U.S. patent number 3,760,180 [Application Number 05/297,017] was granted by the patent office on 1973-09-18 for electron-beam micro-analyzer with an auger electron detector.
This patent grant is currently assigned to Siemens Aktiengesellschaft. Invention is credited to Ulrich Weber.
United States Patent |
3,760,180 |
Weber |
September 18, 1973 |
ELECTRON-BEAM MICRO-ANALYZER WITH AN AUGER ELECTRON DETECTOR
Abstract
An electron-beam micro-analyzer for investigating solid test
specimens and radiation penetrable test specimens is disclosed. The
micro-analyzer has an electron-beam generator for directing a
focussed electron beam unto the test specimen to release Auger
electrons therefrom. An electron spectrometer then separates the
released Auger electrons according to the respective kinetic
energies thereof and an electron detector detects the Auger
electrons of specified energy separated in the spectrometer. The
electron-beam generator has a field-emission point cathode having a
small radius of curvature and an anode having an opening for
passing the electron beam therethrough. A voltage supply applys a
voltage to develop an electric field between the anode and the
point cathode of sufficient strength to excite the cathode to
electron field emission. A deceleration lens disposed intermediate
the anode and the test specimen reduces the velocity of the
electrons of the electron beam passing from the opening of the
anode.
Inventors: |
Weber; Ulrich (Karlsruhe,
DT) |
Assignee: |
Siemens Aktiengesellschaft
(Munich, DT)
|
Family
ID: |
5822311 |
Appl.
No.: |
05/297,017 |
Filed: |
October 12, 1972 |
Foreign Application Priority Data
|
|
|
|
|
Oct 14, 1971 [DT] |
|
|
P 21 51 167.1 |
|
Current U.S.
Class: |
250/305; 850/12;
850/16; 250/311 |
Current CPC
Class: |
H01J
37/252 (20130101); H01J 37/256 (20130101); H01J
37/073 (20130101); G01N 23/227 (20130101); H01J
49/488 (20130101) |
Current International
Class: |
H01J
37/252 (20060101); H01J 49/00 (20060101); H01J
37/073 (20060101); H01J 37/06 (20060101); H01J
37/256 (20060101); H01J 49/48 (20060101); G01N
23/22 (20060101); G01N 23/227 (20060101); G01n
023/20 (); G01t 001/36 () |
Field of
Search: |
;250/49.5A,49.5PE,49.5AE |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Lindquist; William F.
Claims
What is claimed is:
1. An electron-beam micro-analyzer for investigating solid test
specimens and radiation penetrable test specimens comprising an
electron-beam generator for directing a focussed electron beam onto
the test specimen to release Auger electrons therefrom; an electron
spectrometer for separating the released Auger electrons according
to the respective kinetic energies thereof; and an electron
detector for detecting the Auger electrons of specified energy
separated in said spectrometer, said electron-beam generator
defining a space for containing an ultra-high vacuum and comprising
a field-emission point cathode disposed in said space, said point
cathode having a small radius of curvature, an anode having an
opening for passing the electron beam therethrough, voltage supply
means for applying a voltage to develop an electric field between
said anode and said point cathode in the vacuum of sufficient
strength to excite said cathode to electron field emission to
produce the electron beam, and a deceleration lens disposed
intermediate said anode and the test specimen for reducing the
velocity of the electrons of the electron beam passing from said
opening of said anode.
2. The electron-beam micro-analyzer of claim 1, comprising means
for providing a field-free space intermediate said anode and said
decelerating lens.
3. The electron-beam micro-analyzer of claim 1 comprising a first
electrode disposed between said anode and the test specimen and
having an opening for passing the beam therethrough, said electrode
and said anode conjointly constituting said deceleration lens.
4. The electron-beam micro-analyzer of claim 3 wherein the test
specimen is at a given potential, and wherein the analyzer
comprises decelerating voltage supply means connected to said
electrode for adjusting the electric potential of said electrode to
at least approximately the electrical potential of the test
specimen.
