Transcript Metode şi tehnici de studiu a suprafeţelor

Methods and Tehniques in
Surface Science
Prof. Dumitru LUCA
“Alexandru Ion Cuza” University, Iasi, Romania
Outline
• Auger Electron Spectroscopy (AES) – short historic and physical
background,
• How AES measurements are performed,
• Information derived from Auger spectra:
• Methodology
• Data analysis
• Experimental considerations
Short historic of the Auger spectroscopy
• Pierre Auger, in 1925 observed (at first in the cloud chamber, then in
photographic plates) the occurrence of electrons with precisely determined
energies. These electrons have been later named Auger electrons) may serve
to identify their parent atoms.
1953 J. J. Lander – the idea of using the Auger electrons in surface analysis..
Pierre Auger
• The AES has been implemented as an analytic tool in 1967 (Larry Harris), after
increasing the method sensitivity by using differential spectra to discriminate the
tiny Auger peaks in the electronic spectra.
• 1968 – Auger spectrometer with CMA in modern configuration.
• Beam current as low as 1 nA
• Probing depth: 0.5 - 10 nm, and < 10 nm lateral resolution(!!!)
Nowadays:
• Covering a wide range of elements which can be detected (except for H and
He).
• High sensitivity: 100 ppm for most of the elements.
Auger spectra. Expanation of the Auger effect in free atoms
1. The occurrence of an electron vacancy in a
core level (K, L) (core level), by incident
electron, X – ray photons, or ions.
Little information is available for the energy of
the primary and ejected electrons, due to
complex cascade of successive collisions
with the matrix. Therefore the complex
picture in Fig. 1
2. The vacancy is filled by a second electron
coming from an upper energy level.
3. The energy of the emitted electron can serve
for:
- the emission of a X photon (Z > 30)
- ejection of a 3-rd (Auger) electron via a
non-radiative process.
4. The net result: an atom in a double-ionized
state + 2 emitted electrons emisi (the K core
level electron and the Auger electron).
Auger transition nomenclature
Auger electron
Labelling:
KL1L2
AVV
Incident
electron
The 3 letters specify the energy levels implied in the
process of emission of the Auger electrons
KE = EK-EL1-E*L2- j,
EK, EL1, and EL2 – the energy levels mentioned in the labeling (generally
different from the neutral atom, due to the presence of electron vacancies).
Factors influencing the Auger peak area
3keV incident
1. Ionisation cross section
electron beam
LMM
KLL
10 keV incident
electron beam
MNN
Factors influencing the Auger peak area
(cont’d)
2. The Auger yield
• Competition between the Auger
process
and
the
X-ray
fluorescence.
• The probability of occurrence of
the Auger electrons increases
with the decreasing of the
differences between the energy
levels involved in such transitions.
3. Backscattering
Auger electron spectra
1. Direct spectrum
Electrons in the Auger spectra have energy
values between 280 eV (KLL, Zn) si 2100
eV (S).
Example:
The KL1L2 transtion of Si occurs at the
energy of 1600 eV (since the difference
between the energies of L1 and K levels for
Si is1690 eV, while the difference between
L1 and L2 levels is 90 eV).
Auger spectrum
Kinetic Energy
The tiny Auger peaks are difficult to process.
They become visible after 10 x magnification.
After L1 → K de-excitation either an X-ray
photon (Ka of Si), or an Auger electron may
be emitted having energy of approx. 1690 –
90 = 1600 eV.
Auger electron spectra
2. Differential spectrum
More features of the spectra
occur clearly in the dN(E)/dE vs.
K.E.
Even more features occur in the
d[E*N(E)]/dE vs. KE plots.
This is the most used kind of Auger
electron spectrum.
Auger spectra of light elements
(the y-axis differs for different elements)
AES sensitivity
• Electrons emitted in the solid will “escape” in vacuum if they are not inelastically
scattered.
• The scattered electrons vill have energies less energy than Auger electrons, thus they
will occur in the “tail” of the spectrum, towards smaller KE values, along with secondary
electrons.
• Many electrons will fully lose their energy via inelatic collisions in the solid.
• Therefore, only Auger electrons originating in the surface region (which did not
experience inelastic scattering) will be collected by the analyser.
I out  I 0 e

