U.S. patent number 3,710,103 [Application Number 05/204,459] was granted by the patent office on 1973-01-09 for planar retarding grid electron spectrometer.
This patent grant is currently assigned to Varian Associates. Invention is credited to John C. Helmer.
United States Patent |
3,710,103 |
Helmer |
January 9, 1973 |
PLANAR RETARDING GRID ELECTRON SPECTROMETER
Abstract
A retarding grid form of electron spectrometer utilizing a
single grid structure, the retarding grid being planar and being
positioned in a focusing structure that first defocuses the beam of
electrons and then refocuses it onto an electron detector, the
retarding grid being located in the focusing structure near the
point of maximum diameter of the electron beam. The focusing
structure produces very strong accelerating and decelerating fields
near its beam entrance and exit regions, respectively, such that
the retarding and accelerating fields at the retarding grid are
very weak, resulting in sharp lines in the electron spectrum.
Inventors: |
Helmer; John C. (Menlo Park,
CA) |
Assignee: |
Varian Associates (Palo Alto,
CA)
|
Family
ID: |
22757975 |
Appl.
No.: |
05/204,459 |
Filed: |
December 3, 1971 |
Current U.S.
Class: |
250/305 |
Current CPC
Class: |
H01J
49/488 (20130101) |
Current International
Class: |
H01J
49/00 (20060101); H01J 49/48 (20060101); G01t
001/36 () |
Field of
Search: |
;250/49.5AE,49.5ED,49.5PE |
Other References
"Electron Reflection Coefficient at Zero Energy, I, Experiments" by
H. Heil et al. from Tite Physical Review, Vol. 164, Dec., 1967,
pages 881-886 .
"Photoelectron Spectroscopy of the Rare Gases" by J. A. R. Samson
et al. from The Physical Review, Vol. 173, Sept., 1968, pages 80-85
.
"Resolution and Sensitivity Condiserations of an Auger Electron
Spectrometer Based on Display Leed Optics" by N. J. Taylor from The
Review of Scientific Instruments, Vol. 40, June, 1969, pages
792-803 .
"High-Sensitivity Electron Spectrometer" by D. A. Huchital et al.
from Applied Physics Letters, Vol. 16, May 1970, pages
348-351.
|
Primary Examiner: Lindquist; William F.
Claims
What is claimed is:
1. An electron spectrometer comprising means for inducing electron
emission from a sample under analysis, an electron detector means
for measuring electrons impinging thereon, a focusing means of
rotational symmetry about a central axis for focusing electrons
emitted from said sample onto said electron detector means, a grid
positioned in the focusing means in a region of maximum electron
beam diameter and lying substantially normal to the central axis
through said focusing means, and means for applying a variable
voltage to said grid relative to said sample to cause electrons of
selective energies to impinge onto said detector.
2. An electron spectrometer as claimed in claim 1 wherein said
electron beam detector is an electron multiplier.
3. An electron beam spectrometer as claimed in claim 1 including a
second electron beam focusing means positioned between said sample
and said first focusing means for focusing the electrons from said
sample into said first focusing means.
4. An electron spectrometer as claimed in claim 3 including a third
electron beam focusing means positioned between said first focusing
means and said electron detector means for focusing the electrons
exiting from said first focusing means onto said electron
detector.
5. An electron spectrometer as claimed in claim 1 wherein said
focusing means is symmetrical in the central axial direction, said
grid being planar.
6. An electron spectrometer as claimed in claim 5 wherein said grid
is positioned at approximately the midpoint of the central axis
through said focusing means.
7. An electron spectrometer as claimed in claim 5 wherein said
focusing means comprises a plurality of annular electrodes spaced
in axial alignment along said central axis, the inner diameters of
the electrodes increasing in size from the electrodes on either end
of the focusing means to the electrodes at the center of the
focusing means.
8. An electron spectrometer as claimed in claim 1 including means
for applying voltages to said focusing means to create a
substantial electron decelerating field near the entrance end of
said focusing means and a substantial accelerating field near the
exit end of said focusing means, the decelerating and accelerating
field at the grid being weak relative to the entrance decelerating
and exit accelerating fields of said focusing means.
9. An electron spectrometer as claimed in claim 8 wherein said
focusing means is symmetrical in the central axial direction, said
grid being planar.
10. An electron spectrometer as claimed in claim 9 wherein said
grid is positioned at approximately the midpoint of the central
axis through said focusing means.