5. The electron-beam micro-analyzer of claim 3 wherein said first
electrode is configured as a single apertured diaphragm, said
diaphragm being connected to ground potential.
6. The electron-beam micro-analyzer of claim 3 wherein said first
electrode is configured as a single apertured diaphragm and said
voltage supply means applies respective potentials to said anode
and said point cathode to establish said electric field, the
micro-analyzer comprising deceleration voltage supply means
connected to said diaphragm for applying thereto an electric
potential between the potential on said anode and the potential on
said point cathode.
7. The electron-beam micro-analyzer of claim 6 wherein the test
specimen is at a given potential, said deceleration voltage supply
means comprising adjusting means for adjusting the potential
applied to said diaphragm to the potential of the test
specimen.
8. The electron-beam micro-analyzer of claim 6, said deceleration
voltage supply means comprising adjusting means for adjusting the
potential applied to said diaphragm to ground potential.
9. The electron-beam micro-analyzer of claim 3 comprising
decelerating voltage supply means connected to said first electrode
for applying a potential thereto, and a second electrode disposed
between said first electrode and the test specimen and having an
opening for passing the beam therethrough, and second electrode
voltage supply means connected to said second electrode for
applying a potential thereto selected to have a value compared to
the value of the potential on said first electrode to cause the
velocity of the electrons of the electron beam passing from said
opening of said first electrode to increase.
10. The electron-beam micro-analyzer of claim 3 comprising a second
electrode disposed adjacent said first electrode in beam direction,
and a third electrode disposed adjacent said second electrode in
beam direction, said second and third electrodes having respective
openings for passing the electron beam therethrough, said first,
second and third electrodes constituting a three electrode
electrostatic lens.
11. The electron-beam micro-analyzer of claim 3 comprising a
cylindrical tube attached to said anode so as to be electrically
conductive therewith and to be concentric with said opening
thereof, said first electrode being a cylindrical tube spaced an
distance a from and axially aligned with said tube attached to said
anode, said cylinder tubes conjointly constituting said
deceleration lens.
12. The electron-beam micro-analyzer of claim 3, said anode and
said electrode each being configured in the respective regions of
said openings thereof respectively to obtain a minimal imaging
error for said deceleration lens.
13. The electron-beam micro-analyzer of claim 1 comprising a
plurality of electrodes disposed one adjacent the other
intermediate said anode and the test specimen, each of said
electrodes having an opening for passing the beam therethrough,
said plurality of electrodes and said anode conjointly constituting
said deceleration lens.
14. The electron-beam micro-analyzer of claim 13 wherein the test
specimen is at a given potential, and wherein the analyzer
comprises decelerating voltage supply means connected to the one
electrode of said plurality of electrodes disposed adjacent the
test specimen for adjusting the electric potential of said one
electrode to at least approximately the electrical potential of the
test specimen.
15. The electron-beam micro-analyzer of claim 1 comprising an
electrostatic arrangement having a focal length and including at
least two electrostatic lenses, one of said lenses being said
deceleration lens and the other one of said lenses being an
associated electrostatic lens disposed between said deceleration
lens and the test specimen, associated electrostatic supply means
connected to said associated electrostatic lens to focus the same
independently of said deceleration lens, and deceleration voltage
supply means connected to said deceleration lens for adjusting the
potential thereon independently of said associated electrostatic
lens for in turn focussing the electron beam emanating from said
point cathode in said associated electrostatic lens so that any
change in said potential on said deceleration lens will cause no
appreciable change in the focal length of said electrostatic
arrangement.
