FIELD OF THE INVENTION
The invention relates to a charged particle beam device
for inspection system applications, testing system applications, lithography system
applications, electron microscopes and the like. It also relates to methods of operation
thereof. Further, the present invention relates to a charged particle beam device
with a cleaning system. Specifically, it relates to an emitter module, a charged
particle beam device and a method of cleaning and operating a charged particle beam
BACKGROUND OF THE INVENTION
Charged particle beam apparatuses are used in a plurality
of industrial fields. Inspection of semiconductor devices during manufacturing,
exposure systems for lithography, detecting devices and testing systems are only
some of these fields.
In general, there is a high demand for structuring and
inspecting specimens within the micrometer or nanometer scale. On such a small 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
Generally, charged particle beam devices are operated under
vacuum conditions to avoid, e.g. ionization of surrounding gases. In spite of that,
electrons impinging on component surfaces of the device, like extractors, anodes,
apertures or the chamber wall result in an emission of contaminants. Thus, a shower
of residual gas is generated. The residual gas contains molecules which can be hit
by electrons. Thereby, ions, ionized molecules and other particles can be created.
In the case of ions and ionized molecules having a charge which is opposite to the
charge of the charged particles emitted by an emitter, the ions and ionized molecules
in the residual gas are accelerated towards the emitter. As a result, the emitter
can be mechanically deformed from the impingement of the ions and ionized molecules
or these particles can be deposited on the emitter. Thus, emitter noise is introduced.
According to a known solution, a pretreatment is conducted.
A respective apparatus, shown in Fig. 4, will be described in the following. In
Fig. 4, an emitting unit comprising a wire 12 and a field emitter 14 is shown. If
high voltages are applied between the emitter and the extractor 8, the field emitter
emits charged particles, e.g. electrons along optical axis 1. Further, an anode
6 and aperture 7 are provided. These devices are used to avoid a widespread emission,
an acceleration of the charged particles and a beam shaping. Further, condenser
lens 4 can be used to image the electron source or a beam crossover acting as a
virtual electron source.
Before operating the charged particle beam device, it has
to be evacuated. After a certain vacuum level has been reached, an electron gun
42 with a high current floods the chamber with electrons. The emitted electrons
impinging on the walls or other surfaces of parts of the column and additional heat
detach volatile molecules from the surfaces of the column. Thereby, residual gas
is created. The residual gas gets pumped out of the chamber by vacuum pumps.
As a result, within this cleaning step, residual gas in
the form of molecules attached to column surfaces gets pumped out of the column
before the intended use of the charged particle device starts. On the one hand,
ions created during the cleaning step do not damage field emitter 14 and are hardly
deposited thereon. On the other hand, the molecules and ions possibly damaging the
field emitter during intended use are pumped out of the chamber.
However, the known solution is still not satisfactory.
Document JP 55-33719 describes a method for cleaning a
Wehnelt as it is accommodated in vacuum without any disassembling work, through
a process of enabling direct or indirect heating of at least an aperture edge of
the Wehnelt. The method can be described as follows: Install a pair of support electric
poles at certain intervals on an insulated base, and provide an La B6
cathode supported between these support poles via a heat generating block such as
pyrolitic graphite etc. Fix the heat generating block and the cathode together by
pressing, as holding them in this configuration, with a screw. Also install on the
base a pair of support electric poles made of nickel and the like possessing low
heat conductivity and provide, between tips of the electric pole and near the cathode,
a filament to be used for electron beam irradiation- The filament should be, in
this case, made of Phenium etc. and not make reactions with the cathode at high
temperature. By means of this contrivance, cleaning of the Wehnelt can be performed
without the requirement of disassembling work.
SUMMARY OF THE INVENTION
According to aspects of the present invention, an emitter
module according to independent claim 1, a charged particle beam device according
to claim 9 and a method of cleaning and operating a charged particle beam decide
according to independent claim 10 is provided.
According to one embodiment, an emitter module for emitting
charged particle beams is provided. The emitter module comprises a carrier body
insulating high voltage feedthroughs from the surrounding. The emitter further comprises
a charged particle beam emitter for emitting charged particles along an optical
axis and a cleaning emitter for emitting charged particles approximately along the
same optical axis.