11. An electron spectrometer as claimed in claim 1 wherein said
focusing means comprises a first section positioned between the
grid and said sample and including an entrance region for the
electrons and a second region near the grid, and a second section
positioned between the grid and said electron detector and
including an exit region for the electrons and a second region near
the grid, said focusing means creating electron decelerating and
accelerating fields in said entrance and exit regions,
respectively, substantially greater than the electron decelerating
and accelerating fields in said second regions of said first and
second sections, respectively.
12. An electron spectrometer as claimed in claim 11 wherein the
fields in the entrance region and the second region of said first
section are symmetrical with respect to the fields in the exit
region and the second region of said second section,
respectively.
13. An electron spectrometer as claimed in claim 11 wherein the
decelerating and accelerating fields in said first and second
regions are at least ninety percent greater than the fields in said
second regions.
14. An electron spectrometer as claimed in claim 11 wherein said
focusing means is symmetrical in the central axial direction, said
grid being planar and being positioned at approximately the
midpoint of the central axis through said focusing means.
15. An electron spectrometer comprising means for inducing electron
emission from a sample under analysis, an electron detector means
for measuring electrons impinging thereon, an electrostatic
focusing means of rotational symmetry about a central axis for
focusing electrons emitted from said sample onto said electron
detector means, a planar grid positioned in the focusing means and
lying substantially normal to the central axis through said
focusing means, and means for applying a variable voltage to said
grid relative to said sample to cause electrons of selective
energies to impinge onto said detector.
16. An electron spectrometer as claimed in claim 15 wherein said
electrostatic focusing means comprises a plurality of annular
electrodes spaced in axial alignment along said central axis, the
inner diameters of the electrodes increasing in size from the
electrodes on either end of the focusing means to the electrodes at
the center of the focusing means.
17. An electron spectrometer as claimed in claim 15 including means
for applying voltages to said focusing means to create a
substantial electron decelerating field near the entrance end of
said focusing means and a substantial accelerating field near the
exit end of said focusing means, the decelerating and accelerating
field at the grid being weak relative to the entrance decelerating
and exit accelerating fields of said focusing means.
18. An electron spectrometer as claimed in claim 17 wherein said
focusing means is symmetrical in the central axial direction and
said grid is positioned at approximately the midpoint of the
central axis through said focusing means.
19. An electron spectrometer as claimed in claim 15 wherein said
focusing means comprises a first section positioned between the
grid and said sample and including an entrance region for the
electrons and a second region near the grid, and a second section
positioned between the grid and said electron detector and
including an exit region for the electrons and a second region near
the grid, said focusing means creating electron decelerating and
accelerating fields in said entrance and exit regions,
respectively, substantially greater than the electron decelerating
and accelerating fields in said second regions of said first and
second sections, respectively.
20. An electron spectrometer as claimed in claim 19 wherein the
fields in the entrance region and the second region of said first
section are symmetrical with respect to the fields in the exit
region and the second region of said second section,
respectively.
21. An electron spectrometer as claimed in claim 19 wherein the
decelerating and accelerating fields in said first and second
regions are at least ninety percent greater than the fields in said
second regions.
22. An electron spectrometer as claimed in claim 19 wherein said
focusing means is symmetrical in the central axial direction, said
grid being positioned at approximately the midpoint of the central
axis through said focusing means.
Description
BACKGROUND OF THE INVENTION
Chemical analysis spectrometers utilizing the technique of inducing
electron emission from the sample under analysis and measuring the
energies of the electrons emitted to thereby produce an electron
energy spectrum of the sample are in common use. The more
sophisticated forms of induced electron emission spectrometers
employ a magnetic or electrostatic deflection type of energy
analyzer for separation of the electrons into groupings in
accordance with their energies, electrons being detected by a
suitable electron detector such as an electron multiplier. The
analyzer is swept so that at any given instant of time only
electrons of a particular energy group are directed onto the
electron detector. This type of analyzer is therefor very
selective, rendering distinct electron energy peaks or lines
separated along the energy spectrum.