Description
BACKGROUND OF THE INVENTION
The invention relates to an electron-beam micro-analyzer for
investigating a solid or radiation-penetrable test specimen. The
micro-analyzer has an electron-beam generator whose focussed
electron beam is directed onto the test specimen and has an
electron spectrometer in which Auger electrons released at the test
specimen by the electron beam are separated as to their kinetic
energy. The micro-analyzer further has an electron detector for the
detection of the Auger electrons of defined energy which are
separated in the electron spectrometer.
An electron-beam micro-analyzer with a grid-electron spectrometer
is known, for example, from FIG. 1 of the article Auger Electron
Spectroscopy of fcc Metal Surfaces in Journal of Applied Physics,
Vol. 39, Apr. 1968, pages 2425 to 2432. Another electron-beam
micro-analyzer with a 127.degree. cylinder spectrometer is
disclosed in U.S. Pat. No. 3,461,306. Here an improvement of the
ratio of the Auger-electron line intensity to the background
intensity is obtained by electronic differentiation of the recorded
electron energy spectrum; this ratio is of importance for the
sensitivity of the analysis.
Electron-beam micro-analyzers equipped for detection of Auger
electrons will be referred to as auger analyzers in the following.
In auger analyzers it has heretofore been customary to use
electron-beam generators based on the principle of thermionic
emission which have already been used in the field of electron-beam
micro-analysis (ESMA) with X-ray spectrometers. In this connection,
for example, tungsten wires, are heated up to a condition of
thermal electron emission. In principle, this enables large
currents in the primary electron beam to be obtained. In the known
Auger analysers, currents of 10.sup..sup.-6 to 10.sup..sup.-4 A are
produced with these thermionic electron-beam generators, so that
the Auger-electron lines show sufficient contrast against the
background in the recorded electron energy spectrum. These currents
correspond to a diameter of the electron beam striking the test
specimen of 50 to 1,000 .mu. at an energy of the electrons emitted
by the electron beam generator of up to about 3 keV.
The electron-beam micro-analyzers with Auger-electron analysis,
which have become known up to now, do not reach the signal
intensity of the usual electron-beam micro-probes with X-ray
spectrometers, this being the case even for the investigation of
light chemical elements. The signal of the probe, that is, the
number of Auger electrons leaving the surface of the test specimen,
is smaller by a factor of about 1,000 than the number and the
signal of the X-ray quanta leaving the surface of the specimen for
the same diameter of the primary electron beam. This factor 1,000
is determined by the proportion of the emission depth of the X-ray
quanta, which is some few .mu., to the emission depth of the Auger
electrons, which is in the range of about 1 m .mu.. Even in the
case of the light chemical elements, which exhibit a lower X-ray
fluorescence yield than heavy elements, still more X-ray quanta
will leave the surface of the test specimen for this reason than
Auger electrons.
In principle it would be possible to increase the number of Auger
electrons released at the test specimen by increasing the current
of the primary electron beam. With the electron-beam generators
which are based on the principle of thermionic emission, however,
this would automatically be accompanied by an increase of the
diameter of the primary electron beam and therefore by poorer spot
resolution.
However, the spot resolution achieved so far in Auger analyzers
which is of the order of the diameter of the primary electron beam
of 50 to 1,000 .mu., is already considerably poorer than the spot
resolution in ESMA probes with X-ray quantum detection. There, the
resolution is generally confined to a test specimen area of several
.mu.m diameter, which is determined by the emission depth of the
excited X-ray radiation and the diffusion of the exciting primary
electrons in the test specimen. If the diameter of the primary
electron beam is reduced to values below 1 .mu., neither the
emission depth nor the diameter of the diffusion circle of the
primary electrons change. For Auger electrons, on the other hand,
the size of the diffusion circle is about 10 to 100 m.mu..
