ASTM E1127-91(1997)

NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
Designation: E 1127 – 91 (Reapproved 1997)
Standard Guide for
Depth Profiling in Auger Electron Spectroscopy
This standard is issued under the fixed designation E 1127; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope 4.3 In mechanical cratering, a spherical or cylindrical crater
is created in the surface using a rotating ball or wheel. The
1.1 This guide covers procedures used for depth profiling in
sloping sides of the crater are used to improve the depth
Auger electron spectroscopy.
resolution as in angle lapping.
1.2 Guidelines are given for depth profiling by the follow-
4.4 In nondestructive techniques, different methods of vary-
ing:
ing the electron information depth are involved.
Section
Ion Sputtering 6
5. Significance and Use
Angle Lapping and Cross-Sectioning 7
Mechanical Cratering 8
5.1 Auger electron spectroscopy yields information con-
Nondestructive Depth Profiling 9
cerning the chemical and physical state of a solid surface in the
1.3 This standard does not purport to address all of the
near surface region. Nondestructive depth profiling is limited
safety problems, if any, associated with its use. It is the
to this near surface region.
responsibility of the user of this standard to establish appro-
5.2 Ion sputtering is primarily used for depths of less than
priate safety and health practices and determine the applica-
the order of 1 μm.
bility of regulatory limitations prior to use.
5.3 Angle lapping or mechanical cratering is primarily used
for depths greater than the order of 1 μm.
2. Referenced Documents
5.4 The choice of depth profiling methods for investigating
2.1 ASTM Standards:
an interface depends on surface roughness, interface rough-
E 673 Terminology Relating to Surface Analysis 3
ness, and film thickness (1).
E 684 Practice for Approximate Determination of Current
Density of Large-Diameter Ion Beams for Sputter Depth 6. Ion Sputtering
Profiling of Solid Surfaces
6.1 First introduce the specimen into a vacuum chamber
E 827 Practice for Elemental Identification by Auger Elec-
equipped with an Auger analyzer and an ion sputtering gun.
tron Spectroscopy
Align the ion beam using a sputtering target or a Faraday cup,
E 996 Practice for Reporting Data in Auger Electron Spec-
paying careful attention to the relative spot size of the electron
troscopy and X-Ray Photoelectron Spectroscopy
beam, ion beam, and Faraday cup and their respective orien-
tations to ensure accurate convergence of the two beams at the
3. Terminology
specimen surface.
3.1 Definitions:
6.1.1 Place the specimen in front of the Auger analyzer and
3.1.1 For definitions of terms used in this guide, refer to
direct the ion gun towards the analysis area. If the ion beam is
Terminology E 673.
not normal to the specimen surface then possible shadowing of
the analysis area from the ion beam must be considered.
4. Summary of Guide
6.2 Choose the elements to be investigated from previous
4.1 In ion sputtering, the surface layers are removed by ion
experience or from an initial Auger electron spectrum or an
bombardment in conjunction with Auger analysis.
energy-dispersive X-ray spectrum since the latter spectrum can
4.2 In angle lapping, the surface is lapped or polished at a
reveal additional elements present at depths greater than those
small angle to improve the depth resolution as compared to a
that contribute to the Auger electron spectrum (2). Select a
cross section.
specific transition for each element. During the depth profiling,
record the peak-to-peak heights for Auger derivative data, or
peak heights or peak areas for N(E) data. The data may be
This guide is under the jurisdiction of ASTM Committee E-42 on Surface
Analysis and is the direct responsibility of Subcommittee E42.03 on Auger Electron
Spectroscopy and XPS.
Current edition approved Jan. 25, 1991. Published May 1991. Originally
published as E 1127 – 86. Last previous edition E 1127 – 86. The boldface numbers in parentheses refer to the list of references at the end of
Annual Book of ASTM Standards, Vol 03.06. this guide.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
E 1127 – 91 (1997)
gathered during continuous sputtering or between timed sputter modelling of these and other ion-induced phenomena has been
segments. Results may vary between the two techniques. extensively studied and has provided new insights into this
field (21 and 22).