A simplier form of electron spectrometer is the retarded field
spectrometer wherein the emitted electrons are directed in a large
solid angle cone of emission onto a spherical shaped detector
plate, all points on the surface of the plate being equidistant
from the point source of the electrons. A spherical shaped
retarding grid is positioned between the source and the detector
plate and a variable voltage on the grid serves to retard all those
electrons with energy in electron volts less than the voltage on
the grid and allowing all electrons with an energy greater than the
retarding grid voltage to pass through to the detecting plate. The
resulting current in the detector plate is a measure of the energy
of the emitted electrons as established by the sweep voltage
applied to the retarding grid. The spectrum obtained with this
retarding grid spectrometer is a step or integral form of spectrum
as opposed to individual peaks obtained with the deflection type
spectrometer since all of the electrons in the separate energy
groupings with a voltage above that of the retarding grid pass
through the grid and are collected and recorded and these higher
energy electron groups appear as one energy level in the recorder
output. Separation is produced by increasing the retarding voltage
to retard each successive group of electrons in the energy spectrum
and noting the potential step down at which the particular electron
energy group exclusion occurs. Since all of the electrons above the
particular voltage level of the retarding grid pass to the
detecting plate, the retarding grid spectrometer has a very high
background current which creates a noise level high compared to the
noise level of the more selective deflection type spectrometer. As
a result, the advantage of the high current acceptance of the
retarded grid analyzer is more than offset by the high background
current and resultant noise.
One known proposed technique for reducing this background current
comprises placing an electron focusing device, such as an
electrostatic focusing condenser, after the retarding grid to
separate the slower or lower energy electrons from the faster
electrons, focusing the slower electrons onto a suitable detector
such as an electron multiplier.
An additional disadvantage of the retarding field analyzer stems
from the fact that, with a potential applied to the retarding grid,
an electric field is set up between the detecting plate and the
retarding grid; this electric field results in a space modulation
of the potential across the grid as the retarding grid is voltage
modulated for sweep purposes. This space modulation limits the
resolution of the retarding field analyzer. This electric field E
is given by the relationship
E = V/D
where V is the voltage on the retarding grid and D is the distance
between the retarding grid and the detecting plate. The space
modulation .DELTA. V is dependent upon the following
relationship:
.DELTA.V .congruent. E .times. d/4
where d is the spacing between the wires in the mesh of the
retarding grid. From the above relationships,
.DELTA.V = V .times. d/4D
thereby resulting in a resolution limit,
.DELTA.V/V = d/4D .
Also, capacitance between the retarding grid and collector plate
results in a voltage induced in the collector plate as the voltage
on the retarding grid is modulated for sweep purposes and this
induced voltage in the collector serves to mask the current
produced therein by the electrons of interest.
In order to control the capacity of coupling between the retarding
grid and collector and to substantially reduce the effect of the
electric field on the retarding grid, additional grids have been
introduced between the retarding grid and the collector and/or at
the front or the back sides of the retarding grid or both. Typical
forms of such multiple grid devices are shown in articles entitled
"Resolution and Sensitivity Considerations of an Auger Electron
Spectrometer Based on Display LEED Optics" by N.J. Taylor in the
Review of Scientific Instruments, Vol. 40, No. 6, June 1969, pages
792-803, "High-Sensitivity Electron Spectrometer" by D.A. Huchital
et al in Applied Physics Letters, Vol. 16, No. 9, May 1, 1970,
pages 348-351, and "Photoelectron Spectroscopy of the Rare Gases"
by James A. Samson in The Physical Review, Vol. 173, pages 80
through 85.
The introduction of these additional grids, while improving the
performance relative to the noted capacitance and electric field,
results in a decrease in the electron transmission and the
introduction of spurious signals into the spectrometer output due
to secondary electrons. A typical spherical grid employs 1 mil wire
with 100 wires per inch resulting in an electron transparency of
approximately 80 percent for a single grid. Two or more grids in
the electron path will substantially reduce the electron
transparency, for example, to 50 percent or less. The additional
intercepted electrons result in a substantial increase in emission
of secondary electrons from the grids and these secondary electrons
produce spurious signals in the electron detector output.
SUMMARY OF THE PRESENT INVENTION
The present invention provides an electron spectrometer of the
retarding field type wherein a single planar retarding grid is
provided for selection and control of the electrons of desired
energies, the planar retarding grid being positioned within an
electron focusing means in a plane normal to the central axis
through the focusing means. In a preferred embodiment, the planar
grid is positioned in the focusing means in the region where the
electron beam is at maximum diameter. With a symmetrical form of
focusing means as employed in the preferred embodiment, this grid
is located at the mid-section of the focusing structure. The
electrons passing through the planar retarding grid are focused
onto a suitable detector such as an electron multiplier device.
Since only a single retarding grid is employed, electron
transmission is improved over multiple grid forms of electron
spectrometers. In addition, secondary electron emission from the
retarding grid is reduced relative to the multiple grid devices,
thus improving the spectrometer from the standpoint of spurious
output signals.