Thus, the space for a separate cleaning emitter holder
and electrical feedthrough can be avoided. Beyond the easy and small integration
of a cleaning emitter, the cleaning is especially conducted on relevant surfaces,
since the cleaning emission takes the same path through the column. Additionally,
the emitter for intended use can be cleaned and a column alignment might be conducted
making use of the cleaning emitter.
According to one aspect, the cleaning emitter is positioned
behind the charged particle emitter.
According to a further aspect, the cleaning emitter is
a thermionic electron emitter, optionally with a thickness of 50 µm to 500
µm. The cleaning emitter is further optionally made of tungsten or a tungsten
Thus, high currents up to the mA range can be provided
during cleaning. Thereby, the cleaning process is speeded up.
The aspects mentioned above can, even though not limited
thereto, be valuable for charged particle beam devices with field emitters for the
The residual gas, which causes problems particularly with
the high electric fields due to high potentials and the small radius of curvature
of the emitter tip, is reduced, since an improved cleaning has been conducted before.
According to a further aspect, a charged particle beam
device making use of the above described aspects of emitter modules is provided.
Thereby, further, beam shaping means, like apertures, stigmators or the like; guiding
means, like extractors, deflection stages or the like; scanning means; and/or focusing
means like condenser lenses and/or objective lenses or the like might be used in
the charged particle beam device.
Also, a method for using a charged particle device is provided.
Thereby, charged particles are emitted approximately along an optical axis with
a cleaning emitter. Further, molecules, atoms and ions are pumped out of a chamber
of the charged particle device. During intended use, charged particles are emitted
along the same optical axis with a charged particle emitter.
According to a further aspect, the vacuum within the charged
particle device is monitored and the intended use is started depending on the vacuum
According to a further aspect, different methods of applying
voltages during cleaning and intended use are realized. Thereby, during cleaning
the charged particle beam emitter (for intended use) is not biased during cleaning
and is biased during intended use. Further, during cleaning, components influencing
the beam shape and the charged particle acceleration can either be used similarly
to the intended use or can be used such that an increased amount of surfaces as
compared to the intended use is hit by electrons during cleaning.
The invention is also directed to an apparatus 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. Furthermore, the invention is also directed to methods by which
the described apparatus operates or is manufactured. It includes method steps for
carrying out every function of the apparatus.
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. As used herein, like numerals throughout the various
figures represent the same or equivalent features of the present invention. Therein:
DETAILED DESCRIPTION OF THE DRAWINGS
- Fig. 1
- shows a schematic side view of a charged particle beam device making use of
the present invention;
- Fig. 2
- shows a schematic side view of an embodiment of an electron beam device according
to the invention;
- Fig. 3a
- shows a schematic side view of an embodiment of an emitter cleaning filament
module according to the invention;
- Fig. 3b
- shows a schematic perspective view of Fig. 3a;
- Figs. 3c to 3e
- show schematic views of examples of cleaning emitters filament modules; and
- Fig. 4
- shows a schematic side view of a prior-art example of an electron beam device
with a cleaning emitter.
Without limiting the scope of protection of the present
application, in the following the charged particle beam device will exemplarily
be referred to as an electron beam device. The present invention can still be applied
for apparatuses using other sources of charged particles.
Fig. 1 shows one embodiment according to the present invention.
Generally, electron beam devices are operated under vacuum conditions. Therefore,
a vacuum pump is connected to a port of each chamber of the device. Thereby, gas
molecules, which could be ionized by the electron beam, are pumped out of the device.
Without limiting the scope of the invention to systems with several chambers, commonly,
the device is subdivided into different chambers. For example, the vacuum required
for operation of an electron gun has a lower pressure than the vacuum required close
to a specimen. Thus, it is common to provide at least a gun chamber, a chamber for
other beam guiding means in the column and a specimen chamber. The vacuum pumps
are operated most of the time to pump molecules, which get into one of the chambers
during assembly of the electron beam device, during maintenance, through leaks or
through introduction of a new specimen, out of the chamber. Before the intended
use of the device starts, the chambers have to be pumped down to a determined pressure
During intended use, e.g. inspection, imaging, testing
or patterning of specimen 2, electron beam emitter 15 emits an electron beam along
optical axis 1. Thereby, the term "intended use" is to be understood as conducting
measurements or patterning for, e.g inspection, imaging, testing, patterning for
lithography or the like, whatever purpose the electron beam device has. Contrary
thereto, the present invention refers to cleaning steps, alignment steps, calibration
steps or the like as maintenance operations.
