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SEMI F60-0301 © SEMI 2001 7 Figure A1-2 Example of ESCA spectrum of the oxide passive surfac e of sta inless steel A1 -1.5 Illu minated Volume and Surface S ensitivity — The incident x-ray beam penetrates well bel ow the…

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SEMI F60-0301 © SEMI 2001 6
APPENDIX 1
DISCUSSION OF EFFECTS OF DEPTH OF ANALYSIS ON DEPTH
PROFILE ANALYSIS BY ELECTRON SPECTROSCOPY FOR
CHEMICAL ANALYSIS OF PASSIVATED STAINLESS STEEL
NOTE: This appendix is being balloted as an official part of SEMI F## by full letter ballot procedure, but the recommendation in
this appendix are optional and are not required to conform to this standard. This appendix is derived from presentations made to
the SEMI Surface Analysis Task Force.
A1-1.1 The purpose of this appendix is to describe the
ESCA (sometimes referred to as X-Ray Photoelectron
Spectroscopy (XPS)) technique and explain the
interpretation of the depth profile analysis with respect
to the structure and composition of the passive oxide
layer on stainless steel. The effect of the depth of
analysis on the measured chromium to iron ratio, the
oxide thickness, and the depth of enrichment is
discussed. The discussion applies to depth profile
analyses performed by Auger electron spectroscopy,
also.
A1-1.2 ESCA — The ESCA technique makes use of an
X-ray beam for a probe. The surface of interest is
illuminated with X-rays, typically Al kα radiation,
which has a well-defined energy of 1486.6 eV. When
an x-ray photon and an electron interact, the electron
adsorbs all of the energy of the photon, 1486.5 eV; it
cannot react with it partially. The electron then has
sufficient energy to leave the atom and move into the
matrix. The electron’s energy in the matrix is 1486.6
eV minus its binding energy to the atom from which it
escaped. Its binding energy is indicative of the element
or compound from which it came and the orbital from
which it was removed. Such an electron is termed a
“photoelectron”.
A1-1.3 In a typical ESCA instrument, a
monochromatic beam of x-rays is focused at a point of
interest on a surface. A cloud of electrons is generated
at the surface by the x-rays. This electron cloud is
made up of secondary electrons, Auger electrons and
photoelectrons. (NOTE: The generation of Auger
electrons is a byproduct of the process. If the
photoelectron was an inner shell electron, when it
leaves the atom it creates an ion in an excited state.
Once the ion is created it can de-excite via the Auger
process. The presence of Auger electrons in an ESCA
survey can be used to determine what elements were
present.) The mechanisms of ESCA photoelectron and
Auger electron generation are illustrated in Figure A1-
1.
Figure A1-1
Schematic representation of the ESCA
photoelectron and Auger electron generation
processes.
A1-1.4 An electron lens is focused at the same point as
the x-ray beam and a portion of the electron cloud
enters the lens. From the lens, the electrons enter an
electron analyzer that measures the number of electrons
as a function of their kinetic energy. The kinetic energy
is subtracted from the incident x-ray photon energy, and
the resulting binding energy is then plotted as the x-axis
with the number of electrons at each energy on the y-
axis. An example is shown in Figure A1-2. The
binding energy can be indicative not only of the
element from which the electron came but also the
chemical bonding state of the element.
SEMI F60-0301 © SEMI 20017
Figure A1-2
Example of ESCA spectrum of the oxide passive
surface of stainless steel
A1-1.5 Illuminated Volume and Surface Sensitivity —
The incident x-ray beam penetrates well below the
surface. Photoelectrons and Auger electrons are
generated all through this illuminated volume. The
inelastic mean free path (IMFP) of an electron in a solid
is the mean distance through the solid that an electron
can travel before losing some of its energy (suffering an
inelastic interaction). The IMFP is a function of the
electron’s energy and the matrix through which it is
traveling. Electrons with an energy of approximately 1
keV have an IMFP of about 10 Å. That means that a 1
keV electron will travel, on average, 10 Å before
interacting with the matrix and giving up some of its
energy. In other words it will be scattered to lower
energy and will not be part of the photoelectron or
Auger electron line for its element. The IMFP is a
statistical parameter and an electron may travel several
IMFPs before having a collision. The “several IMFPs”
is called the escape depth of the electron, or the depth
of analysis, as shown in Figure A1-3. Although ESCA
and Auger are considered surface analytical tools, their
depth of analysis is on the order of 50 Å, or about one
to two times the depth of the oxide film found on a
typical passivated stainless steel surface.
