Note: Descriptions are shown in the official language in which they were submitted.
PHN. 8128.
1061477
The invention relates to an electron micros-
cope, comprising an electron gun for generating an illu-
minating electr~n beam and a devioe for the apparent
enlargement of the aperture of the illuminating electron
beam.
Electron microscope, notably high-resolution
types, have a drawback in that, due to the c~mparatively
small aperture of the illuminating electron beam, the
depth of focus of the image is so large that exact
adjustment of an image plane is impeded thereby.
British Patent Specification 687,207 which
issued to MetrDpolitan-Vickers Electrical Cbmpany
Limited and published on February 11, 1953 describes
an electron microscope in which it is attempted to eli-
minate this drawback by superposing an alternating image
having a comparatively high frequency on the focussing
field of the condenser lens. The focal point of the
condenser lens is then displaced over a given traject
along the opkical axis of the electron microscope. This
re~llts in a variable object distan oe for the objective
lens, a varying aperture for the illuminating electron
beam thus being r~l;zed, so that optimum focus can be
more readily adjusted. However, this is acco~panied by
an additional contribution to spherical aberation, a
restriction in the freedom of choice of the exposure
intensity in the object, and a variation of the real
beam aperture across the object. ~ue to the varying
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` 10~1477
P~N. 8128.
. 1-8-1976.
brightness in the image, this method cannot be used for
making recordings.
The invention has for i-ts object to provide
an electron microscope in which a comparatively large
apparent aperture can be realized without giving rise
to the said drawbacks, To this end, an electron micros-
cope of the kind set forth according to the invention
is characterized in that the device ~or the apparent
enlargement o~ the aperture comprises a beam deflection
device which includes a power supply source ~or tilting
the illuminating electron beam about a point situated
in an object plane.
In an electron microscope in accordance with
the invention, the beam current is not at all influenced
for introducing a quasi large aperture, because the
transverse dimen~ion of the electron beam remains constant
in the object plane, and no axial shift of the object
point in the illuminating beam occurs either. ~riewed
in the transverse direction, an object of the illuminat-
ing beam describes a rather arbitrary figure about the
optical axis. The transverse dimensions of this figur~
as well as the registration in the figure can be chosen
substantially at random, provided that the tilting point
is always situated in the object plane. Because no de-
focussing of the beam occurs, the illumination intensity
in the image plane is constant, and the pursued apparent
coherence increase of the illuminating beam can also be
used during the making or studying of recordings. In a
10~477 PHN. 8128.
1-8-1976.
preferred embodiment it is possible, for the non-axi~l
passing of the e]ectron beam through the objective lens,
to apply a given correction, coupled to the degree of
transverse displacement of the beam, to the excitation
of the relevant lens.
Some preferred embodiments of an electron
microscope in accordance with the invention will be
described in detail hereinafter with reference to the
drawing. The drawing diagrammatically shows an electron
microscope comprising an electron gun 1 which preferably
comprises a field emission source 2 for generating an
electron beam having a comparativcly high density or
a comparatively small cross-section. Opposite the emis-
sion source 2 there is arranged~an anode 3 and, proceed-
ing in the direction of the electron beam, a succession
of a beam alignment device 4~ a condens~r lens 5, a beam
wobbler 6, an objective lens 7 comprising pole shoes 8
an object 9 comprising an object adjustment device 10,
an intermediate lens 11, and a projection lens 12. An
image space 13 of the electron microscope accommodates
a camera 14 and a phosphor screen 15 which can be ob-
served through a window 16. All electron-optical ele-
ments are accommodated in a housing 17 which comprises,
besides the window 16, a passage 18 for applying the
necessary potentials and vacuum duct 19, comprising a
connection 20 for a pumping device 20.
A power supply source 21 for the wobbler is
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1061477 PHN. 81Z8.
1-8-1976.
constructed in accordance with the invention so that
a tilting movement about a point 22 in the object can
be imparted to an electron beam. The power supply source
is preferably also controlled by the high voltage to
be applied; this is diagrammatically shown by way of
the connection line 23.
A beam wobbler usually comprises two sets of
electromagnetic coils whereby an electron beam is first
deflected in known manner from the optical axis 24 and
subsequently deflected again to a given point on the
optical axis, The latter point on the optical axis is
referred to as tho -tilting point in which the bea~ is
incident from all directions within a conical circum-
ference. ~ base plane of this co~ne is Ol is not scanned
by the beam; consequently, viewed in the time, the beam
can describe only the conical surface of fill the
entire cone. The base surface of the cone may have
any shape, For reasons of s~nmetry, a circle or a rec-
tangle is preferably used as the base. Notably in the
case of a circle, the beam can then describe a helix,
for example, by utilizing power supply by means of two
sine wave generators, powered slightly out of phase,
whilst in the case of a rectangle, the movement of
the beam is that of a scanning bcam, as in a television
system, by way of a corresponding power supply.
If the wobbler is actuated in this manner,
the object is exposed, measured in the time, with an.
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1061477 PHN. 8128.
1-8-1976.
.
aperturc which is given by the apex of the cone and
which, obviously, may amount to as much as, for example,
1000 times the aperture of the electron beam itself.
During this beam deflection, the beam current
is not influenced and the object point is not shifted
in the axial direction for the exposure. Like in the
said British Patent Specification 687,207, correction
for spherical aberration is also possible. To this end,
it is merely necessary in an electron microscope in
accordance with the invention to control the appropri-
ate lens, in this case, for example, the objective lens,
in dependence of the radial position of the electron
beam. Contrax~y to the correction in the known electron
microscope, the beam in the electron microscope in ac-
cordance with the invention is n~t wider, but only does
not pass paraxially through the lens. On the basis of
a paraxial beam path of the non-deflected electron beam~
the spherical aberration which would occur due to the
deflection can thus be fully compensated for. As a
result of the addition of an adjustable control member
for the power supply of the deflection device, a pre-
ferred embodiment of an electron microscope in accord-
ance with the invention enables operation with a conical
exposure with a presettable apex or series of apexes.
Exposure is also possible from a fixed point or a series
of fixed points outside the optical axis. As resultS the
nature of the exposure can be adapted, for example, to
special properties of the object to be examined.
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