FIELD OF THE INVENTION
This invention relates to a charged particle beam device
for the examination of specimen. In particular, this invention relates to the beam
column used for guiding the charged particle beam.
BACKGROUND OF THE INVENTION
Beams of negatively or positively charged particles can
be used for the examination of specimen. Compared to optical light, the resolving
power of a beam of charged particles is several magnitudes higher and allows for
the examination of much finer details. In charged particle beam devices, electric
or magnetic fields, or a combination thereof, act upon the beam in a manner analogous
to that in which an optical lens acts upon a light beam. In particular, any electric
or magnetic field which is symmetrical about an axis is capable of forming either
a real or a virtual charged particle image. Hence, an axially symmetric electric
or magnetic field is analogous to a spherical lens. Furthermore, similar to light-optics,
apertures are also used in charged particle devices. The primary use of these apertures
is to limit the diameter of the beam of charged particles or to eliminate stray
or widely scattered particles. However, in charged particle devices, apertures are
an easy target for contamination caused by hydrogen carbons.
Charged particles on their path from particle source to
the specimen to be examined are strongly scattered by all forms of matter including
air. Hence, the entire instrument must, in general, be evacuated. Nevertheless,
the presence of hydrocarbon molecules in vacuum chambers is virtually unavoidable.
These are commonly formed in vacuum chambers of the charged particle devices as
a result of hydrocarbons and silicon oils migrating from vacuum pumps or evaporated
from vacuum seals. Radiation of hydrocarbon molecules with charged particles leads
to cracking of bonds and to the creation of carbon double bonds. The formation of
carbon double bonds, in turn, results in cross linking and the final product will
be a carbonaceous polymerized substance. In particular, the edges of apertures which
serve to limit the diameter of a particle beam, as e.g. described in document
EP 0 797 236. are exposed to particle radiation. At these edges, carbon-rich
films or contamination needles easily form and grow into the openings which changes
the shape of the passing beam.
Furthermore, these contaminations protruding into the openings
are getting charged by the particle beam. The impinging particles are absorbed by
the protrusions which could primarily be classified as insulators. The charge build
up causes the passing particle beam to deflect and results in imaging artifacts.
Due to the constant accumulation of charge, the voltage of a contamination increases
until it reaches the break down point. In this moment, a sudden discharge will take
place and the imaging artifact, caused by charging, disappears. The subsequent absorption
of charged particles will again build up the voltage of the contamination until
it reaches the break down point. Consequently, this results in a periodical artifact
of image flickering. Additionally, there is the artifact caused by the slowly growing
contamination layer at the edge of an aperture which steadily narrows the diameter
of the passing beam.
In some devices multi apertures have been used for obtaining
a variety of preselected beam diameters. A plate comprising several apertures with
distinct diameters is placed between particle source and specimen. The beam of charged
particles is then guided through one of these apertures in order to reduce it to
a desired diameter before it impinges onto the specimen to be examined. Without
limiting the scope of the invention, the following explanations will primarily concentrate
on the use of electrons as charged particles. An impinging beam of electrons with
a given electron density and a bigger beam diameter causes more primary electrons
to hit the target. The higher number of interactions between primary electrons and
target result, in general, in an increase of secondary products being detected and,
consequently, in a higher imaging contrast. On the other hand, a smaller beam diameter
with fewer primary electrons getting absorbed by the target causes lesser charging
and allows for focusing the beam to a smaller diameter in the sample plane.
In particular small apertures of multi aperture units require
frequent cleaning due to high intensity radiation of their edges. Furthermore, frequent
cleaning is necessary because of the large influence a contamination spot of given
size has with respect to the total surface of a small apertures, . For cleaning,
the part of the electron beam column containing the multi aperture unit needs to
be opened and its vacuum broken. After cleaning, time consuming realignment and
adjustment steps are necessary before the electron beam device is fully operational
again. Since this procedure results in considerable machine down-time, it is desirable
to increase the interval at which such apertures need to be cleaned.
In the past, a variety of attempts have been made to reduce
contamination of apertures e.g. use of hydrocarbon free vacuum and appropriate prior
cleaning of vacuum chamber and aperture unit. In another attempt, apertures were
heated during machine running time. The increased Brownian movement of the molecules
at the edges of the apertures prevent the formation of contamination layers thereon.
Yet, the installation of heaters in the vicinity of apertures is burdensome and
SUMMARY OF THE INVENTION
The present invention intends to provide an improved charged
beam column for examining a specimen with a charged particle beam. According to
one aspect of the present invention, there is provided an apparatus as specified
in claim 1.