The invention is based on the recognition of the fact that in Auger
analyzers, the resolution capability of electron-beam
micro-analyzers with X-ray spectrometers is not only matched for
solid or radiation-permeable test specimens, but can be improved
substantially and the sensitivity of analysis (signal intensity)
attained so far with regard to the light elements can be increased
decisively if it is possible to find an electron-beam generator, in
which the diameter of the primary electron beam can be further
decreased below the values of the magnitudes of the emission depth
of the X-ray radiation or the diffusion circle of the X-ray
radiation, because the emission current density of the
electron-beam generator per solid angle (directionality) and the
Auger electron yield obtained therewith at the test specimen are
considerably better than with known-electron-beam generators. The
lowest possible spot resolution is desired particularly if the test
specimen is to be scanned by the primary electron beam in raster
fashion and the Auger electron measuring signal is to be used to
produce an image of the distribution of chemical elements on the
surface of the specimen.
If in the known Auger analyzers merely the diameter of the primary
electron beam would be reduced, one would admittedly obtain a
finer, and therefore better spot resolution, but the measuring
signal would be at the detection limit or below. The reason for
this is that a reduction of the diameter d of the primary electron
beam by a given factor, for example, by the factor 10 or 1,000, is
accompanied, according to the equation
I = k .sup.. U .sup.. d .sup.8/3
where k is a constant and U the accelerating voltage of the primary
electrons, by a decrease of the current I by a considerably larger
factor, in the present example by the factor 500 and 2.5 .sup..
10.sup.5, respectively. In order to reach in the known Auger
analyzers the resolution capability of the ESMA probes or to
improve it by a factor of 10, the diameter of the primary electron
beam must be reduced by at least a factor of 10 or 100 to values
between 0.5 and 10 .mu.. Reducing the diameter, however, is not
sufficient; at the same time, measures must be taken to cancel the
loss in intensity of the measuring signal which occurs
therewith.
SUMMARY OF THE INVENTION
It is an object of the invention to provide an electron-beam
micro-analyzer with an Auger electron detector of the kind
mentioned above that is equipped with a beam generator of high
directivity for low-energy electrons for obtaining the highest
possible yield of Auger electrons at the test specimens.
The electron-beam micro-analyzer of the invention for investigating
solid test specimens and radiation penetrable test specimens has as
a feature an electron-beam generator that directs a focussed
electron beam unto the test specimen to release Auger electrons
therefrom. An electron spectrometer is provided for separating the
released Auger electrons according to the respective kinetic
energies thereof and an electron detector detects the Auger
electrons of specified energy separated in the spectrometer. The
electron-beam generator defines a space for containing an ultrahigh
vacuum and includes a field-emission point cathode disposed in said
space, the point cathode having a small radius of curvature. An
anode has an opening for passing the electron-beam therethrough and
a voltage supply applys a voltage to develop an electric field
between the anode and the point cathode in the vacuum of sufficient
strength to excite the cathode to electron field emission to
produce the electron-beam. An electrostatic device in the form of a
deceleration lens disposed intermediate the anode and the test
specimen reduces the velocity of the electrons of the electron-beam
passing from the opening of the anode.
With such an Auger analyzer, it is possible to obtain a spot
resolution which is at the theoretical limit and which is given by
the size of the diffusion circle of the Auger electrons of about 10
to 100 m .mu..
As investigations described by A.V.Crewe, J.Wall and L.M.Walter in
the Journal of Applied Physics (1968), Vol. 39, pages 5861 to 5868
have shown, directivity values can be achieved with field-emission
cathode systems which are superior to thermal electron-beam
generators with tungsten hairpin cathodes for the same accelerating
voltage of 20 kV by a factor of approximately 10.sup.3. Whereas in
the conventional electron-beam generators the electrons are emitted
from a relatively large area (crossover diameters of 30 to 100 .mu.
at about 30 kV to 3 kV), the electrons in a point cathode with a
point having, for example, a radius of curvature of 50 m .mu. are
apparently field-emitted from the center of a sphere whose diameter
is only about 3 m .mu. because of the electric field which emanates
radially from the tip. Depending on the choice of the radius of
curvature of the point cathode, its distance from the anode and the
diameter of the aperture of the anode, the high voltage required
for field emission is between about 3 and 10 kV. Currents of
10.sup..sup.-7 A can be obtained in the primary electron-beam with
field-emission point cathodes. The use of a field-emission point
cathode generally necessitates an ultra-high vacuum with a pressure
of 10.sup..sup.-10 to at most 10.sup..sup.-8 Torr.
