Transcript Microscopy and Surface Analysis

Microscopy and Surface Analysis 2
Lecture Date: March 17th, 2008
Reading Assignments for Microscopy and
Surface Analysis
 Skoog, Holler and Nieman, Chapter 21, “Surface
Characterization by Spectroscopy and Microscopy”
 Hand-out Review Article:
C. R. Brundle, J. F. Watts, and
J. Wolstenholme, “X-ray Photoelectron and Auger
Electron Spectroscopy”, in Ewing’s Analytical
Instrumentation Handbook, 3rd Ed. (J. Cazes, Ed.),
Marcel-Dekker 2005.
Introduction to the Solid State
 In solids, atomic and molecular energy levels broaden into
bands that in principle contain as many states as there
are atoms/molecules in the solid.

Bands may be separated by a band gap with energy Eg
P.A. Cox, "The Electronic Structure and Chemistry of Solids" Oxford University Press, 1987.
C. Kittel, Solid-state Physics, 7th Ed, Wiley, 1999.
W. A. Harrison, Electronic Structure and the Properties of Solids, Dover, 1989.
Energy Bands in the Solid State
 Bands are continuous and delocalized over the material
 Band “widths” are determined by size of orbital overlap


The highest-energy filled band (which may be only
partially filled) is called the valence band
The lowest-energy empty band is called the conduction
band
P.A. Cox, "The Electronic Structure and Chemistry of Solids" Oxford University Press, 1987.
C. Kittel, Solid-state Physics, 7th Ed, Wiley, 1999.
W. A. Harrison, Electronic Structure and the Properties of Solids, Dover, 1989.
The Workfunction: A Barrier to Electron Emission
 How does the electronic arrangement in solids affect
surfaces? In particular, how can an electron be removed?
Free electron!


For some electron being removed, its energy just as it
gets free is EV
The energy required to remove the electron is the
workfunction  (typically several eV)
P.A. Cox, "The Electronic Structure and Chemistry of Solids" Oxford University Press, 1987.
C. Kittel, Solid-state Physics, 7th Ed, Wiley, 1999.
W. A. Harrison, Electronic Structure and the Properties of Solids, Dover, 1989.
The Workfunction: A Barrier to Electron Emission
 Workfunctions vary from <2 eV for alkali metals to >5 eV
for transition metals.

Material
Crystal State
Workfunction (eV)
Na
polycrystalline
2.4
Cu
polycrystalline
4.4
Ag
polycrystalline
4.3
Au
polycrystalline
4.3
Pt
polycrystalline
5.3
W
polycrystalline
4.5
W(111)
single crystal
4.39
W(100)
single crystal
4.56
W(110)
single crystal
4.68
W(112)
single crystal
4.69
The workfunction is the ‘barrier” to electron emission –
like the wall in the particle-in-a-box concept.
Data from CEM 924 Lectures presented at MSU (2001).
Basic Considerations for Surface Spectroscopy
 Common sampling “modes”
– Spot sampling
– Raster scanning
– Depth profiling
 Surface contamination:
– The obvious contamination/alteration of surfaces that can be the
result of less-than careful sample preparation
– Solid surfaces can adsorb gases:
 At 10-6 torr, a complete monolayer of a gas (e.g. CO) takes just 3
seconds to form.
 At 10-8 torr, monolayer formation takes 1 hour.
– Most studies are conducted under vaccuum – although there are
newer methods that don’t require this.
D. M. Hercules and S. H. Hercules, J. Chem. Educ., 1984, 61, 403.
Surface Spectrometric Analysis
 Surface spectrometric techniques:
–
–
–
–
X-ray fluorescence (from electron microscopy)
Auger electron spectrometry
X-ray photoelectron spectrometry (XPS/UPS)
Secondary-ion mass spectrometry (SIMS)
 Depth profiling – if you are going to study surfaces with
high lateral resolution (e.g. using microscopy), then
wouldn’t it be nice to obtain information from various
depths within the sample?
The Basic Idea Behind Surface Spectrometry
Photons, electrons,
ions: they can go in
and/or out!!!
Leads to lots of
techniques, and lots
of acronyms!
Primary
photon
electron
ion
Secondary
photon
electron
ion
Surface
Primary
Secondary
Name of Technique
photon (X-ray/UV)
electron
XPS (ESCA) and UPS
photon (X-ray) or electron
electron
Auger electron spec. (AES)
ion
ion
SIMS (secondary ion MS)
photon
ion
LMMS (laser microprobe MS)
electron
Photon (X-ray)
SEM “electron microprobe”
Electron Microprobes and X-ray Emission
 Electron microscopy (usually SEM) can also be used to
perform X-ray emission analysis in a manner similar to
X-ray fluorescence analysis
– see the X-ray spectrometry lecture for details on the spectra
 The electron microprobe (EM) is the commonly used
name for this type of X-ray spectrometry