The emitted electron beam is further guided with the following
components. The electrons are extracted by a first extraction electrode and accelerated
along the optical axis 1 towards the specimen. A first electrostatic condenser 4
might be used to focus the electron beam. With thermal emitters a Wehnelt grid 9
(or Wehnelt cylinder) might be used, which is biased to a slightly more negative
potential than the emitter itself. Thus, electrons do not move in arbitrary directions
as compared to the optical axis. Instead, the electrons move along the optical axis
and are focused. Extractor 8 and anode 6 have e.g. a potential of 3 kV with respect
to the emitter. Thereby, electrons of the electron beam emitter are accelerated
towards specimen 2. Condenser lens 4 and aperture 7 are used to further shape the
electron beam. Additionally, depending on the strength of the focusing field of
condenser 4, more or less of the electron beam is suppressed by aperture 7. Thus,
the beam current applied to the specimen can be adjusted with the strength of the
focusing field of the condenser.
Objective lens 5 focuses the electron beam on specimen
2. Thereby, an electron beam spot of a few nanometers can be achieved. This beam
spot can either be used to image the specimen or write a pattern thereon. Additional
components (not shown) for deflecting the beam, adjusting the beam position relative
to optical axis 1 or scanning the beam over an area of the specimen can be used.
Further, the specimen, which is located on specimen stage 3, can be moved two-dimensionally
in relation to the optical axis by moving the specimen stage.
However, as described within the "background of the invention",
electrons impinging on any components like a Wehnelt grid, extractor 8, anode 6
or the like cause the release of molecules from the surfaces hit by the electrons.
Thereby, a shower of residual gas is created. This residual gas can be ionized by
the electron beam. Positively charged ions of the ionized residual gas are accelerated
towards emitter 15. The ions, accelerated to high energies, can either be deposited
on the electron beam emitter or mechanically deform the emitter. Thereby, the electron
beam current is influenced by these ions and current noise and/or damage of the
Thus, before the intended use described above, a pretreatment
of the electron beam device is required. Thereby, cleaning emitter 16 is used to
produce electrons. These electrons also impinge on the surfaces of Wehnelt cylinder
9, extractor 8, anode 6 and other components. The residual gas created can be pumped
out of the device with vacuum pumps. In contrast to the intended use, electron beam
emitter 15 is not negatively biased during the cleaning operation. Thus, ions from
the ionized residual gas are not accelerated directly towards electron beam emitter
Compared to the known solution previously described with
respect to Fig. 4, cleaning emitter 16 does not require extra space within the device.
Additionally, electrons, which are emitted by the cleaning emitter to clean the
surfaces, do travel approximately along optical axis 1. Thereby, especially the
surfaces, which might be hit by the electron beam during the intended use, are cleaned.
Thus, the cleaning step concentrates on the surfaces relevant for the intended use.
This is explained in more detail with respect to Fig. 2.
Within Fig. 2, two chambers of the electron beam device are shown. This is: the
gun chamber 10a, and a further chamber 10b. In the case where an electron gun 42
(see Fig. 4) would be used, only a view of the components that might be hit by electrons
during the intended use would be cleaned. If electron gun 42 was located similarly
to Fig. 4, e.g. the upper side of extractor 8 and aperture 7 would not be cleaned
by direct impingement of electrons. Further, cleaning of components outside of gun
chamber 10a is more difficult. The surfaces of extractor 8, anode 6, aperture 7
and the like are particularly critical concerning the creation of residual gas during
the intended use.
Thus, positioning the cleaning emitter approximately on
the optical axis makes a cleaning of all relevant surfaces possible. Thereby, the
term approximately is to be understood as positioning the cleaning emitter substantially
on optical axis 1. Small lateral displacements from the optical axis preferably
below 2 mm, even more preferably below 0.5mm are used.
