FIELD OF THE INVENTION
The invention relates to charged particle beam devices for inspection
system applications, testing system applications, lithography system applications
and the like. It also relates to methods of operation thereof. Further, the present
invention relates to high current density particle beam applications. Specifically,
the present invention relates to charged particle beam devices and methods of using
charged particle devices.
BACKGROUND OF THE INVENTION
Charged particle beam apparatuses have many functions in a plurality
of industrial fields, including, but not limited to, inspection of semiconductor
devices during manufacturing, exposure systems for lithography, detecting devices
and testing systems. Thus, there is a high demand for structuring and inspecting
specimens within the micrometer and nanometer scale.
Micrometer and nanometer scale process control, inspection or structuring,
is often done with charged particle beams, e.g. electron beams, which are generated
and focused in charged particle beam devices, such as electron microscopes or electron
beam pattern generators. Charged particle beams offer superior spatial resolution
compared to, e.g. photon beams due to their short wavelengths.
Besides resolution, throughput is an issue of such devices. Since
large substrate areas have to be patterned or inspected, throughput of for example
larger than 10 cm2/min and, therefore, high probe currents in the range
of 100 nA or higher, are desirable.
However, particle-particle interaction (Boersch effect) limits the
resolution for high beam currents. Especially for low voltage applications, that
are applications with the beam energy around or below 1 keV, particle interaction
limits the resolution for high beam currents.
One approach for reducing the particle-particle interaction for a
lithography systems was proposed by US patent no. 6,635,891. Therein, a hollow-beam
apparatus utilizing a ring aperture in a crossover is suggested.
However, the hollow beam is difficult to shape and the charged particle
interaction might not be significantly reduced.
SUMMARY OF THE INVENTION
The present invention provides an improved charged particle system.
Thereby, the particle-particle interaction is intended to be reduced. According
to aspects of the present invention, a charged particle beam device according to
independent claims 1, 5 and 8 and a method of operating a charged particle beam
device according to independent claim 25 are provided.
Further advantages, features, aspects and details of the invention
are evident from the dependent claims, the description and the accompanying drawings.
According to one aspect a charged particle beam device is provided.
The device comprises: an emitter for emitting charged particles; an aperture arrangement
with at least one aperture for blocking a part of the emitted charged particles,
whereby the aperture arrangement forms a multi-area sub-beam charged particle beam
with a cross-section-area and a cross-section-circumference, whereby a ratio between
the cross-section-circumference and the cross-section-area is increased by at least
15% as compared to the ratio between a cross-section-circumference and a cross-section-area
of a circular beam with the same cross-section-area as the multi-area sub-beam charged
particle beam; an objective lens for focusing the multi-area sub-beam charged particle
beam onto the same location within the focal plane.
According to another aspect a charged particle beam device is provided,
whereby the device comprises: an emitter for emitting charged particles; an aperture
arrangement with at least one aperture for blocking a part of the emitted charged
particles, whereby the aperture arrangement forms a multi-area sub-beam charged
particle beam with a cross-like shape; and an objective lens for focusing the at
least two independent charged particle beams onto the same location within the focal
According to a still further aspect, a charged particle beam device
is provided. The device comprises an emitter for emitting charged particles; an
aperture arrangement with at least two apertures for separating the emitted charged
particles into at least two independent charged particle beams; and an objective
lens for focusing the at least two independent charged particle beams, whereby the
independent charged particle beams are focused onto the same location within the
Thereby, each independent charged particle beam (bundle) can be interaction
optimized and/or an interaction between the bundles can be avoided. Thus, the charged
particle current density on a specimen can be increased, without too much drawback
due to the Boersch-effect.
It may be understood that "focusing onto the same location within
the focal plane" means to generate one continuous particle probe in the focal plane.
It has to be noted that, unlike many state of the art devices, according
to an aspect of the present invention, no image of the aperture arrangement is generated
on the specimen. The probe generated on the specimen is the image of a source, virtual
source or a crossover. Thus, one charged particle spot is generated in the focal
plane of the objective lens of the scanning type charged particle beam device.