As other investigations have shown, optimum Auger-electron yield is
not obtained if the electrons of the primary electron beam
impinging on the test specimen have the highest possible energy,
and therefore also not at the high, optimal accelerating voltages
of electron-beam micro-analysis (ESMA) in the energy range between
10 and 50 keV. The primary-beam energy required for optimal
excitation of the Auger electrons is much lower and is given by
three times the energy which is required to ionize an atom of an
element for releasing an Auger electron in an atom shell; it is
therefore between 0.5 and 3 keV. This optimum excitation energy is
achieved by focussing the primary electron beam onto the test
specimen by means of an electrostatic deceleration lens. The
deceleration lens reduces the energy of the electrons in the
primary electron beam which, due to the high accelerating voltage
required for field emission at the electron-beam generator, is too
high for optimal excitation. The energy of these electrons is
reduced by the deceleration lens to values in the range of a few
keV in the space in front of the test specimen. The high
directivity of an electron-beam generator with a field-emission
point cathode can thereby be utilized for an electron-beam
micro-analyzer with Auger-electron detection.
According to a further embodiment of the invention, a field-free
space is provided between the anode and the deceleration lens. This
affords the advantage that the field emission current can be
adjusted independently of the operating condition of the
decelerating lens.
In still another embodiment of the invention, at least one first
electrode having an aperture is arranged between the anode and the
test specimen. The first electrode is combined with the anode to
form a decelerating lens; this embodiment is distinguished by its
simple configuration.
By means of a special configuration of the anode and the electrodes
in the region of their respective apertures and/or their thickness,
it is possible to achieve a minimal imaging error of the
electrostatic device. The configuration required therefor can be
calculated theoretically. Applicable calculations for a
two-electrode lens of the acceleration type have already been
carried out. In this connection, reference may be had to
J.W.Butler, Sixth International Congress for Electron Microscopy
(Kyoto),191 (1966).
Although the invention is illustrated and described herein as an
electron-beam micro-analyzer with an Auger electron detector, it is
nevertheless not intended to be limited to the details shown, since
various modifications may be made therein within the scope and the
range of the claims. The invention, however, together with
additional objects and advantages will be best understood from the
following description and in connection with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
Embodiments of the electron-beam micro-analyser of the invention
together with potential diagrams are illustrated in the following
FIGS. described below. Similar or like components are designated by
the same reference numeral in each FIG. in which they appear.
FIG. 1 is a schematic diagram of an electron-beam micro-analyzer
according to the invention wherein the electron beam generator is
equipped with a field-emission point cathode. An anode and a
diaphragm conjointly define a decelerating lens.
FIG. 2 is a schematic diagram of an alternate embodiment of the
micro-analyzer of the invention. This diagram depicts a
field-emission point cathode, an anode and a test specimen as well
as two electrodes arranged between the anode and the test
specimen.
FIG. 3 shows the electric potential distribution between the
field-emission point cathode and the test specimen for the
micro-analyzer according to FIG. 2.
FIG. 4 is a schematic diagram of another embodiment of the
micro-analyzer of the invention. There, three electrodes are
arranged between the anode and the test specimen.
FIG. 5 shows the electric potential distribution between the
field-emission point cathode and the test specimen according to the
arrangement of FIG. 4.
FIG. 6 is a schematic diagram of still another embodiment of the
micro-analyzer according to the invention. This embodiment includes
a cylindrical decelerating lens arranged between the field-emission
point cathode and the test specimen.
FIG. 7 shows the electric potential distribution between the
field-emission point cathode and the test specimen for the
embodiment of FIG. 6.
DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION
FIG. 1 illustrates an electron-beam micro-analyzer in which a
primary electron beam 1 is generated by field emission at a
field-emission point cathode 2. The point cathode 2 consists, for
example, of tungsten and has, at its tip, a radius of curvature of
only 50 m.mu.. The point cathode 2 is welded to a wire loop 3 which
can be connected via leads 4 and a switch 5 to a heater-current
source 6. By turning on the heater-current source 6 contamination
on the point cathode 2 can be heated out.
The point cathode 2 is situated in a space 7 kept at ultra-high
vacuum. The space 7 is bounded by electric insulators 8 and 9 which
can be of ceramic material for example and an anode 10 having an
aperture 11. The diameter of the aperture 11 can be adjusted by
means of a diaphragm, now shown, to 100 .mu. for example. The
diaphragm determines the aperture error of the imaging system which
is formed by the electrodes 10 and 17. The electron beam focus
attainable on the test specimen is determined, as far as its lower
limit is concerned, by the aperture error.
The point cathode 2 is connected to a voltage supply means
comprising the adjustable bias source 12. The point cathode 2 is
connected to the minus terminal of source 12. The positive terminal
of the source 12 is grounded. The voltage of the bias source 12 can
be adjusted, for example, (to a value of between 0.5 kV and 3kV).
The point cathode 2 is therefore at a potential which is negative
relative to ground.
The voltage supply means can also comprise an adjustable
high-voltage source 13. The anode 10 is connected to the positive
terminal of the adjustable high-voltage source 13. The negative
terminal of source 13 is tied to ground. The high-voltage source 13
is adjustable, for example, to a value between 3 kV and 10kV; this
enables a value to be obtained between 3.5 kV and 13 kV for the
voltage between the anode 10 and the point cathode 2, this value of
voltage is sufficient to obtain a field strength of at least
10.sup.7 V/cm directly at the tip of the point cathode and to
thereby produce cold-field emission at the point cathode 2.
The electron beam 1 leaving the point cathode 2 because of the
field emission arrives in the further space 14 through the aperture
11 of the anode 10 with a kinetic energy between 3.5 keV and 13
keV. The space 14 is enclosed by a spacer 15 of insulating material
and a diaphragm which serves as an electrode 17 having an aperture
16. The diaphragm 17 can be set to an electric potential which can
be adjusted to any value, for example, between +200 and -200 V
relative to ground by means of decelerating voltage supply means in
the form of an adjustable decelerating-voltage source 18. The
diaphragm 17 is therefore at least approximately at ground
potential, that is, at an electric potential between that of the
point cathode 2 and that of the anode 10. The electrons entering
the space 14 through the aperture 11 of the anode 10 are thereby
greatly decelerated. The anode 10 and the diaphragm 17 are combined
in this manner to form a decelerating lens. Whereas the electrons
still have a kinetic energy in the range between 3.5 keV and 13 keV
when they pass through the aperture 11, their energy after passing
the aperture 16 has only a value in the range between 0.5 and 3
keV, if the diaphragm is, for example, exactly at ground potential.
This, however, is the energy range of the primary electron beam, at
which an optimum Auger-electron yield can be obtained.
After passing through the aperture 16, the electron beam 1 arrives
at a deflection system 36, which is shown in FIG. 1 as a magnetic
deflection system and is controlled by a deflecting arrangement
(not shown). An electrostatic deflecting system can, of course,
also be used. The deflection system 36 deflects the decelerated
electron beam 1 in raster fashion over a test specimen 19. This
specimen 19 is attached on a movable specimen manipulator 20 and is
connected essentially to ground potential. The test specimen 20 is
therefore practically at the same potential as the diaphragm
17.
