U.S. patent number 3,699,331 [Application Number 05/175,575] was granted by the patent office on 1972-10-17 for double pass coaxial cylinder analyzer with retarding spherical grids.
Invention is credited to Paul W. Palmberg.
United States Patent |
3,699,331 |
Palmberg |
October 17, 1972 |
**Please see images for:
( Certificate of Correction ) ** |
DOUBLE PASS COAXIAL CYLINDER ANALYZER WITH RETARDING SPHERICAL
GRIDS
Abstract
A charged particle analyzer is provided wherein a retarding grid
consisting of two spaced screen members formed of spherical
sections having a concentric center located at the surface of the
sample being analyzed is utilized to control the entering velocity
of the charged particles into a double pass, coaxial cylinder
analyzer. In a specific embodiment, the invention is utilized in an
electron spectroscopy for chemical analysis (ESCA) apparatus.
Inventors: |
Palmberg; Paul W. (Minneapolis,
MN) |
Family
ID: |
22640776 |
Appl.
No.: |
05/175,575 |
Filed: |
August 27, 1971 |
Current U.S.
Class: |
250/305;
850/9 |
Current CPC
Class: |
H01J
49/282 (20130101); H01J 49/484 (20130101); H01J
49/482 (20130101) |
Current International
Class: |
H01J
49/28 (20060101); H01J 49/00 (20060101); H01J
49/26 (20060101); H01J 49/48 (20060101); H01j
037/26 () |
Field of
Search: |
;250/41.9ME,49.5PE,49.5AE |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
High Sensitivity Auger Electron Spectrometer by Palmberg, Applied
Physics Letter Oct. 15, 1969 Vol. 15 No. 8..
|
Primary Examiner: Lawrence James W.
Assistant Examiner: Dixon; Harold A.
Claims
I claim:
1. An analyzer for determining the energy distribution of charged
particles being emitted from a source, comprising:
a. first and second metallic cylinders, the first of said cylinders
having an outer diameter less than that of the inner diameter of
the second cylinder and being positioned interior to the second
cylinder, each of said cylinders having a common axis;
b. disc-shaped aperture means positioned interior to and
intermediate the ends of said first cylinder in a plane generally
perpendicular to said axis and dividing said analyzer into first
and second stages, said disc having an opening therethrough at said
axis, the opening defining the source of charged particles for said
second stage;
c. said first cylinder defining a plurality of annularly spaced
openings near each end of each stage of said first cylinder;
d. a retarding grid arrangement including first and second
spherical section shaped screens positioned adjacent a first end of
said first cylinder, said screens being spaced from one another and
having a common center of symmetry at the source of charged
particles and along said axis, said screens being in electrical
isolation from one another, the second of said screens being in
electrical contact with said first cylinder;
e. means for applying an electrical potential between said first
screen and said second screen and for maintaining a predetermined
potential between said first and second cylinders; and
f. charged particle detecting means positioned at the opposite end
of said first cylinder.
2. An analyzer in accordance with claim 1 wherein said cylinders
are held in spaced relation by annular electrically insulating
spacers positioned at each end of the cylinders and at a point
midway along the length thereof.
3. An analyzer in accordance with claim 2 wherein the surfaces of
the spacers facing the interior space intermediate the first and
second cylinders are coated with a high resistance conductive
material which is in electrical contact with each of said
cylinders.
4. An analyzer in accordance with claim 3 wherein the conductive
coating includes a plurality of annular relatively high
conductivity rings concentric with said axis to provide a plurality
of equi-potential regions in said conductive coating.
5. An analyzer in accordance with claim 1 wherein the screens
forming the retarding grid are spaced about 0.1 inches apart.
6. An analyzer in accordance with claim 4 wherein the screens are
formed from line wire of about 100 lines per inch and about 80
percent transparency.