Also, the planar grid is well spaced from other parallel surfaces
within the focusing structure and, therefore, there is no problem
with capacitive coupling to the electron detector region. The
electric field at the grid is very weak and is not a resolution
limiting factor as with prior retarding grid spectrometers.
The novel form of focusing means and planar retarding grid renders
an output in the form of sharp peaks or lines, with trailing edges
nearly as sharp as leading edges, much the same as the deflection
type spectrometers, as contrasted with the integral or step type
spectrum or the sharp leading edge-slow cut off sharp trace
obtained with certain forms of retarding grid analyzers.
The manufacture of this new spectrometer is simple, with relatively
few components; the grid, being planar, is easily fabricated.
DESCRIPTION OF THE DRAWINGS
FIG. 1 is a longitudinal cross-section view of the novel retarding
grid form of electron spectrometer.
FIG. 2 is a plot of the equipotential lines and the electron
trajectories for an analyzer of the type shown in FIG. 1.
FIGS. 3(A) and 3(B) illustrate examples of expected and actual
spectrums, respectively, obtained with this spectrometer.
FIGS. 4 and 5 are illustrations of the electron trajectories in
this spectrometer under two different modes of operation.
FIG. 6 is a trace of the electron spectrum from argon obtained with
the analyzer of the present invention.
FIG. 7 is a trace similar to that of FIG. 6 with an expanded
scale.
FIG. 8 is a diagram illustrating two electron beam paths from a
source plane to a retarding grid.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to FIG. 1, the electron spectrometer comprises a
sample chamber 11 which contains the sample to be analyzed, in this
specific example argon gas fed into the chamber 11 through gas
inlet 12. Means 13 is provided for irradiating the sample with a
beam of ultraviolet from a helium discharge via an aperture 14 in
the side of the chamber to ionize the gas and produce the
photoelectrons to be analyzed. As with other known electron
analyzers, other gases or solids may be analyzed, and other
irradiation may be employed such as X-rays for producing the
photoelectron or Auger electron emission desired.
The emitted electrons pass out from the sample chamber 11 via a
small aperture 15 in an aperture plate 16 in the chamber and pass
into a first Einzel lens comprising three spaced-apart annular
copper electrodes 17, 18 and 19. The three electrodes are affixed
together in axial alignment by three longitudinally extending
support rods 21 and suitable insulators 22, the rods being
positioned 120.degree. apart around the central axis of the
structure. The three rods 21 are secured within the end electrode
of the main focusing structure for support and in turn serve to
mount the sample chamber 11. The two outermost electrodes 17 and 19
in the Einzel lens are grounded while the middle electrode 18 is
coupled to a source 23 of negative voltage, for example, -8 volts
as indicated in the drawings. The Einzel lens serves to adjust the
launching angle of the electrons into the main focusing
structure.
The main focusing structure comprises a plurality of annular copper
electrodes affixed in axial alignment by support rods 24 spaced
90.degree. apart around the structure and suitable insulators 25.
The first or entrance section of this focusing apparatus includes
the four electrodes 26, 27, 28 and 29 while the second or exit
section of the focusing apparatus includes the four electrodes 26',
27', 28' and 29', the latter four electrodes forming a mirror image
of the first four electrodes. The electrodes 26 and 26' have inner
diameters smaller than those of electrodes 27 and 27', with
electrodes 28, 28', 29 and 29' having equal inner diameters and
larger than those of electrodes 27 and 27'.
In one embodiment built and operated, the electrodes 26 and 26' had
inner diameters of 1 inch and were 3/8 inch thick; electrodes 27
and 27' had 2 inch inner diameters and were 1 inch thick, and
electrodes 28, 28', 29 and 29' had inner diameters of 4 inches and
were 1 inch thick.
A planar 100 mesh circular grid 31 is positioned at the midpoint of
the main focusing structure between the entrance or decelerating
region and the exit or accelerating region and normal to the
central axis through the structure. The grid is supported on the
rods 24 and suitably insulated from the other electrodes in the
structure.
Another Einzel lens including annular plates 32 and 33 is mounted
at the exit of the main focusing section and serves to direct the
electrons into an electron detector, in this case an electron
multiplier device 34. A small opaque area 35 at the center of the
grid 31 serves to prevent light from the source 15 from impinging
upon the electron multiplier and giving a false registration of
electrons.
The entire spectrometer device is enclosed in a stainless steel
envelope structure 36 and evacuated by suitable vacuum pumps, not
shown, in accordance with standard electron emission spectrometer
techniques.