Through the incidence of the electron beam 1, Auger electrons 21 as
well as back-scatter and secondary electrons 22 are released at the
surface of the test specimen 19, the normal of which, different
from the position shown, is preferably inclined with respect to the
primary electron beam 1. The kinetic energy of the Auger electrons
21, which are used for the analysis, is in the energy range below
about 1,000 eV, and that of the other electrons is in the energy
range of O to E.sub.p, where E.sub.p is the energy of the primary
electrons at the test specimen 19. The released electrons 21 and 22
get into an electron spectrometer, which in FIG. 1 is shown as a
hemispherical spectrometer 23. Any type of electron spectrometer
can be used in the invention. The hemisphere spectrometer 23
comprises a first grid 24 which is at ground potential as is the
test specimen 19. The spectrometer 23 also has a second grid 25 to
which a negative potential, relative to ground, can be applied by
means of an adjustable voltage source 26. This voltage source 26 is
adjusted to a fixed voltage value in the range of 0 to 1 kV, so
that Auger electrons and other electrons with an energy which is
lower than the adjusted voltage value, cannot overcome the second
grid 25. The voltage source 26 is adjusted for the Auger-electron
line in the energy sepctrum of that chemical element, the
distribution of which on the surface of the test specimen 19 is to
be determined and recorded.
Electrons which overcome the second grid 25, arrive at a
hemispherical detector surface 27 which is kept at a positive
potential relative to the second grid 25. For this purpose a
voltage source 28 with a collecting voltage of, for example 0.2 kV
is provided.
A modulation oscillator 29 generates an alternating-current voltage
of, for example, 2 kHz which is superimposed by means of a
transformer 30 on the potential of the test specimen with an
amplitude of, for example, 1 to 10 V. The load resistor 31 thereby
receives a modulated signal, which is fed via a blocking capacitor
32 to a detection apparatus 33. The latter comprises a
phase-sensitive rectifier whose reference input is fed with the
alternating-current voltage of the modulation oscillator 29. The
measuring signal determined by the detection apparatus 33 is fed to
a picture tube 34; this measuring signal now originates essentially
from Auger electrons and the voltage source 26 is adjusted to the
energy of these electrons. Here the measuring signal is used for
displaying the distribution on the surface of the test specimen of
that chemical element, to the Auger-electron energy of which the
voltage source 26 is adjusted. The line scan of the picture tube 34
is synchronized with the motion of the electron beam 1 (not shown)
via the leads 35 by the deflection arrangement which controls the
deflection system 36.
FIG. 2 is a schematic diagram of an Auger analyzer in which, in
contrast to the Auger analyzer according to FIG. 1, a second
electrode 37 having an aperture 38 is additionally arranged between
the first electrode 17 and the test specimen. As in the Auger
analyzer according to FIG. 1, the first electrode 17 is combined
with the anode 10 to form a deceleration lens. This becomes clear
from FIG. 3 where the electric potential P(d) of the Auger analyzer
according to FIG. 2 is shown as a function of the distance d; for
simplicity, straightline segments are shown. Negative potential
values are plotted in the upward direction.
The point cathode 2 is again at a negative potential relative to
the ground potential of the test specimen 19, for example, at -0.5
kV. A positive potential of, for example, +3 kV is applied to the
anode 10. The first electrode 17 can now be at a negative potential
of -0.4 kV for example. This potential can be adjusted toward
larger values, which is indicated by the vertical arrow.
The second electrode 37 is preferably at ground potential. Its
potential can, however, likewise be varied within narrow limits,
which is indicated by the double arrow. As the potential of the
second electrode 37 is higher than that of the first electrode 17,
it acts as a post-acceleration electrode. The velocity of the
electrons in the beam 1 emanating from the aperture 16 of the first
electrode 17 is therefore again increased somewhat. The deciding
factor for the optimum kinetic energy of the electrons in the
electron beam 1 at the point of the test specimen 19 is only the
potential difference between the point cathode 2 and the test
specimen 19.