7. An electron spectroscopy chemical analysis system
comprising:
a. first and second metallic cylinders, the first of said cylinders
having an outer diameter less than that of the inner diameter of
the second cylinder and being positioned interior to the second
cylinder, each of said cylinders having a common axis;
b. disc-shaped aperture means positioned interior to and
intermediate the ends along the length of said first cylinder in a
plane perpendicular to said axis and dividing said analyzer into
first and second stages, said disc having an opening at said axis,
the opening defining the source of charged particles for said
second stage;
c. said first cylinder defining a plurality of annularly spaced
openings near each end of each stage of said first cylinder;
d. a retarding grid arrangement including first and second
spherical section shaped screens positioned adjacent the first end
of said first cylinder, said screens being spaced from one another
and having a common center of symmetry along said axis, said
screens being in electrical isolation from one another, the second
of said screens being in electrical contact with said first
cylinder;
e. means for positioning a sample to be analyzed so that a portion
thereof is along said axis and at the center of symmetry of said
screens;
f. X-ray source means arranged to direct a beam of X-rays onto said
sample to be analyzed;
g. means for applying electrical potential between said first
screen and said second screen and for maintaining constant
predetermined potential between said first and second cylinders;
and
h. electron detecting means positioned at the opposite end of said
first cylinder.
Description
The present invention is directed to apparatus useful in analyzing
the energy distribution of charged particles being emitted from a
test sample after being bombarded with some form of high energy
radiation such as protons, electrons, or X-ray. While an analyzer
in accordance with the invention is anticipated to find a wide
variety of use, it will be described with particularity in its use
in the field of electron spectroscopy for chemical analysis
(ESCA).
In ESCA type analyses, a sample to be analyzed is irradiated with
generally monochromatic soft X-rays. These may be Al K.alpha.
X-rays which have an energy of about 1486 eV. Other sources of
activation may be used, such as an electron gun or high energy
photons. The sample, in turn, gives off emitted electrons which
have an energy range determined by the elements that the sample is
made of. When the energy distribution is determined with
sufficiently high resolution, one can also establish the chemical
environment which a particular atom has based upon the energy
distribution of the emitted electrons.
It is toward the production of an apparatus having an improved
degree of resolution and sensitivity that the present invention is
directed. The objects of the invention are accomplished through a
combination of a specially constructed retarding grid and a double
pass cylindrical mirror analyzer.
IN THE DRAWINGS
FIG. 1 is a side elevational view, mostly in cross-section, of an
analyzer and certain peripheral equipment in accordance with the
invention;
FIG. 2 is a front elevational view of an aperture disc for use in
the apparatus of FIG. 1;
FIG. 3 is a front elevational view of a second aperture for use in
the apparatus of FIG. 1;
FIG. 4 is a front elevational view of a field termination plate
utilized in the apparatus of FIG. 1;
FIG. 5 is a side elevational view of a grid forming mold; and
FIG. 6 is a side cross-sectional view of the cylinder analyzer of
FIG. 1 with a schematic illustration of circuitry for use
therewith.
The resolution of a cylinder analyzer is determined by the geometry
of the instrument. This means that (.DELTA.E)/E is equal to a
constant determined by the geometry of the instrument. If one
reduces the energy of electrons entering the cylinder analyzer, one
can increase the resolution of the instrument. For example, if the
instrument has a designed resolution of 0.3 percent, this means
that for electrons having an initial energy of 1000 eV, the
resolution will be 3 eV. By decreasing the energy of electrons from
an initial energy of 1000 eV to 100 eV, with a retarding grid in
accordance with the invention, one can increase the resolution of
the measurement from 3 eV to 0.3 eV. This resolution proves
adequate for ESCA measurements. I have found, however, that it is
critical that the field utilized to retard the electrons being
analyzed be one of high uniformity and radially directed to the
source of electrons. To provide this highly uniform field, the
retarding grid assembly in accordance with my invention is
constructed of two concentric spherical sections, as will be
described hereinbelow.
Ideally, an analysis should be performed on electrons being emitted
from a point source on the sample being analyzed. If this ideal was
actually the case, the cylinder analyzer would perform in an
optimum manner, as focusing of the electrons analyzed would be less
of a problem. However, in virtually every form of analysis
utilizing a cylinder analyzer, this ideal condition is not met. In
particular, when one utilized X-rays as the irradiating source, it
is extremely difficult to concentrate the X-rays into a region that
could be considered a point source. Imaging of the source of the
electrons in the sample then becomes a problem, and the resolution
of the instrument is decreased. By utilizing a retarding grid
construction in accordance with my invention in conjunction with a
double pass analyzer, as will be described in greater detail below,
my invention accomplishes its purposes of providing high resolution
even though it uses a relatively large source. Through use of my
invention, one can irradiate a large region of the sample without
adversely affecting the analysis, as only those X-ray excited
electrons within a well-defined region are able to pass through
both regions of the analyzer. The double grid increases the virtual
source size by deflecting electrons which do not originate from the
ideal point source. Those electrons which are deflected during
passage through the grids will not have proper trajectories to pass
through both analyzers.