Both WDS and EDS
detectors are used (as in
XRF), elemental mapping
Not particularly surface
sensitive!
Electron Microprobes: X-ray Emission
Electron Spectroscopy


Electron spectroscopy – measuring the energy of
electrons.
Major forms:
– Auger electron spectroscopy
– X-ray/UV photoelectron spectroscopy
– Electron energy loss spectroscopy (EELS)
Electron Spectroscopy: Surface Sensitivity

Electrons can only escape from shallow depths in the
surface of a sample, because they will undergo
collisions and lose energy.
XPS/AES region,
electrons that have
not been inelastically
scattered from
shallow regions
(mostly excitation of
conduction-band
electrons)
Deep electrons that
undergo inelastic
collisions but lose
energy (exciting e.g.
phonons)
Auger Electron Spectrometry (AES)
 The Auger process can also be a source of spectral
information. Auger electrons are expelled from
atomic/molecular orbitals and their kinetic energy is
characteristic of atoms/molecules
 However, since it is an electron process, analysis of
electron energy is necessary!
– This is unlike the other techniques we have discussed, most of
which measure photon wavelengths or energy
 Auger electron emission is a three-step (three electron)
process, that leaves an atom doubly-ionized
AES: Basic Mechanism
See Figure 21-7 in Skoog, et al. for a related figure.
AES: Basic Mechanism
 Auger electrons are created from outer energy levels (i.e.
less-tightly bound electrons, possibly valence levels).
This example
would be called a
LMM Auger
electron. Other
Common types
are denoted KLL
and MNN.
AES: Efficiency of Auger Electron Production
 Two competing
processes:
– X-ray fluorescence
– Auger electron emission
 Auger electrons
predominate at lower
atomic number (Z)
 Photoelectron emission
does not compete!
K 
number of K photons produced
number of K shell vacancies created
 Auger  1   K
Top Figure from Strobel and Heineman, Chemical Instrumentation, A Systematic Approach, Wiley, 1989.
AES: Spectrometer Design
 AES instruments are designed like
an SEM – often they are
integrated with an SEM/EDXA
system
 Unlike an SEM, AES instruments
are designed to reach higher
vacuum (10-8 torr)
Electron
detector
Electron
Gun
Energy
analyzer
– Helps keep surfaces clean and free
from adsorbed gases, etc…
 Basic components:
–
–
–
–
–
Electron source/gun
Electron energy analyzer
Electron detector
Control system/computer
Ion gun (for depth profiling)
Auger
electrons
Sample
AES (and XPS): Electron Energy Analyzers

Two types of electron energy analyzers (also used in XPS):
Electrons only pass if
their KE is:
KEelectron  ke V
R1 R2
k 2
R2  R12
Concentric hemispherical analyzer
(higher resolution) – better resolution, mostly
for XPS/UPS
Cylindrical mirror analyzer
(higher efficiency)
More common for AES
(Right) Diagram from http://www.cea.com/cai/auginst/caiainst.htm
(Left) Diagram from Strobel and Heineman, Chemical Instrumentation, A Systematic Approach, Wiley, 1989.
AES: Detectors

More sophisticated detectors are needed to detect low
numbers of Auger electrons. Two types of electronmultiplier detectors:
Discrete dynode
Continuous dynode