According to a further aspect, the present invention also
provides a method as specified in claim 11.
Further advantageous, features, aspects and details of
the invention are evident from the dependent claims, the description and the accompanying
drawings. The claims are intended to be understood as a first non-limiting approach
to define the invention in general terms.
According to prefered aspect there is provided a charged
particle beam column with a first vacuum chamber. The charged particle beam device
further comprises a particle source for providing a beam of charged particles and
a multi aperture unit with at least two beam defining apertures for shaping the
beam of charged particles. The particle source and the beam defining apertures are
located within the first vacuum chamber. A separation unit for isolating a second
vacuum chamber from the first vacuum chamber whereby the separation unit comprises
a path aperture for the charged particle beam is arranged between the first and
second vacuum chamber. A first deflecting unit directs the beam of charged particles
through one of the beam defining apertures and a second deflecting unit directs
the beam of charged particles through the path aperture.
According to a preferred aspect of the present invention,
the first and/or second deflecting unit is located outside the first vacuum chamber.
This allows the use of smaller vacuum chambers and disposes of the need to provide
sealing members for cables and connectors used to operate the deflecting units.
In a further preferred aspect of the present invention,
the second deflecting unit guiding the beam of charged particles through the pass
aperture comprises two stages. The first one guides the beam coming through a selected
beam defining apertures back to the optical axis. Then, the second stage guides
it along the optical axis or, alternatively, in close vicinity to the optical axis.
Advantageously, this allows the beam to be directed onto a trajectory close to the
optical axis even before it passes through the path aperture of the separation unit.
The second deflection unit already guides the beam to a direction so that it propagates
towards the target or specimen without having to be directed again by a third deflecting
According to a still further aspect of the present invention,
there is provided a third deflection unit for directing the beam of charged particles
through the objective lens after it has passed through path aperture of the separation
unit. In some preferred embodiments, the trajectory of the beam is tilted with respect
to the optical axis after it has passed the first and second deflection unit. The
third deflection unit, advantageously, compensates excessive tilt angles and redirects
the charged particle beam so that the angle between the optical axis and the beam
is reduced. Preferably, the third deflection unit is located in between separation
unit and specimen.
In a preferred embodiment according to the invention, the
third deflection unit comprises two deflection stages. The first stage of the deflection
unit redirects the beam towards the optical axis and, subsequently, the second stage
guides the beam along the optical axis or in close vicinity thereto. This preferred
embodiment does not only allow for a reduction of the angle between optical axis
and beam, it also allows to the beam to propagate along, or in close vicinity, to
the optical axis. Furthermore, in case the optical axes defined by path aperture
and objective lens are not coaxial, the second stage allows the beam to be redirected
beam along the optical axis as defined by the objective lens.
In another preferred embodiment, the vacuum in the first
vacuum chamber is higher than the vacuum in the second chamber. Advantageously,
the multi aperture located in the first vacuum chamber is kept at a higher vacuum.
Thus, the time span in which contaminants develop at the path aperture and start
to negatively influence the trajectory of the charged particle beam is slowed down
by specifically reducing the number of hydrocarbons in this area. At the same time
it is not necessary to maintain the vacuum level of the first vacuum chamber in
parts of the beam column where contaminants have lesser disturbing influence.
In still another preferred embodiment, the first vacuum
chamber is kept at ultra high vacuum. At this vacuum level, the number of hydrocarbons
in the vicinity of the path defining apertures are drastically reduced which increases
machine running times.
The invention is also directed to methods by which the
described apparatus operates. It includes method steps for carrying out every function
of the apparatus. 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:
DESCRIPTION OF THE PREFERRED EMBODIMENTS
- Fig. 1 is a schematic vertical cross section of a beam column for a charged
particle beam device comprising a first, a second and a third double stage deflection
unit. The first and second deflection unit are arranged outside a first vacuum chamber.
- Fig. 2 is a schematic vertical cross section of a second embodiment of a beam
column for a charged particle beam device comprising a first, a second and a third
double stage deflection unit. The first and second deflection unit are arranged
within a first vacuum chamber.
- Fig. 3 is a schematic vertical cross section of a third embodiment of a beam
column for a charged particle beam device comprising a first and a second double
stage deflection unit. The first and second deflection unit are arranged within
the first vacuum chamber.