Turning first to FIG. 1, there is illustrated in primarily
cross-sectional view an analyzer in accordance with the present
invention. The apparatus in accordance with FIG. 1 includes an
inner cylinder 11 and an outer cylinder 12, which are held in
concentric alignment with one another by end ceramic plates 13 and
a center ceramic plate 14. Cylinders 11 and 12 are constructed of a
non-magnetic metal like copper, as are other metallic portions
unless noted otherwise. Surrounding substantially the entire
construction is a magnetic shielding member 15 which may be mu
metal.
Inner cylinder 11 is provided with a series of annular openings 16,
17, 18 and 19 around the periphery thereof. Small regions 20 of the
original cylinder wall are retained to support sections of the
cylinder intermediate openings 16, 17, 18 and 19. Openings 16, 17,
18 and 19 are desirably covered by a fine mesh metallic screen
which is desirably of about 100 lines per inch and has a
transparency of about 80 percent. This screen permits the majority
of the electrons or other charged particles being analyzed to pass
through the screen while substantially eliminating electric field
fringing due to the discontinuity of the walls produced by these
openings.
One end of the inner cylinder 11 is partially closed by grids 21
and 22, which will be described in greater detail below. These
grids are sphere segments having a common center of symmetry. At
the center of symmetry is located the sample 23 (shown
schematically) which is to be analyzed by the instrument. That
portion of the sample to be analyzed is located at the center of
symmetry of the grid members 21 and 22, and also on line with the
axis of cylinders 11 and 12. A source of irradiation energy, such
as an X-ray source, shown schematically as 24, is positioned to
direct X-rays to strike as closely as possible to the symmetry
point on the sample. The irradiation gives rise to emission of
electrons from the surface layers of the sample, whose energies are
functions of the composition of the material and of the chemical
environment in which the elements are associated. Some of the
electrons emitted will follow the curved path illustrated by the
dotted line 25 through the analyzer.
Turning to FIG. 5, there is illustrated a form for shaping of the
grid members 21 and 22. Grid members 21 and 22 are formed of fine
wire screen of the same type utilized in covering the openings of
holes 16, 17, 18 and 19. The fine stainless steel screen is shaped
over a block generally designated 26 of a material such as Teflon.
The block will have a radius of curvature corresponding to that
desired for the individual grid. In the instance of grid 21, the
radius of curvature will, of course, be larger than that of grid
22. As typical dimensions, grid 21 will have a radius of about 1
inch, while grid 22 will have a radius of about 0.9 inches. These
are suitable dimensional configurations for a cylinder analyzer
wherein the inner tube has a diameter of about 2 inches, and the
outer cylinder has an inner diameter of about 4 3/8 inches. For
such a system, the overall length would be approximately 11
inches.
In the forming of the screens 21 and 22, a shoulder 27 is provided
on the forming block so as to produce a flanged portion on the
screen. Suitable die means are provided for pressing the screen
member onto block 26 and shoulder 27 to produce the final
configuration.
Referring again to FIG. 1, it will be seen that grid 22 is fastened
by means such as welding to a metal plate 43 which is in electrical
contact with outer magnetic shielding member 15. Shielding member
15 is electrically insulated from the balance of the assembly by
ceramic 13. The inner grid 21 is in electrical contact with and
mounted to a flange 28 projecting into the center of cylinder 11.
Grid 21 and grid 22 are maintained in electrical isolation from one
another by means of ceramic disc 13.
Spacing elements 13 and 14 are provided to maintain cylinders 11
and 12 in fixed relationship to one another and to provide a
special field termination means to prevent field fringing. Each of
these discs 13 and 14 is desirably formed of some electrically
insulating material, which is preferably ceramic. Alumina or quartz
are particularly suitable for the purpose. These discs are provided
on the inner surface thereof with a conductive coating 29, having a
high resistivity of about 30 megohms. This high resistance coating
aids in the reduction of field fringing, which adversely effects
the paths of the electrons as they pass through the analyzer.