According to a further aspect, the charged particle beam device is
provided in form of a SEM. Typically, the invention is applied to high current (typically
>= 50 nA for 1 keV) SEMs.
Consequently, according to another aspect, the charged particle device
further comprises a detector. As an example, this detector might be position above
the objective lens.
As a further consequence, according to an even further aspect, the
opening of the objective lens typically has a maximal diameter of 20 mm.
According to another aspect, a charged particle beam device is provided,
whereby the at least two independent charged particle beams have a distance with
respect to each other such that no interaction occurs between the at least two independent
charged particle beams. Thereby, typically for a further aspect of the present invention,
the at least two independent charged particle beams have a distance with respect
to each other between 1/2 of the diameter of the at least two apertures and two
times the diameter of the at least two apertures. Typically the distance and the
diameter may be about the same size. This may absolutely be in the range of 5 to
Consequently, the apertures are formed and are positioned with respect
to each other such that a bundle-interaction can be neglected or at least limited.
According to another aspect of the present invention, a charged particle
beam device is provided, whereby the at least two apertures have an elongated shape
with a long axis and short axis. Thereby, the long axis is arranged radially with
respect to an optical axis of the charged particle beam device.
Thus, it is possible to realize the independent charged particle beams
in a manner such that an optional aberration correction element can be provided,
which can be more easily realized than state of the art aberration correction elements.
Typically, according to an even further aspect, this aberration correction element
is provided as a spherical aberration correction element, and even more typically,
optionally, by an octopole element.
According to another aspect of the present invention, a charged particle
beam device is provided, whereby the at least two apertures are arranged rotational-symmetrical
to an optical axis of the charged particle beam device. And especially, according
to yet another aspect, the aperture arrangement comprises four apertures.
With respect to the first of these aspects, a homogeneous (uniform)
illumination of the aperture arrangement, which is a further optional aspect of
the present invention, can be realized more easily. With respect to the second of
these aspects, the above-mentioned aberration correction can advantageously be applied
by providing four apertures. However, this condition is not obligatory for an inspection
or measurement apparatus. For an inspection or measurement apparatus also a non-uniform
illumination is possible.
According to a further aspect, if independent charged particle beams
are formed the number of independent charged particle beams is smaller than 50,
preferably smaller than 17 and even more preferably, smaller than 5.
According to an even further aspect, the charged particle beam device
further comprises a scanning unit for scanning the charged particle beam over the
specimen. Typically, according to another aspect, this scanning unit is positioned
after the front focal plane of the objective lens. According to a further optional
aspect this position would be around the center (in direction of the optical axis)
of the objective lens.
Thereby, one alternative to provide the scanning unit is an electrostatic
scanning unit, that is especially advantageous for high current applications. In
the event of other applications also magnetic or electrostatic-magnetic scanning
units may be used.
The above mentioned aspects can be combined with any other feature,
aspect, detail, or combination of features, aspects and details, which promote the
possibility to reduce particle-particle interaction effects for high current densities
on a specimen.
According to another aspect of the present invention, a method of
operating a charged particle beam device is provided. The method comprises the steps:
illuminating an aperture arrangement having at least two apertures, whereby at least
two independent charged particle beams are generated; and focusing the at least
two independent charged particle beams with an objective lens onto the same location
of a specimen.
According to yet another aspect, a method of operating a charged particle
beam device is provided, whereby the at least two apertures are provided on a circle
around an optical axis of the charged particle device. And according to another
aspect, optionally, the method can be provided, whereby the aperture arrangement
is illuminated such that the at least two apertures are homogeneously illuminated.
Thereby, similar imaging conditions for the independent charged particle
beams can be realized. Consequently, aberration correction means, focusing means,
and other means for guiding, shaping, or influencing the bundles can be applied
similarly for all beam bundles.
Making use of the above aspects, it is possible, according to another
aspect of the present invention, to include the step of interaction-optimizing each
of the at least two independent charged particle beams.