Both types of detector are also used in XPS/UPS!!!
AES: Surface Analysis
 AES is very surface sensitive (10-50 Ǻ) and its reliance on
an electron beam results in excellent lateral resolution
 Electron beam does not
have to be monochromatic
– Note: an X-ray beam can
also be used for AES, but is
less desirable b/c it cannot
currently be focused as tightly
(as is the case in XPS)

Auger electrons typically
have energies of < 1000
eV, so they are only
emitted from surface
layers.
Diagram from http://www.cea.com/cai/auginst/caiainst.htm
AES: Spectral Interpretation
 AES Electron Kinetic Energies* versus Atomic Number
(Most intense peaks only. Valid for CMA-type analyzers.)
*Data is from J.C. Vickerman (Ed.), "Surface Analysis: The Principal Techniques“, John Wiley and Sons, Chichester, UK, 1997 .
Image from http://www.cem.msu.edu/~cem924sg/KineticEnergyGraph.html (accessed 12-Nov-2004)
AES: Typical Spectra
AES: Elemental Surface
Analysis



Very common application of
AES - elemental surface
analysis
For true surface analysis,
AES is better than SEM/X-ray
emission (electron
microprobe) because it is
much more surface sensitive
AES can be easily made
quantitative using standards.
Image from http://www.cem.msu.edu/~cem924sg/ (accessed 12-Nov-2004)
AES: Chemical Shifts


Chemical information (i.e.
on bonding, oxidation
states) should be found
in Auger spectra because
the electron energy
levels are sensitive to the
chemical environment.
In practice, it is not
(usually) there because
too many electron energy
levels are involved – it is
difficult to calculate and
simulate Auger spectra.
X-ray Photoelectron Spectrometry (XPS)
 Photoelectron spectroscopy is used for solids, liquids and
gases, but has achieved prominence as an analytical
technique for solid surfaces
 XPS: “soft” x-ray photon energies of 200-2000 eV for
analysis of core levels
 UPS: vacuum UV energies of 10-45 eV for analysis of
valence and bonding electrons
 Photoelectric effect:
Proposed by A. Einstein (1905),
harnessed by K. Siegbahn (1950-1970) to develop XPS
XPS: Basic Concepts
 Like in AES, photoelectrons can not escape from depths
greater than 10-50 A inside a material
 Schematically, the photoelectron process is:
A  h  A  e
*
atom or molecule

cation
 Like in AES, the kinetic energy of the emitted electron is
measured in a spectrometer
XPS: Review of X-ray Processes
XPS: Photoelectron Emission and Binding Energy
 The kinetic energy of the emitted electron can be related
to the “binding energy”, or the energy required to remove
an electron from its orbital.
– Higher binding energies mean tighter binding – e.g. as atomic
number goes up, binding energies get tighter because of
increasing number of protons.
Ebinding  h  IP
Ebinding  h  BE  w
http://www.chem.qmw.ac.uk/surfaces/scc/scat5_3.htm
(gas)
(solid)
XPS: Binding Energy
 The workfunction w is usually linked to the spectrometer

(if the sample is electrically connected)
In gases, the BE is directly related to IP
– Ionization potential – the energy required to take an electron out
of its orbital all the way to the “vacuum” (i.e. far away!)
– PE spectroscopy on gases is used to check the accuracy of
modern quantum chemical calculations
 In conducting solids the workfunction is involved
 Koopman’s Theorem: binding energy = -(orbital energy)

