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Патент USA US3100265

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Aug. 6, 1963
A. P. WILSKA
3,100,260
ELECTRON LENS FOR REDUCTION OF' SPHERICAL ABERRATION
Filed Nov. l5, 1961
W.E
Wif_/7:@
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United States Patent O ”
l
CC
Patented Aug. 6, 1963
2
the pole pieces ‘19 and 211 is substantially rotationally
3,160,260
ELECTRÜN LENS FÜR REDUCTION GF
SPERICAL ABERRATIÜN
symmetric with respect to the electron beam axis 17.
Located within the magnetic field, that is, within the
gap between the pole pieces 19 and 21, is an electrode
Alvar P. Wiiska, Tucson, Ariz., assignor to Philips Electronies and Pharmaceutical industries Corp., New York,
N.Y., a corporation of Maryland
Filed Nov. 15, 1961, Ser. No. 152,531
15 Claims. (Cl. 250-495)
system comprising an elongated, cylindrical, central elec
This invention relates to an electron lens system for
nular electrode 26 which, in the embodiment shown, is
trode Z4 that operates as a central stop to prevent the
central portion of the electron beam 18 from continuing
along the axis 17. Surrounding the central electrode 2.4
and concentric therewith and spaced therefrom is an an
reducing the spherical aberration that is present in all
magnetic electron lenses.
In accordance with the present invention an electro
shorter than the central electrode in a direction meas
ured along the axis 17.
A difference of potential is maintained between the
static lens is placed in the lens region of a magnetic lens
central electrode 24 and the annular electrode 26 and
in order to correct the spherical aberration of an annular
this difference of potential is polarized so as to make the
region of the electron beam. The electrostatic lens com
annular electrode negative with respect to the central
electrode 24. To give some ‘idea of the magnitude of the
prises a central electrode, which is located on the elec
tron beam axis within the magnetic iield of the mag
potential between these two electrodes, it has been found
netic lens, and an annular electrode surrounding the cen
that a voltage of about 300 bolts is suiiicient to provide
tral electrode and operated so as to be electrically nega 20 satisfactory spherical aberration correction of an elec
tive with respect to it. The central electrode is cylin
tron beam in which the electrons have been accelerated
drical and serves as an elongated central stop and the
by a voltage ñeld created by making the anode 16 about
electric field strength between the central electrode and
15,000 volts positive with respect to the `cathode `13. In
the annular electrode decreases logarithmically, as meas
this environment, and with the central electrode operating
ured along a radius perpendicular to the beam axis. This 25 at substantially the same voltage as the anode 16», a suit
radially directed electric field causes greater convergence
able electrostatic field is established between the central
in 'the inner parts than in the outer parts of the hollow i electrode 24 and the annular electrode 26 to correct for
electron beam that passes through the gap between the
spherical aberration of the electron beam or at least of
central electrode and the annular electrode which is op
that annular portion of the beam which is able to pass
posite to the effect of spherical aberration produced by 30 through the gap between the central electrode and the
annular electrode.
the magnetic lens. In order properly to define the iield,
an additional electrode surrounds the annular electrode
The effect of the field between the two electrodes is
except in the gap between the latter and the central elec
such as to deñect more strongly toward the axis 17 those
trode. The additional electrode has annular portions
electrons which come closer to the annular electrode 26
spaced from each end of the annular electrode and the 35 than those electrons that pass through the gap in regions
inner edges of these annular portions form annular gaps
closer to the central electrode 24. By appropriate ad
with the outer edges of the ends `of the central electrode
justments of both the magnetic and the electric com
to deiine the portion of the beam that will be acted upon
ponents of the field within the gap between the pole
by the electric iield.
pieces 19’ and 211 substantially complete compensation of
The invention will be described in greater detail in 40 spherical aberration can be achieved for a narrow annular
Zone of electrons.
connection with the drawings, in which:
`FIG. l is a simpliiied representation of an electron
In order to define more precisely the dimensions of the
beam device having two electron 'lenses with spherical
annular zone of electrons to be atfected by the ñeld be
aberration-reducing electron systems in .accordance with 45 tween electrodes 2.4 and 26, an additional electrode Z7 is
the present invention; and
placed around the electrode 26. As may be seen, the
electrode 27 consists of two annular discs 28» and 29
which are spaced from opposite ends »of the electrode 26
in FIG. 1.
and the perimeters of which are joined by a short, hol
In FIG. 1 an electron beam `device 11 enclosed within 50 low tube ‘31 so that the annular electrode Z6 is sur
a chamber bounded by a wall 12 includes an electron
rounded on three sides by the additional electrode 2.7.