According to another aspect of the present invention, a method of
operating a charged particle beam device is provided, whereby the charged particles
are energized to impinge on the specimen with an energy below 3keV. Additionally,
the step of correcting spherical aberrations, which are introduced by guiding the
at least two independent charged particle beams off-axis, may be included.
According to another aspect, an image area of the specimen is imaged
by scanning the charged particle probe over the specimen. Thereby, the objective
lens is excited to generate a first focal length. Further, the image area is imaged
at least a second time, whereby the objective lens is excited to generate at least
a second focal length. The set of focus series measurements is superposed to create
a 3-dimensional image.
This aspect may advantageously be applied with the independent beam
bundles, since the relatively large objective angle results in a depth of focus.
The invention is also directed to apparatuses for carrying out the
disclosed methods, including apparatus parts for performing each of the described
method steps. These method steps may be performed by way of hardware components,
a computer programmed by appropriate software, by any combination of the two, or
in any other manner.
BRIEF DESCRIPTION OF THE DRAWINGS
Some of the above indicated and other more detailed aspects of the
invention will be described in the following description and partially illustrated
with reference to the figures. Therein:
DETAILED DESCRIPTION OF THE DRAWINGS
- Figs. 1a to 1c
- show schematic side views of three charged particle beam columns illustrating
some effects relating to particle-particle interaction in charged particle beam
- Fig. 2
- shows a schematic side view of a charged particle beam device according to one
embodiment of the present invention;
- Figs. 3a to 3d
- schematically show aperture arrangements of embodiments of the present invention;
- Figs. 4a to 4d
- show schematic side views of embodiments relating to aperture arrangement positions
within a charged particle beam device; and
- Fig. 5
- shows a schematic side view of charged particle beam devices according to embodiments
of the present invention, whereby an aberration correction element is included;
- Fig. 6
- shows a schematic side view of a charged particle beam device according to one
embodiment of the present invention, whereby an aberration correction element and
a detector are included;
- Figs. 7a and 7b
- show schematic side views of a charged particle beam device, whereby possible
detection schemes to be combined with embodiments of the present invention are illustrated;
- Figs. 8a to 8d
- show schematic top views of further aperture arrangements according to the present
Without limiting the scope of protection of the present application,
in the following the charged particle beam device or components thereof will exemplarily
be referred to as an electron beam device or components thereof. Thereby, the electron
beam might especially be utilized for inspection or lithography. The present invention
can still be applied for apparatuses and components using other sources of charged
particles and/or other secondary and/or backscattered charged particles to obtain
a specimen image.
Within the following description of the drawings, the same reference
numbers refer to the same components. Generally, only the differences with respect
to the individual embodiments are described.
Within fig. 1a an electron beam device is shown. Emitter 12 emits
an electron beam 11 which is extracted by extractor 13 and accelerated by anode
14. Condenser lens 15 focuses electron beam 11. In the example shown in fig. 1a,
aperture 16 for shaping the electron beam is positioned above the crossover. The
electron beam, which is focused on specimen 19 by objective lens 18, can be scanned
over the specimen by scan deflectors 17.
Independent of specific embodiments, the objective lenses are typically
provided in form of rotational symmetric objective lenses.
As denoted with reference sign "L", the embodiment shown in fig. 1a
has a predetermined distance between emitter 12 and specimen 19. Even though having
the same distance L between the emitter and the specimen, fig. 1b shows an embodiment
with a first improvement. This embodiment avoids a crossover within the optical
column of the charged particle device. Thereby, no intermediate image is generated.
Generally, within the crossover, additional energy width is created.
This reduces the performance by increased chromatic aberration. Nevertheless, particle
optical systems having a dominant interaction limitation might also tolerate a crossover
Within the embodiment shown in fig. 1c, firstly, a variety of measures
are taken to reduce the interaction of the charged particles. These measures, even
though combined within one embodiment, can also be individually used for reducing
the particle-particle interaction.
Within fig. 1c, the distance between the emitter and the specimen
is reduced as indicated by L*. Thereby, the electron beam path, along which an electron-electron
interaction can limit the resolution, is shortened. Further, the primary electron
beam can be accelerated to a higher potential within the column and decelerated
by a retarding lens or the like. Aperture 16, which shapes electron beam 11, is
arranged closer to the emitter 12 as compared to the embodiments shown in figs.