– Orbital energies can be calculated from Hartree-Fock
Another definition for XPS binding energy: the minimum energy
required to move an inner electron from its orbital to a region away
from the nuclear charge. Absorption edges result from this same
effect
XPS: Sources
 Monochromatic sources using electrons
fired at elemental targets that emit x-rays.
– Can be coupled with separate post-source
monochromators containing crystals, for high
resolution (x-ray bandwidth of <0.3 Å)
 XPS Sources (hit core electrons):
– Mg Ka radiation: h = 1253.6 eV
– Al Ka radiation: h = 1486 eV
– Ag La radiation: h = 2984.3 eV
 UPS Sources (hit valence electrons):
– He(I) radiation: h = 21.2 eV (~58.4 nm)
h = 23 eV (~53.7 nm)
– He(II) radiation: h = 41 eV (~30.4 nm)
 Focusing the spot and lateral resolution 10-m diameter spots are now possible
A Thermo-Electron
Dual-anode (Al/Mg)
XPS source
XPS: Spectral Interpretation
 Orbital binding energies can be interpreted based on
correlation tables, empirical trends and theoretical
analysis.
 Peaks appear in XPS spectra for distinguishable atomic
and molecular orbitals.
 Auger peaks also appear in XPS spectra – they are easily
distinguished by comparing the XPS spectra from two
sources (e.g. Mg and Al Ka lines). The Auger peaks
remain unchanged w.r.t. kinetic energy, while the XPS
peaks shift.
XPS: Binding Energy Ranges
 XPS Photoelectron Binding Energies versus Atomic Number (Z)
*Data from C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder and G.E. Muilenberg, Eds., "Handbook of X-ray Photoelectron Spectroscopy,"
Perkin-Elmer Corp., Flying Cloud, MN, 1979.
Image from http://www.cem.msu.edu/~cem924sg/BindingEnergyGraph.html (accessed 12-Nov-2004)
XPS: Typical Spectra
 An XPS survey spectrum of stainless steel:
Spectrum image from http://www.mee-inc.com/esca.html
XPS: Typical Spectra
 An expanded XPS spectrum of the C1s region of PET:
Spectrum image from http://www.mee-inc.com/esca.html
XPS: Chemical Shifts
 Peaks appear in XPS
spectra for distinguishable
atomic and molecular
orbitals.
 Effects that cause chemical
shifts in XPS spectra:
– Oxidation states
– Covalent structure
– Neighboring electron
withdrawing groups
– Anything else that can affect
ionization/orbital energies
XPS: Depth Profiling
 Option 1:
Sputtering techniques
– Disadvantage – can damage the surface
– Advantage – wide range of depths can be
sampled (just keep sputtering), e.g. 100 A
 Option 2:
normal
electron
Angle-resolved XPS (AR-XPS)
– Reducing the photoelectron take-off angle
grazing
(measured from the sample surface) reduces
the depth from which the XPS information is
obtained. XPS is more surface sensitive for
grazing take-off angles than for angles close to
the surface normal (longer PE paths).
– The most important application of angle
resolved XPS (AR-XPS) is in the estimation of
the thickness of thin films e.g. contamination,
implantation, sputtering-altered and segregation
layers.
Sample
For more on AR-XPS, see Briggs and Seah, Practical Surface Analysis, 2nd Ed., Vol. 1. “Auger and X-ray Photoelectron Spectroscopy,” Wiley, 1990, pp. 183-186, 244-250
Depth Profiling with Angle-Resolved XPS


AR-XPS data is often acquired by tilting the specimen
Example: gallium arsenside with a thin oxide layer on its surface:
bulk
electron
Grazing angle
(X-ray takeoff angle)
surface
(grazing)
Sample
AR-XPS figure from C. R. Brundle, J. F. Watts and J. Wolstenholme, in Ewing’s Analytical Instrumentation Handbook 3 rd Ed., Dekker 2005.
XPS: Applications
 A modern application of XPS – study the nature of PEG as a surface
coating to prevent biofouling in biosensors
– Biofouling: the tendency of proteins to adsorb to silicon-based surfaces
 XPS can be used, with AFM, to observe the coating of PEG onto
silicon surfaces (PEG-silane coupling) - Increased C 1s C-O signal
indicates greater grafting density
S. Sharma, et al., “XPS and AFM analysis of antifouling PEG interfaces for microfabricated silicon biosensors”, Biosensors and Bioelectronics, 20 227–239 (2004).
XPS: Quantitative Applications
 Quantitative XPS is not as widely used as the qualitative
version of the technique.
 Variations in instrument parameters and set-up have
traditionally caused problems with reproducibility
 Using internal standards, XPS can achieve quantitative
accuracies of 3-10% in most cases (and getting better
every year, as more effort is put into this type of analysis)
AES and XPS: Combined Systems
 Dual Auger/XPS systems are very common, also
combined with a basic SEM
– Note - SAM = scanning Auger microprobe
 Auger is seen as complementary to XPS with generally