If the aperture 16 in the first electrode 17 is chosen large
enough, the potential of the first electrode 17 can also be made
smaller, that is, more negative than the potential of the point 2
cathode because of the field penetration which takes place in that
case.
In FIG. 2, the anode 10 is combined with the first and the second
electrodes, 17 and 37, to form an electrostatic lens. If the
aperture 16 of the first electrode 17 is chosen smaller than the
apertures 11 and 38 of the anode 10 and the second electrode 37,
respectively, and if short mutual spacings are chosen, an
asymmetrical three-electrode lens is obtained which is known per se
and in which the field emission current can be adjusted via the
potential of the anode 10, and the deceleration via the potential
of the first electrode 17.
The advantage of the Auger analyzer shown in FIG. 2 with the
potential pattern P(d) shown in FIG. 3 over the Auger analyzer
according to FIG. 1 is that the properties of the beam generating
system can be varied to a larger extent; these properties include
the field emission potential, energy of the primary beam and focal
length.
In FIG. 4, an associated electrostatic lens is shown between the
anode 10 and the test specimen 19; this lens consists of three
electrodes 17, 37 and 39 with approximately equally large
respective apertures 16, 39 and 40, which in turn are larger than
the aperture 11 of the anode 10. This arrangement is provided for a
large working distance between the third electrode 39 and the test
specimen 19. As will be seen from the corresponding potential
distribution P(d) in FIG. 5, the first electrode 17, which may have
selectively a slightly positive or a slightly negative potential,
as desired, but preferably ground potential, again acts as a
decelerating electrode. The third electrode 39 has preferably the
same potential as the first electrode 17, that is, ground
potential. The potential of the second electrode 37, which is shown
in FIG. 5 as negative, can be varied. Thereby, the focal length of
the electrostatic lens 17, 37, 39 can be adjusted. If the potential
of the second electrode 37 is chosen more positive, a longer focal
length results, but if it is chosen more negative the focal length
is shortened.
The anode 10 and the first electrode 17 can image the point
electrode 2 electron-optically in the aperture 38 of the second
electrode 37. Additionally, the lens system consisting of the
electrodes 17, 37 and 39 can image the point cathode 2 on the test
specimen 19 through focussing. Focussing on the test specimen 19 is
thereby largely independent of the potential of the anode 10
adjusted to obtain field emission and the decelerating potential
adjusted between the anode 10 and the first electrode 17. The
arrangement then functions according to the principle of a
light-optical elastic lens. The arrangement shown in FIG. 2 can
also be configured in the manner of a light-optical elastic
lens.
It is fundamental for the configuration of the electrostatic
arrangement as an elastic lens that it consists of at least two,
separately adjustable electrostatic lenses and that the electron
beam 1 emanating from the point cathode 2 is focussed within the
second lens in such a manner that the focal length of the overall
electrostatic arrangement is virtually not influenced if the
emission potential at the first lens is varied.
In FIG. 6, the electron-beam generator of an Auger analyzer is
shown schematically which corresponds with respect to the electric
circuit arrangement and also otherwise in large measure to the
Auger analyzer shown in FIG. 1. At the anode 10, a cylindrical tube
41 is attached here in an electrically conducting manner. The tube
41 and a cylindrical tube 42 as the first electrode conjointly
define a decelerating lens. The cylindrical tube 42 is aligned with
the tube 41 and is arranged at a distance a as shown in FIG. 6. The
corresponding potential plot in FIG. 7 shows that the electrons of
the primary electron beam 1 are accelerated and are also
decelerated again over a relatively short distance. By changing the
potential of the anode 10, the focal length of the decelerating
lens can be adjusted.
Deviating from the arrangement of FIG. 6, the interior space 43 of
the cylindrical tube 42 and the interior space 44 of the tube 41
can be made non-cylindrical, although rotation-symmetrical with
respect to the electron beam 1, to obtain minimum imaging
errors.
* * * * *