The invention relates to a charged particle beam apparatus, comprising
a particle source for generating a particle beam, a multiple electrode system for
collimation and/or acceleration of the particle beam, and a device for a spatial
separation of electrons and ions.
An apparatus of this kind is known from EP 209389. In an apparatus
described therein, ions formed by the electron beam are removed from the beam path
of the electron beam by means of a cascade system of electrodes. For the separation
use is made of the difference in kinetic energy between electrons desired for further
use and nondesired ions.
An electrode system of this kind, however, will also have an effect
on the electron beam. In order to avoid the adverse effects of such influencing,
use is made of a cascade system of, for example 8 alternately positively and negatively
charged pairs of deflection plates. Because the adverse effects on the electron
beam can thus be reduced, they cannot be fully eliminated in this manner. Moreover,
the separating system has a comparatively complex construction and requires very
high precision as regards positioning and control.
It is an object of the invention to mitigate the described drawbacks;
to achieve this, an apparatus of the kind set forth in accordance with the invention
is characterized in that the particle separating device comprises a magnetic deflection
device which cooperates with an electro-static deflection device.
A beam deflection system included in a multiple electrode system
of an apparatus in accordance with the invention deflects the entire beam of charged
particles away from an optical axis, the beam being deflected back by a magnetic
beam alignment system so that it is again made to extend along the optical axis,
preferably at a comparatively short distance before an object to be irradiated.
For the invention it is irrelevant whether the magnetic field and
the electrostatic field act in the same space or a different locations and in which
sequence they are arranged. A set up comprising first an electrostatic deflection
field and, situated comparatively near the object, a magnetic redeflection field
offers the advantage that the beam to be effectively used extends again along the
optical axis only over a slight distance, or a main ray of the redeflected beam
is directed transversely of an object to be irradiated.
In a preferred embodiment, the electrostatic deflection system is
formed by an adapted electrode construction in a multiple acceleration system of
the apparatus. The electrostatic deflection system notably comprises a conically
arranged electrode system. This is achieved notably by imparting a conical shape
to one of the electrode systems. To the invention it is irrelevant whether the
charged particle beam to be effectively used is an electron beam or an ion beam.
Preferably, the entire beam is deflected away from the optical axis and the beam
of particles to be used is made to extend along the optical axis again. The latter,
however, is not a condition for the invention, because use can also be made of
a beam which is incident at an angle or of an object which is arranged so as to
be inclined, for example transversely with respect to the incident beam.
Some preferred embodiments in accordance with the invention will
be described in detail hereinafter with reference to the drawing.
The sole Figure of the drawing diagrammatically shows an apparatus
in accordance with the invention, comprising a particle source 2 which is accommodated
in a source housing 4 and which comprises an emitter 6, for example a filament
wire, an emission needle, a lanthanum hexabromide emitter, a semiconductor emitter
and the like as an electron source (for example, see EP 192244), or it comprises
an ion source, for example as described in EP 80170, EP 37455 or US 3,731,089.
Of the source only a first anode 8 with an aperture 10 is shown. Using the anode,
and possibly a grid, the beam current can be controlled. The source housing 4 can
be coupled, via a flange coupling 12, to an electrode system housing 14
which is similarly connected to an object housing 18 by way of a flange coupling
16. An electrode system 19 of the present embodiment comprises a system of acceleration,
deflection or collimation electrodes 20, an electrode 201 which is situated nearest
to the source comprising an anode 22 having an aperture 24 wherethrough a particle
beam 25 enters the apparatus. For the deflection of the beam 25 a conical electrode
system 26 is arranged between two successive electrodes 20. The electrode system
26 in this case comprises a conical disc 28 and a conical disc 29, which discs
are mounted against successive electrodes 202 and 203 of the electrode system 19.
Facing surfaces of the conical discs preferably extend in parallel. Also viewed
from a point of conductivity, the conical discs may be integral with the relevant
electrodes 20, in which case they will carry corresponding potentials. Beam deflection
is thus realized with only minor modifications to the acceleration system and its
The degree of beam deflection is determined by the cone angle which
amounts to, for example from 10° to 15°, and the cross-section of the beam aperture
which amounts to, for example 20 mm. Electrostatic controls are not required in
this respect and the potentials of the two modified electrodes 20 need in principle
not be changed.
Alternatively, the conical discs 28 and 29 can also be arranged so
as to be insulated from the electrodes 20 and be provided with electrical power
supply leads for an electrostatically controllable degree of beam deflection. Using
a conical electrode system, the beam 25 is deflected, for example through an angle
30 and enters an object space 33, via a final electrode 204, at said angle
with respect to the optical axis 31 of the apparatus. In order to prevent beam
disturbances, the final electrode may alternatively be non-axially arranged or
be provided with a larger aperture 34. It is to be noted that the beam deflection
angle 30 is comparatively small and amounts to, for example only a few degrees.
The beam shift with respect to the optical axis 31 at the area of the electrode
204 then amounts to at the most approximately 1 mm in the case of a small distance
of, for example 10 mm between the electrode 204 and the deflection point 32 of
For example, the electron beam is directed as a pure electron beam
45 from the deflected beam 25 to an object 46 by means of a magnetic beam alignment
system 40 comprising two deflection coils 42 and 44 which act in mutually perpendicular
directions. The object 46 is mounted on an object carrier 48 whereby the object
can be translated and possibly also rotated and tilted. Signals generated by the
beam 45, for example secundary electrons, X-rays, reflected electrons and transmitted
electrons, can be measured by means of detectors 50. Via the object carrier,
electrons intercepted by the object can also be measured.
Ions present in the beam 25 are not deflected or only slightly deflected
by the beam alignment system and can be intercepted, for example by means of a
For the selection of a pure ion beam from a beam which also contains,
for example electrons, in principle the same approach can be used; however, preferably
the electrons are deflected again for interception by a diaphragm, an object to
be irradiated being oriented about a main ray of the electrostatically deflected
beam. It may then be advantageous to reverse the position of the electrostatic
deflection and the magnetic deflection, viewed in the direction of the beam path.