As further aid to reducing the field fringing within the device, I
construct the ceramic discs 13 and 14 in such a way that the
surfaces interior to the analyzer have a plurality of concentric
relatively high conducting annular rings, as is best seen in FIG.
4. In FIG. 4 there is generally illustrated a ceramic disc 13 in
front elevational view, which has been provided with a series of
concentric conductors in annular form and identified 30 through 34,
respectively. These rings are concentric with one another and with
the axis of the ceramic plate 13. Rings 30 through 34 are desirably
formed by metallizing the surface of the ceramic discs 13 or 14 and
then, by photolithographic masking and etching, removing the
intermediate metal between the rings. The rings will desirably be
about 0.005 inch in width and a few microns in thickness. Suitable
metals include gold-chromium alloys that have been vacuum sputtered
onto the surface of the ceramic. The function of these rings is to
provide equi-potential regions on the discs so as to minimize the
effects caused by regions of resistivity that are not uniform
within film 29.
As an alternative to the construction shown in FIG. 1, the spacer
13 at the sample end may be in the form of a truncated cone rather
than a flat washer shape as shown. The forward edge of cylinder 12
would then be offset away from the sample, thus allowing a more
simple positioning for the activation sources 24. These features
are described in greater detail in my copending application filed
of even date herewith entitled FIELD TERMINATION PLATE.
It should be noted with regard to FIG. 1 that the intermediate
ceramic disc 14 has been treated on opposite surfaces thereof in a
manner analogous to that described with regard to FIG. 4. It should
also be appreciated that the conductive coating 29 is in electrical
contact with cylinders 11 and 12 at the inner and outer extremities
thereof. This can be readily achieved by metallizing the inner and
outer edges of the ceramic discs 13 and 14.
At a point midway between the ends of inner cylinder 11, I provide
an intermediate aperture disc 35. The aperture disc 35 is secured
in fixed relationship to cylinder 11 by a mounting means generally
indicated as 36. Disc 35 is shown in front elevational view in FIG.
2 and consists of a thin metallic disc (desirably of molybdenum) of
about 0.003 inches thickness, having an opening 37 through the
center thereof of about 0.036 inches diameter. This narrow aperture
37 acts as a filter to provide essentially a near point source of
electrons for the second stage of the analyzer.
At the opposite end of the analyzer from sample 23 is a second set
of aperture members mounted within a suitable mounting means
generally identified 38. The end apertures 39 and 40 correspond to
the configurations shown in FIGS. 3 and 2, respectively. Disc 40 is
substantially identical to that of FIG. 2, both in thickness and
dimension, while disc 39 is, as shown in FIG. 3, formed of a thin
molybdenum disc of about 0.003 inches thickness, having a plurality
of annular ring segment openings 41 that are positioned concentric
with, but off of, the central axis of the disc. The function of
such an off-axis aperture in disc 39 is described in the copending
application of Bohn et al., Ser. No. 68,983, for AUGER ELECTRON
SPECTROSCOPY.
As a final termination, there is provided a tubular member 42,
which functions in the invention as the first dynode of a
photomultiplier.
It will now be apparent to the reader that the interaction of the
construction in accordance with the invention will provide an
instrument capable of precise determination of the energy
distribution of X-ray excited electrons over a wide energy range.
The spherical grids 21 and 22, due to their configuration and
positioning, act to controllably decrease the energy of the
electrons passing therethrough so as to provide a resolution of
measurement down into a range necessary for determining the atomic
number and chemical state of the atom from which the electron
originated. By use of the double pass cylindrical tube analyzer in
accordance with the invention, the source of electrons is redefined
as the electrons pass through the intermediate aperture 37. The
second stage of the double pass analyzer has, by virtue of the
aperture 37, a "source" at a precisely known distance from the
detector 42. Thus the energies of the electrons passing through the
second stage is precisely defined by the potential between inner
and outer cylinders 11 and 12. The detector for the electrons is a
electronmultiplier, of which element 42 forms the first dynode. The
double pass arrangement further provides a construction wherein
magnetic effects are essentially eliminated by the interior
position of the entire second stage of the double pass analyzer.
The absence of magnetic interference further provides a higher
degree of precision of the overall instrument.