The position of cleaning emitter 16 in Figs. 1 and 2 is
above electron beam emitter 15. Thus, from a perspective of the specimen towards
the emitters, the cleaning emitter is located behind the electron beam emitter for
the intended use. Further, from a perspective of the electron beam emitter 15 (12+14)
towards the specimen, cleaning emitter 16 would be located behind the electron beam
emitter. Thus, the expression "cleaning emitter 16 is located behind electron beam
emitter 15" is to be understood such that the two emitters are positioned with respect
to each other approximately along one axis, whereby an electron beam emitted towards
the specimen during the intended use does not need to pass the other emitter, whereas
electrons emitted towards the specimen during cleaning have to pass the other emitter.
As already describe above, before the intended use, a cleaning
step making use of cleaning emitter 16 is conducted. This is not to be understood
as conducting the cleaning step before every measurement or patterning operation,
but as conducting the cleaning step e.g. once after assembly, after maintenance
or after any operation that could introduce contamination to the interior of the
device before the intended use starts. Additionally, the cleaning step could be
used on a regular basis, e.g. every 100 to 1000 measurements, before the regular
use starts. Thus, the cleaning step is only required before the intended use in
the case where the surfaces hit by the electron beam during intended use are contaminated
with molecules, atoms, ions or the like. However, according to a different usage
of the cleaning emitter, the cleaning process can be in operation all the time.
Alternatively, the cleaning process can only be disabled during the intended use
of the charged particle device. Thus, an improved cleanness can be achieved. In
case the cleaning is disabled during the intended use, acceleration of ions or ionized
molecules from the residual gas towards e.g. a field emitter tip can be avoided.
As already described above, within the cleaning step, cleaning
emitter 16 emits electrons which impinge on the surfaces of the electron beam device,
namely, the Wehnelt grid 9, the extractor 8, anode 6, parts of the housing or the
like. Thereby, a shower of residual gas is released from the respective surfaces.
This residual gas is then pumped out of the gun chamber 10a, chamber 10b or other
chambers of the device.
Thereby, a further aspect has to be considered. As shown
in Fig. 2, the electron beam emitter (12+14) for intended use is a field emitter
comprising wire 12 and field emitter tip 14. Generally, field emitter tip 14 is
welded to wire 12. For field emission, a high potential is applied to the field
emitter. Due to the small radius of curvature of the emitter tip, high electrical
fields are obtained. Thereby, electrons can escape from the emitter tip surface.
However, these high electrical fields would also act on ionized atoms or molecules,
in the case of an insufficient cleaning of the chamber. Due to the small radius
of curvature, the ionized atoms and molecules would be focused on the field emitter
tip. Thus, the presented cleaning embodiment and the method of operating thereof
is especially valuable for field emitters. During the operation of cleaning emitter
16, field emitter (12+14) does not need to be biased. Thus, there is no focusing
force towards emitter tip 14 acting on the ionized atoms and molecules. However,
the present invention is not limited to the usage of the cleaning unit for devices
with field emitters.
A further aspect to be considered refers to the biasing
of the components that are surrounding the emitters. According to one usage of the
cleaning unit, the emitter, the Wehnelt grid 9, the extractor 8 and the aperture
6 are biased such that electrons from the cleaning emitter are accelerated less
as compared to the intended use. Thereby, an acceleration of ionized atoms and molecules
is also reduced. Thus, damage introduced by the ionized atoms and molecules during
the cleaning operation can be reduced. In view of the above, the usage of a thermionic
emitter as a cleaning emitter might be advantageous, since a thermionic emitter
can be biased to voltages about one magnitude below the voltages required for field
However, according to another usage of the cleaning unit,
the respective components are biased similarly as compared to the conditions during
intended use. Thereby, the electrons of cleaning emitter 16 are guided through the
column on substantially the same path and with essentially the same beam shape as
the electron beam during the intended use, since the voltages applied to the respective
components influence the electron beam path and the electron beam shape. Thus, applying
similar voltages and/or control signals to the respective components increases the
cleaning of those surfaces that are hit by electrons during the intended use. Thus,
it can also be advantageous if the cleaning emitter is biased to voltages, which
are comparable to e.g. the voltage of a field emitter 14 applied during intended
On the one hand, the two usages described above can be
chosen depending on the needs for best intended use. On the other hand, the two
usages relating to the voltages applied to the respective components can be combined
such that first a cleaning of a large surface area is conducted and afterwards a
cleaning of the surfaces most relevant for the intended use is conducted.