- Fig. 4 is a schematic vertical cross section of a fourth embodiment of a beam
column for a charged particle beam device comprising a first, a second and a third
double stage deflection unit. The first and second deflection unit are arranged
within the first vacuum chamber
- Fig. 5 shows a top view of a multi aperture unit with several beam defining
apertures arranged in a circle around the center beam defining aperture located
in the middle of the circular plate.
An exemplary embodiment of an electron beam column used
in an apparatus for examining specimen is shown in Fig. 1. Electron beam 4 comes
from electron source 3 e.g. a Schottky emitter or a tungsten hairpin. An anode arranged
below attracts and accelerates the electrons and a condenser lens concentrates them
into a fine beam. Both, anode and condenser lens, are not shown in the drawings
since they are not crucial for understanding the principals of the invention. A
first deflection unit 11 deflects the e-beam path from optical axis 9 defined by
electron source 3 and guides the e-beam through one of the beam defining openings
6 in multi aperture 5.
In the embodiment shown, multi aperture 5 is a circular
flat disc having several beam defining openings 6 with a specified diameter. Before
the e-beam passes through one of these openings, it generally has a diameter bigger
than the diameter of the opening. Thus, the beam defining aperture only allows electrons
to pass who's distance from the e-beam axis is smaller than the aperture diameter
and eliminates the rest. The first deflection unit is capable of directing e-beam
4 through any one of the beam defining apertures 6 in multi aperture unit 5 thereby
determining the beam diameter and, consequently, the current of the e-beam. The
selection of a specific beam defining aperture is the choice of the user and based
on his intentions. In view of the present invention, it is not required to arrange
beam defining apertures 6 in a specific pattern on multi aperture unit 5. It is,
however, preferred to have sufficient distance between outer rims of adjacent apertures
so that electrons flying on the outmost trajectories of the e-beam do not incidentally
pass through neighboring apertures. On the other hand, the distances between outer
rims of adjacent apertures should not be too wide. This permits deflections of the
e-beam with smaller angles since the center of beam defining apertures 6 are arranged
closer to the center of multi aperture unit 5. Furthermore, it is preferred to have
one beam defining aperture located in the center of multi aperture unit 5. If a
certain application requires the use of the center aperture, then it is not necessary
to deflect the e-beam which can remain on the optical axis.
A second deflection unit 12 redirects the beam after its
diameter has been defined by one of apertures 6 and guides it through path aperture
8 of separation unit 7. The separation unit 7 is located between a first vacuum
chamber 1 and a second vacuum chamber 2 and separates the two vacuums existing in
each respective chamber. Thereby, the vacuum in the first chamber 1 is higher than
the vacuum in the second chamber 2 or, in other words, the pressure in the second
chamber is higher than the pressure in the first chamber. This reduces the number
of hydrocarbon molecules in the surroundings of the beam defining apertures thus
slowing down the formation of contaminants. In preferred embodiments, the vacuum
in the first chamber 1 is at least five times as high as the vacuum in the second
chamber. In even more preferred embodiments, the vacuum in the first chamber is
at least 10 times as high as the vacuum in the second chamber. By establishing a
pressure difference of about one magnitude, the number of hydrocarbon molecules
present in the first chamber, compared with the number of hydrocarbon molecules
present in the subsequent second chamber, is considerably reduced which results
in even more enhanced running times of the charged particle-beam device.
Alternatively, it is preferred to establish in the first
vacuum chamber 1 an ultra high vacuum of higher than 10-7 mbar. This
reduces the number of all molecules including hydrocarbon molecules per cm-3
to not more than 109 and, consequently, slows down the formation of contamination
spots. In even more preferred embodiments, an additional high vacuum of about 10-4
mbar to about 10-7. mbar is established in vacuum chamber 2. The additional
existence of a high vacuum in the second vacuum chamber adjacent to the first vacuum
chamber enhances machine running time even more. It is of course possible and advantageous
to combine the aspects of establishing vacuum gradients as described in the preceding
paragraph with the aspects of establishing an ultra high vacuum in the first vacuum
chamber as described in the present paragraph.