1a and 1b. Since aperture 16 decreases the beam current by blocking parts of the
emitted electrons, there is less particle interaction of the electron beam traveling
through the column. By means of aperture 16, the size of the beam bundle can be
optimized with respect to geometrical aberration and interaction limitation. Additionally,
a plurality of columns of fig. 1c can be arrayed. Thereby, the sum of the electron
beam current can be increased since there is no interaction between electron beams
of neighboring columns, which may further be shielded with respect to each other.
An embodiment according to the invention is shown in fig. 2. Therein,
an electron beam device with an optical axis 1 is shown. The electrons emitted by
emitter 12 under emission angle αem are extracted by extractor
13 and by anode 14. Aperture arrangement 26 blocks parts of the electrons, whereby
a plurality of electron beams (beam bundles) are generated. These beam bundles travel
separated from aperture arrangement 26 to specimen 19. Each of these beam bundles
are independent electron beams. They are interaction limited and interaction optimized,
respectively. However, the independent electron beams are separated from each other
by a distance such that no interaction occurs between the individual electron beams.
The beam bundles (independent electron beams) travel independently
through the optical system. Thereby, the electron beams indicated by reference numbers
21 a and 21b in fig. 2 pass through condenser lens 15 and scan deflectors 17 in
order to be focused by objective lens 18 into a common probe on the specimen. The
charged particle beam bundles are focused onto the same location. The location may
be defined as having a maximal dimension of 200 nm. Thus, objective lens 18 focuses
the independent electron beams onto the same focal point in the focal plane.
Independent of specific embodiments, an aspect of the invention may
generally be described as follows. Charged particles are emitted from a single emitter.
Further, at least two independent beam bundles are generated from the charged particles
emitted by the single emitter. The independent beam bundles are guided through the
column substantially parallel and are focused by one objective lens into one charged
Independent of specific embodiments described herein, the lenses and
especially the objective lens may either be electrostatic, magnetic or compound
electrostatic-magnetic. Thereby, also high precision lenses as described in European
patent application No. 03025353.8 by Frosien may be used as one option.
As already described with respect to fig. 1b, without being limited
thereto, a beam path without a crossover can be considered advantageous. Since each
of the electron beam bundles 21a and 21b is optimized with regard to the beam current,
the electron current density on the specimen can be increased n-times by providing
n electron beam bundles.
Thereby, aperture arrangement 26 is arranged close to the emitter
so that a separation into independent electron beams takes place as soon as possible
within the column. Within the embodiment shown in fig. 2, the aperture arrangement
and thereby the beam bundles are arranged symmetrically with respect to optical
axis 1. Thus, the optical imaging characteristic of the electron beam column is
similar for electron beam 21a and 21b.
However, generally, it has to be considered disadvantageous to provide
an electron beam path with electron beams traveling significantly off-axis. Yet,
the aperture arrangement 26 as shown in fig. 2 has off-axial aperture locations.
Thereby, spherical aberrations will be increased. Generally, the diameter of the
electron probe can be described as:
Dprobe=(D2 sperical+D2 chromatic+D2 interaction)1/2
The spherical aberration depends on the third order of aperture angle
Thus, providing off-axis beam bundles is contradictory to the general
teaching of minimizing possible aberrations. However, the system as shown in fig.
2 can be better described by:
Dsperical enlargement = 3Cs αB α2
whereby α is the angle between the center of the aperture
and the optical axis. Thus, spherical aberrations increase merely with the second
order of the angle between aperture and optical axis.
Especially for applications with high current density, which are interaction
dominated anyway, spherical aberrations can be neglected up to a certain degree.
Nevertheless, the distance D/2 of the electron bundle from optical axis 1 should
be as small as possible without introducing any interaction between the independent
electron beams. Typical design criteria could be that the aperture openings and
the distances between the aperture are in the same order of magnitude.
