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SEMI F72-1102 © SEMI 2002 6 in Figure A1-2. Although Auger an d ESCA are considered surface analytical tools, t heir depth of analysis is on the order of 50 angstroms, or about one to three times the dept h of the oxide …

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SEMI F72-1102 © SEMI 2002 5
APPENDIX 1
DISCUSSION OF EFFECTS OF DEPTH OF ANALYSIS ON DEPTH
PROFILE ANALYSIS BY AUGER ELECTRON SPECTROSCOPY FOR
SURFACE CHEMISTRY ANALYSIS OF PASSIVATED STAINLESS
STEEL
NOTE: The material in this appendix is an official part of SEMI F72 and was approved by full letter ballot procedures. This
appendix was derived from presentations made to the Stainless Steel and Surface Analysis Workshop
2
.
2 Stainless Steel and Surface Analysis Workshop, March, 1999; Interpratation of Depth Profile Analysis Data, Dave Harris, Charles Evans and
Associates.
A1-1
A1-1.1 The purpose of this appendix is to describe the
Auger Electron Spectroscopy (AES) 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 also applies to depth
profile analyses performed by ESCA (Electron
Spectroscopy for Chemical Analysis, sometimes
referred to as X-Ray Photoelectron Spectroscopy, or
XPS).
A1-1.2 Auger — The mechanism of Auger electron
generation is illustrated in Figure A1-1. The energy of
the Auger electron is determined by the energies of the
electron energy levels participating in the generation
process and are unique for each chemical element.
Thus the elements present in the analyzed region may
be determined by analysis of the energies of emitted
Auger electrons.
K or 1s
Auger Electron
L
2
,
3
or 2p
L
1
or 2s
Electron
Energy
Levels
Electron Hole
1
.
Electron from higher energy
level drops into electron hole
created by incident electron or
photon
2. Energy released by 1
“kicks out” Auger Electron
from hi
g
her ener
gy
level
Figure A1-1
Schematic Representation of Auger Electron
Generation Process
A1-1.3 The Auger technique makes use of an electron
beam for primary excitation, typically of energy in the
range 2 to 5 keV. The electron beam may by defocused
to analyze a relatively large area of the surface, of the
order of micrometers, or focussed and rastered to
“map” the distribution of elements on the surface. An
electron energy analyzer is focused at the same point as
the incident electron beam, and the Auger electrons that
escape the surface are detected and analyzed. The
analysis is commonly presented as the derivative of the
electron signal intensity N as a function of energy E,
dN(E)/dE, of the spectrum to enhance the visibility of
the small Auger electron peaks relative to the
background. An example of an Auger electron
spectroscopy spectrum of a stainless steel surface is
shown in Figure 1 of this test method.
A1-2 Illuminated Volume and Surface
Sensitivity
A1-2.1 The incident electron beam penetrates well
below the surface. 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 typical of Auger and ESCA
electrons, approximately 1 keV, have an IMFP of about
10 angstroms. That means that a 1 keV electron will
travel, on average, 10 angstroms 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
SEMI F72-1102 © SEMI 2002 6
in Figure A1-2. Although Auger and ESCA are
considered surface analytical tools, their depth of
analysis is on the order of 50 angstroms, or about one to
three times the depth of the oxide film found on a
typical passivated stainless steel surface.
Figure A1-2
The Electron Escape Depth of Low Energy
Electrons in Some Metals
A1-3 Depth of Analysis Function
A1-3.1 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 A1-3, 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
angstrom level will be detected.
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 10111213141516171819202122232425
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 10111213141516171819202122232425
Angstroms
Figure A1-3
These Two Graphs Represent the “Depth of
Analysis” That Will be Seen When Analyzing 1 keV
Electrons. The Top Curve is the 5λ Curve and
Bottom the 3 λ.
A1-4 Modeling
A1-4.1 Figure A1-4 is the composition profile of a
“perfect” oxide. It represents 25 angstroms of pure
Cr
2
O
3
on an atomically flat 316L stainless steel surface,
with no surface or interfacial contamination present.
Figures A1-5a and b 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 angstroms. This must be
appreciated when interpreting these depth profiles.
SEMI F72-1102 © SEMI 2002 7
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 A1-4
Composition Versus Depth of a Pure Cr
2
O
3
Layer
on Stainless Steel
A1-4.2 The second factor to notice is the thickness of
the oxide. Historically, the FWHM (Full Width Half
Maximum; the width of a peak measured at half its
maximum height) of the oxygen profile has been used
as the measure of oxide thickness. It is apparent in
Figures A1-4 and A1-5 that this underestimates the
oxide thickness by 15 to 20 %.
A1-4.3 It should also be noted that the Cr:Fe ratio
measured from Figures A1-4 and A1-5 has its
maximum value at the initial surface, decreasing as the
depth profile progresses, whereas the actual Cr:Fe ratio
in Figure A1-4 is infinite down to 25 angstroms. Most
actual depth profiles exhibit a maximum of the Cr:Fe
ratio at some depth below the initial surface, referred to
as the depth of maximum enrichment. This is a
consequence of variation in the actual composition of
the oxide, possibly having a higher concentration of Fe
near the surface, a phenomenon commonly termed a
“detached iron oxide layer”, or of contamination on the
surface, generally hydrocarbons. These are discussed
in the next section
A1-5 Effects of Detached Iron Oxide Layer
and Surface Contamination
A1-5.1 The model composition profile of Figure A1-6
shows 3 angstroms of pure Fe
2
O
3
over 22 angstroms of
pure Cr
2
O
3
on 316L stainless steel. This is a model of a
detached iron oxide layer. Figure A1-7 is the
theoretical depth profile of this model derived assuming
a depth of analysis of 5λ. Note that the initial Cr value
in the profile is higher than the Fe value, even though
the surface is pure Fe
2
O
3
. This is a consequence of the
depth of analysis detecting the Cr from levels beneath
the surface. The Cr:Fe ratio for this model has its
maximum value at 3 angstroms below the initial
surface. The theoretical depth profile using a depth of
analysis of 3λ is very similar
Convoluted 5 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
20
40
60
80
100
120
140
Cr Fe Ni Mo O C Cr:Fe
Figure A1-5a
Theoretical Depth Profile of Figure 5 Assuming 5λ
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 A1-5b
Theoretical Depth Profile of Figure 5 Assuming 3λ
Depth of Analysis
A1-6 Effects of Hydrocarbon Contamination
A1-6.1 The model composition profile of Figure A1-8
shows 3 angstroms of pure carbon over 22 angstroms of
pure Cr
2
O
3
on 316L stainless steel, representing an
idealized model of the adsorbed hydrocarbon
contamination generally found on stainless steel
surfaces exposed to the atmosphere. Figure A1-9 is the
theoretical depth profile of this model assuming a depth
of analysis of 5λ. In this case the O, Cr and Fe atomic
concentration values are reduced by the presence of the
carbon until the depth profiling proceeds past the
carbon, but they have the same relative values (ie: same
Cr:Fe ratio) versus depth as derived in the
uncontaminated model. The maximum of the Cr:Fe
ratio is seen to be at the initial surface. Although the
actual oxide thickness is less in this model, the FWHM
measure of the oxide thickness from the depth profile is
the same due to the presence of the carbon layer. Note