6.2.1 One source of their difference is due to the presence of
6.8.1 It should be determined for each specimen if compo-
ion-induced electrons during continuous sputter depth profil-
sitional changes or other sputter effects are likely to occur. It
ing, especially at low-electron kinetic energies, that can be-
may be possible to minimize these effects in some instances by
come comparable in intensity to the electrons induced by the
adjusting the sputtering parameters.
probing incident electron beam. Unless one or the other of the
6.9 Ion guns used in Auger analysis are normally self-
excitation beams is modulated and detected synchronously
contained units capable of producing a focused beam of ions.
these two types of emitted electrons are difficult to distinguish.
The specimen is not used as an anode for the gun. Many ion
These ion-induced electrons usually form a featureless back-
guns are able to raster the ion beam. A rastered ion beam will
ground that rises steeply as their kinetic energy decreases, but
produce a more uniform ion current distribution on the
sometimes ion-induced Auger peaks might be present whose
specimen surface in the region of analysis.
lineshape may be different from those produced by the electron
6.10 If the ion gun is equipped with a restricted orifice for
beam (3). As a result, care must be taken during continuous
the sputter gas flow, then the vacuum pumps may be left on
sputtering to ensure reliable results. Another source of differ-
during sputtering, removing most of the sputtered gases. If not,
ence is due to the buildup of adsorbed species during the data
then the chamber must be back filled with gas and provisions
acquisition time in the discontinuous sputter depth profile
for removing the sputtered active gases must be considered.
mode (4). If portions of the ion-eroded surface expose very
Titanium sublimation is effective in removing these gases.
reactive phases, then Auger peaks due to adsorbed species, for
6.11 Noble gas ions are normally used in sputtering and the
example, oxygen or carbon, or both, will appear in the spectra
most commonly used gas is argon. Xenon is occasionally used
and mask the actual depth distribution.
with high beam energies when rapid sputtering is needed.
6.2.2 It is advisable when analyzing an unknown specimen
Active gases such as oxygen and metal ions are used in special
to periodically examine survey scans to detect any new
circumstances.
elements that were not present in the initial survey scan and to
6.11.1 Ion energies commonly used for depth profiling
determine if any of the Auger peaks have been displaced
using noble gases are in the range from 1 to 5 keV where lower
outside of their analysis windows (5).
ion energies are usually preferred for improved depth resolu-
6.3 Crater-edge profiling of the sputter-formed crater by
tion. Higher ion energies usually can be obtained with higher
using Auger line scans is a technique similar to the analysis of
ion currents and less preferential sputtering.
the mechanically formed craters in Section 8 (6). Forming the
6.11.2 Ion beam current density can be measured by a
crater by sputtering may introduce the additional complications
Faraday cup or by following Practice E 684.
of ion-induced damage and asymmetric crater dimensions.
6.11.3 The sputter rate is needed to calibrate the depth scale
6.4 If specimen rotation is used to reduce ion-induced when depth profiling using ion sputtering. Several reference
roughness, then the rotational speed, rotation axis runout
standards are available for this purpose. One reference material
relative to ion beam sputtered area or wobble and data consists of 30 and 100-nm thick tantalum pentoxide films
acquisition rate should be reported (7 and 8).
(23). Another reference material is an alternating nickel and
chromium thin film structure; each layer is nominally 50-nm
6.5 Identify the elements in the survey scans using Practice
thick.
E 827.
6.6 The Auger data and the sputtering conditions should be
7. Angle Lapping and Cross-Sectioning
reported as described in Practice E 996.
7.1 In cross-sectioning, polish the specimen perpendicular
6.7 There is extensive information available in the literature
to the interface, while in angle lapping, polish the specimen at
on the effects of ion bombardment on solid surfaces (9-14).
an angle to increase the depth resolution as shown in Fig. 1
6.8 Special care must be exercised whenever specimen
(24). Polishing usually includes the use of silicon carbide
temperature changes are present because effects due to surface
papers, diamond paste, and alumina. Use progressively finer
diffusion, surface segregation or diffusion limited bulk pro-
polishing particles to obtain the desired surface finish. Possible
cesses such as point defect migration can occur and dramati-
limitations of the techniques include smearing of material
cally alter the specimen composition, even over depths larger
across the interface, surface roughness, and the electron probe
than the ion beam penetration depth which is typically a few
diameter limiting the spatial resolution.