The annular zone of electrons that is able to pass
source, or cathode, 13 together wit-h a control electrode,
or grid, 14. An accelerating anode 16 is connected to a
through the electrode system impinges upon an object `31.?,
suitable source of potential to draw electrons `from the
which may be, for example, a specimen in an electron
cathode 13 along a direction generally parallel to an axis 55 microscope. Thereafter the annular electron beam 18,
17. ‘If necessary, additional electrodes or magnetic fields
having been modified by the specimen 32, is magniiied by
may be provided, as is well known in the electron beam
a second electron lens in the region between the magnetic
art, to form the electrons into a beam 18.
pole pieces 33 and v34». This lens system is, in principle,
The electron beam 18 travels into the lens region of a
the same as the lens system which was just described, and
magnetic iield defined by pole pieces 19 and 21 which 60 in addition to the pole pieces 33` and 34 and the rotational
are at opposite magnetic polarities in order to provide
ly symmetrical axial magnetic field therebetween 4there is
an «axial magnetic field in the axial space between the
an electrode system comprising a central cylindrical elec
pole pieces. For the sake of simplicity, it may be as
trode 36 surrounded by an annular electrode 37 which,
sumed that the pole pieces 19 and 21 are parts of a struc
in turn, is surrounded on three `sides by an additional elec
ture which is permanently magnetized with north and 65 trode 3S »comprising annular discs 39 and 41 joined by a
south poles as indicated by the letters N and S, although
short tube 42. The additional electrode -38 and the central
it will be understood that the magnetic polarization may
electrode 36 operate at the same electrical potential while
be induced by current flowing in a coil of wire properly
the annular electrode 37 `operates at a potential which
situated with respect to the pole pieces 19 and 21. Both
is negative ywith respect thereto.
of the pole pieces 19 and 21 have central apertures de 70
The magnified electron image of the specimen 32 may
FIG. 2 shows a more detailed cross-sectional View of
the electrode structure for use in a lens of the type shown
iined by walls 22 and 23, respectively, through which the
electron beam 18 passes, and the magnetic field between
be further magniiied by additional electron lenses, if de
sired, and may be rendered visible by being directed
3,100,260
3
against a fluorescent screen 43 at the opposite end of the
device ‘.11 from the origin of the electron beam 18.
FIG. 2 shows the electrode system of FIG. l in greater
detail. The central electrode -124 is in the form of a
solid metal post having an outwardly extending flange 44
at its upper end. The upper surface of the flange 44 is
ñat While the lower surface of the ñange has a frusto-
beam axis; a central electrode located on said beam axis
Within said magnetic field; and a second electrode annu
strike the tapered surface. Therefore, if bits of dust be
come attached to the tapered surface, they do not become
charged by the electrons of the beam. A similar frusto
conical flange 46 is located at the other end of the elec
rotationally symmetric magnetic ñeld along an electron
ceramic posts 47 and 48. Post 48 is formed with a chan
nel through it to permit an electrode lead 49 to be con
nected to the annular electrode 126 in order to operate
beam travels lalong a beam axis, an electron lens on said
axis comprising: means for forming a substantially rota
larly spaced from said central electrode and substantially
coaxial therewith, said second electrode being operable
at a negative voltage with respect to said central electrode
to cause electrons passing through the annular space be
tween said electrodes to be converged toward said axis
more strongly the closer said electrons are to said central
conical shape. The edge of the flange is therefore rela
electrode.
tively sharp and electrons which strike the flange either
2. An electron lens comprising means for producing a
land on top of it or at the very edge, but they cannot 10
beam axis; an elongated, cylindrical, central electrode lo
cated on said ‘beam axis Within said lmagnetic: field; and
a second electrode annularly spaced from a central region
trode 124 and is provided with a tapered under-surface 15 of said ‘central electrode and substantially coaxial therewith, said second electrode being operable at a negative
similar to that of flange 44 and for the same purpose.
voltage with respectV to said central electrode to cause
The annular electrode 126 is in the form of a metal ring
electrons passing through the annular space -between said
which surrounds the central area of the central electrode
electrodes to be converged toward said axis more strongly
17A and is spaced at some distance from the central elec
trode and is substantially concentric therewith. The an 20 the closer said electrons are to said central electrode.
3. In an electron beam device in which the `electron
nular electrode is supported by insulating means, such as
tionally symmetric magnetic ñeld on said axis; a cylin
this electrode at the desired voltage.