The voltage source 23 supplies the desired DC voltages to the
various elements, and typical values of voltage are shown on the
leads from the voltage source to the elements. The sweep voltage,
e.g. -10 to +10 volts, for this spectrometer is applied to the
sample chamber 11 so that, once the voltages for the focusing
structure and retarding grid have been set up, no changes are made
therein during the particular analysis. It should be noted that the
voltages shown are only illustrative, and other values are utilized
for different samples or different analyses.
In operation, as the electrons emitted from the sample enter the
main focusing structure at source point 37, they are subjected to a
very strong decelerating field such that, by the time they pass
from the region within the focus electrode 27 and into the region
defined by the two focus electrodes 28 and 29, the electrons have
lost about 98 percent of their initial energy, and they approach
the retarding grid 31 with very low energy. This can better be seen
by reference to FIG. 2 which shows the equipotential lines in a
focusing structure similar to that of FIG. 1 with an electron
starting from a source with 10ev energy and moving to a retarding
grid held at -10 volts. It is noted that both the -9.9 and -9.8
volt equipotential lines are spaced a considerable distance from
the plane of the retarding grid, and that a substantial
deceleration of the electron from 0v to -9.8v has occurred near the
entrance region of the focusing structure.
Those electrons with an energy greater than zero volts will pass
through the retarding grid 31 and those with less energy will be
retarded and turned back. The electrons in the slightly higher
energy grouping passing through the retarding grid will be
accelerated in the second or exit region of the focusing means and
will be focused into a focal point and then into the exit Einzel
lens and into the electron multiplier.
As with the other forms of retarding grid analyzer, it should be
expected that the recorded spectrum would be integral or step-like
in form, or that the electron line would have a sharp leading edge
and a slow trailing edge similar to the spectrum shown in FIG. 2 of
the Huchital et al article cited above. However, the spectrum
actually obtained with this new retarding grid analyzer has a
trailing edge which is almost as sharp as the leading edge, giving
a spectrum more closely resembling that obtained with the
deflection type spectrometers. An example of the expected form of
slow trailing edge spectrum is shown in FIG. 3(A) and a
corresponding spectrum of the sharp line form actually obtained is
illustrated in FIG. 3(B).
This sharp delineation of the energy groupings occurs because the
deflection of the electrons leaving the grid 31 in the rear section
of the focusing structure occurs at a very low energy. The change
in kinetic energy of the electron relative to the energy at which
the trajectory is bent determines how far off from its original
trajectory it will move; the faster electrons will rapidly move off
focus and will not pass to the electron multiplier; the slower
electrons will deflect very rapidly and thus move off focus.
Therefore, a sharp energy line is obtained for slow electrons at
the focusing energy.
In order to more clearly understand the process taking place in
this analyzer, reference is made to FIG. 4 which illustrates the
electron beam flow through the analyzer from source to detector
through the grid. The source is assumed to be at 10ev and the grid
at -10v. The field in the focusing structure produces a radial
deceleration to nearly zero energy in the initial decelerating
region, during which period the electron trajectories are nearly
straight paths. This is followed by a region of electron deflection
at very low energy, i.e. 0.1ev, in which the electrons are brought
to rest on the grid at normal incidence.
If now the source energy is increased by a very small amount, i.e.
to 10.1ev, leaving the grid at -10v, then in the deflection region
the electron energy will be increased from 0.1ev to 0.2ev, a 100
percent increase. The percent change in radius of curvature R goes
as
.DELTA.R/R .about. .DELTA. E/E = 0.1ev/0.1ev = 1
in this example. Therefore, a change of source energy of only 1
percent will cause a major change in the electron trajectory such
that the electron with increased energy goes substantially off the
path to the detector as shown in the drawing, giving a very sharp
trailing edge to the spectral line.
The preceding discussion assumed that the leading edge of the
spectral line is produced by the retarding field grid cutoff. In
general, this may not be the case but may only be one mode of
operation. Another mode of operation is shown in FIG. 5 which
illustrates the electron trajectory between source and detector,
where the source and detector are at 10ev and the grid is at a
-9.8v. In this case, the source electrons at 10ev pass the grid and
are focused onto the detector by appropriate adjustment of the
focusing voltage. In this example, it is quite possible to focus
the beam at the detector under conditions such that the beam passes
through the grid with a particular kinetic energy > 0. If the
beam, under focused conditions, passes through the grid with a
kinetic energy equal to or greater than the observed linewidth,
then the line shape will be determined by the deflection
characteristics of both sides, as indicated in the drawing, and the
grid cutoff will not be observed. In the illustration, source
electrons at both 10.1ev and 9.9ev pass the grid and are deflected
from the path to the deflector, whereas those source electrons at
10ev impinge on the detector. The diameter of central disc 35 may
be chosen to confine the paths of transmitted electrons to a region
of high radius of curvature within the focusing structure.
