRRAS
Polarization Modulation Fourier Transform Infrared Reflection Absorption Spectroscopy
BaF2 prism
S 
Kerr Cell
2( I s  I p )
Is  I p
Electrolyte (D2O) µm
Organic layer nm
Electrode surface
Electronically modulate polarization at 150 kHz.
 2.3 A
PM FTIRRAS Spectrum - Pyridine
0.002
0.30V
S
-0.30V
-0.55V
-0.60V
Pyridine bound to Au(111)
changes orientation with cell
potential
-0.65V
-0.85V
1650
1600
1450
Wavenumber / cm
-1
1400