Embodiments of aperture arrangements will now be described with respect
to figs. 3a to 3d. Within fig. 3a, two apertures 36a are arranged with the 2-fold
symmetry around the optical axis. Thereby, the electron bundles have a distance
D1. The sizes of the apertures are indicated by reference sign S1.
The apertures are arranged on a virtual circle around optical axis 1. Within fig.
3b, three apertures are arranged in 3-fold symmetry. By increasing the number of
apertures, the distance between the electron beams is decreased unless the size
of the apertures is reduced.
Thus, for the aperture arrangement 26b within fig. 3b, the distance
D2 between the electron bundles is realized by having an aperture size
S2 which is smaller than aperture size S1. The apertures 36
of aperture arrangement 26c shown in fig. 3c are further reduced in size in order
to realize distance D3 between the electron beams. The four apertures
in fig. 3c are arranged in 4-fold symmetry around optical axis 1.
A further embodiment that differs from the above-mentioned embodiments
is shown in fig. 3d. Thereby, a ring is segmented to form apertures 36. Thereby,
aperture arrangement 26d includes eight apertures with 8-fold symmetry around optical
The above-described embodiments of figs 3a to 3d are examples for
aperture arrangements with different distances D1 to D4, whereby
each distance is sufficiently large in order not to have any inter-bundle interaction.
Nevertheless, aperture arrangements with different numbers of apertures can be realized.
Further, the symmetrical formation around optical axis 1 is a typical arrangement
having some advantages, which will be described in the following. However, the present
invention is not limited thereto.
The symmetrical formation of the apertures can be considered advantageous
with regard to the emission characteristics of the gun. Typical emission angles,
e.g. from TFE, have a value of αemision = 10-1 rad, which shows
homogeneous current density. Accordingly, the apertures should be arranged within
this emission angle to be illuminated homogeneously. This results in homogeneous
current density of all electron beam bundles. Therefore, the above-mentioned symmetry
may be considered advantageous in order to have the same illumination for all apertures.
Generally, without reference to any of the specific embodiments, the
distance of the apertures from the optical axis should be as small as possible to
keep the spherical aberration contribution as small as possible. However, the distance
has to be made large enough that different electron bundles do not interact with
Other embodiments according to the present invention are shown in
figs. 8a to 8d. Therein, 4-fold symmetric aperture arrangements 86a to 86 d are
shown. This rotational symmetry by 90° has the advantage, that the resulting spherical
aberrations can be corrected with an octupole element that may be present in typical
charged particle beam inspection or testing devices. However, also an aperture arrangement
with a 2-fold symmetry (as described in fig. 3a) might be used and the aberrations
introduced may also be easily corrected.
Fig. 8a shows an aperture arrangement with four apertures 36a to 36d.
The apertures have rectangular shape. Typically the longer dimension of the aperture
is radially orientated. Thereby, the introduced spherical aberrations have advantageously
correctable spherical aberrations.
A further embodiment (see Fig. 8a) also has 4 apertures. However,
the apertures do extend further towards an optical axis, which would be located
essentially in the middle of the aperture arrangement 86b.
Within Fig. 8c, the apertures are further extended towards the center.
Thereby a cross-shaped aperture is realized. Such an aperture arrangement does no
longer form independent electron beams (bundles). However, the same inventive concept
does still apply. The electron-electron-interaction is reduced since the ratio of
the circumference of the aperture and the area of the aperture is increased as compared
to a circular aperture. Electrons passing through the aperture at the ends of the
four arms of the cross do not interact with electrons from a neighboring arm because
of their distance.
The aperture of aperture arrangement 86c forms a multi-area sub beam
electron beam. That is, there are sub-beams in multiple areas, which are, however,
connected to form a single beam.
Since the distance between the arms (36a-36d) of the cross-shape increases
with the distance from the center, it is also possible to realize a shape according
to fig. 8d. Therein, the arms of the cross get broader with the distance from the
center of the cross-shaped aperture.