25 drical central electrode within said iield and coaxial with
said beam axis; a first annular electrode concentrically
The insulating posts 47 and 48 are, in turn, supported
spaced from said central electrode and coaxial therewith
by a ‘hollow cylinder `131, which is part of an additional
and negatively electrically polarized with respect to said
electrode 127. The additional electrode 12.7 has two an
central electrode; and a second annular electrode com
nular disc portions 12S and 129, the latter being formed
integrally with the cylindrical portion 131 Iwhile the disc 30 prising a first annular portion axially spaced from said
first annular electrode in one direction along said beam
portion 128 is bolted to the other end of the cylindrical
axis and a second annular portion axially spaced from
portion 131 by a plurality of machine screws 51. Both
said ñrst annular electrode in the opposite direction along
of the disc portions 12S and 129 have central apertures
said beam axis, said second -annular electrode having
«which ,are concentric -with the axis 17 that passes through
the center of the electrodes 124 and i126. Furthermore, 03 substantially the same electrical potential as said central
electrode and being coaxial therewith.
both of the annular disc portions ‘12S and -129 are formed
4. The electrode system of claim 3 in which the central
so that the edges of their respective apertures have tapered
apertures of Said first and second annular portions are
surfaces 52 and 53, respectively. These tapered surfaces
smaller than the central opening in said first annular
are provided for the :same purpose as the tapered surfaces
of the ñanges 44 and `46 and in addition the sharp edge 40 electrode whereby said first annular electrode is shielded
on both axial ends by -said first and second annular por
of the disc portion 128 is substantially coplanar with the
sharp edge of the flange 44 while the sharp edge of the
tions.
5. ln -an electron beam device in which the electron
disc portion 129 is substantially coplanar Iwith the sharp
beam travels along a beam axis, an electron lens on
edge of the lower flange 46. These edges deñne the an
said axis comprising: a rotationally symmetric magnetic
nular zone of the electron beam 18 that can pass through
lield substantially coaxial with said beam axis; and an
the electrode system.
electrode structure located Within said magnetic iield and
The electrode system is supported from the wall 1-2 of
l
the electron beam device on a flat plate 54 which has a
central opening 56 into which an axial extension 57 of
comprising a central `electrode on said beam axis, a first
annular electrode surrounding said central electrode and
the central electrode 124 is inserted. A plurality of aper 50A electrically insulated therefrom, said first annular elec
trode being electrically negative with respect to said cen~
tures 58 in the plate 54 permits the electrons of the an
tral electrode, a second annular electrode substantially
nular zone of the electron beam 18 to pass through and
coaxial with said central electnode and axially displaced
to continue toward the utilization area of the device.
from said ñrst annular electrode in one direction along
These apertures 58 may be of circular cross-section, if
desired, or they may be in the form of radial tins, provided 55 said beam axis, and a third annular electrode substantially
coaxial with said central electrode and axially displaced
that they are suiiiciently strong to support the central
electron rigidly. The upper surface of the plate S4 has
vfrom said first annular electrode in the opposite direction
an annular groove 59 directly over the apertures 58 for
«along said beam axis, lsaid second and third annular
electrodes being at substantially the same electrical poten
the purpose of removing the upper surface of the ñns or
of the plate as Afar as possible, consonant with require 60 tial as said central electrode.
’
ments of rigidity, from the region of the electrode system
6. >In an electron beam device in which the electron
so that the electric iield with the electrode system will
beam travels along a beam axis, an electron lens on said
be substantially unalîected ‘by the plate 54. Since the
axis comprising: a rotationally symmetric magnetic field
electrodes 124 and «127 are normally operated at the same
substantially coaxial with said beam axis; and an elec
electrical potential, the metal plate 54 serves as a suitable 65 trode structure located within said magnetic ñeld and
means for connecting the proper potential to the central
comprising an electron-impermeable central electrode on
electrode 124.y
said beam axis, a íirst annular electrode surrounding the
While this invention Vhas been described in specific
central region of said central electrode and spaced and
terms, it will be realized by those skilled in the art that
electrically insulated therefrom, said first annular elec
the embodiments shown do not constitute limitations on 70 trode being electrically negative with respect to said cen
the invention but on the contrary the true scope of the
tral electrode, -a second annular electrode coaxial with
said central electrode and axially displaced from said
invention is determined by the ‘following claims.
ñrst annular electrode in one direction along said beam
What is claimed is:
axis; and a third :annular electrode coaxial with said
l. An electron lens comprising means for producing a
rotationally symmetric magnetic iield along an electron 75 central electrode and axially displaced from said tirst an~
3,100,2eo
5
nular electrode in the `opposite direction along said beam
axis, said second and third annular electrodes being at
substantially the Same electrical potential as said central
electrode.