d

MNN

3
0


0
Background
Auger electrons
x

e dx
x

e dx

1  e3
 0.95
1
95% of the electrons leaving the surface
originate in a layer 3  depth.
Experimental arrangement
Instrumentation
Electron gun
Electron
detector
Cylindrical mirror
analyser (CMA)
• UHV chamber
• Specimen import unit
• Electronics
• Computer and software
Specimen
Scanning Auger Microscopy (SAM)
AES
Auger Electron Spectroscopy
SAM
Scanning Auger Microscopy: Same instrument can provide SEM
imaging, Auger spectra and chemical Auger mapping.
Applications
• 1keV incident electron beam →
penetration depth of about 15 Å.
Focussing &
scanning system of
the incident e-beam
• Verification of surface
contamination freshly prepared in
UHV.
• Investigation of the thin film growth
process + elemental analysis.
• Depth profiling of concentration of
chemical elements.
Specimen
Qualitative analysis
Elemental identification procedure
1.
First, the main Auger peak positions are
identified.
2.
These values are correlated with the
listed values in the Auger spectra book
or standard tables. The main chemical
elements are thus identified.
3.
The identified element and transition
are labelled in the spectrum (close to
the negative jump in the differential
spectrum).
4.
The procedure is repeated for so-far
unidentified peaks.
E0 = 3keV
The Auger spectrum of a sample under
investigation
Qualitative analysis
Example:
From the differential AES spectrum
Ni, Fe and Cr have been identified.
Ni
Fe
Cr
Information concerning chemical composition
• Peak shape and the energy values,
corresponding to maxima contain
information on the nature of the
environment, due to addition relaxation
effects during the Auger process
• A full theoretical model is difficult to
construct.
• In practice, Auger spectra of standard
samples are used and the results are
drawn from spectra comparison.
SAM
SEM surface image
SAM image:
red =Al; blue = F; magenta = Al + F
Al+F+O
Red = Al; green = O
red =Al; blue = F; green = O
SEM and Auger images of an aluminium oxide surface, in absence and presence of
fluorine contramination.
Quntitative analysis
dEN(E)/dE vs. E
1. Measuring the peak-to-peak height
N(E) vs. E
2. Measuring the peak area
(after background subtraction)
Factors affecting the peak intensity
For a homogeneous sample, the Auger peak intensity is given by:
I i  I P  Ni  i   i  (1  r )    cos  F  T  D  R
Ii: Intensity of the detected current, due to the ABC Auger transition of the i element,
IP: Incident beam current,
Ni: Concentration of the element i in the surafce,
i: Ionization cross-section on the A-level of the element i by the electrons from the incident beam,
i: Probability of the Auger ABC transition of the element i,
r: ionization cross-section on the A-level of the element i by the electrons scattered in previous
processes,
: mean free path for inelastic collisions,
: incidence angle of the primary beam,
F: Correction factor, dependent on the entrance solid angle in the analyzer,
T: Transfer function of the analyzer,
D: Detection efficiency,
R: Roughness factor of the surface.
Remarks
1. Deriving Ni from the previous equation is rather difficult, due to large number of implied parameters,
2. In applications, empirical methods are used, which leave from: (a) utilization of standard
specimens; (b) utilization of sensitivity factors.
Quantitative analysis using standard specimens
Advantages:
No need to know “obscure” physical quantities:
• ionization cross-section, i on the A-level of the
element i by the electrons in the primary beam,
• the Auger yield,
• backscattering cross-section and electron escape
depth values.
Drawbacks:
• Necessity to prepare standard samples,
• Valid only for homogeneous samples,
• Quite low accuracy.
Quantitative analysis using sensitivity factors
• Measurements are done under the same conditions on standard samples to cancel the
correction factors, associated to the set-up particularities:
I pur
Ix

N x  S x N pur  S pur
Sensitivity factors, Si, have been measured for certain energy values of the electrons in the
primary beam. They are tabulated for all the chemical elements.
• The atomic concentration of the elment a in the sample with N elements can be derived using the
equation:
Xa 
Na

 Ni
i 1.. N
I a / Sa
 I i / Si
()
i 1.. N
The method is semi-quantitative, since the back-scattering effects and the escape depth of
the electrons are negliged.
Drawbacks of empirical methods
They do not include the so-called matrix effect of the sample:
the inelastic mean free path (),
the back-scattering factor (r),
chemical effects on the peak shape in the Auger spectra,
effect of the surface roughness.
All these drawbacks result in errors of about 15%. The errors can be diminished to
1% by using standard samples with the same matrix, to derive Si.
Fe
Example
Cr
Ni
Peak-to-peak height:
Fe
Cr @ 529eV:
Fe @ 703eV:
Ni @ 848eV:
4.7
10.1
1.5
Si
0.32
0.20
0.27