Turning now to FIG. 6, there is illustrated in cross section the
analyzer in accordance with FIG. 1, with the principal portions
thereof being shown in essentially schematic form. Electrical
connections are made as shown to the various portions of the
analyzer. The outer mu metal shield 15 is connected by a lead 44 to
ground and to one side of a voltage programmable power supply. The
outer cylinder 12 is electrically connected by lead 45 to a first
floating power supply 47, while inner cylinder 11 is connected by
means of lead 46 to the output of a second floating power supply
48. It will be noted that grid 21 is at the same potential as inner
cylinder 11, while grid 22 will be at the same potential as mu
metal shield 15, which is at ground. With power supplies 47 and 48
joined to cylinders 11 and 12 as shown, it will be appreciated that
the potential of cylinder 12 will be at a value equal to the sum of
power supplies 47, 48 and 50 while the potential of cylinder 11
will be at the potential of power supplies 48 and 50 alone.
A digital ramp generator 49 supplies the timing logic both to the
voltage programmable power supply 50 and to the pulse counting
electronics and digital logic unit. It also supplies a signal to an
X-Y recorder as schematically illustrated in the figure. As the
receiving element for the charged particles (electrons) which pass
through the detector, there is an electron multiplier, which is
shown as having a power supply, with its output of the multiplier
going to a pulse counting electronic and digital logic unit. A
digital to analog converter is utilized to convert the signal which
then passes to the X-Y recorder as the Y axis signal.
Various commercial units are available for each of the units of
electronics utilized in FIG. 6. As illustrative of suitable units,
the floating power supplies 47 and 48 may conveniently be Hewlett
Packard Models 6209B. The voltage programmable power supply 50 may
be Model OPS--2000 of the Kepco Company of Flushing, New York. A
digital ramp generator 49 is conveniently a laboratory computer
PDP8/E, and a D/A convertor available from Digital Equipment
Corporation of Maynard, Massachusetts. A conventional X-Y recorder
can be utilized such as Model 7004B of the Hewlett Packard
Company.
In operation, the system of FIG. 6 functions substantially as
follows. Sample 23 is irradiated by suitable means so as to emit
electrons. These electrons then pass through screens 22 and 21,
which are maintained at ground and at some suitable elevated
negative potential, respectively. A scanning potential is applied
to grid 21 to slow down the electrons passing between the two grids
to a value such that the resolution of the instrument can detect
the necessary differences in electron energy required for the ESCA
measurement. The voltage V.sub.p applied between the inner cylinder
11 and outer cylinder 12, where 12 is negative with respect to 11,
is the magnitude of the voltage that determines the pass energy
E.sub.p of the double pass analyzer:
E.sub.p = k V.sub.p
where k is a constant and depends upon the diameters of the inner
and outer cylinders of the analyzer. The k is fixed for a
particular analyzer geometry. In order to scan the range of
energies of the electrons being emitted by the sample, one changes
the potential of inner cylinder 11 and thus grid 21 to suitably
slow down the electrons. A constant differential between the two
cylinders is maintained by means of a floating power supply 47.
Alternatively, one may hold the potential between grids 22 and 21
at a constant value and scan by changing the potential between
cylinders 11 and 12. However, under such a condition the energy
spread, .DELTA.E, of the instrument changes and makes measurements
more complex.
Floating power supplies 47 and 48 are chosen so that their ratios
of voltage are such that the energy of the electrons incident on
grids 21 and 22 and passed by the analyzer is equal to V.sub.e, the
voltage of the voltage programmable power supply. It should be
appreciated that power supply 48 will be positive with respect to
power supply 50 so that the scans can be made down to zero
energy.
As the electrons emitted from sample 23 pass through the grid
arrangement 22-21 and enter the analyzer via opening 16, they are
deflected by the more negative potential of outer cylinder 12 and
follow the path generally marked 25. In this first stage of the
analyzer, the electrons are partially resolved and generally are
focused on plate 35 containing the aperture 37. Aperture 37
functions to further resolve the electrons into a nearly point
source of definite size and distance from the ultimate receiver 42.
In the second stage of the analyzer, the electrons which pass
through opening 37 are further resolved and analyzed by passage
through openings 18 and back through 19 and are ultimately focused
by means of the double aperture system 39 and 40. The resulting
analysis has a high signal to noise ratio and permits far greater
precision of measurements than has been known with instruments
known heretofore.
* * * * *