An embodiment of an electron emitter-cleaning filament
module is described with respect to Figs. 3a and 3b. Fig. 3a shows a side view of
an emitter module. The emitter module comprises an carrier body 32. The carrier
body comprises insulating material to provide an insulation for the feedthrough
34 of the high voltage wires. The high voltage for the electron beam emitter 15
and the cleaning emitter 16 is applied to contact pins 33. These contact pins are
connected to wire 12 of the electron beam emitter 15 and to the filament of cleaning
emitter 16. As shown in Fig. 3b, the electron beam emitter 15 for the intended use
is a field emitter with field emitter tip 14 connected to wire 12. Cleaning emitter
16 is a thermionic emitter made of tungsten, Lanthanum Hexaboride or the like. Thereby,
high currents can be achieved. Therefore, a tungsten filament has a thickness of
at least 50 µm. The filament thickness can be between 50 µm and 500 µm.
Preferably, it is between 100 µm and 200 µm. Thus, the filament is strong
enough to allow heating to high temperatures as compared to other usages of tungsten
filaments. The high currents achieved thereby enable a fast cleaning.
As can be seen from Fig. 3b, the tip of filament 16 and
the field emitter tip 14 are positioned closely together. Thereby, the aspect of
cleaning essentially the same surfaces during the cleaning process as compared to
the surfaces hit by electrons during intended use can be further improved. However,
the heat emitted by filament 16 during cleaning has to be considered. Since field
emitter tip 14 is generally welded to wire 12, the exposure to heat might damage
this connection. Thus, the distance between the tip of cleaning filament 16 and
the field emitter tip 14 should be between 200 µm and 5 mm and is preferably
between 500 µm and 2 mm.
Examples of cleaning emitter modules which are useful for
understanding the invention are described with respect to Figs. 3c to 3e. Fig. 3c
shows an example comparable to Fig3a. However, the tungsten wire of cleaning emitter
16 is formed such that two tips adjacent to electron beam emitter 15 are formed.
Thus, in contrast to the embodiment of Fig. 3a the cleaning electrons are not emitted
behind electron beam emitter 15, but are emitted next to the electron beam emitter.
In spite of the different geometry, all aspects relating to distances between the
two emitters and to distances between the optical axis and the cleaning emitters
The same applies for the example shown in Fig. 3d. Therein,
two cleaning emitters 16 are provided. Comparable to Fig. 3c, the cleaning emitters
16 are located next to electron beam emitter 15. In contrast to Fig. 3c, the two
cleaning emitters are biased via separate wires.
A further example is shown in Fig. 3e. Therein, two modifications
as compared to the previous embodiments and examples are illustrated. These modifications,
namely the construction of carrier body and the arrangement of cleaning emitter
16, can be combined independent of each other with other examples described within
In Fig. 3e, carrier body 32 comprises three components
32a, 32b and 32c. However, these components are arranged such that they form one
carrier body, which can e.g. during maintenance of an electron beam device be replaced
in one piece. Thus, the carrier body can be made of a single piece unit with feed
throughs 34 for e.g. the emitter wire, can be made of several pieces with feed throughs
or can be made of several pieces with feed throughs at the intersection of the several
In Fig. 3e, cleaning emitter 16 is provided in the form
of a ring around field emitter tip 14. Thus, cleaning electrons can be emitted over
a wide area in the vicinity of the optical axis. Thus, many cleaning electrons can
be provided close to the optical axis. The radius of the cleaning emitter ring can
be in the range of 100 µm to 2 mm and is preferably between 200 µm and
The terms and expressions which have been employed herein
are used as terms of description and not of limitation, and there is no intention,
in the use of such terms and expressions, of excluding any equivalents of the features
shown and described or portions thereof. Having thus described the invention in
detail, it should be apparent that various modifications can be made in the present
invention without departing from the scope of the following claims.
Generally, providing a cleaning filament integrated in
the same emitter module used for the charged particle emission during the intended
use provides an improved cleaning over known solutions.