Another aspect of the present invention will now be described with
reference to the embodiments shown in figs 4a to 4d. As already mentioned with respect
to fig. 1c, the beam path along which electron-electron interaction can occur should
be short. A position of the aperture arrangement closer to the emitter 12 results
in a shorter beam path along which the electrons can interact, and a longer beam
path along which the electron beams are separated. Therefore, it may be considered
advantageous if any of the embodiments shown in figs 4a to 4d are realized. Within
fig. 4a, the aperture arrangement 26 is directly positioned after anode 14. The
term "directly positioned" should herein be understood as having no space-consuming
components, like lenses or the like, between anode 14 and aperture arrangement 26.
Within fig. 4b, a separation of the electrons into independent electron
beam bundles is realized even closer to the emitter as compared to fig. 4a. Thereby,
the anode is formed to incorporate the aperture arrangement 44 by having at least
two openings. Within fig. 4c, the extractor is formed to incorporate the aperture
arrangement 43, whereby a separation of the electrons into independent electron
beam bundles is realized still closer to the emitter.
As described above, on the one hand the aperture arrangement should
be close to the emitter. On the other hand, the apertures should be within a homogeneous
emission angle of the gun and the distance between the independent beam bundles
should be sufficiently large. Thus, providing a characteristic emission angle of
a certain gun, the aperture arrangement's distance from the emitter and the aperture
distances can be optimized. Typical values are in the range of 10-1 rad for the
emission angle (half angle) results for an aperture arrangement distant from the
source or the virtual source in a 1mm-radius illuminated circle. For a 1mm distant
aperture arrangement, the illuminated circle has a radius of 100 µm. Therein, apertures
with a dimension of e.g. 10 µm can be positioned.
A further embodiment is shown in fig. 4d. Within this embodiment,
the aperture arrangement 26 of fig. 4a has an additional opening. This opening is
positioned such that the electron beam passing through this opening travels substantially
along the optical axis. The on-axis opening of aperture arrangement 46 of fig. 4d
may for example be used for an additional measurement mode. Thereby, the electron
beam column can be operated with an on-axis electron beam. This on-axis electron
beam mode may be used for adjustment purposes or in the event low current measurements
should be conducted.
Nevertheless, the on-axis electron beam bundle may also be used in
combination with other electron beam bundles to realize a high current density measurement
mode as long as electron beam bundle interaction is sufficiently small.
Within the embodiments shown in fig. 5, a further optional feature
is included. As described above, at least two independent electron beams (bundles)
are separated such that no significant interaction may occur. Since each independent
beam can be interaction optimized, the beam current density on the specimen can
be increased n times for n electron beam bundles. Thereby, spherical aberrations
are introduced because of the off-axis beam paths. Depending on the operation conditions,
the spherical aberrations may be neglected or may not be neglected. For some applications,
e.g. with very high current densities, the size limitation of the electron probe
on the specimen will still be dominated by interaction effects. However, other applications
may have operation conditions for which aberration limitations will have a significant
effect on the resolution of the electron beam device.
In the event that spherical aberrations may not be neglected, a spherical
aberration correction element 52 (see fig. 5) can be included in the system. Within
fig. 5, the electrons emitted by emitter 12 pass through the same elements as described
with respect to fig. 2. After passing through condenser lens 15, the independent
electron beams pass through correction element 52. Scanning deflector 57 is located
below the correction element and may, for example, be positioned within the objective
lens 18. The independent electron beam bundles are focused by objective lens 18
onto the same area of specimen 19. Thereby, one single electron probe is realized
on the specimen.
The spherical aberration correction element 52 can for example be
a sixtupole element as known from US patent No. 4,414,474. However, according to
one optional aspect of the present invention, other configurations can be used.
For an aperture arrangement 26 having only four electron beam bundles, an octopole
element can be used. Thereby, according to yet another optional aspect of the present
invention, one single octopole element is sufficient for correcting the spherical
aberrations of four electron beam bundles. Such a single element, which is commonly
not used for spherical operation corrections, is easier to realize and is especially
suitable for certain beam bundle arrangements. This will be described in more detail
with respect to fig. 6,.