Figure A1-3
The electron escape depth of low energy electrons in
some metals.
A1-1.6 Depth of Analysis Function — There has been
some discussion with respect to the “Depth of Analysis
Function”. There is one school that believes that it is 5
IMFPs and another that uses 3 IMFPs. These two
functions will be referred to as 5λ and 3λ respectively.
It will be seen that it makes little difference which
function is used even though the depth of the oxide
films is of the order of the functions. These functions
are seen in Figure A1-4, which shows the proportion of
the total signal generated from each depth below the
surface. The y-axis is in decimal and the x-axis is in
angstroms from the surface. The first point of the 5λ
curve is 0.16 at the 1
st
angstrom level. That means that
16% of the signal comes from the 1
st
angstrom, 14%
from the 2
nd
angstrom level and so on down to 1% from
the 25
th
angstrom level. Thus elements that do not
appear on the surface or in the first 24 angstroms in
from the surface, but do exist at the 25 Å level will be
detected.
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x 10
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FCEPSSe1.spe
Binding Energy (eV)
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-O KLL
-O KL
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-Fe LMM
-Fe2p3
-Cr2p3
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-N1s
-C1s
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-Fe3s
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5 Lambda
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0.18
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
Angstroms
3 Lambda
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0.18
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
Angstroms
Figure A1-4
These two graphs represent the “Depth of Analysis”
that will be seen when analyzing 1 keV electrons.
The top curve is the 5λ
λλ
λ curve and bottom the 3λ
λλ
λ.
A1-1.7 Modeling — Figure A1-5 is the composition
profile of a “perfect” oxide. It represents 25 Å of pure
Cr
2
O
3
on an atomically flat 316L stainless steel surface,
with no surface or interfacial contamination present.
Figures A1-6a and A1-6b are the theoretical depth
profiles derived by assuming a depth of analysis of 5λ
and 3λ respectively. The first factor to note is that Fe,
Ni and Mo appear most or all of the way through the
depth profile of the oxide due to the contribution of the
metal substrate to the detected signal, the “depth of
analysis” effect as introduced above. 1 keV electrons
can escape from as deep as 25 Å. This must be
appreciated when interpreting these depth profiles.
Theoretical
0
10
20
30
40
50
60
70
80
0 5 10 15 20 25 30 35 40 45
Angstroms
Atomic Concentration
Cr Fe Ni Mo O C
Figure A1-5
Composition versus depth of a pure Cr
2
O
3
layer on
stainless steel.
Convoluted 5 Lambda
0
10
20
30
40
50
60
70
80
90
100
0 5 10 15 20 25 30 35 40 45
Atomic Concentration
Cr Fe Ni Mo O C Cr:Fe
Figure A1-6a
Theoretical depth profile of Figure A1-5 assuming
5λ
λλ
λ depth of analysis.
Convoluted 3 Lambda
0
10
20
30
40
50
60
70
80
90
100
0 5 10 15 20 25 30 35 40 45
Angstroms
Atomic Concentration
0
100
200
300
400
500
600
700
Cr Fe Ni Mo O C Cr:Fe
Figure A1-6b
Theoretical depth profile of Figure A1-5 assuming
3λ
λλ
λ depth of analysis.
A1-1.8 The second factor to notice is the thickness of
the oxide. Historically, the FWHM of the oxygen
profile has been used as the measure of oxide thickness.