nanometers (15 and 16). The concept of preferential sputtering
7.2 In angle lapping mount the specimen on a flat gage
in multielement, single-phase specimens has altered signifi-
block and measure the angle with a collimator. The accuracy
cantly so that chemical effects such as surface segregation are
depends on the flatness of the specimen. In practice an angle of
considered to be at least as important as physical effects such
0.1° can be accurately measured.
as mass differences in the evolution of the near surface
composition during sputter depth profiling (17-20). Since the
Available from the National Physical Laboratory, Teddington, Middlesex,
probing depths in Auger electron spectroscopy are usually
England. Listed as Certified Reference Material NPL No. S7B83, BCR No. 261.
smaller than the ion-penetration depth these effects are very
Available from the National Institute of Standards and Technology, U.S.
important in any interpretation of Auger signal intensity in
Department of Commerce, Gaithersburg, MD 20899. Listed as NIST Standard
terms of composition during ion-beam profiling. Computer Reference Material 2135.
NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
E 1127 – 91 (1997)
NOTE 1—In practice, the angle u is much smaller than shown, being of
the order of 1°
FIG. 2 Cross Section of Specimen After Ball-Cratering Using a
FIG. 1 Cross Section of Angle-Lapped Specimen
Sphere of Radius, R, to a depth, d
7.3 The depth, d, is given by the following equation:
d 5 Y tan u (1)
where:
D = the diameter of the crater,
where (in Fig. 1) u is the lapped angle and Y is the distance
R = the radius of the ball, and
from the edge.
R =>> D/2.
7.4 The depth resolution, Dd, is given by the following
8.1.4 The Auger analysis is the same as described in 7.5 and
equation:
7.6.
Dd5DY tan u (2)
8.1.5 The depth at any point in the analysis, Z, is given by
where DY includes the electron beam diameter and uncer-
the following equation (1):
tainties in position that may be due to errors in specimen or
2 2 2 1/2 2 2 1/2
Z 5 ~R 2 x 1 Dx 2 D /4! 2 ~R 2 D /4! (4)
electron beam positioning.
7.5 Auger analysis can include line scans and point analysis
where x is the lateral distance from the crater edge. The
along the lapped surface. Perform the analysis by either
depth may also be given by the approximation as follows:
moving the specimen using micrometer adjustments or by
Z 5 x~D 2 x!/2R (5)
electronically moving the electron beam.
7.6 Ion sputtering (Section 6) is often used in conjunction
8.1.6 The depth resolution, DZ, is given by the following
with angle lapping to remove contaminants and to investigate
equation:
interfaces beneath the lapped surfaces.
DZ5Dx tan u (6)
7.7 Consideration should be given if specimen mounting
where Dx includes the electron beam diameter and other
methods, for example, plastic embedding media, are used
uncertainties in lateral position and u is the taper angle. In
which may employ high vapor pressure materials. Out-gassing
contrast to angle lapping (Section 7), the taper angle, which is
of the media as well as trapped gases between the media and
defined as the angle between the surface and the tangent to the
the specimen may require complete removal of the mounting
crater, varies in value along the crater wall. Its value is given
materials prior to analysis.
by the following equation:
8. Mechanical Cratering
sinu5 ~0.5D 2 x!2/R (7)
8.1 Ball Cratering:
The best resolution is when u is the smallest at the crater
8.1.1 First mount the specimen in a device where a rotating
bottom.
steel ball can be placed against its surface. Commercial
8.2 Radial Sectioning—A technique similar to ball cratering
apparatus is available that uses a rotating shaft with a notch that
that uses a cylindrical grinding tool instead of a spherical one
holds the ball and spins it. The rotational speed and the force
(26).
against the specimen can be adjusted (25).
8.1.2 Coat the ball with an abrasive material to improve the
9. Nondestructive Depth Profiling
cratering rate. In practice diamond paste is used with a particle
size of 0.1 to 1 μm. The larger particle sizes will give the most
9.1 Methods for nondestructive depth profiling with Auger
rapid cratering rates and the finer particle sizes will give the
electron spectroscopy are based upon varying the effective
smoothest crater wall surface. The coarser pastes can be used
electron escape depth from the specimen and are limited to
first to form the crater and the fine pastes can be used to smooth
characterizing the outermost 2 to 5 nm.
the crater wall. As with cross-sectioning and angle lapping,
9.2
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