7. An electron lens comprising means for producing a
rotationally symmetric magnetic field along an electron
beam axis; a cylindrical central electrode located `on said
vbeam axis within said magnetic iield; a second electrode
annularly spaced from a central region of said -central
electrode and substantially coaxial therewith, said second
electrode being operable at a negative voltage with re~
spect to said central electrode to cause electrons passing
through the annular space between said electrodes to be
converged toward said axis more strongly the closer said
ö
outside of said hollow cylindrical electrode, said third
electrode having substantially the same electrical poten
tial as said central electrode with Athe centers of the aper
tures of both of said disc portions being located on said
beam axis.
l0. rThe electron lens of claim 9 lin which said central
electrode has la radial llange at each end thereof sub
stantially coplanar with said lirst anfd second disc por
tions, respectively.
ll. The electron lens of claim l0A in which each of
said radial flanges has a ilat surface facing in the direc
tion of origin `of the electron beam and ya conical surface
facing in the opposite direction and tapered inwardly, and
each of said annular disc portions has a centnal »aperture
electrons are to said central electrode; and a third elec
trode electrically connected to said central electrode and
defined by an edge which is substantially flat in the direc
comprising a tubular portion surrounding said second
wardly tapered surface extending longitudinally along the
electrode and insulated therefrom and a pair `of annular
discs extending inwardly toward said central electrode to
form a pair of annular electron beam gaps therewith.
8. An electron lens comprising means for producing a
tion facing the origin of `the electron beam and an out
thickness of said annular disc portions.
l2. in an electron beam `device in which the electrons
travel along a beam axis, an electron lens on said axis
comprising: a rotationally »symmetric magnetic field sub
stantially coaxial with said beam axis; a cylindrical elec
tron~impermeable central electrode having an axis sub
located on said beam axis within said magnetic field; a
stantially coincident with said beam axis and being Ilo
pair 0f laterally extending :ñanges near each end of said 25 cated within said magnetic held; la first hollow electrode
central electrode; a second electrode annularly »spaced
surrounding la central region of said central electrode and
from a central region of said central electrode and sub
electrically negative with .respect :to said central electrode;
stantially coaxial therewith, said second electrode being
a third electrode comprising a lirst Aannular disc portion
rotationally symmetric magnetic ñeld along an electron
beam axis; an elongated, cylindrical, central electrode
operable at a negative voltage With respect to said cen
adjacent to one end of said central electrode, a second
tral electrode to cause electrons passing through the 30 annular disc portion adjacent »to the lother `end of said
annular space between said electrodes >to be converged
central electrode, both of said Iannular `disc portions being
toward said axis more strongly the closer said electrons
axially spaced from and insulated from said ñrst hollow
are to said central electrode; and a third electrode corn
electrode, and la tubular portion of said third electrode
prising a tubular portion substantially coaxial with said
second electrode and annularly spaced therefrom and a
pair of annular discs extending inwardly from said tubular
portion and substantially coplanar -with said flanges to
form annular electron beam gaps therewith.
9. In an electron beam device in which the electron
beam travels along a beam axis, an electron lens ron said
joining the periphenies tof said ñr‘st and second disc por
tions, said tubular portion encircling and being annularly
spaced from said ñrst hollow electrode; a conductive
member; a pin extending Ifrom said central electrode into
sai-d conductive member to support said central electrode;
an annular groove in the surface ‘of said conductive mem
ber facing said central electrode; and a plurality Vof `open
axis comprising: a rotationally symmetric magnetic field
ings extending from the bottom of said groove through
substantially coaxial with said beam axis; -a cylindrical
said conductive member to permit electrons of said beam
electron-impermeable central electrode having an axis
to pass therethrough.
substantially -coincident with said beam axis and being
13. rThe electron lens ‘of claim l2 «in which said con
located within said magnetic ñeld; a ñrst hollow cylin 45 ductive member comprises a `support for said third elec
drical electrode surrounding a central region of said cen
trode.
tral electrode and concentrically spaced therefrom and
14. rlîhe electron lens of claim l2 in which said open
coaxial therewith and electrically negative with respect
to said central electrode; and a third electrode compris
ing a ñrst annular disc portion substantially coplanar
with one end of said central electrode yand axially spaced
from said hollow cylindrical electrode in one direction
along said beam axis, a second annular disc portion sub
stantially coplanar with the yother end of said central
electrode and axially spaced from said hollow cylindrical
electrode in the opposite direction along said beam axis,
and a portion joining said ñrst and second 4disc portions
ings iare circular in cross-section :and are spaced apart
in a circle which is coaxial with said axis.
l5. The electron lens of claim l2 in which said open
ings ia-re wedge~shaped and are separated by radial wall
sections formed out Áof `said conductive member.
References Cited in the file of this patent
UNITED STATES PATENTS
2,862,129
‘Van Dorsten ________ __ Nov. 25, 1958
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