better lateral resolution
Both are extreme surface sensitive techniques:
– AES better elemental quantitative analysis
– XPS contains more chemical information
 Also, remember that Auger peaks are often seen in XPS
spectra (and are hence useful analytically) – they can be
identified by changing source, so that the X-ray peaks
shift (the Auger peaks do not).
Comparison of XPS, AES and Other Techniques
Characteristic
AES
XPS
SEM/X-ray EM
SIMS
Elemental range
Li and higher Z
Li and higher Z
Na and higher Z
All Z
Specificity
Good
Good
Good
Good
Quantification
With calibration
With calibration
With calibration
Correction
necessary
Detection limits
10-2 to 10-3
10-2 to 10-3
10-3 to 10-8
10-3 to 10-8
Lateral resolution
(um)
0.05
~1000
0.05
1
Depth resolution
(nm)
0.3-2.5
1-3
1000-50000
0.3-2
Organic samples
No
Yes
Yes*
Yes
Insulator samples
Yes*
Yes
Yes*
Yes*
Structural
information
Elemental
Elemental and
Chemical
Elemental
Elemental and
Chemical
Destructiveness
Low
Very Low
Medium
Medium
(atomic fraction)
* = yes, with compensation for the effects of sample charging
SIMS = secondary ion mass spectrometry, discussed in the “Ion and Particle Spectrometry” Lectures.
See Strobel and Heineman, Chemical Instrumentation, A Systematic Approach, Wiley, 1989, pg. 832.
XPS: New Applications
 A recent report in Chem. Commun. (2005) by Peter
Licence and co-workers describes the use of XPS to
study ionic liquids
 Normal liquids evaporate under ultrahigh vacuum (UHV),
ionic liquids do not (they have a vapor pressure of nearly
zero!)
 Why?
Ionic liquids have become important for
electrochemistry, catalysis, etc…
 See C&E News Oct. 31, 2005, pg 10.
Optional Homework Problems (for Study!)
Skoog, Holler and Nieman, Chapter 21.
Problems: 21-1, 21-2, 21-4, and 21-8.
Further Reading
Electron Microscopy and Electron Microprobe/X-ray Emission Analysis
1. J. I. Goldstein et al., Scanning Electron Microscopy and X-ray Microanalysis, 3rd Ed., Kluwer Academic,
2003.
2. J. J. Bozzola et al., Electron Microscopy: Principles and Techniques for Biologists, 2nd Ed., Jones and
Bartlett, 1998.
3. J. W. Edington, N. V Philips, Practical Electron Microscopy in Materials Science, Eindhoven, 1976.
Electron Microscopy and Electron Diffraction/Electron Energy Loss Spectroscopy
4. A. Engel and C. Colliex, “Application of scanning transmission electron microscopy to the study of
biological structure”, Current Biology 4, 403-411 (1993). (STEM and EELS)
5. W. Chiu and M. F. Schmid, “Electron crystallography of macromolecules”, Current Biology 4, 397-402
(1993). (ED and LEED)
6. W. Chiu, “What does electron cryomicroscopy provide that X-ray crystallography and NMR cannot?”,
Annu. Rev. Biophys. Biomol. Struct., 22, 233-255 (1993). (Electron Cryomicroscopy/Imaging)
7. L. Tang and J. E. Johnson, “Structural biology of viruses by the combination of electron cryomicroscopy
and X-ray crystallography”, 41, 11517-11524 (2002). (Electron Cryomicroscopy/Imaging)
Optical Microscopy
8. R. H. Webb, "Confocal optical microscopy“, Rep. Prog. Phys. 59, 427-471 (1996).
Force Microscopy:
9. R. J. Hamers, “Scanned probe microscopies in chemistry,” J. Phys. Chem., 100, 13103-13120 (1996).
Further Reading
Surface Spectrometric Methods (XPS and AES)
10. T. L. Barr, Modern XPS, Boca Raton: CRC Press (1994).
11. M. Thompson, M. D. Baker, A. Christie, and J. F. Tyson, Auger Electron Spectroscopy, New York: Wiley
(1985).
12. N. H. Turner, “X-ray Photoelectron and Auger Electron Spectroscopy”, Applied Spectroscopy Reviews,
35 (3), 203-254 (2000).