Beam defining apertures 6 of multi aperture unit 5 are
all arranged in vacuum chamber 1 which, preferably, also includes electron source
3. It is within the scope of the invention to place multi aperture plate 5 completely
within vacuum chamber 1. In case the need arises to replace or clean plate 5, vacuum
chamber 1 must be opened. If only the part of multi aperture plate 5 which contains
beam defining apertures 6 is arranged within vacuum chamber 1 and the outer parts
of the plate extend beyond it, then it is possible to insert the plate from the
side without opening the whole chamber. Such an arrangement, however, requires ultra
high vacuum seals between multi aperture plate 5 and the walls of vacuum chamber
1. Still, for replacement of the plate, it is necessary to break the vacuum.
Vacuum chamber 2 is located adjacent to vacuum chamber
1. Path aperture 8 of separation unit 7 provides a constant fluid communication
between the two chambers and gases are exchanged. The difference in vacuum is maintained
by vacuum pumps (not shown in fig. 1) which are connected to each chamber. The vacuum
pump connected to chamber 1 needs to evacuate gas at a rate which compensates the
gas flow streaming from chamber 2 into chamber 1.
The vacuums in the beam column reduce interactions between
gas molecules and electrons thus allowing the e-beam to travel along predictable
paths. Nevertheless, due to the partial pressure of hydrocarbons and silicon oils
from vacuum pumps and the grease of vacuum seals and fingerprints, hydrocarbon molecules
are always present in the vacuum chambers. Radiation of same results in formation
of polymerized contaminations. This happens, in particular, at the edges of beam
defining apertures 6 which are exposed to higher e-beam radiation. By having these
beam defining apertures located within a separate vacuum it is possible to considerably
reduce the formation of contaminants. The diameter of path aperture 8 of separation
unit 7 is preferably higher than the diameter of each one of the beam defining apertures.
Hence, after the e-beam diameter has been delimited by one of apertures 6, the electron
radiation intensity at the edges of path aperture 8 is lower than the one at the
edges of beam defining apertures 6. Since the slimmed down beam contains less electrons
which are all confined to an area smaller than one of the path aperture, they do
not hit the edge of the aperture itself Advantageously, this compensates for the
higher concentration of carbon molecules in the vicinity of path aperture 8.
Separation unit 7 can be formed out of a circular flat
disc with path aperture 8 in its center. The flat disc is then arranged between
vacuum chambers 1 and 2 whereby vacuum seals are interposed at the contact areas
of the respective chamber walls and the flat disc. This allows separation unit 7
to be replaced if necessary. Alternatively, it is possible to provide a wall separating
vacuum chamber 1 and 2 with a path opening. In such an arrangement it is not necessary
to use vacuum seals. The path aperture is used to isolate the two vacuum chambers.
Fig. 1 shows the first and second deflection unit located
outside vacuum chamber 1. The smaller vacuum chambers are easier to evacuate and
to maintain at a high vacuum level. Also, cables and wires necessary for operating
the deflection units don't need to be put through the vacuum chamber walls and the
respective cable and wire passages don't need to be equipped with vacuum seals.
A further advantage is that the inner layout of vacuum chamber 1 does not need to
include holding and alignment means for the deflection units.
The deflection of a charged particle beam is achieved by
applying transverse electrostatic and magnetic fields. Charged particles move around
parabolic trajectories in a uniform electrostatic field E of a parallel plate capacitor
and around a circle in a uniform magnetic field B. Thus, in principal, all kinds
of electric and magnetic field producing equipment can be used for deflection units.
In the embodiment shown in fig. 1, with the deflection units being located outside
the vacuum chamber, attention has to be paid to the influence the materials used
for the chamber wall will have on the deflection fields. Preferably, non magnetic
materials are used together with deflection units operating with magnetic fields
and non conductive materials are used together with deflection units operating with
After the e-beam passes through path aperture 8 it enters
vacuum chamber 2. It is, for the present invention, not important which part of
the electron optic or the beam column is arranged within vacuum chamber 2. Furthermore,
it is not important if the e-beam on its way from vacuum chamber 1 to its target
passes through additional vacuum chambers. Therefore, parts of the following explanations
concentrate on the description of arrangements of further deflection units for guiding
the e-beam to its target without further discussing specific locations of these
In general, it is desirable to let the e-beam intersect
the optical axis at a small angle. A third deflection unit 13 located below separation
unit 7 preferably redirects e-beam 4 passing through path aperture 8 and guides
it through objective lens 10. In fig. 1 the optical axis defined by objective lens
10 is not coaxial with the axis defined by path aperture 8. In order to avoid imaging
artifacts, it is important to let the e-beam pass through the objective lens close
to its optical axis. Depending on the lateral displacement between the two axes,
it is sometimes possible to guide e-beam 4 in a single deflection step through objective
lens 10. In such case, e-beam 4 passes through the objective lens with a slight
tilt in respect to the optical axis of the objective lens. To avoid the tilt, it
is preferred to use a double deflection step. In a first step, the e-beam is deflected
so that it intersects the optical axis of objective lens 10 and, subsequently, in
a second step the e-beam is deflected again so that it propagates directly along
the optical axis of the objective lens. It is also possible to redirect the e-beam
in the second step so that it does not propagate directly on the optical axis of
objective lens 10 but more or less parallel to and in direct vicinity of same.