The spherical aberration correction element 52 can either be magnetic,
electrostatic or combined electrostatic-magnetic. Within fig. 6, an electrostatic
correction element 52 with electrodes 62 is shown. This octopole correction element,
that could as well be magnetic or electromagnetic, has a field distribution that
matches especially well with the aperture arrangement 26 shown in fig. 6.
The aperture arrangement 26 has four rectangular apertures, whereby
the longer sides are radially orientated. The slit-like apertures result in electron
beam bundle-shapes which can advantageously be corrected in two directions by a
single octopole element.
Additionally to the components that have already been described with
respect to other embodiments, fig. 6 shows a detector 61. Within the present invention,
the electron beam bundles of the primary electron beam travel independently of each
other. Nevertheless, the independent electron beams meet in the same focus. Thus,
it is possible to use conventional detection schemes. Within fig. 6, a detector
61 in the form of a ring positioned above objective lens 18 is shown. Detector 61
collects backscattered electrons, secondary electrons, other charged particles,
or photons released from specimen 19 on impingement of the primary electron beams.
Further detection schemes will next be described with respect to fig.s
7a and 7b. Figure 7a shows an electron beam column wherein the electrons pass substantially
along a straight optical axis from the emitter to the specimen. Above the objective
lens, a Wien filter element 73 is provided. The Wien filter element has, when excited,
magnetic and electric fields such that primary electrons with a predetermined energy
pass through the filter element 73 in an undisturbed manner. However, charged particles
having a direction different from the primary electrons are deflected by the combination
of the electric and magnetic field. As a result, secondary particles that are released
from the specimen and that have passed from the specimen through the objective lens
are deflected to detector 61a.
Another embodiment regarding a detection scheme is shown in fig. 7b.
Therein, two deflectors 74a and 74b are utilized. Without being limited thereto,
the deflectors of this example are indicated as magnetic deflectors. Emitter 12
emits electron beam 11, which is extracted by extractor 13 and further shaped by
anode 14. The independent electron beam bundles formed by aperture arrangement 26
are deflected in a first deflector 74a after passing through condenser lens 15.
Thereby, the independent electron beams travel nonparallel to the first optical
axis 1a and the second optical axis 1b. Second deflector 74b deflects the independent
electron beams to travel substantially parallel to the second optical axis 1b. Thereafter,
the independent electron beams are focused into a common electron probe by objective
lens 18. Additionally, depending on the desired impingement location on the specimen,
scanning deflector 57 can be used for scanning the independent electron beams over
the specimen. The secondary and/or backscattered charged particles that have passed
through the objective lens and travel upwardly in the electron beam column are deflected
by second deflector 74b. Since the deflection of the magnetic deflector depends
on the direction of the secondary and/or backscattered charged particles, these
particles are deflected towards detector 61b.
As described with respect to the exemplary embodiments above, providing
several independent electron beams having no beam-interaction can increase the beam
current density on the specimen. Thereby, the interaction limitation of the resolution
can be overcome. As described with respect to fig.s 1a to 1c, other measures can
also be used for reducing the interaction limitation. For all of the above-described
embodiments, aspects and/or details, the features described with respect to fig.
1c can be added individually or in any combination of these features. That is, one,
two, three, four, or five measures can be arbitrarily chosen from the following
group: the charged particles can be accelerated to higher energies, several columns
can be arrayed, the column length can be reduced, each charged particle beam bundle
can be interaction optimized, and charged particles, which are blocked for beam
shaping reasons or the like within the charged particle column, are blocked as close
to the emitter as possible, whereby a possible interaction between charged particles
is reduced to a shorter path.
A further aspect will now be described with reference to fig. 2. In
view of the independent electron beams having distance D and, therefore, traveling
off-axis, the defective aperture angle αobj is relatively large
as compared to systems with an on-axis electron beam. Thereby, the depth of focus
is reduced. This effect may be used for the construction of 3-dimensional images.
According to this aspect, the specimen area to be inspected is measured several
times. Objective lens 18 is excited to have different focal length within the several
measurements. Each of the different focal lengths refers to one focus layer. Thereby,
a set of focus series measurements is obtained. These focus series measurements
can be superposed. Thereby, a 3-dimensional image is obtained.