Thus, this new analyzer may be viewed as a new type of deflection
analyzer which in one limit of its operation becomes a new form of
retarding field analyzer whose resolution is determined by the
cutoff properties of the grid. In contrast with other types of
deflection analyzers which require an inner or coaxial electrode to
terminate the transverse field from an outer electrode, the present
device uses a grid to establish a uniform electron potential over
the cross-section of a cylindrical (hollow) lens. The grid corrects
the non-uniformity of the lens potential across the transverse
plane so that the entire electron beam may be brought to a
predetermined low and uniform kinetic energy.
A typical line for argon radiated with helium ultraviolet light is
shown in FIG. 6, and also in FIG. 7 but on an expanded scale, with
typical values of grid and sweep voltages shown. The general
formula is
-V.sub.g = (h.nu. - E.sub.b)/(e) - V.sub.s
where V.sub.g is the retarding grid voltage, e is the electron
charge, h.nu. is the photon energy of the ultraviolet light in
electron volts, E.sub.b is the binding energy of the
photoelectrons, and V.sub.s is the sweep voltage applied to the
sample.
The prior art suggested to utilize a planar grid in a retarding
grid electron spectrometer and to position the grid slightly in
front of the exit focal point before the electron detector region
where the electron beam has been well converged. Such an analyzer
is described in an article entitled "Electron Reflection
Coefficient at Zero Energy. I. Experiments" by H. Heil et al. in
The Physical Review, Vol. 164, No. 3, Dec. 15, 1967, pages 881-886.
Referring to FIG. 8, there is shown two electron trajectories from
a source 41 with radius y.sub.1 through a focusing means 42 to a
retarding grid 43 with a radius y.sub.2. From Abbe's sine law,
.sqroot.E.sub.1 y.sub.1 sin .theta..sub.1 = .sqroot. E.sub.2
y.sub.2 sin .sigma..sub.2
where E.sub.1 and E.sub.2 are the kinetic energies of the electron
at the source 41 and grid 43, respectively, .theta..sub.1 is the
angle at which the electron leaves the source relative to the
central axis 44, and .theta..sub.2 is the angle at which the
electron approaches the grid sin .theta. 43 relative to the central
axis. In operation of a retarding grid analyzer, the electrons with
normal trajectories are brought to rest at the grid. The finite
source radius y.sub.1 causes non-normal .theta..sub.2 paths and,
for a retarding grid analyzer, we have
.theta..sub.2 = .pi. /2, E.sub.2 = .DELTA. E
where .DELTA.E is the limiting resolution of the analyzer due to
y.sub.1. Therefore,
E.sub.1 y.sub.1.sup.2 sin.sup.2 .theta. = .DELTA. E
y.sub.2.sup.2
or
.DELTA.E/E = (y.sub.1.sup.2 /y.sub.2.sup.2) sin.sup.2
.theta..sub.1.
Thus, the best resolution is obtained with the greatest value of
y.sub.2 which means that the retarding grid should be placed near
the maximum electron beam diameter as contrasted with a position
nearer the focal point, and therefore grid 31 is so positioned in
the spectrometer of FIG. 1.
It is noted that, although the decelerating section and
accelerating section of the focusing structure on either side of
the retarding grid 31 in FIG. 1 are symmetrical, this relationship
is only preferred, and the two focusing sections may be
non-symmetrical. Also, the retarding grid has been shown at the
exact midpoint and, again, although this is preferred, the grid may
be positioned near the midpoint and preferably near the region of
maximum beam diameter for reasons given above.
The advantages of this form of planar retarding grid analyzer
include the fact that the electric field at the grid is very weak
and under the normal requirements of
.DELTA.E/E = 10.sup..sup.-2 to 10.sup..sup.-3
the mesh of the grid does not limit the resolution. Conversely, the
ultimate resolution capability is higher than can be achieved with
prior designs.
Since only a single grid is employed, scattering or interception is
reduced as well as secondary emission from the grid, thus resulting
in lower background noise. The sharp refocusing of the higher
energy electrons passing the grid reduces the background, giving
sharp peaks in the spectrum. The larger spacing between the
retarding grid and the detector means permits modulation of the
grid, if desired, without suffering from capacitive coupling to the
detector.
* * * * *