Fig. 2 shows a further embodiment according to the invention.
Different to the above described beam column, the first and second deflection unit
are now located within first vacuum chamber 1. The smaller distance between deflection
units and e-beam to be deflected allows the use of deflection units operating with
weaker electromagnetic fields. Furthermore, a much larger variety of materials may
be used for producing the walls of vacuum chamber 1 since their influence on the
deflection fields are considerably smaller.
An alternative arrangement of deflection units is shown
in fig. 3. Here, second deflection unit 12 comprises two deflection stages 12a and
12b. This embodiment is preferably used in beam columns in which the optical axis
defined by path aperture 8 and the optical axis defined by objective lens 10 are
coaxial. The e-beam, after having been delimited by beam defining aperture 6, diverges
from optical axis 9. In a first step, deflection unit 12a redirects e-beam and guides
it back towards the optical axis. At the point of intersection or, in case the e-beam
does not intersect with optical axis 9, at a point where the e-beam passes optical
axis 9 in close vicinity, deflection unit 12b redirects e-beam 4 so that it propagates
along optical axis 9 or in close vicinity to optical axis 9 and more or less parallel
to it. The expression "more or less parallel" within the meaning of this invention
includes deviations in which the e-beam still passes the objective lens close to
its center without causing excessive imaging artifacts.
The provision of a double stage second deflection unit
12a and 12b disposes, in certain applications, the provision of a third deflection
stage 13, since e-beam 4 already propagates in a direction close to the optical
axis. Nevertheless, even in a beam column with a double stage second deflection
unit 12a and 12b, it is in some applications preferred to have an additional double
stage third deflection unit 13a and 13b. The provision of such allows, after the
beam has passed path aperture 8, conducting a parallel shift of the e-beam.
Fig. 4 shows a still further embodiment according to the
invention. Here, a single stage second deflection unit 12 is combined with a double
stage third deflection unit 13. Such an arrangement is advantageously used in beam
columns in which the optical axes defined by path aperture 8 and objective lens
10 are coaxial. Compared with the arrangement shown in fig. 3, an additional deflection
stage has to be used. On the other hand, the part of the beam column between separation
unit 7 and objective lens 10 is in many devices more spacious than the upper part
of the beam column and allows more options in installing the third deflection units.
It is within the scope of the present invention to arrange the first and second
deflection units, as shown in figs. 3 and 4, outside vacuum chamber 1 as well.
Fig. 5 shows a top view of a multi aperture unit with several
beam defining apertures arranged in a circle around a center beam defining aperture
which is located in the middle of the circular plate. It is possible to use a multi
aperture blade with a larger diameter without changing the arrangement of beam defining
apertures 6 in respect to each other and without changing their dimensions. This
increases the outer rim of multi aperture plate 5 and adapts it to various sizes
of vacuum chambers. With a distance of about 5 cm between charged particle source
and multi aperture plate and aperture sizes between 5µm and 50 µm the
beam current can be varied by a factor of 100.
Fig. 6 shows the embodiment of fig. 3 including one modification.
Separation unit 7 comprises a further separation means 7a which is also provided
with a path aperture 8a. The space created between the two plates can function as
independent vacuum chamber 14 connected to an own vacuum pump (not shown). The installment
of an additional vacuum chamber 14 allows better isolation between the first and
second vacuum chamber. Gas molecules moving from the higher pressure second vacuum
chamber 2 through path aperture 8a in direction of lower pressure vacuum chamber
1 get trapped in isolation vacuum chamber 14. Caught in this chamber, these molecules
can be evacuated before they enter the first vacuum chamber. Hence, it is easier
to maintain a desired vacuum gradient between the first and second vacuum chamber.
The double path aperture described above can be used in all other embodiments according
to the present invention, in particular in those shown if figs. 1, 2 and 4.
The double path aperture, compared with a single path aperture,
still provides better isloation even so it is not connected to a vacuum pump. In
this case the space between the two plates does not function as a vacuum chamber,
however, gas particles still need to pass both apertures before they can contribute
to the pressure in the adjacent chamber. This preferred embodiment does not require
an additional vacuum chamber.